Transition-metal phosphosulfides represent an emerging category of earth-abundant electrocatalyst materials, and some metal phosphosulfides have been shown to outperform the corresponding sulfides and phosphides. To fully realize the potential and benefit energy storage and conversion, it is necessary to study the chemistry of unknown phosphosulfide materials. In this article, we report on the materials chemistry of iron phosphosulfides. We systematically investigate the materials synthesis, solid state chemistry, surface structures, and electrocatalytic properties of iron phosphosulfide nanoparticles supported on carbon nanotubes. Two types of iron phosphosulfide nanomaterials, adopting either the FeS or the FeP crystal structure, have been successfully synthesized by two distinct synthetic routes designed in accordance with the different thermodynamic properties of the two structures. The compositions (i.e., P/S ratios) of the phosphosulfides can be adjusted within certain ranges without phase separation occurring. We discover that all the phosphosulfide nanoparticles exhibit higher P/S ratios on the surface than in the bulk and that the presence of P atoms suppresses the oxidation of Fe and S atoms on the surface. We further find that there is a positive correlation between the P content of the iron phosphosulfide nanomaterials and their electrocatalytic activity for the hydrogen evolution reaction, which renders high-performance electrocatalysts for hydrogen production and the understanding that the Fe atoms coordinated by P atoms are the most active catalytic sites in the materials.Keywords: electrocatalysis; hydrogen evolution reaction; iron phosphosulfides; phosphorus-rich surface; solid state chemistry;

Bimetallic electrocatalysts can improve the activity and selectivity over their monometallic counterparts by tuning the structure, morphology, and composition. However, there scarcely was a systematic model to understand the structural effect relationship on CO2 electrochemical reduction reaction, especially for a product tuning process by introduction of a second metal to grow into outer layers. Herein, we report a structure-controlled model of the growth process of Ag@Cu bimetallic nanoparticles that are fabricated by a polyol method, that is, reducing mixtures of Ag+ and Cu2+ (excess amount) in ethylene glycol (reducing agent) in the presence of polyvinylpyrrolidone. Structural characterizations reveal that a series of Ag@Cu NPs are tuned from the Ag core, Cu modified Ag, to the Cu outer shell by controlling the heating time (0–25 min). Moreover, highly selective catalysts with the tuning reduction products from carbon monoxide to hydrocarbons can be realized. Different from the “dilution” effects between Ag and Cu, the volcanic curve for carbon monoxide production is detected for the introduction of Cu and the peak point is the Ag@Cu-7 electrocatalyst (heating time is 7 min). Similarly, interestingly, when the Cu cladding layer continuously grows, the hydrocarbons are not a simple proportional addition and optimized at Ag@Cu-20 (heating time is 20 min). The geometric effects dominantly account for the synergistic effect of CO product and control the surface activity to hydrocarbons. This study serves as a good starting point to tune the energetics of the intermediate binding to achieve even higher selectivity and activity for core–shell structured catalysts.

The design and development of materials for electrochemical energy storage and conversion devices requires fundamental understanding of chemical interactions at electrode/electrolyte interfaces. For Li–S batteries that hold the promise for outperforming the current generation of Li ion batteries, the interactions of lithium polysulfide (LPS) intermediates with the electrode surface strongly influence the efficiency and cycle life of the sulfur cathode. While metal oxides have been demonstrated to be useful in trapping LPS, the actual binding modes of LPS on 3d transition metal oxides and their dependence on the metal element identity across the periodic table remain poorly understood. Here, we investigate the chemical interactions between LPS and oxides of Mn, Fe, Co, and Cu by combining X-ray photoelectron spectroscopy and density functional theory calculations. We find that Li–O interactions dominate LPS binding to the oxides (Mn3O4, Fe2O3, and Co3O4), with increasing strength from Mn to Fe to Co. For Co3O4, LPS binding also involves metal–sulfur interactions. We also find that the metal oxides exhibit different binding preferences for different LPS, with Co3O4 binding shorter-chain LPS more strongly than Mn3O4. In contrast to the other oxides, CuO undergoes intense reduction and dissolution reactions upon interaction with LPS. The reported findings are thus particularly relevant to the design of LPS/oxide interfaces for high-performance Li–S batteries.

One major challenge to the electrochemical conversion of CO2 to useful fuels and chemical products is the lack of efficient catalysts that can selectively direct the reaction to one desirable product and avoid the other possible side products. Making use of strong metal/oxide interactions has recently been demonstrated to be effective in enhancing electrocatalysis in the liquid phase. Here, we report one of the first systematic studies on composition-dependent influences of metal/oxide interactions on electrocatalytic CO2 reduction, utilizing Cu/SnOx heterostructured nanoparticles supported on carbon nanotubes (CNTs) as a model catalyst system. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO2 electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic acid-dominant. In the Cu-rich regime, SnOx dramatically alters the catalytic behavior of Cu. The Cu/SnOx–CNT catalyst containing 6.2% of SnOx converts CO2 to CO with a high faradaic efficiency (FE) of 89% and a jCO of 11.3 mA·cm–2 at −0.99 V versus reversible hydrogen electrode, in stark contrast to the Cu–CNT catalyst on which ethylene and methane are the main products for CO2 reduction. In the Sn-rich regime, Cu modifies the catalytic properties of SnOx. The Cu/SnOx–CNT catalyst containing 30.2% of SnOx reduces CO2 to formic acid with an FE of 77% and a jHCOOH of 4.0 mA·cm–2 at −0.99 V, outperforming the SnOx–CNT catalyst which only converts CO2 to formic acid in an FE of 48%.Keywords: CO2 reduction reactions; electrocatalysis; metal/oxide interactions; tunable product selectivity;

Transition-metal-based molecular complexes are a class of catalyst materials for electrochemical CO2 reduction to CO that can be rationally designed to deliver high catalytic performance. One common mechanistic feature of these electrocatalysts developed thus far is an electrogenerated reduced metal center associated with catalytic CO2 reduction. Here we report a heterogenized zinc–porphyrin complex (zinc(II) 5,10,15,20-tetramesitylporphyrin) as an electrocatalyst that delivers a turnover frequency as high as 14.4 site–1 s–1 and a Faradaic efficiency as high as 95% for CO2 electroreduction to CO at −1.7 V vs the standard hydrogen electrode in an organic/water mixed electrolyte. While the Zn center is critical to the observed catalysis, in situ and operando X-ray absorption spectroscopic studies reveal that it is redox-innocent throughout the potential range. Cyclic voltammetry indicates that the porphyrin ligand may act as a redox mediator. Chemical reduction of the zinc–porphyrin complex further confirms that the reduction is ligand-based and the reduced species can react with CO2. This represents the first example of a transition-metal complex for CO2 electroreduction catalysis with its metal center being redox-innocent under working conditions.

AbstractA surface-restructuring strategy is presented that involves self-cleaning Cu catalyst electrodes with unprecedented catalytic stability toward CO2 reduction. Under the working conditions, the Pd atoms pre-deposited on Cu surface induce continuous morphological and compositional restructuring of the Cu surface, which constantly refreshes the catalyst surface and thus maintains the catalytic properties for CO2 reduction to hydrocarbons. The Pd-decorated Cu electrode can catalyze CO2 reduction with relatively stable selectivity and current density for up to 16 h, which is one of the best catalytic durability performances among all Cu electrocatalysts for effective CO2 conversion to hydrocarbons. The generality of this approach of utilizing foreign metal atoms to induce surface restructuring toward stabilizing Cu catalyst electrodes against deactivation by carbonaceous species accumulation in CO2 reduction is further demonstrated by replacing Pd with Rh.

High-performance lithium–sulfur batteries are widely and intensively pursued, owing to their projected high energy density and low cost. However, realizing the stable cycling of a sulfur cathode with good discharging/charging rate capability under high sulfur content and high sulfur loading conditions remains a major challenge. Confining the dissolvable lithium polysulfide intermediates while addressing the intrinsic low electrical conductivity of sulfur is a key approach toward solving the problem. This work presents the design of a pomegranate-structured sulfur cathode material with high electrochemical performance. To synthesize the material, mesoporous carbon particles with ferrocene decoration are infiltrated with sulfur and then wrapped into secondary particles by dendrimer-linked graphene oxide. In the designed structure, the mesoporous carbon serves as a conductive matrix and porous host for sulfur species; ferrocene provides polar sites to bind lithium polysulfides chemically; the dendrimer-linked graphene oxide encapsulation layers further confine leaching of polysulfides and ferrocene into the electrolyte. With the three components providing triple confinement of the polysulfides, the material with a high sulfur content of 75.7 wt% exhibits excellent cycling stability and good rate capability. A capacity of 826 mA h g−1 can be delivered at 1.0C with an average decay of only 0.010% per cycle over 1200 cycles. With a high S mass loading of 4 mg cm−2, the cathode can still be cycled at 0.5C for 300 cycles with a capacity decay as low as 0.038% per cycle.

Dendrite growth and low coulombic efficiency are two major factors that limit the utilization of Li metal electrodes in future generations of high-energy-density rechargeable batteries. This article reports the first study on metal–organic framework (MOF) materials for boosting the electrochemical performance of Li metal electrodes and demonstrates the power of molecular-structure functionalization for realizing desirable ion transport and Li metal nucleation and growth. We show that dendrite-free dense Li deposition and stable Li plating/stripping cycling with high coulombic efficiency are enabled by modifying a commercial polypropylene separator with a titanium-based MOF (NH2-MIL-125(Ti)) material. The NH2-MIL-125(Ti)-coated-separator renders Li|Cu cells that can run for over 200 cycles at 1 mA cm−2–1 mA h cm−2 with average coulombic efficiency of 98.5% and Li|Li symmetric cells that can be cycled at 1 mA cm−2–1 mA h cm−2 for more than 1200 h without short circuiting. The superior cycling stability is attributed to the amine substituents in the NH2-MIL-125(Ti) structure which induce increased Li+ transference numbers and uniform and dense early-stage Li deposition.

Lithium–sulfur batteries (Li–S batteries) have attracted intense interest because of their high specific capacity and low cost, although they are still hindered by severe capacity loss upon cycling caused by the soluble lithium polysulfide intermediates. Although many structure innovations at the material and device levels have been explored for the ultimate goal of realizing long cycle life of Li–S batteries, it remains a major challenge to achieve stable cycling while avoiding energy and power density compromises caused by the introduction of significant dead weight/volume and increased electrochemical resistance. Here we introduce an ultrathin composite film consisting of naphthalimide-functionalized poly(amidoamine) dendrimers and graphene oxide nanosheets as a cycling stabilizer. Combining the dendrimer structure that can confine polysulfide intermediates chemically and physically together with the graphene oxide that renders the film robust and thin (<1% of the thickness of the active sulfur layer), the composite film is designed to enable stable cycling of sulfur cathodes without compromising the energy and power densities. Our sulfur electrodes coated with the composite film exhibit very good cycling stability, together with high sulfur content, large areal capacity, and improved power rate.

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal–phosphide core–shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron–iron phosphide (Fe@FeP) core–shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53–110 mV to reach catalytic current densities of 10–100 mA cm–2.Keywords: core−shell nanostructures; electrocatalysis; hygrogen evolution reaction; iron phosphide; Metal−phosphide interaction;

AbstractA surface-restructuring strategy is presented that involves self-cleaning Cu catalyst electrodes with unprecedented catalytic stability toward CO2 reduction. Under the working conditions, the Pd atoms pre-deposited on Cu surface induce continuous morphological and compositional restructuring of the Cu surface, which constantly refreshes the catalyst surface and thus maintains the catalytic properties for CO2 reduction to hydrocarbons. The Pd-decorated Cu electrode can catalyze CO2 reduction with relatively stable selectivity and current density for up to 16 h, which is one of the best catalytic durability performances among all Cu electrocatalysts for effective CO2 conversion to hydrocarbons. The generality of this approach of utilizing foreign metal atoms to induce surface restructuring toward stabilizing Cu catalyst electrodes against deactivation by carbonaceous species accumulation in CO2 reduction is further demonstrated by replacing Pd with Rh.

Exploration of heterogeneous molecular catalysts combining the atomic-level tunability of molecular structures and the practical handling advantages of heterogeneous catalysts represents an attractive approach to developing high-performance catalysts for important and challenging chemical reactions such as electrochemical carbon dioxide reduction which holds the promise for converting emissions back to fuels utilizing renewable energy. Thus, far, efficient and selective electroreduction of CO2 to deeply reduced products such as hydrocarbons remains a big challenge. Here, we report a molecular copper-porphyrin complex (copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin) that can be used as a heterogeneous electrocatalyst with high activity and selectivity for reducing CO2 to hydrocarbons in aqueous media. At −0.976 V vs the reversible hydrogen electrode, the catalyst is able to drive partial current densities of 13.2 and 8.4 mA cm–2 for methane and ethylene production from CO2 reduction, corresponding to turnover frequencies of 4.3 and 1.8 molecules·site–1·s–1 for methane and ethylene, respectively. This represents the highest catalytic activity to date for hydrocarbon production over a molecular CO2 reduction electrocatalyst. The unprecedented catalytic performance is attributed to the built-in hydroxyl groups in the porphyrin structure and the reactivity of the copper(I) metal center.

•Correlations between surface capping and electrocatalytic kinetics•A useful model catalyst system comprising Pt nanoparticles with well-defined size, shape and various surface capping•Surface capping can affect both the activity and selectivity of oxygen reduction reaction positively, neutrally or negatively.Organic and polymer capping agents are prevailingly used in the synthesis of metal nanocrystals to render size and shape controls for desirable catalytic properties. A general assumption in the electrocatalysis field is that the capping agents block active sites and hinder catalytic turnover. However there have been a number of experimental results suggesting otherwise. Investigation of the fundamental correlations between the surface capping and the catalytic kinetics of metal nanoparticles is of paramount importance yet still remains challenging in large part due to structural changes induced by capping agent removal or synthesis using different capping agents. Our approach involves a unique catalyst system comprising of 1.7 nm Pt nanoparticles with and without various surface capping. We find that surface capping affects both activity and selectivity of electrocatalytic oxygen reduction reaction. The influences can be positive, neutral or negative. The five capping agents studied fall into three groups. Polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP) cappings do not change the onset potential or product selectivity, but increase the catalytic current density. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) cappings do not change the onset potential or product selectivity, but slightly decrease the catalytic current density. Oleylamine (OA) capping significantly decreases the onset potential and the catalytic current density as well as change the product selectivity by favoring a high percentage of 2-electron reduction.

Strong metal/oxide interactions have been acknowledged to play prominent roles in chemical catalysis in the gas phase, but remain as an unexplored area in electrocatalysis in the liquid phase. Utilization of metal/oxide interface structures could generate high performance electrocatalysts for clean energy storage and conversion. However, building highly dispersed nanoscale metal/oxide interfaces on conductive scaffolds remains a significant challenge. Here, we report a novel strategy to create metal/oxide interface nanostructures by growing mixed metal oxide nanoparticles on carbon nanotubes (CNTs) and then selectively promoting migration of one of the metal ions to the surface of the oxide nanoparticles and simultaneous reduction to metal. Employing this strategy, we have synthesized Ni/CeO2 nanointerfaces coupled with CNTs. The Ni/CeO2 interface promotes hydrogen evolution catalysis by facilitating water dissociation and modifying the hydrogen binding energy. The Ni/CeO2–CNT hybrid material exhibits superior activity for hydrogen evolution as a result of synergistic effects including strong metal/oxide interactions, inorganic/carbon coupling, and particle size control.

The rechargeable lithium–sulfur battery is a promising option for energy storage applications because of its low cost and high energy density. The electrochemical performance of the sulfur cathode, however, is substantially compromised because of fast capacity decay caused by polysulfide dissolution/shuttling and low specific capacity caused by the poor electrical conductivities of the active materials. Herein we demonstrate a novel strategy to address these two problems by designing and synthesizing a carbon nanotube (CNT)/NiFe2O4–S ternary hybrid material structure. In this unique material architecture, each component synergistically serves a specific purpose: The porous CNT network provides fast electron conduction paths and structural stability. The NiFe2O4 nanosheets afford strong binding sites for trapping polysulfide intermediates. The fine S nanoparticles well-distributed on the CNT/NiFe2O4 scaffold facilitate fast Li+ storage and release for energy delivery. The hybrid material exhibits balanced high performance with respect to specific capacity, rate capability, and cycling stability with outstandingly high Coulombic efficiency. Reversible specific capacities of 1350 and 900 mAh g–1 are achieved at rates of 0.1 and 1 C respectively, together with an unprecedented cycling stability of ∼0.009% capacity decay per cycle over more than 500 cycles.

Dendrite growth and low coulombic efficiency are two major factors that limit the utilization of Li metal electrodes in future generations of high-energy-density rechargeable batteries. This article reports the first study on metal–organic framework (MOF) materials for boosting the electrochemical performance of Li metal electrodes and demonstrates the power of molecular-structure functionalization for realizing desirable ion transport and Li metal nucleation and growth. We show that dendrite-free dense Li deposition and stable Li plating/stripping cycling with high coulombic efficiency are enabled by modifying a commercial polypropylene separator with a titanium-based MOF (NH2-MIL-125(Ti)) material. The NH2-MIL-125(Ti)-coated-separator renders Li|Cu cells that can run for over 200 cycles at 1 mA cm−2–1 mA h cm−2 with average coulombic efficiency of 98.5% and Li|Li symmetric cells that can be cycled at 1 mA cm−2–1 mA h cm−2 for more than 1200 h without short circuiting. The superior cycling stability is attributed to the amine substituents in the NH2-MIL-125(Ti) structure which induce increased Li+ transference numbers and uniform and dense early-stage Li deposition.

High-performance lithium–sulfur batteries are widely and intensively pursued, owing to their projected high energy density and low cost. However, realizing the stable cycling of a sulfur cathode with good discharging/charging rate capability under high sulfur content and high sulfur loading conditions remains a major challenge. Confining the dissolvable lithium polysulfide intermediates while addressing the intrinsic low electrical conductivity of sulfur is a key approach toward solving the problem. This work presents the design of a pomegranate-structured sulfur cathode material with high electrochemical performance. To synthesize the material, mesoporous carbon particles with ferrocene decoration are infiltrated with sulfur and then wrapped into secondary particles by dendrimer-linked graphene oxide. In the designed structure, the mesoporous carbon serves as a conductive matrix and porous host for sulfur species; ferrocene provides polar sites to bind lithium polysulfides chemically; the dendrimer-linked graphene oxide encapsulation layers further confine leaching of polysulfides and ferrocene into the electrolyte. With the three components providing triple confinement of the polysulfides, the material with a high sulfur content of 75.7 wt% exhibits excellent cycling stability and good rate capability. A capacity of 826 mA h g−1 can be delivered at 1.0C with an average decay of only 0.010% per cycle over 1200 cycles. With a high S mass loading of 4 mg cm−2, the cathode can still be cycled at 0.5C for 300 cycles with a capacity decay as low as 0.038% per cycle.

Making defects, structuring and incorporating transition-metal elements have all been demonstrated as effective strategies to enhance intrinsic activity toward the hydrogen evolution reaction (HER), but how to integrate all these merits into one system is still a challenge. An amorphous Co–Mo–S ultrathin film fabricated via low-temperature sulfurization, with rich defects, hierarchical structuring and transition metal doping, shows excellent HER performance and good working stability in acidic media. Therefore, the low-temperature sulfurizing method and hierarchical nanoarrays are extremely important to construct highly active and stable electrocatalytic gas-evolution electrodes.