Co-reporter:Mikaela Görlin, Jorge Ferreira de Araújo, Henrike Schmies, Denis Bernsmeier, Sören Dresp, Manuel Gliech, Zenonas Jusys, Petko Chernev, Ralph Kraehnert, Holger Dau, and Peter Strasser
Journal of the American Chemical Society 2017 Volume 139(Issue 5) pp:2070-2082
Publication Date(Web):January 12, 2017
DOI:10.1021/jacs.6b12250
Ni–Fe oxyhydroxides are the most active known electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes and are therefore of great scientific and technological importance in the context of electrochemical energy conversion. Here we uncover, investigate, and discuss previously unaddressed effects of conductive supports and the electrolyte pH on the Ni–Fe(OOH) catalyst redox behavior and catalytic OER activity, combining in situ UV–vis spectro-electrochemistry, operando electrochemical mass spectrometry (DEMS), and in situ cryo X-ray absorption spectroscopy (XAS). Supports and pH > 13 strongly enhanced the precatalytic voltammetric charge of the Ni–Fe oxyhydroxide redox peak couple, shifted them more cathodically, and caused a 2–3-fold increase in the catalytic OER activity. Analysis of DEMS-based faradaic oxygen efficiency and electrochemical UV–vis traces consistently confirmed our voltammetric observations, evidencing both a more cathodic O2 release and a more cathodic onset of Ni oxidation at higher pH. Using UV–vis, which can monitor the amount of oxidized Ni+3/+4 in situ, confirmed an earlier onset of the redox process at high electrolyte pH and further provided evidence of a smaller fraction of Ni+3/+4 in mixed Ni–Fe centers, confirming the unresolved paradox of a reduced metal redox activity with increasing Fe content. A nonmonotonic super-Nernstian pH dependence of the redox peaks with increasing Fe content—displaying Pourbaix slopes as steep as −120 mV/pH—suggested a two proton–one electron transfer. We explain and discuss the experimental pH effects using refined coupled (PCET) and decoupled proton transfer–electron transfer (PT/ET) schemes involving negatively charged oxygenate ligands generated at Fe centers. Together, we offer new insight into the catalytic reaction dynamics and associated catalyst redox chemistry of the most important class of alkaline OER catalysts.
Co-reporter:Camillo Spöri;Jason Tai Hong Kwan;Dr. Arman Bonakdarpour; Dr. David P. Wilkinson; Dr. Peter Strasser
Angewandte Chemie 2017 Volume 129(Issue 22) pp:6088-6117
Publication Date(Web):2017/05/22
DOI:10.1002/ange.201608601
AbstractDieser Aufsatz befasst sich mit den technischen Herausforderungen, wissenschaftlichen Grundlagen, neuen Entwicklungen und Perspektiven auf dem Gebiet der Stabilität und Degradation von OER-Katalysatoren (OER=Sauerstoffentwicklungsreaktion) an Elektrolyseuranoden in saurer Umgebung. Vorrangig wird der Betrieb auf Basis von Membran-Elektrode-Einheiten betrachtet. Zuerst wird der Begriff “Katalysatorstabilität” diskutiert; weitere Themen sind die aktuellen Leistungsziele sowie die Hauptmechanismen der Katalysatordegradation und Strategien zu deren Verminderung. Anschließend werden geeignete In-situ-Methoden für die Untersuchung der Katalysatordegradation bewertet und Entwicklungen bei der Abstimmung der OER-Katalysatorstabilität beschrieben. Abschließend wird die Bedeutung allgemeingültiger Kennzahlen für die Stabilität diskutiert, und es wird ein umfassender beschleunigter Alterungstest vorgeschlagen, der vergleichbare Leistungsdaten für verschiedene Laborbedingungen und Katalysatortypen liefert. Ziel ist es, die Beziehungen zwischen Struktur, Zusammensetzung und Stabilität von OER-Katalysatoren bei verschiedenen Betriebsbedingungen aufzuzeigen.
Co-reporter:Dr. Nina Erini;Vera Beermann;Martin Gocyla;Manuel Gliech;Dr. Marc Heggen;Dr. Rafal E. Dunin-Borkowski; Peter Strasser
Angewandte Chemie 2017 Volume 129(Issue 23) pp:6633-6638
Publication Date(Web):2017/06/01
DOI:10.1002/ange.201702332
AbstractDirect ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO2. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3Ni1Rhx nanoparticles plays a crucial mechanistic role for this behavior.
Co-reporter:Dr. Nina Erini;Vera Beermann;Martin Gocyla;Manuel Gliech;Dr. Marc Heggen;Dr. Rafal E. Dunin-Borkowski; Peter Strasser
Angewandte Chemie International Edition 2017 Volume 56(Issue 23) pp:6533-6538
Publication Date(Web):2017/06/01
DOI:10.1002/anie.201702332
AbstractDirect ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO2. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3Ni1Rhx nanoparticles plays a crucial mechanistic role for this behavior.
Co-reporter:Peter Strasser
Accounts of Chemical Research 2016 Volume 49(Issue 11) pp:2658
Publication Date(Web):October 24, 2016
DOI:10.1021/acs.accounts.6b00346
Nanomaterial science and electrocatalytic science have entered a successful “nanoelectrochemical” symbiosis, in which novel nanomaterials offer new frontiers for studies on electrocatalytic charge transfer, while electrocatalytic processes give meaning and often practical importance to novel nanomaterial concepts. Examples of this fruitful symbiosis are dealloyed core–shell nanoparticle electrocatalysts, which often exhibit enhanced kinetic charge transfer rates at greatly improved atom-efficiency. As such, they represent ideal electrocatalyst architectures for the acidic oxygen reduction reaction to water (ORR) and the acidic oxygen evolution reaction from water (OER) that require scarce Pt- and Ir-based catalysts. Together, these two reactions constitute the “O-cycle”, a key elemental process loop in the field of electrochemical energy interconversion between electricity (free electrons) and molecular bonds (H2O/O2), realized in the combination of water electrolyzers and hydrogen/oxygen fuel cells.In this Account, we describe our recent efforts to design, synthesize, understand, and test noble metal-poor dealloyed Pt and Ir core–shell nanoparticles for deployment in acidic polymer electrolyte membrane (PEM) electrolyzers and PEM fuel cells. Spherical dealloyed Pt core–shell particles, derived from PtNi3 precursor alloys, showed favorable ORR activity. More detailed size–activity correlation studies further revealed that the 6–8 nm diameter range is a most desirable initial particle size range in order to maximize the particle Ni content after ORR testing and to preserve performance stability. Similarly, dealloyed and oxidized IrOx core–shell particles derived from Ni-rich Ir–Ni precursor particles proved highly efficient oxygen evolution reaction (OER) catalysts in acidic conditions. In addition to the noble metal savings in the particle cores, the Pt core–shell particles are believed to benefit in terms of their mass-based electrochemical kinetics from surface lattice strain effects that tune the adsorption energies and barriers of elementary steps. The molecular mechanism of the kinetic benefit of the dealloyed IrOx particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrOx shells that generate preactive oxygen centers.
Co-reporter:Vera Beermann, Martin Gocyla, Elena Willinger, Stefan Rudi, Marc Heggen, Rafal E. Dunin-Borkowski, Marc-Georg Willinger, and Peter Strasser
Nano Letters 2016 Volume 16(Issue 3) pp:1719-1725
Publication Date(Web):February 8, 2016
DOI:10.1021/acs.nanolett.5b04636
Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt–Ni octahedral nanoparticles with high activities up to 1.14 A mgPt–1 combined with improved performance and shape stability compared to previous bimetallic Pt–Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt–Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt–Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt–Ni nanoparticles.
Co-reporter:Mikaela Görlin; Petko Chernev; Jorge Ferreira de Araújo; Tobias Reier; Sören Dresp; Benjamin Paul; Ralph Krähnert; Holger Dau
Journal of the American Chemical Society 2016 Volume 138(Issue 17) pp:5603-5614
Publication Date(Web):March 31, 2016
DOI:10.1021/jacs.6b00332
Mixed Ni–Fe oxides are attractive anode catalysts for efficient water splitting in solar fuels reactors. Because of conflicting past reports, the catalytically active metal redox state of the catalyst has remained under debate. Here, we report an in operando quantitative deconvolution of the charge injected into the nanostructured Ni–Fe oxyhydroxide OER catalysts or into reaction product molecules. To achieve this, we explore the oxygen evolution reaction dynamics and the individual faradaic charge efficiencies using operando differential electrochemical mass spectrometry (DEMS). We further use X-ray absorption spectroscopy (XAS) under OER conditions at the Ni and Fe K-edges of the electrocatalysts to evaluate oxidation states and local atomic structure motifs. DEMS and XAS data consistently reveal that up to 75% of the Ni centers increase their oxidation state from +2 to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst under OER catalysis. The Fe centers consistently remain in the +3 state, regardless of potential and composition. For mixed Ni100–xFex catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 sharply increases from ∼30% to 90%, suggesting that Ni atoms largely remain in the oxidation state +2 under catalytic conditions. To reconcile the apparent low level of oxidized Ni in mixed Ni–Fe catalysts, we hypothesize that a kinetic competition between the (i) metal oxidation process and the (ii) metal reduction step during O2 release may account for an insignificant accumulation of detectable high-valent metal states if the reaction rate of process (ii) outweighs that of (i). We conclude that a discussion of the superior catalytic OER activity of Ni–FeOOH electrocatalysts in terms of surface catalysis and redox-inactive metal sites likely represents an oversimplification that fails to capture essential aspects of the synergisms at highly active Ni–Fe sites.
Co-reporter:Hyung-Suk Oh, Hong Nhan Nong, Tobias Reier, Arno Bergmann, Manuel Gliech, Jorge Ferreira de Araújo, Elena Willinger, Robert Schlögl, Detre Teschner, and Peter Strasser
Journal of the American Chemical Society 2016 Volume 138(Issue 38) pp:12552-12563
Publication Date(Web):August 23, 2016
DOI:10.1021/jacs.6b07199
Co-reporter:Ana Sofia Varela, Wen Ju, Tobias Reier, and Peter Strasser
ACS Catalysis 2016 Volume 6(Issue 4) pp:2136
Publication Date(Web):January 20, 2016
DOI:10.1021/acscatal.5b02550
In the present study we demonstrate that the activity and selectivity of copper during CO2 electrochemical reduction can be tuned by simply adding halides to the electrolyte. Comparing the production rate and Faradaic selectivity of the major products as a function the working potential in the presence of Cl–, Br–, and I–, we show that the activity and selectivity of Cu depends on the concentration and nature of the added halide. We find that the addition Cl– and Br– results in an increased CO selectivity. On the contrary, in the presence of I– the selectivity toward CO drops down and instead methane formation is enhanced up to 6 times compared with the halide-free electrolyte. Even though Br– and I– can induce morphology changes of the surface, the modification in the catalytic performance of Cu is mainly attributed to halides adsorption on the Cu surface. We hypothesizes that the adsorption of halides alters the catalytic performance of Cu by increasing the negative charge on the surface according to the following order: Cl– < Br– < I–. In the case of adsorbed I–, the induced negative charge has a remarkably positive effect favoring the protonation of CO. These results present an easy way to enhance CH4 production during the CO2RR on Cu. Furthermore, understanding this effect can contribute to the design of new and more efficient catalysts.Keywords: CO2 reduction; copper; electrocatalysis; electrolyte effect; halides; iodide
Co-reporter:Lin Gan, Marc Heggen, Chunhua Cui, and Peter Strasser
ACS Catalysis 2016 Volume 6(Issue 2) pp:692
Publication Date(Web):December 22, 2015
DOI:10.1021/acscatal.5b02620
We performed in situ transmission electron microscopy of phase-segregated octahedral Pt–Ni alloy fuel cell nanocatalysts under thermal annealing to study their morphological stability and surface compositional evolution. The pristine octahedral Pt–Ni nanoparticles (NPs) showed Pt-rich corners/edges and slightly concave Ni-rich {111} facets. Time-resolved image series unequivocally revealed that upon annealing up to 500 °C, the Pt-rich surface atoms at the corners/edges diffused onto and subsequently covered the concave Ni-rich {111} surfaces, leading to perfectly flat Pt-rich {111} surfaces with Ni-rich subsurface layers. This was further corroborated by in situ aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. Our results propose a feasible approach to construct shaped Pt alloy nanoparticles with Pt-rich {111} surfaces and Ni-rich subsurface layers that are expected to be catalytically active and stable for the oxygen reduction reaction, thus providing important implications for rational synthesis of durably highly active shaped Pt alloy fuel cell electrocatalysts.Keywords: concave nanoparticles; in situ TEM; octahedral Pt−Ni nanoparticles; oxygen reduction reaction; surface composition; thermal annealing
Co-reporter:Prashanth W. Menezes, Arindam Indra, Arno Bergmann, Petko Chernev, Carsten Walter, Holger Dau, Peter Strasser and Matthias Driess
Journal of Materials Chemistry A 2016 vol. 4(Issue 25) pp:10014-10022
Publication Date(Web):06 Jun 2016
DOI:10.1039/C6TA03644A
The fabrication and design of earth-abundant and high-performance catalysts for the oxygen evolution reaction (OER) are very crucial for the development and commercialization of sustainable energy conversion technologies. Although spinel catalysts have been widely explored for the electrochemical oxygen evolution reaction (OER), the role of two geometrical sites that influence their activities has not been well established so far. Here, we present more effective cobalt–zinc oxide catalysts for the OER than ‘classical’ Co3O4. Interestingly, the significantly higher catalytic activity of ZnCo2O4 than that of Co3O4 is somewhat surprising since both crystallize in the spinel-type structure. The reasons for the latter remarkable difference of ZnCo2O4 and Co3O4 could be deduced from structure–activity relationships of the bulk and near-surface of the catalysts using comprehensive electrochemical, microscopic and spectroscopic techniques with a special emphasis on the different roles of the coordination environment of metal ions (octahedral vs. tetrahedral sites) in the spinel lattice. The vital factors influencing the catalytic activity of ZnCo2O4 over Co3O4 could be directly attributed to the higher amount of accessible octahedral Co3+ sites induced by the preferential loss of zinc ions from the surface of the ZnCo2O4 catalyst. The enhanced catalytic activity is accompanied by a larger density of metal vacancies, defective sites and hydroxylation. The results obtained here clearly demonstrate how a surface structural modification and generation of defects of catalysts can enhance their OER performance.
Co-reporter:Peter Strasser, Stefanie Kühl
Nano Energy 2016 Volume 29() pp:166-177
Publication Date(Web):November 2016
DOI:10.1016/j.nanoen.2016.04.047
•This review highlights the recent advances of the preparation of dealloyed Pt nanoparticle electrocatalysts for the oxygen reduction reaction.•Furthermore, it gives an overview about their structural as well as electrocatalytic properties.•It is an invited review.Dealloyed Pt core-shell nanoparticles constitute the most active and stable bimetallic oxygen reduction catalysts for low-temperature fuel cells. Here, we review recent advances on their preparation, structural characterization, and electrocatalytic performance. Starting with bimetallic metal overlayer model systems, for which we illustrate fundamental principles of the ORR activity enhancements of dealloyed nanoparticles, we discuss progress in our understanding of structure-activity relations of dealloyed nanoparticle catalysts, both in idealized liquid-electrolyte cell formats and more realistic Membrane Electrode Assemblies (MEAs).
Co-reporter:Stefanie Kühl
Topics in Catalysis 2016 Volume 59( Issue 17-18) pp:1628-1637
Publication Date(Web):2016 October
DOI:10.1007/s11244-016-0682-z
We review the fundamental principles, the preparation and catalytic performance of dealloyed Pt core–shell electrocatalysts for the electroreduction of molecular oxygen. This reaction is key to the efficiency of all fuel cell cathodes, as the oxygen electrocatalysis exhibits much larger kinetic overpotentials compared to typical fuel cell anode reactions. We discuss structural surface lattice strain in metal overlayers and show that they serve as models for nanostructured core–shell catalysts. We address preparation pathways with particular emphasis on the dealloying routes. Trends in reactivity of different dealloyed Pt core–shell catalysts are compared with a focus on the dealloyed Pt–Ni alloy system. Size effects are discussed. Practical catalytic performance data in automotive fuel cells and under automotive fuel cell conditions is provided and contrasted to other state-of-art catalyst concepts. This review concludes that dealloyed Pt core–shell cathode catalysts are currently the most attractive commercialization candidate for automotive applications.
Co-reporter:Dr. Fabio Dionigi;Dr. Tobias Reier;Zarina Pawolek;Manuel Gliech ;Dr. Peter Strasser
ChemSusChem 2016 Volume 9( Issue 9) pp:962-972
Publication Date(Web):
DOI:10.1002/cssc.201501581
Abstract
Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.
Co-reporter:Rosa M. Arán-Ais, Fabio Dionigi, Thomas Merzdorf, Martin Gocyla, Marc Heggen, Rafal E. Dunin-Borkowski, Manuel Gliech, José Solla-Gullón, Enrique Herrero, Juan M. Feliu, and Peter Strasser
Nano Letters 2015 Volume 15(Issue 11) pp:7473-7480
Publication Date(Web):October 6, 2015
DOI:10.1021/acs.nanolett.5b03057
Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition, and shape is limited requiring trial and error. Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under coreduction “one-step” conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content, we develop a “two-step” route and track the evolution of the composition and morphology of the particles at the atomic scale. To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt1.5M, PtM, and PtM1.5 (M = Ni + Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles.
Co-reporter:Tobias Reier; Zarina Pawolek; Serhiy Cherevko; Michael Bruns; Travis Jones; Detre Teschner; Sören Selve; Arno Bergmann; Hong Nhan Nong; Robert Schlögl; Karl J. J. Mayrhofer
Journal of the American Chemical Society 2015 Volume 137(Issue 40) pp:13031-13040
Publication Date(Web):September 10, 2015
DOI:10.1021/jacs.5b07788
Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir–Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir–Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir–Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.
Co-reporter:Hyung-Suk Oh;Hong Nhan Nong
Advanced Functional Materials 2015 Volume 25( Issue 7) pp:1074-1081
Publication Date(Web):
DOI:10.1002/adfm.201401919
The M-doped tin oxides (M = Sb, F, and In) to be used as catalyst support are synthesized by using templating process with tetradecylamine (TDA) as the template, combined with a hydrothermal (HT) method to improve its thermal stability. The obtained materials are characterized by XRD, SAXS, TEM, EDX, SEM, and BET to study microstructure and physical properties, which have a mesoporous structure, small particle size, and high surface area (125–263 m2 g–1). The materials show an overall conductivity of 0.102–0.295 S cm–1. Repetitive potential cycling is employed to characterize the electrochemical properties and stability. The M-doped tin oxides are highly electrochemical stable compared to carbon black. From the observed results, it can be concluded that the combination of TDA and HT treatment are an effective synthetic method for designing mesoporous M-doped tin oxide as catalyst supports.
Co-reporter:Hyung-Suk Oh, Hong Nhan Nong, Tobias Reier, Manuel Gliech and Peter Strasser
Chemical Science 2015 vol. 6(Issue 6) pp:3321-3328
Publication Date(Web):19 Mar 2015
DOI:10.1039/C5SC00518C
Reducing the noble-metal catalyst content of acid Polymer Electrolyte Membrane (PEM) water electrolyzers without compromising catalytic activity and stability is a goal of fundamental scientific interest and substantial technical importance for cost-effective hydrogen-based energy storage. This study presents nanostructured iridium nanodendrites (Ir-ND) supported on antimony doped tin oxide (ATO) as efficient and stable water splitting catalysts for PEM electrolyzers. The active Ir-ND structures exhibited superior structural and morphological properties, such as particle size and surface area compared to commercial state-of-art Ir catalysts. Supported on tailored corrosion-stable conductive oxides, the Ir-ND catalysts exhibited a more than 2-fold larger kinetic water splitting activity compared with supported Ir nanoparticles, and a more than 8-fold larger catalytic activity than commercial Ir blacks. In single-cell PEM electrolyzer tests, the Ir-ND/ATO outperformed commercial Ir catalysts more than 2-fold at technological current densities of 1.5 A cm−2 at a mere 1.80 V cell voltage, while showing excellent durability under constant current conditions. We conclude that Ir-ND/ATO catalysts have the potential to substantially reduce the required noble metal loading, while maintaining their catalytic performance, both in idealized three-electrode set ups and in the real electrolyzer device environments.
Co-reporter:Prashanth W. Menezes, Arindam Indra, Diego González-Flores, Nastaran Ranjbar Sahraie, Ivelina Zaharieva, Michael Schwarze, Peter Strasser, Holger Dau, and Matthias Driess
ACS Catalysis 2015 Volume 5(Issue 4) pp:2017
Publication Date(Web):February 16, 2015
DOI:10.1021/cs501724v
Future advances in renewable and sustainable energy require advanced materials based on earth-abundant elements with multifunctional properties. The design and the development of cost-effective, robust, and high-performance catalysts that can convert oxygen to water, and vice versa, is a major challenge in energy conversion and storage technology. Here we report cobalt oxide nanochains as multifunctional catalysts for the electrochemical oxygen evolution reaction (OER) at both alkaline and neutral pH, oxidant-driven, photochemical water oxidation in various pH, and the electrochemical oxygen reduction reaction (ORR) in alkaline medium. The cobalt oxide nanochains are easily accessible on a multigram scale by low-temperature degradation of a cobalt oxalate precursor. What sets this study apart from earlier ones is its synoptical perspective of reversible oxygen redox catalysis in different chemical and electrochemical environments.Keywords: cobalt oxide; morphology; multifunctional catalyst; oxygen evolution; oxygen reduction; water oxidation
Co-reporter:Guang-Ping Hao, Nastaran Ranjbar Sahraie, Qiang Zhang, Simon Krause, Martin Oschatz, Alicja Bachmatiuk, Peter Strasser and Stefan Kaskel
Chemical Communications 2015 vol. 51(Issue 97) pp:17285-17288
Publication Date(Web):05 Oct 2015
DOI:10.1039/C5CC06256J
Exploring the role of surface hydrophilicity of non-precious metal N-doped carbon electrocatalysts in electrocatalysis is challenging. Herein we discover an ultra-hydrophilic non-precious carbon electrocatalyst, showing enhanced catalysis efficiency on both gravimetric and areal basis for oxygen reduction reaction due to a high dispersion of active centres.
Co-reporter:Nina Erini, Paul Krause, Manuel Gliech, Ruizhi Yang, Yunhui Huang, Peter Strasser
Journal of Power Sources 2015 Volume 294() pp:299-304
Publication Date(Web):30 October 2015
DOI:10.1016/j.jpowsour.2015.06.042
•The electrooxidation of EtOH on PtRhSn/C catalysts was investigated in acidic media.•Small metal nanoparticles with sizes in the range of 6–12 nm were synthesized.•Variation of pressure and carbon supporting conditions during polyol synthesis.•Ambient pressure conditions result in catalysts with higher EOR mass activities.The present work explores the effect of autoclave-based autogenous-pressure vs. ambient pressure conditions on the synthesis and properties of carbon-supported Pt–Rh–Sn nanoparticle electrocatalysts. The Pt–Rh–Sn nanoparticles were characterized by X-ray spectroscopy, electron microscopy and mass spectroscopy and deployed as catalysts for the electrocatalytic ethanol oxidation reaction. Pt–Rh–Sn catalysts precipitated with carbon already present showed narrow particle size distribution around 7 nm, while catalysts supported on carbon after particle formation showed broader size distribution ranging from 8 to 16 nm, similar metal loadings between 40 and 48 wt.% and similar atomic ratios of Pt:Rh:Sn of 30:10:60. The highest ethanol oxidation activity at low overpotentials associated with exceptionally early ethanol oxidation onset potential was observed for ambient-pressure catalysts with the active ternary alloy phase formed in presence of the carbon supports. In contrast, catalysts prepared under ambient pressure in a two-step approach, involving alloy particle formation followed by particle separation and subsequent deposition on the carbon support, yielded the highest overall mass activities. Based on the observed synthesis–activity correlations, a comparative assessment is provided of the synthetic techniques at high vs. low pressures, and in presence and absence of carbon support. Plausible hypotheses in terms of particle dispersion and interparticle distance accounting for these observed differences are discussed.
Co-reporter:Dr. Prashanth W. Menezes;Dr. Arindam Indra;Dr. Nastaran Ranjbar Sahraie;Arno Bergmann;Dr. Peter Strasser;Dr. Matthias Driess
ChemSusChem 2015 Volume 8( Issue 1) pp:164-171
Publication Date(Web):
DOI:10.1002/cssc.201402699
Abstract
Recently, there has been much interest in the design and development of affordable and highly efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts that can resolve the pivotal issues that concern solar fuels, fuel cells, and rechargeable metal-air batteries. Here we present the synthesis and application of porous CoMn2O4 and MnCo2O4 spinel microspheres as highly efficient multifunctional catalysts that unify the electrochemical OER with oxidant-driven and photocatalytic water oxidation as well as the ORR. The porous materials were prepared by the thermal degradation of the respective carbonate precursors at 400 °C. The as-prepared spinels display excellent performances in electrochemical OER for the cubic MnCo2O4 phase in comparison to the tetragonal CoMn2O4 material in an alkaline medium. Moreover, the oxidant-driven and photocatalytic water oxidations were performed and they exhibited a similar trend in activity to that of the electrochemical OER. Remarkably, the situation is reversed in ORR catalysis, that is, the oxygen reduction activity and stability of the tetragonal CoMn2O4 catalyst outperformed that of cubic MnCo2O4 and rivals that of benchmark Pt catalysts. The superior catalytic performance and the remarkable stability of the unifying materials are attributed to their unique porous and robust microspherical morphology and the intrinsic structural features of the spinels. Moreover, the facile access to these high-performance materials enables a reliable and cost-effective production on a large scale for industrial applications.
Co-reporter:Nina Erini;Stefan Rudi;Vera Beermann;Paul Krause; Ruizhi Yang; Yunhui Huang; Peter Strasser
ChemElectroChem 2015 Volume 2( Issue 6) pp:903-908
Publication Date(Web):
DOI:10.1002/celc.201402390
Abstract
Alloying Pt with highly oxophilic transition metals such as Rh, Ni, or Sn has been a promising strategy to modify the electrocatalytic surface properties of Pt in order to supply active oxygen-containing species for ethanol electrooxidation. A new, highly active, ternary single-phased fcc PtRhNi/C nanoparticle electrocatalyst for the electrocatalytic oxidation of ethanol (EOR) is reported and its morphology (XRD and TEM), composition (inductively coupled plasma optical emission spectroscopy), and electrochemical activity are discussed in comparison with the state-of-art PtRhSn/C electrocatalyst. The EOR activity of the PtRhNi/C material outperformed the benchmark PtRhSn/C material in acidic and alkaline media, showing high stability, especially in alkaline media. The higher intrinsic EOR activity of the Ni-containing electrocatalyst lends support to the notion that surface NiOx is an excellent oxygenate-supplying catalyst component for the oxidation of ethanol.
Co-reporter:Dr. Ana Sofia Varela;Dr. Nastaran RanjbarSahraie;Julian Steinberg;Wen Ju;Dr. Hyung-Suk Oh ;Dr. Peter Strasser
Angewandte Chemie International Edition 2015 Volume 54( Issue 37) pp:10758-10762
Publication Date(Web):
DOI:10.1002/anie.201502099
Abstract
This study explores the kinetics, mechanism, and active sites of the CO2 electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon-based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO2 to CO/H2 mixtures outperforming a low-area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low-cost, abundant electrocatalysts for synthetic fuel production from CO2.
Co-reporter:Dr. Ana Sofia Varela;Dr. Nastaran RanjbarSahraie;Julian Steinberg;Wen Ju;Dr. Hyung-Suk Oh ;Dr. Peter Strasser
Angewandte Chemie 2015 Volume 127( Issue 37) pp:10908-10912
Publication Date(Web):
DOI:10.1002/ange.201502099
Abstract
This study explores the kinetics, mechanism, and active sites of the CO2 electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon-based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO2 to CO/H2 mixtures outperforming a low-area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low-cost, abundant electrocatalysts for synthetic fuel production from CO2.
Co-reporter:Nastaran Ranjbar Sahraie ; Jens Peter Paraknowitsch ; Caren Göbel ; Arne Thomas
Journal of the American Chemical Society 2014 Volume 136(Issue 41) pp:14486-14497
Publication Date(Web):September 15, 2014
DOI:10.1021/ja506553r
The synthesis and characterization of functionalized carbon using variable doping profiles are presented. The hybrids were obtained from nitrile-functionalized ionic precursors and a ferric chloride mediator. This way, novel nitrogen doped and nitrogen–sulfur, nitrogen–phosphorus, and nitrogen–boron codoped carbon hybrids with a morphology containing microporous nanometer-sized particles were obtained. As-prepared heteroatom doped carbons exhibited superior electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline and acid electrolytes. In particular, both the heteroatom type and iron were found to play crucial roles in improving the catalytic activity of functionalized carbon. It is worth noting that sulfur–nitrogen codoped functionalized materials synthesized in the presence of ferric chloride showed higher activity and stability in comparison to those of the commercial state-of-the-art Pt catalyst in alkaline electrolyte. Moreover, in acid electrolyte, sulfur–nitrogen codoped catalyst rivaled the activity of Pt with a stability outperforming that of Pt. Our X-ray photoelectron spectroscopy (XPS) investigation revealed a distinctive atomic structure in nitrogen–sulfur codoped material in comparison to other codoped catalysts, most likely explaining its superior electrocatalytic activity. This work presents a novel toolbox for designing advanced carbon hybrids with variable heteroatom doping profiles which presents tunable and enhanced ORR performance.
Co-reporter:Chunhua Cui ; Lin Gan ; Maximilian Neumann ; Marc Heggen ; Beatriz Roldan Cuenya
Journal of the American Chemical Society 2014 Volume 136(Issue 13) pp:4813-4816
Publication Date(Web):March 4, 2014
DOI:10.1021/ja4124658
Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt–Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations.
Co-reporter:Rulle Reske ; Hemma Mistry ; Farzad Behafarid ; Beatriz Roldan Cuenya
Journal of the American Chemical Society 2014 Volume 136(Issue 19) pp:6978-6986
Publication Date(Web):April 18, 2014
DOI:10.1021/ja500328k
A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2–15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity–selectivity–size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (∼2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification.
Co-reporter:Hemma Mistry ; Rulle Reske ; Zhenhua Zeng ; Zhi-Jian Zhao ; Jeffrey Greeley ; Peter Strasser ;Beatriz Roldan Cuenya
Journal of the American Chemical Society 2014 Volume 136(Issue 47) pp:16473-16476
Publication Date(Web):October 17, 2014
DOI:10.1021/ja508879j
The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1–8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles.
Co-reporter:Arindam Indra ; Prashanth W. Menezes ; Nastaran Ranjbar Sahraie ; Arno Bergmann ; Chittaranjan Das ; Massimo Tallarida ; Dieter Schmeißer ; Peter Strasser ;Matthias Driess
Journal of the American Chemical Society 2014 Volume 136(Issue 50) pp:17530-17536
Publication Date(Web):November 21, 2014
DOI:10.1021/ja509348t
Catalytic water splitting to hydrogen and oxygen is considered as one of the convenient routes for the sustainable energy conversion. Bifunctional catalysts for the electrocatalytic oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are pivotal for the energy conversion and storage, and alternatively, the photochemical water oxidation in biomimetic fashion is also considered as the most useful way to convert solar energy into chemical energy. Here we present a facile solvothermal route to control the synthesis of amorphous and crystalline cobalt iron oxides by controlling the crystallinity of the materials with changing solvent and reaction time and further utilize these materials as multifunctional catalysts for the unification of photochemical and electrochemical water oxidation as well as for the oxygen reduction reaction. Notably, the amorphous cobalt iron oxide produces superior catalytic activity over the crystalline one under photochemical and electrochemical water oxidation and oxygen reduction conditions.
Co-reporter:Hong Nhan Nong, Lin Gan, Elena Willinger, Detre Teschner and Peter Strasser
Chemical Science 2014 vol. 5(Issue 8) pp:2955-2963
Publication Date(Web):23 May 2014
DOI:10.1039/C4SC01065E
A family of dealloyed metal–oxide hybrid (M1M2@M1Ox) core@shell nanoparticle catalysts is demonstrated to provide substantial advances toward more efficient and less expensive electrolytic water splitting. IrNi@IrOx nanoparticles were synthesized from IrNix precursor alloys through selective surface Ni dealloying and controlled surface oxidation of Ir. Detailed depth-resolved insight into chemical structure, composition, morphology, and oxidation state was obtained using spectroscopic, diffraction, and scanning microscopic techniques (XANES, XRD, STEM-EDX, XPS), which confirmed our structural hypotheses at the outset. A 3-fold catalytic activity enhancement for the electrochemical oxygen evolution reaction (OER) over IrO2 and RuO2 benchmark catalysts was observed for the core-shell catalysts on a noble metal mass basis. Also, the active site-based intrinsic turnover frequency (TOF) was greatly enhanced for the most active IrNi@IrOx catalyst. This study documents the successful use of synthetic dealloying for the preparation of metal-oxide hybrid core-shell catalysts. The concept is quite general, can be applied to other noble metal nanoparticles, and points out a path forward to nanostructured proton-exchange-electrolyzer electrodes with dramatically reduced noble metal content.
Co-reporter:Nina Erini, Rameshwori Loukrakpam, Valeri Petkov, Elena A. Baranova, Ruizhi Yang, Detre Teschner, Yunhui Huang, Stanko R. Brankovic, and Peter Strasser
ACS Catalysis 2014 Volume 4(Issue 6) pp:1859
Publication Date(Web):April 25, 2014
DOI:10.1021/cs500147p
Novel insights in the synthesis–structure–catalytic activity relationships of nanostructured trimetallic Pt–Rh–Sn electrocatalysts for the electrocatalytic oxidation of ethanol are reported. In particular, we identify a novel single-phase Rh-doped Pt–Sn Niggliite mineral phase as the source of catalytically active sites for ethanol oxidation; we discuss its morphology, composition, chemical surface state, and the detailed 3D atomic arrangement using high-energy (HE-XRD), atomic pair distribution function (PDF) analysis, and X-ray photoelectron spectroscopy (XPS). The intrinsic ethanol oxidation activity of the active Niggliite phase exceeded those of earlier reports, lending support to the notion that the atomic-scale neighborhood of Pt, Rh, and Sn is conducive to the emergence of active surface catalytic sites under reaction conditions. In situ mechanistic Fourier transform infrared (in situ FTIR) analysis confirms an active 12 electron oxidation reaction channel to CO2 at electrode potentials as low as 450 mV/RHE, demonstrating the favorable efficiency of the PtRhSn Niggliite phase for C–C bond splitting.Keywords: electrocatalysis; electrochemistry; ethanol oxidation reaction; fuel cells; HE-XRD; in situ FTIR; PDF analysis
Co-reporter:Panagiotis Trogadas, Thomas F. Fuller, Peter Strasser
Carbon 2014 Volume 75() pp:5-42
Publication Date(Web):August 2014
DOI:10.1016/j.carbon.2014.04.005
Carbon has unique characteristics that make it an ideal material for use in a wide variety of electrochemical applications ranging from metal refining to electrocatalysis and fuel cells. In polymer electrolyte fuel cells (PEFCs), carbon is used as a gas diffusion layer, electrocatalyst support and oxygen reduction reaction (ORR) electrocatalyst. When used as electrocatalyst support, amorphous carbonaceous materials suffer from enhanced oxidation rates at high potentials over time. This drawback has prompted an extensive effort to improve the properties of amorphous carbon and to identify alternate carbon-based materials to replace carbon blacks. Alternate support materials are classified in carbon nanotubes and fibers, mesoporous carbon, multi-layer graphene (undoped and doped with metal nanoparticles) and reduced graphene oxide. A comparative review of all these supports is provided. Work on catalytically active carbon hybrids is focused on the development of non-precious metal electrocatalysts that will significantly reduce the cost without sacrificing catalytic activity. Of the newer electrocatalysts, nitrogen/metal-functionalized carbons and composites are emerging as possible contenders for commercial PEFCs. Nitrogen-doped carbon hybrids with transition metals and their polymer composites exhibit high ORR activity and selectivity and these catalytic properties are presented in detail in this review.
Co-reporter:Serhiy Cherevko, Tobias Reier, Aleksandar R. Zeradjanin, Zarina Pawolek, Peter Strasser, Karl J.J. Mayrhofer
Electrochemistry Communications 2014 Volume 48() pp:81-85
Publication Date(Web):November 2014
DOI:10.1016/j.elecom.2014.08.027
•In-situ dissolution analysis by ICP-MS coupled to an electrochemical flow cell•Quantification of dissolved iridium catalyst and titanium support during OER•Activity and stability of electrocatalyst depend on nature of Ir oxide species.•There is no simple relationship between activity for OER and stability.The electrochemical stability of thermally prepared Ir oxide films is investigated using a scanning flow cell (SFC)–inductively coupled plasma mass-spectrometer (ICP-MS) setup under transient and stationary potential and/or current conditions. Time-resolved dissolution rates provide important insights into critical conditions for material breakdown and a fully quantitative in-situ assessment of the electrochemical stability during oxygen evolution reaction (OER) conditions. In particular, the results demonstrate that stability and OER activity of the IrOx catalysts strongly depend on the chemical and structural nature of Ir oxide species and their synthesis conditions.Figure optionsDownload full-size imageDownload as PowerPoint slide
Co-reporter:Rameshwori Loukrakpam, Qiuyi Yuan, Valeri Petkov, Lin Gan, Stefan Rudi, Ruizhi Yang, Yunhui Huang, Stanko R. Brankovic and Peter Strasser
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 35) pp:18866-18876
Publication Date(Web):23 Jul 2014
DOI:10.1039/C4CP02791D
Efficient catalytic C–C bond splitting coupled with complete 12-electron oxidation of the ethanol molecule to CO2 is reported on nanoscale electrocatalysts comprised of a Pt monolayer (ML) and sub-monolayer (sML) deposited on Au nanoparticles (Au@Pt ML/sML). The Au@Pt electrocatalysts were synthesized using surface limited redox replacement (SLRR) of an underpotentially deposited (UPD) Cu monolayer in an electrochemical cell reactor. Au@Pt ML showed improved catalytic activity for ethanol oxidation reaction (EOR) and, unlike their Pt bulk and Pt sML counterparts, was able to generate CO2 at very low electrode potentials owing to efficient C–C bond splitting. To explain this, we explore the hypothesis that competing strain effects due to the Pt layer coverage/morphology (compressive) and the Pt–Au lattice mismatch (tensile) control surface chemisorption and overall activity. Control experiments on well-defined model Pt monolayer systems are carried out involving a wide array of methods such as high-energy X-ray diffraction, pair-distribution function (PDF) analysis, in situ electrochemical FTIR spectroscopy, and in situ scanning tunneling microscopy. The vibrational fingerprints of adsorbed CO provide compelling evidence on the relation between surface bond strength, layer strain and morphology, and catalytic activity.
Co-reporter:Aleksandar R. Zeradjanin, Nadine Menzel, Wolfgang Schuhmann and Peter Strasser
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 27) pp:13741-13747
Publication Date(Web):31 Mar 2014
DOI:10.1039/C4CP00896K
The faradaic selectivity of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on the industrially important Ti–Ru–Ir mixed metal oxide is discussed. Absolute evolution rates as well as volume fractions of Cl2 and O2 were quantified using differential electrochemical mass spectrometry (DEMS), while the catalyst surface redox behavior was analyzed using cyclic voltammetry. The spatial inhomogeneity of the surface catalytic reaction rate was probed using Scanning Electrochemical Microscopy (SECM). Although the nature of the competition between electrochemical discharging of chloride ions and water molecules remains elusive on a molecular scale, new insights into the spatial reactivity distribution of the CER and OER were obtained. Oxidation of water is the initial step in corrosion and concomitant deactivation of the oxide electrodes; however, at the same time the nature of interaction between the oxide surface and water is used as a rational indicator of selectivity and catalytic activity. An experimental procedure was established that would allow the study of selectivity of a variety of different catalyst materials using polycrystalline electrode surfaces.
Co-reporter:Stefan Rudi;Chunhua Cui;Lin Gan
Electrocatalysis 2014 Volume 5( Issue 4) pp:408-418
Publication Date(Web):2014 October
DOI:10.1007/s12678-014-0205-2
This study intends to provide some insight in the up-to-date elusive assessment of a correct choice of method for estimating the active surface area of Pt alloy nanoparticle catalysts. Taking PtNi3 nanoparticles as an example, we have compared three types of electrochemically active surface area (ECSA) data, CO-ECSA, Hupd-ECSA, and Hupd/CO-ECSA, which were evaluated from CO stripping and underpotentially deposited hydrogen stripping steps applied at different times along a reference catalyst activity test protocol. Considering a total of six different detailed voltammetric test protocols, we address Pt alloy particle size effects, analyze the effect of the time of application of CO and hydrogen stripping, and study their effect on the Pt mass and Pt surface-specific activities for the oxygen reduction reaction (ORR). In a discussion of the ratio of CO charge to hydrogen charge, it is shown that this quantity is more complex than previously thought and not associated with a specific surface structure. The Hupd/CO-ECSA data are found to be a reasonable balance for the estimate of surface area normalized, so-called specific catalytic ORR activities.
Co-reporter:Lin Gan;Chunhua Cui;Marc Heggen;Stefan Rudi;Fabio Dionigi
Science 2014 Volume 346(Issue 6216) pp:1502-1506
Publication Date(Web):19 Dec 2014
DOI:10.1126/science.1261212
Abstract
Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.
Co-reporter:Lin Gan, Marc Heggen, Rachel O’Malley, Brian Theobald, and Peter Strasser
Nano Letters 2013 Volume 13(Issue 3) pp:1131-1138
Publication Date(Web):January 29, 2013
DOI:10.1021/nl304488q
Nanoporosity is a frequently reported phenomenon in bimetallic particle ensembles used as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. It is generally considered a favorable characteristic, because it increases the catalytically active surface area. However, the effect of nanoporosity on the intrinsic activity and stability of a nanoparticle electrocatalyst has remained unclear. Here, we present a facile atmosphere-controlled acid leaching technique to control the formation of nanoporosity in Pt–Ni bimetallic nanoparticles. By statistical analysis of particle size, composition, nanoporosity, and atomic-scale core–shell fine structures before and after electrochemical stability test, we uncover that nanoporosity formation in particles larger than ca. 10 nm is intrinsically tied to a drastic dissolution of Ni and, as a result of this, a rapid drop in intrinsic catalytic activity during ORR testing, translating into severe catalyst performance degradation. In contrast, O2-free acid leaching enabled the suppression of nanoporosity resulting in more solid core–shell particle architectures with thin Pt-enriched shells; surprisingly, such particles maintained high intrinsic activity and improved catalytic durability under otherwise identical ORR tests. On the basis of these findings, we suggest that catalytic stability could further improve by controlling the particle size below ca. 10 nm to avoid nanoporosity. Our findings provide an explanation for the degradation of bimetallic particle ensembles and show an easy to implement pathway toward more durable fuel cell cathode catalysts.
Co-reporter:Dr. Lin Gan;Stefan Rudi;Dr. Chunhua Cui ;Dr. Peter Strasser
ChemCatChem 2013 Volume 5( Issue 9) pp:2691-2694
Publication Date(Web):
DOI:10.1002/cctc.201300235
Abstract
Thermal annealing is an important and widely adopted step during the synthesis of Pt bimetallic fuel-cell catalysts, although it faces the inevitable drawback of particle sintering. Understanding this sintering mechanism is important for the future development of highly active and robust fuel-cell catalysts. Herein, we studied the particle sintering during the thermal annealing of carbon-supported Pt1–xNix (PtNi, PtNi3, and PtNi5) nanoparticles, a reported recently class of highly active fuel-cell catalysts. By using high-resolution transmission electron microscopy, we found that annealing at an intermediate temperature (400 °C) effectively increased the extent of alloying without particle sintering; however, high-temperature annealing (800 °C) caused severe particle sintering, which, unexpectedly, was strongly dependent on the composition of the alloy, thus showing that a higher Ni content resulted in a higher extent of particle sintering. This result can be ascribed to the solid-state transformation of the carbon support into graphene layers, catalyzed by Ni-richer catalyst, which, in turn, promoted particle migration/coalescence and, hence, more-significant sintering. Therefore, our results provide important insight for the synthesis of carbon-supported Pt-alloy fuel-cell catalysts.
Co-reporter:Mehtap Oezaslan, Frédéric Hasché, and Peter Strasser
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 19) pp:3273-3291
Publication Date(Web):September 12, 2013
DOI:10.1021/jz4014135
Pt-based core–shell nanoparticles have emerged as a promising generation of highly active electrocatalysts to accelerate the sluggish kinetics of oxygen reduction reaction (ORR) in fuel cell systems. Their electronic and structural properties can be easily tailored by modifying the Pt shell thickness, core composition, diameter, and shape; this results in significant improvements of activity and durability over state-of-the-art pure Pt catalysts. Prompted by the relevance of efficient and robust ORR catalysts for electrochemical energy conversion, this Perspective reviews several concepts and selected recent developments in the exploration of the structure and composition of core–shell nanoparticles. Addressing current achievements and challenges in the preparation as well as microscopic and spectroscopic characterization of core–shell nanocatalysts, a concise account of our understanding is provided on how the surface and subsurface structure of multimetallic core–shell nanoparticles affect their reactivity. Finally, perspectives for the large-scale implementation of core–shell catalysts in polymer exchange membrane fuel cells are discussed.
Co-reporter:Rulle Reske, Matteo Duca, Mehtap Oezaslan, Klaas Jan P. Schouten, Marc T. M. Koper, and Peter Strasser
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 15) pp:2410-2413
Publication Date(Web):July 9, 2013
DOI:10.1021/jz401087q
The catalytic activity and selectivity of the electrochemical CO2 reduction on Cu overlayers with varying atomic-scale thickness on Pt was investigated. Hydrogen, methane, and ethylene were the main products. Beyond an activity improvement with increasing copper layer thickness, we observed that the thickest 15 nm Cu layer behaved bulk-like and resulted in high relative faradaic selectivities for hydrocarbons. With decreasing Cu layer thickness, the formation of methane decreased much faster than that of ethylene. As a result, the relative faradaic selectivity of the technologically useful product ethylene increased sharply. The selectivity ratios between methane and ethylene were independent of electrode potential on a Cu monolayer. A combination of geometric tensile strain effects and electronic effects is believed to control the surface reactivity and product distribution on the copper surfaces. This study highlights the general strategy to tune product distributions on thin metal overlayers.Keywords: CO2; electrocatalysis; lattice strain; selectivity;
Co-reporter:Xenia Tuaev, Stefan Rudi, Valeri Petkov, Armin Hoell, and Peter Strasser
ACS Nano 2013 Volume 7(Issue 7) pp:5666
Publication Date(Web):June 27, 2013
DOI:10.1021/nn402406k
When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core–shell structures. Structural evolution of PtNi6 and PtNi3 NP catalysts during their electrochemical activation and catalysis was studied by in situ anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. Ex situ high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au3Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi6. Pt-enriched core–shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi3 alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core–shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles.Keywords: ASAXS; fuel cell catalysis; in situ characterization; pair distribution function; PtNi alloys
Co-reporter:Chunhua Cui, Lin Gan, Hui-Hui Li, Shu-Hong Yu, Marc Heggen, and Peter Strasser
Nano Letters 2012 Volume 12(Issue 11) pp:5885-5889
Publication Date(Web):October 12, 2012
DOI:10.1021/nl3032795
We demonstrate how shape selectivity and optimized surface composition result in exceptional oxygen reduction activity of octahedral PtNi nanoparticles (NPs). The alloy octahedra were obtained by utilizing a facile, completely surfactant-free solvothermal synthesis. We show that the choice of precursor ligands controls the shape, while the reaction time tunes the surface Pt:Ni composition. The 9.5 nm sized PtNi octahedra reached a 10-fold surface area-specific (∼3.14 mA/cmPt2) as well as an unprecedented 10-fold Pt mass based (∼1.45 A/mgPt) activity gain over the state-of-art Pt electrocatalyst, approaching the theoretically predicted limits.
Co-reporter:Lin Gan, Marc Heggen, Stefan Rudi, and Peter Strasser
Nano Letters 2012 Volume 12(Issue 10) pp:5423-5430
Publication Date(Web):September 14, 2012
DOI:10.1021/nl302995z
Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy line profiles with Ångstrom resolution, we uncover novel core–shell fine structures in a series of catalytically active dealloyed PtxNi1–x core–shell nanoparticles, showing the formation of unusual near-surface Ni-enriched inner shells. The radial location and the composition of the Ni-enriched inner shells were sensitively dependent on the initial alloy compositions. We further discuss how these self-organized Ni-enriched inner shells play a key role in maintaining surface lattice strain and thus control the surface catalytic activity for oxygen reduction.
Co-reporter:Xenia Tuaev, Jens Peter Paraknowitsch, René Illgen, Arne Thomas and Peter Strasser
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 18) pp:6444-6447
Publication Date(Web):15 Mar 2012
DOI:10.1039/C2CP40760D
A homogeneous coating of nitrogen-doped carbon on carbon nanotubes is performed using ionic liquids. The N-doped material is employed as a support for nanoparticles. Electrochemical degradation behavior is monitored in situ and compared to an unmodified material. The strongly enhanced stability is explained on the basis of a Pt–nitrogen interaction.
Co-reporter:Katharina Mette;Arno Bergmann;Dr. Jean-Philippe Tessonnier;Dr. Michael Hävecker;Dr. Lide Yao;Dr. Thorsten Ressler;Dr. Robert Schlögl;Dr. Peter Strasser;Dr. Malte Behrens
ChemCatChem 2012 Volume 4( Issue 6) pp:851-862
Publication Date(Web):
DOI:10.1002/cctc.201100434
Abstract
Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnOx/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as-prepared state of a 6.5 wt. % MnOx/CNT sample could be comprehensively characterized by comparison to an unsupported MnOx reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as-prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnOx compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post-treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5 wt. % MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state-of-art MnOx catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions.
Co-reporter:Dr. Chengfei Yu;Dr. Edward F. Holby;Dr. Ruizhi Yang;Dr. Michael F. Toney; Dane Morgan;Dr. Peter Strasser
ChemCatChem 2012 Volume 4( Issue 6) pp:766-770
Publication Date(Web):
DOI:10.1002/cctc.201200090
Co-reporter:Stefan Rudi;Xenia Tuaev
Electrocatalysis 2012 Volume 3( Issue 3-4) pp:265-273
Publication Date(Web):2012 December
DOI:10.1007/s12678-012-0098-x
The synthesis, structural, and compositional characterization as well as the electrocatalytic oxygen reduction (ORR) activity of a number of carbon-supported PtxNi1−x (x = 1.00–0.14) nanoparticles in acidic electrolyte are reported. A number of different low-temperature colloidal synthesis routes were employed to prepare monodisperse, single-phase Pt-Ni alloy nanoparticles. The catalysts were characterized using XRD, TEM, and ICP-OES techniques, subsequently electrochemically dealloyed and, in their dealloyed state, tested for their Pt mass-based ORR activity, specific Pt surface area-based ORR activity. Additional 4,000 voltage cycles were applied to investigate the durability of the electrocatalysts in terms of their electrochemically active surface area and their final ORR activity. It is found that Pt-Ni alloys exhibit a distinctly different dealloying and ORR stability behavior compared to Pt-Co or Pt-Cu alloy nanoparticles. In particular, Pt-Ni alloys require longer cycling times to unfold their full ORR activity. A distinct ORR activity maximum was uncovered for Pt-Ni nanoparticle alloys with initial Ni contents in the neighborhood of 70–75 at.% consistent with results from dealloyed macroscopic Pt-Ni thin films.
Co-reporter:Jean Sanabria-Chinchilla ; Koichiro Asazawa ; Tomokazu Sakamoto ; Koji Yamada ; Hirohisa Tanaka
Journal of the American Chemical Society 2011 Volume 133(Issue 14) pp:5425-5431
Publication Date(Web):March 22, 2011
DOI:10.1021/ja111160r
We report the discovery of a highly active Ni−Co alloy electrocatalyst for the oxidation of hydrazine (N2H4) and provide evidence for competing electrochemical (faradaic) and chemical (nonfaradaic) reaction pathways. The electrochemical conversion of hydrazine on catalytic surfaces in fuel cells is of great scientific and technological interest, because it offers multiple redox states, complex reaction pathways, and significantly more favorable energy and power densities compared to hydrogen fuel. Structure−reactivity relations of a Ni60Co40 alloy electrocatalyst are presented with a 6-fold increase in catalytic N2H4 oxidation activity over today’s benchmark catalysts. We further study the mechanistic pathways of the catalytic N2H4 conversion as function of the applied electrode potential using differentially pumped electrochemical mass spectrometry (DEMS). At positive overpotentials, N2H4 is electrooxidized into nitrogen consuming hydroxide ions, which is the fuel cell-relevant faradaic reaction pathway. In parallel, N2H4 decomposes chemically into molecular nitrogen and hydrogen over a broad range of electrode potentials. The electroless chemical decomposition rate was controlled by the electrode potential, suggesting a rare example of a liquid-phase electrochemical promotion effect of a chemical catalytic reaction (“EPOC”). The coexisting electrocatalytic (faradaic) and heterogeneous catalytic (electroless, nonfaradaic) reaction pathways have important implications for the efficiency of hydrazine fuel cells.
Co-reporter:Mehtap Oezaslan, Frédéric Hasché, and Peter Strasser
Chemistry of Materials 2011 Volume 23(Issue 8) pp:2159
Publication Date(Web):March 29, 2011
DOI:10.1021/cm103661q
Bimetallic alloy nanoparticles exhibit a complex, (for the most part) poorly understood, crystallographic phase behavior, rarely following their macroscopic counterparts. We have studied formation kinetics, time scales of individual processes, compositional changes, and particle growth rates of Pt bimetallic alloy nanoparticles. We chose the Pt−Cu system, because of its technological importance as a precursor for core−shell nanoparticle electrocatalysts. We provide correlation of annealing control parameters, such as heating rate, temperature, and time, with microscopic alloy structure, composition, and particle size. We have clarified the roles of annealing temperature and time in the alloy formation process and traced out entire Vegard-type structure composition relationships over a wide temperature range. We have found that, during heating ramps, the annealing temperature essentially controls the Cu content of the resulting disordered Pt−Cu lattices. Increasing annealing times, in contrast, leads primarily to particle growth. Phase ordering occurs only during cooling. Our insight offers practical synthetic guidelines toward single-phase ordered and disordered PtCu3 alloy nanoparticles with optimized particle dispersion.Keywords: alloy formation; bimetallic catalyst; in situ high temperature XRD; particle growth; phase transformation; PtCu3;
Co-reporter:Prasanna Mani, Ratndeep Srivastava, Peter Strasser
Journal of Power Sources 2011 Volume 196(Issue 2) pp:666-673
Publication Date(Web):15 January 2011
DOI:10.1016/j.jpowsour.2010.07.047
Dealloyed Pt bimetallic nanoparticles are highly active electrocatalysts for the electroreduction of molecular oxygen (ORR), the key barrier to more efficient polymer electrolyte membrane fuel cells (PEMFCs). Most previous studies of dealloyed Pt alloys focused on the structure and mechanism of dealloyed Pt–Cu bimetallic materials. Also, stability concerns related to Cu prompted the search for alternative non-noble metal components for dealloying.Here, we report on a comparative study of dealloyed binary PtM3 (M = Co, Cu, Ni) electrocatalyst for use in PEMFC cathodes. We also study synergistic effects of a third metal in ternary PtNi3M (M = Co, Cu, Fe, Cr) cathode electrocatalysts. All catalyst precursor materials were prepared by an impregnation, freeze-drying, annealing route. After deployment of the catalyst precursor in single PEM cells, the active dealloyed form of the catalysts was obtained through a voltammetric dealloying protocol. Dealloyed binary PtM3 catalysts showed more than a threefold activity improvement for ORR for M = Co, Cu, and close to a threefold improvement for M = Ni in terms of the Pt-mass activity (A mgPt−1) of the single fuel cell, compared to a 45 wt% Pt/C reference cathode catalyst. Improvements in specific surface area normalized activities (A cmPt−2) followed those in Pt-mass activity. All ternary catalysts, except the Fe containing one, showed clearly improved catalytic ORR performance compared to PtNi3, in particular PtNi3Co and PtNi3Cu. A previously unachieved four- to fivefold activity improvement in real single MEAs was observed.Near-surface (XPS) and bulk (EDS/ICP) compositional characterizations suggested that the degree of dealloying of Pt–Co and Pt–Ni binary precursors is lower than that of Pt–Cu compounds. Pt–Co and Pt–Ni still showed 15–20 at.% non-noble metal near the surface and in the bulk of the dealloyed particles, whereas, under the chosen dealloying conditions, Pt–Cu formed core–shell structures with a Pt-rich surface and a Pt–Cu core. Of the selectively characterized Pt–Ni–Co and Pt–Ni–Cu ternaries, the near-surface composition of dealloyed Pt–Ni compounds showed an atomic ratio of about 1:1, compared to about 5:1 in the bulk, pointing to a Ni enrichment at the surface with only small residual amounts of Co or Cu.Our study highlights a number of novel active cathode catalyst compositions and underscores the sensitive dependence of the ORR activity of dealloyed Pt binary and ternary nanoparticle electrocatalysts on the nature and initial composition of the non-noble alloy component.
Co-reporter:Frédéric Hasché, Mehtap Oezaslan and Peter Strasser
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 46) pp:15251-15258
Publication Date(Web):2010/10/29
DOI:10.1039/C0CP00609B
Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m2 g−1 and with a comparable Pt mean particle size were subjected to electrode potential cycling in a “lifetime” stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a “start-up” stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the “lifetime” regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the “start-up” regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.
Co-reporter:Holger Dau Dr.;Christian Limberg Dr.;Tobias Reier;Marcel Risch;Stefan Roggan Dr. Dr.
ChemCatChem 2010 Volume 2( Issue 7) pp:724-761
Publication Date(Web):
DOI:10.1002/cctc.201000126
Abstract
Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.
Co-reporter:P. Strasser
Chemie Ingenieur Technik 2009 Volume 81( Issue 5) pp:573-580
Publication Date(Web):
DOI:10.1002/cite.200800506
Abstract
Die elektrochemische Umwandlung von Sauerstoff zu Wasser sowie ihre Umkehrreaktion gehören im Moment zu den größten Herausforderungen der Wissenschaft und Forschung zum Zwecke einer effizienten und nachhaltigen Nutzung elektrochemischer Energieumwandlungsprozesse basierend auf Brennstoffzellen oder der Elektrolyse von Wasser. Reines Platin – der am häufigsten verwendete Kathodenkatalysator für die Sauerstoffreduktion in Brennstoffzellen – ist teuer und nicht ausreichend aktiv und stabil. Vielversprechend ist ein neues Katalysatorkonzept basierend auf einem sehr Pt armen nanostrukturierten Kern-Schale-Prinzip. Durch selektives partielles Entlegieren, d. h. elektrochemische Entfernung von unedlen Metallatomen aus der Oberfläche einer nichtedelmetallreichen Pt-Legierung („Dealloying”), wird eine Pt reiche Schale auf einem nichtedelmetallreichen Kern von definierbarer Dicke gebildet, die eine gezielte Steuerbarkeit von katalytischer Aktivität ermöglicht. Eine Reduzierung der Pt-Menge in der Zellkathode um mehr als 80 % ist dabei möglich.
Co-reporter:P. Strasser Dr.;M. Oezaslan Dipl.-Chem.;F. Hasche Dipl.-Chem.;S. Koh M. Sc.;C. Yu M. Sc.;R. Srivastava M. Sc.
Chemie Ingenieur Technik 2008 Volume 80( Issue 9) pp:
Publication Date(Web):
DOI:10.1002/cite.200750626
No abstract is available for this article.
Co-reporter:Manuel Gliech, Arno Bergmann, Camillo Spöri, Peter Strasser
Journal of Energy Chemistry (March 2016) Volume 25(Issue 2) pp:278-281
Publication Date(Web):1 March 2016
DOI:10.1016/j.jechem.2016.01.002
Mixed metal oxides in the nanoscale are of great interest for many aspects of energy related research topics as water splitting, fuel cells and battery technology. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. While monometallic oxides have been studied in much detail, reliable synthetic recipes targeting specific crystal structures of mixed metal oxide nanoparticles are largely missing. Yet, in order to meet the requirements for a broad range of technical implementation it is necessary to tailor the properties of mixed metal oxides to the particular purpose. Here, we present a study on the impact of the nature of the gas environment on the resulting crystal structure during a post-synthesis thermal heat treatment of manganese–cobalt oxide nanoparticles. We monitor the evolution of the crystal phase structure as the gas atmosphere is altered from pure nitrogen to synthetic air and pure oxygen. The particle size and homogeneity of the resulting nanoparticles increase with oxygen content, while the crystal structure gradually changes from rocksalt-like to pure spinel. We find the composition of the particles to be independent of the gas atmosphere. The manganese–cobalt oxide nanoparticles exhibited promising electrocatalytic activity regarding oxygen evolution in alkaline electrolyte. These findings offer new synthesis pathways for the direct preparation of versatile utilizable mixed metal oxides.An investigation on the influence of gas atmosphere during synthesis on the crystal structure of non noble MMOs for energy conversion and storage.Download high-res image (184KB)Download full-size image
Co-reporter:Xenia Tuaev, Stefan Rudi and Peter Strasser
Catalysis Science & Technology (2011-Present) 2016 - vol. 6(Issue 23) pp:NaN8288-8288
Publication Date(Web):2016/10/11
DOI:10.1039/C6CY01679K
This study explores how the morphology of nanostructured carbons impacts the morphological stability of supported Pt fuel cell nanoparticle catalysts under extended potential cycling. Using in situ small angle X-ray scattering (SAXS), we monitor the evolution of key structural parameters of four different Pt/carbon catalyst couples, involving carbons with vastly different porosity characteristics (hollow carbons, nanotubes, and carbon blacks). In line with the size of supported Pt nanoparticles, the intrinsic specific electrochemical oxygen reduction reaction (ORR) activities of all samples were comparable. However, counter to common sense, a non-monotonic trend between the carbon surface area and the ORR mass-based activity, coupled with a similar relative loss in the electrochemical surface area (ECSA), was observed. This is explained in terms of a varying effective ECSA, which is sensitively dependent on the morphology of the support. In situ SAXS monitoring revealed a mainly coalescence-based increase in mean particle size for the low surface area carbon nanotubes. In contrast, the highly microporous hollow carbons showed strongly enhanced particle stability where Ostwald ripening accounted for the observed coarsening. Altogether, our study provides new atom-scale insights into Pt/C fuel cell catalyst stability. Based on this study, supports of intermediate surface area provide the best compromise between activity and size stability, while highly graphitized or highly nanoporous supports are detrimental.
Co-reporter:Rameshwori Loukrakpam, Qiuyi Yuan, Valeri Petkov, Lin Gan, Stefan Rudi, Ruizhi Yang, Yunhui Huang, Stanko R. Brankovic and Peter Strasser
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 35) pp:NaN18876-18876
Publication Date(Web):2014/07/23
DOI:10.1039/C4CP02791D
Efficient catalytic C–C bond splitting coupled with complete 12-electron oxidation of the ethanol molecule to CO2 is reported on nanoscale electrocatalysts comprised of a Pt monolayer (ML) and sub-monolayer (sML) deposited on Au nanoparticles (Au@Pt ML/sML). The Au@Pt electrocatalysts were synthesized using surface limited redox replacement (SLRR) of an underpotentially deposited (UPD) Cu monolayer in an electrochemical cell reactor. Au@Pt ML showed improved catalytic activity for ethanol oxidation reaction (EOR) and, unlike their Pt bulk and Pt sML counterparts, was able to generate CO2 at very low electrode potentials owing to efficient C–C bond splitting. To explain this, we explore the hypothesis that competing strain effects due to the Pt layer coverage/morphology (compressive) and the Pt–Au lattice mismatch (tensile) control surface chemisorption and overall activity. Control experiments on well-defined model Pt monolayer systems are carried out involving a wide array of methods such as high-energy X-ray diffraction, pair-distribution function (PDF) analysis, in situ electrochemical FTIR spectroscopy, and in situ scanning tunneling microscopy. The vibrational fingerprints of adsorbed CO provide compelling evidence on the relation between surface bond strength, layer strain and morphology, and catalytic activity.
Co-reporter:Prashanth W. Menezes, Arindam Indra, Arno Bergmann, Petko Chernev, Carsten Walter, Holger Dau, Peter Strasser and Matthias Driess
Journal of Materials Chemistry A 2016 - vol. 4(Issue 25) pp:NaN10022-10022
Publication Date(Web):2016/06/06
DOI:10.1039/C6TA03644A
The fabrication and design of earth-abundant and high-performance catalysts for the oxygen evolution reaction (OER) are very crucial for the development and commercialization of sustainable energy conversion technologies. Although spinel catalysts have been widely explored for the electrochemical oxygen evolution reaction (OER), the role of two geometrical sites that influence their activities has not been well established so far. Here, we present more effective cobalt–zinc oxide catalysts for the OER than ‘classical’ Co3O4. Interestingly, the significantly higher catalytic activity of ZnCo2O4 than that of Co3O4 is somewhat surprising since both crystallize in the spinel-type structure. The reasons for the latter remarkable difference of ZnCo2O4 and Co3O4 could be deduced from structure–activity relationships of the bulk and near-surface of the catalysts using comprehensive electrochemical, microscopic and spectroscopic techniques with a special emphasis on the different roles of the coordination environment of metal ions (octahedral vs. tetrahedral sites) in the spinel lattice. The vital factors influencing the catalytic activity of ZnCo2O4 over Co3O4 could be directly attributed to the higher amount of accessible octahedral Co3+ sites induced by the preferential loss of zinc ions from the surface of the ZnCo2O4 catalyst. The enhanced catalytic activity is accompanied by a larger density of metal vacancies, defective sites and hydroxylation. The results obtained here clearly demonstrate how a surface structural modification and generation of defects of catalysts can enhance their OER performance.
Co-reporter:Hyung-Suk Oh, Hong Nhan Nong, Tobias Reier, Manuel Gliech and Peter Strasser
Chemical Science (2010-Present) 2015 - vol. 6(Issue 6) pp:NaN3328-3328
Publication Date(Web):2015/03/19
DOI:10.1039/C5SC00518C
Reducing the noble-metal catalyst content of acid Polymer Electrolyte Membrane (PEM) water electrolyzers without compromising catalytic activity and stability is a goal of fundamental scientific interest and substantial technical importance for cost-effective hydrogen-based energy storage. This study presents nanostructured iridium nanodendrites (Ir-ND) supported on antimony doped tin oxide (ATO) as efficient and stable water splitting catalysts for PEM electrolyzers. The active Ir-ND structures exhibited superior structural and morphological properties, such as particle size and surface area compared to commercial state-of-art Ir catalysts. Supported on tailored corrosion-stable conductive oxides, the Ir-ND catalysts exhibited a more than 2-fold larger kinetic water splitting activity compared with supported Ir nanoparticles, and a more than 8-fold larger catalytic activity than commercial Ir blacks. In single-cell PEM electrolyzer tests, the Ir-ND/ATO outperformed commercial Ir catalysts more than 2-fold at technological current densities of 1.5 A cm−2 at a mere 1.80 V cell voltage, while showing excellent durability under constant current conditions. We conclude that Ir-ND/ATO catalysts have the potential to substantially reduce the required noble metal loading, while maintaining their catalytic performance, both in idealized three-electrode set ups and in the real electrolyzer device environments.
Co-reporter:Guang-Ping Hao, Nastaran Ranjbar Sahraie, Qiang Zhang, Simon Krause, Martin Oschatz, Alicja Bachmatiuk, Peter Strasser and Stefan Kaskel
Chemical Communications 2015 - vol. 51(Issue 97) pp:NaN17288-17288
Publication Date(Web):2015/10/05
DOI:10.1039/C5CC06256J
Exploring the role of surface hydrophilicity of non-precious metal N-doped carbon electrocatalysts in electrocatalysis is challenging. Herein we discover an ultra-hydrophilic non-precious carbon electrocatalyst, showing enhanced catalysis efficiency on both gravimetric and areal basis for oxygen reduction reaction due to a high dispersion of active centres.
Co-reporter:Hong Nhan Nong, Lin Gan, Elena Willinger, Detre Teschner and Peter Strasser
Chemical Science (2010-Present) 2014 - vol. 5(Issue 8) pp:NaN2963-2963
Publication Date(Web):2014/05/23
DOI:10.1039/C4SC01065E
A family of dealloyed metal–oxide hybrid (M1M2@M1Ox) core@shell nanoparticle catalysts is demonstrated to provide substantial advances toward more efficient and less expensive electrolytic water splitting. IrNi@IrOx nanoparticles were synthesized from IrNix precursor alloys through selective surface Ni dealloying and controlled surface oxidation of Ir. Detailed depth-resolved insight into chemical structure, composition, morphology, and oxidation state was obtained using spectroscopic, diffraction, and scanning microscopic techniques (XANES, XRD, STEM-EDX, XPS), which confirmed our structural hypotheses at the outset. A 3-fold catalytic activity enhancement for the electrochemical oxygen evolution reaction (OER) over IrO2 and RuO2 benchmark catalysts was observed for the core-shell catalysts on a noble metal mass basis. Also, the active site-based intrinsic turnover frequency (TOF) was greatly enhanced for the most active IrNi@IrOx catalyst. This study documents the successful use of synthetic dealloying for the preparation of metal-oxide hybrid core-shell catalysts. The concept is quite general, can be applied to other noble metal nanoparticles, and points out a path forward to nanostructured proton-exchange-electrolyzer electrodes with dramatically reduced noble metal content.
Co-reporter:Xenia Tuaev, Jens Peter Paraknowitsch, René Illgen, Arne Thomas and Peter Strasser
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 18) pp:NaN6447-6447
Publication Date(Web):2012/03/15
DOI:10.1039/C2CP40760D
A homogeneous coating of nitrogen-doped carbon on carbon nanotubes is performed using ionic liquids. The N-doped material is employed as a support for nanoparticles. Electrochemical degradation behavior is monitored in situ and compared to an unmodified material. The strongly enhanced stability is explained on the basis of a Pt–nitrogen interaction.
Co-reporter:Frédéric Hasché, Mehtap Oezaslan and Peter Strasser
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 46) pp:NaN15258-15258
Publication Date(Web):2010/10/29
DOI:10.1039/C0CP00609B
Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m2 g−1 and with a comparable Pt mean particle size were subjected to electrode potential cycling in a “lifetime” stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a “start-up” stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the “lifetime” regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the “start-up” regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.
Co-reporter:Aleksandar R. Zeradjanin, Nadine Menzel, Wolfgang Schuhmann and Peter Strasser
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 27) pp:NaN13747-13747
Publication Date(Web):2014/03/31
DOI:10.1039/C4CP00896K
The faradaic selectivity of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on the industrially important Ti–Ru–Ir mixed metal oxide is discussed. Absolute evolution rates as well as volume fractions of Cl2 and O2 were quantified using differential electrochemical mass spectrometry (DEMS), while the catalyst surface redox behavior was analyzed using cyclic voltammetry. The spatial inhomogeneity of the surface catalytic reaction rate was probed using Scanning Electrochemical Microscopy (SECM). Although the nature of the competition between electrochemical discharging of chloride ions and water molecules remains elusive on a molecular scale, new insights into the spatial reactivity distribution of the CER and OER were obtained. Oxidation of water is the initial step in corrosion and concomitant deactivation of the oxide electrodes; however, at the same time the nature of interaction between the oxide surface and water is used as a rational indicator of selectivity and catalytic activity. An experimental procedure was established that would allow the study of selectivity of a variety of different catalyst materials using polycrystalline electrode surfaces.
