Eutrophication of water bodies caused by the excessive phosphate discharge has constituted a serious threat on a global scale. It is imperative to exploit new advanced materials featuring abundant binding sites and high affinity to achieve highly efficient and specific capture of phosphate from polluted waters. Herein, water stable Zr-based metal organic frameworks (MOFs, UiO-66) with rational structural design and size modulation have been successfully synthesized based on a simple solvothermal method for effective phosphate remediation. Impressively, the size of the resulting UiO-66 particles can be effectively adjusted by simply altering reaction time and the amount of acetic acid with the purpose of understanding the crucial effect of structural design on the phosphate capture performance. Representatively, UiO-66 particles with small size demonstrates 415 mg/g of phosphate uptake capacity, outperforming most of the previously reported phosphate adsorbents. Meanwhile, the developed absorbents can rapidly reduce highly concentrated phosphate to below the permitted level in drinking water within a few minutes. More significantly, the current absorbents display remarkable phosphate sorption selectivity against the common interfering ions, which can be attributed to strong affinity between Zr–OH groups in UiO-66 and phosphate species. Furthermore, the spent UiO-66 particles can be readily regenerated and reused for multiple sorption–desorption cycles without obvious decrease in removal performance, rendering them promising sustainable materials. Hence, the developed UiO-66 adsorbents hold significant prospects for phosphate sequestration to mitigate the increasingly eutrophic problems.Keywords: adsorption; phosphate; selectivity; size modulation; UiO-66;

Developing bifunctional oxygen electrocatalysts with superior catalytic activities of oxygen reduction reaction (ORR) and oxygen revolution reaction (OER) is crucial to their practical energy storage and conversion applications. In this work, we report the fabrication of Co/CoxSy@S,N-codoped porous carbon structures with various morphologies, specific surface areas, and pore structures, derived from controllably grown Co-based metal–organic frameworks with S- and N-containing organic ligands (thiophene-2,5-dicarboxylate, Tdc; and 4,4′-bipyridine, bpy) utilizing solvent effect (e.g., water and methanol) under room temperature and hydrothermal conditions. The results demonstrate that Co/CoxSy@S,N-codoped carbon fibers fabricated at a pyrolytic temperature of 800 °C (Co/CoxSy@SNCF-800) from Co-MOFs fibers fabricated in methanol under hydrothermal conditions as electrocatalysts exhibit superior bifunctional ORR and OER activities in alkaline media, endowing them as air cathodic catalysts in rechargeable zinc–air batteries with high power density and good durability.Keywords: bifunctional oxygen electrocatalysts; S, N dual organic ligands; solvent effect; Zn−air battery; [Co(Tdc)(bpy)]n;

•An ordered and vertically-aligned TiNR array can grow on BDD electrode.•A p-n heterojunction can form between TiNR array and BDD.•The TiNR/BDD electrode has a high photoelectrocatalytic activity.•The TiNR/BDD electrode will be a versatile material for the sensing of organics.Rutile TiO2 nanorod (TiNR) arrays were fabricated on a boron-doped diamond (BDD) substrate by a simple hydrothermal synthesis method. A fluorine-doped tin oxide (FTO) electrode grown with TiNR arrays was also prepared using the same technology for comparison. Field-emission scanning electron microscopy results show that oriented TiNR arrays can grow vertically on the surface of BDD and FTO electrodes. TiNR arrays grown on both electrodes had the same length (3 μm). In comparison with the TiNR/FTO electrode, the TiNR/BDD electrode demonstrated a higher photoelectrocatalytic activity for the degradation of water and organic compounds, which is mostly attributed to the formation of a p-n heterojunction between the TiNR arrays and BDD at high potential, apart from the density of TiNR. A linear relationship between the photoelectrocatalytic current and the organic concentration can be observed on both electrodes. However, the linear range between net photoelectrocatalytic current values and organic compound concentrations for the TiNR/BDD electrode are much greater than those for the TiNR/FTO electrode, which makes the TiNR/BDD electrode a versatile material for the photocatalytic degradation and sensing of organic compounds.Download high-res image (110KB)Download full-size image

•Material for dye-sensitized solar cells for the first time.•The composite film shows superior electrocatalytic activity.•The dye-sensitized solar cell with In2.77S4@Cc CE exhibits high power conversion efficiency.An In2.77S4@conductive carbon (In2.77S4@Cc) composite was successfully synthesized via a simple two-step fabrication approach. When it was used as a counter electrode (CE) material for dye-sensitized solar cells (DSSCs), the In2.77S4@Cc composite electrode shows superior electrocatalytic activity for the reduction of I3− to I− in a DSSC. Under one sun (100 mW/cm2) illumination, the dye-sensitized solar cell with the In2.77S4@Cc CE exhibits high photovoltaic conversion efficiency (η = 8.71%), comparable to commercial Pt-based DSSC (η = 8.75%). The results indicate that the prepared In2.77S4@Cc composite CE has the potential for low-cost and highly efficient counter electrodes.

Due to their controllable morphologies, tunable porous structures, diverse compositions and easy fabrication, metal–organic frameworks (MOFs) are an ideal class of precursor material to develop high performance carbon-based materials for energy applications. In this work, two-dimensional (2D) Co/Ni MOFs nanosheets with a molar ratio of Co2+ to Ni2+ of 1 : 1 were first synthesized at room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (4,4′-Bpy) as organic linkers. As a precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 °C in N2 atmosphere to incorporate 2D CoNi alloy nanoparticles into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m2 g−1, a porous structure, and good conductivity. Interestingly, it was found that the 2D Co/Ni MOFs nanosheets can be directly used as electrode materials for supercapacitors, delivering a specific capacitance of 312 F g−1 at 1 A g−1, whereas CoNi@SNC derived from its MOFs precursor as an electrode material for supercapacitors exhibits a much higher specific capacitance (1970, 1897 and 1730 F g−1 at 1, 2, 5 A g−1, respectively) with long cycling life (retaining 95.1% of the value at 10 A g−1 after 3000 cycles) and excellent rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 W h kg−1 at a power density of 0.8 kW kg−1 with lifetime stability up to 4000 charge–discharge cycles (capacitance retention of ∼90.6%). The results demonstrate that electrochemical activation-generated CoNi oxides/oxyhydroxides on the surface of the CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements are the electrochemical active species of the CoNi@SNC-constructed supercapacitor. Additionally, the high performance of the CoNi@SNC-constructed supercapacitor can be collectively attributed to its relatively high surface area, which is favourable for the exposure of electrochemical active sites; its porous structure, which promotes redox-related mass transport; and the combination of CoNi alloy nanoparticles with graphitic carbon, which functions as an electron collector to improve electron transfer.

•Vapour-phase hydrothermal method was utilized to synthesize nanocrystalline Ni2P on CFC.•Ni2P/CFC exhibited bifunctional electrocatalytic activities of UOR and HER.•Ni2P/CFC constructed two-electrode system was established for simultaneous H2 production and urea decomposition.Ni2P nanocrystallines grown on carbon fiber cloth (Ni2P/CFC) was successfully achieved by a facile vapour-phase hydrothermal method. The as-prepared Ni2P/CFC, as an electrocatalyst, exhibited superior electrocatalytic activities toward urea oxidation reaction (UOR) with a potential of 1.42 V (vs. RHE) delivering a current density of 10 mA cm−2 and hydrogen evolution reaction (HER) with overpotentials of 90 and 155 mV at current densities of 10 and 100 mA cm−2 in alkaline media. On this basis, a Ni2P/CFC constructed two-electrode system for high-efficiency H2 production and simultaneous urea decomposition was therefore established using commercial urea as reaction source. Besides, such two-electrode system as proof of concept study was also evaluated using human urine as urea source for highly efficient H2 generation with a rate of 0.35 μM min−1 at an applied potential of 1.48 V, delivering a current density of 10 mA cm−2.Download high-res image (287KB)Download full-size image

Through the slight dissolution of a Pt mesh counter electrode, Pt-deposited carbon fiber cloth (Pt/CFC) was successfully fabricated by successive cyclic voltammetry (CV) scanning from −0.77 V to 0.20 V (vs. RHE) in 0.5 M H2SO4 electrolyte. In the process of successive CV scanning, the electrocatalytic activity of CFC toward the hydrogen evolution reaction (HER) becomes better and better owing to Pt deposition on CFC, and after 1000 CV cycles, the onset potential of CFC is identical to that of the commercial Pt/C catalyst (0 V, vs. RHE) with high current densities at low overpotentials (η10, η100, and η170 = 4.5, 34.5, and 43.5 mV at current densities of 10, 100 and 170 mA cm−2). Interestingly, it was found that as-prepared Pt/CFC exhibits decreased HER activity with an onset potential of −0.26 V (vs. RHE) when Pt mesh was changed to bare CFC as the counter electrode, mainly due to the lower Pt loading amount (0.5 mg cm−2) on CFC compared to commercial Pt/C (Pt loading amount of 1.0 mg cm−2) and the inert CFC counter electrode (not like the Pt mesh counter electrode with slight dissolution in acidic media during CV scanning). Generally, there is great competition between the HER from water splitting and hydrogenation reduction of organic matter using the same HER-active electrocatalyst under identical experimental conditions. In this work, Pt/CFC was, therefore, evaluated for hydrogenation reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) using water as the hydrogen donor in a CFC counter electrode configurated two-compartment reaction system. The results demonstrate that PNP can be effectively converted into PAP with a conversion efficiency of 83.1% and a Faradaic efficiency of 9.9% after electrocatalytic reaction for 12 h at an overpotential of −0.023 V. The findings of this work indicate that Pt counter electrodes are prone to dissolving, resulting in Pt deposition on the working electrode in acidic media, readily causing the false appearance of a non-Pt working electrode with high HER activity. Additionally, the Pt-deposited working electrode produced by this electrochemical approach is efficient for hydrogenation reduction of organic substances into high value-added chemicals using water as the hydrogenation donor.

A Mn-incorporated Ni(OH)2/carbon fiber cloth (Mn-Ni(OH)2/CFC) fabricated via a room-temperature solution route exhibited superior electrocatalytic activities of the oxygen reduction and urea oxidation reactions, delivering 12–21% energy saving in the charging process of Mn-Ni(OH)2/CFC assembled Zn–air batteries in the presence of 0.5 M urea compared to the battery without urea.

In this study, we first synthesized Co9S8@N-doped porous carbon (Co9S8@NC) using shrimp-shell derived carbon nanodots as a carbon/nitrogen source in the presence of CoSO4 by a one-step molten-salt calcination method. This was followed by low-temperature phosphorization in the presence of NaH2PO2, whereby Co9S8@N,P-doped porous carbon (Co9S8@NPC) was finally obtained using the Co9S8@NC as a precursor. The results demonstrated that the molten-salt calcination approach can effectively create a pyrolytic product with a porous structure and improve the material’s surface area, which is favourable for electrocatalysis-related mass transport and the exposure of catalytic active sites during electrocatalysis. As an electrocatalyst, Co9S8@NPC exhibits higher catalytic activity for the hydrogen evolution reaction (HER) than Co9S8@NC in an alkaline medium. Among all the investigated Co9S8@NPC catalysts, Co9S8@NPC-10 (mass ratio of NaH2PO2 to Co9S8@NC = 10 : 1) displays the best HER activity with an overpotential of 261 mV at 10 mA cm−2 in the alkaline medium. Interestingly, Co9S8@NPC-10 also displays good catalytic activity for the oxygen evolution reaction (OER) in this study. Owing to its bifunctional catalytic activity towards the HER and OER, the fabricated Co9S8@NPC-10 was simultaneously used as an anode and cathode material to generate O2 and H2 from overall water splitting in the alkaline medium, exhibiting a nearly 100% faradaic yield. This study would be helpful to the design and development of high performance non-precious metal electrocatalysts to be applied in overall water splitting to produce H2 and O2.

Controllable synthesis of metal–organic framework (MOF) materials with tunable morphologies, sizes, compositions and pore structures is critically important for MOF materials and their pyrolysis derivatives’ applications in environmental and energy fields. Here we report the synthesis of Co-MOF crystals with controllable morphologies, sizes and S/N ratios in water/NaOH and ethylene glycol/NaOH systems using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (Bpy) as S, N dual organic ligands by a “pillar-layer” assembly method. Water and ethylene glycol with different viscosities result in various crystallization processes of [Co(Tdc)(Bpy)]n crystals in the corresponding reaction system, thus respectively obtaining three-dimensional (3D) Co-MOF ([Co(Tdc)(Bpy)]n) bulk and cuboid structures in water/NaOH and ethylene glycol/NaOH reaction systems. The as-prepared Co-MOF crystals in two different reaction systems were further pyrolytically treated at 800 °C in a N2 atmosphere to obtain Co9S8@S,N-doped carbon materials with different surface areas, pore distributions and S/N doping ratios. As electrocatalysts, the Co9S8@S,N-doped carbon cuboid (Co9S8@SNCC) obtained in the ethylene glycol/NaOH system demonstrates superior bifunctional electrocatalytic activities toward both oxygen reduction and evolution reactions resulting from Co9S8 and S, N doping in the carbon structure providing catalytic active sites, better than that of Co9S8@S,N-doped carbon bulk (Co9S8@SNCB) obtained in the water/NaOH system and comparable to commercial Pt/C and RuO2 catalysts. Owing to its high surface area and porous structure, Co9S8@SNCC also exhibits great potential as the electrode material for application in supercapacitors, with high performance and recycling stability.

Co/CoO nanoparticles immobilized on Co–N-doped carbon were successfully developed using shrimp-shell derived N-doped carbon nanodots as precursors by a combined approach of polymerization and pyrolysis, as electrocatalysts exhibiting trifunctional catalytic activities toward oxygen reduction, oxygen evolution and hydrogen evolution reactions and high performance in rechargeable zinc–air batteries.

In this work, we have successfully prepared sandwich-like structured N-doped porous carbon@graphene composites (N-PC@G) derived from sandwich-like structured zeolitic imidazolate framework@graphene oxide (ZIF-8@GO). ZIF-8@GO was obtained by in situ controllable growth of ZIF-8 nanocrystals on both surfaces of graphene oxide (GO) sheets with different contents. Experimental results demonstrate that N-PC@G-0.02 (representing GO amount of 0.02 g in reaction precursors) obtained at 900 °C possesses high surface area (1094.3 m2 g−1), bimodal-pore structure (micropores and mesopores) and high graphitization degree, exhibiting great potential as a bifunctional electrocatalyst for both ORR and OER. Compared to commercial Pt/C catalyst, the N-PC@G-0.02 shows superior electrocatalytic activity with onset and half-wave potentials of 1.01 V and 0.80 V (vs. RHE), respectively, better durability and high resistance to methanol crossover effect toward ORR in alkaline media. Also, the metal-free N-PC@G-0.02 also exhibits high electrocatalytic activity of OER, comparable to commercial RuO2 catalyst. The superior ORR and OER performance could be due to a synergistic effect between ZIF-8 derived porous carbon and graphene with regard to structure and composition of N-PC@G-0.02 with high surface area, porous structure, and suitable N doping level and type, boosting the catalytic active sites, mass transport and electron transfer.

Development of cheap, abundant and metal-free N-doped carbon materials as high efficiency oxygen reduction electrocatalysts is crucial for their practical applications in future fuel cell devices. Here, three-dimensional (3D) N-doped porous carbon (NPC) materials have been successfully developed by a simple template-assisted (e.g., SiO2 spheres) high temperature pyrolysis approach using shrimp-shell derived N-doped carbon nanodots (N-CNs) as carbon and nitrogen sources obtained through a facile hydrothermal method. The shrimp-shell derived N-CNs with a product yield of ∼5% possess rich surface O- and N-containing functional groups and small nanodot sizes of 1.5–5.0 nm, which are mixed with surface acidification treated SiO2 spheres with an average diameter of ∼200 nm in aqueous solution to form a N-CNs@SiO2 composite subjected to a thermal evaporation treatment. The resultant N-CNs@SiO2 composite is further thermally treated in a N2 atmosphere at different pyrolysis temperatures, followed by acid etching, to obtain 3D N-doped porous carbon (NPC) materials. As electrocatalysts for oxygen reduction reaction (ORR) in alkaline media, the experimental results demonstrate that 3D NPC obtained at 800 °C (NPC-800) with a surface area of 360.2 m2 g−1 exhibits the best ORR catalytic activity with an onset potential of −0.06 V, a half wave potential of −0.21 V and a large limiting current density of 5.3 mA cm−2 (at −0.4 V, vs. Ag/AgCl) among all NPC materials investigated, comparable to that of the commercial Pt/C catalyst with an onset potential of −0.03 V, a half wave potential of −0.17 V and a limiting current density of 5.5 mA cm−2 at −0.4 V. Such a 3D porous carbon ORR electrocatalyst also displays superior durability and high methanol tolerance in alkaline media, apparently better than the commercial Pt/C catalyst. The findings of this work would be valuable for the development of low-cost and abundant N-doped carbon materials from biomass as high performance metal-free electrocatalysts.

Combining TiO2 photocatalysts with noble metal nanoparticles (NPs) has been deemed an effective method to enhance visible-light-induced photocatalytic activity. Herein, we report that the mesoporous hollow TiO2–Au–TiO2 (MHTAT) sandwich structured nanocomposite was synthesized using resorcinol–formaldehyde resin polymer nanospheres as templates, followed by TiO2 coating, Au NP deposition, and external TiO2 coating, and then hydrothermal treatment and calcination for post-treatment of template removal. The MHTAT nanocomposites have enhanced visible light harvesting efficiency through their unique hierarchical nanostructure and high surface-to-volume ratio, which are beneficial to enhancing visible photocatalytic performance. The Au NPs were sandwiched between two shells of TiO2 in the MHTAT nanocomposites, which formed close Schottky contact for electron transfer through the interface between TiO2 and Au in the photocatalytic reaction. The photocatalytic activity of MHTAT sandwiched nanocomposites was fully demonstrated in degradation reactions of a number of organic compounds under visible light irradiation, suggesting their intriguing potential as effective visible-light-induced photocatalysts.

The facet-dependent photocatalytic performance of TiO2 nanocrystals has been extensively investigated due to their promising applications in renewable energy and environmental fields. However, the intrinsic distinction in the photocatalytic oxidation activities between the {001} and {101} facets of anatase TiO2 nanocrystals is still unclear and under debate. In this work, a simple photoelectrochemical method was employed to meaningfully quantify the intrinsic photocatalytic activities of {001} and {101} faceted TiO2 nanocrystal photoanodes. The effective surface areas of photoanodes with different facets were measured based on the monolayer adsorption of phthalic acid on TiO2 photoanode surface by an ex situ photoelectrochemical method, which were used to normalize the photocurrents obtained from different faceted photoanodes for meaningful comparison of their photocatalytic activities. The results demonstrated that the {001} facets of anatase TiO2 nanocrystals exhibited much better photocatalytic activity than that of {101} facets of anatase TiO2 nanocrystals toward photocatalytic oxidation of water and organic compounds with different functional groups (e.g., –OH, –CHO, –COOH). Furthermore, the instantaneous kinetic constants of photocatalytic oxidation of pre-adsorbates on {001} faceted anatase TiO2 photoanode are obviously greater than those obtained at {101} faceted anatase TiO2 photoanode, further verifying the higher photocatalytic activity of {001} facets of anatase TiO2. This work provided a facile photoelectrochemical method to quantitatively determine the photocatalytic oxidation activity of specific exposed crystal facets of a photocatalyst, which would be helpful to uncover and meaningfully compare the intrinsic photocatalytic activities of different exposed crystal facets of a photocatalyst.二氧化钛纳米晶的光催化性能具有晶面依赖效应,在可再生能源和环境领域已经展示出广阔的应用前景。然而,已有报道关于{001}和{101}晶面暴露的锐钛矿二氧化钛纳米晶的光催化活性本质区别还一直不是很清楚,甚至存在争议。本文采用一种简单的光电化学方法有意义地量化{001}和{101}晶面暴露的锐钛矿二氧化钛纳米晶的本征光催化活性。基于邻苯二甲酸在二氧化钛表面强的单分子层吸附特性,采用非原位光电化学方法测量了不同晶面暴露的二氧化钛光阳极有效表面积,进而将光电化学方法测定的光电流进行归一化处理,从而比较了{001}和{101}晶面暴露的锐钛矿二氧化钛光阳极的光催化活性。结果表明,{001}晶面暴露的锐钛矿二氧化钛纳米晶相比{101}晶面暴露的锐钛矿二氧化钛纳米晶,在光催化氧化水和有机物中表现出较高的光催化氧化活性。通过光电化学方法测定的光催化氧化瞬时动力学常数结果进一步证明了{001}晶面暴露的锐钛矿二氧化钛拥有更高的光催化氧化活性。本研究提供了一种简便的光电化学方法用于定量测量特定暴露晶面光催化剂光催化活性,有利于揭示和比较不同暴露晶面光催化剂的本征光催化活性。

•Natural biomass as reaction precursor;•Hydrothermal fabrication of nitrogen-doped carbon nanodots;•Self-assembly formation of N-doped carbon nanodots@nanospheres nanocomposite;•Superior electrocatalytic activity toward oxygen reduction reaction;In this work, nitrogen-doped carbon nanodots (N-CNDs) with sizes of 2–6 nm were successfully synthesized by hydrothermal treatment of natural biomass (e.g., fresh grass) at 180 °C for 10 h. The synthesized carbon nanodots were subsequently immobilized onto functionalized microporous carbon nanospheres (MCNSs) with an average diameter of ∼100 nm and a surface area of 241 m2 g−1via a simple hydrothermal process to self-assembly form a carbon-based nanocomposite (N-CNDs@MCNSs) owing to the presence of oxygen (O)-containing surface functional groups. As electrocatalyst for oxygen reduction reaction (ORR) application, our experimental results demonstrated that sole N-CNDs could not form stable electrocatalyst film for ORR measurement owing to their high water dispersion property, while the N-CNDs@MCNSs exhibited high electrocatalytic activity with an onset potential of −0.08 V, superior durability and high resistance to methanol cross-over effect, comparable to commercially available Pt/C electrocatalyst.

In this work, chitosan whiskers (CWs) were first extracted using low-cost and earth-abundant crab shells as materials by a series of chemical processes, and then assembled into chitosan whisker microspheres (CWMs) via a simple photochemical polymerization approach. Subsequently, a cementite (Fe3C) nanocrystal@N-doped graphitic carbon (Fe3C@NGC) nanocomposite was successfully fabricated by high temperature pyrolysis of CWMs adsorbed with ferric acetylacetonate (Fe(acac)3) at 900 °C. It was found that a suitable growth atmosphere generated inside CWMs during high temperature pyrolysis is critically important to form Fe3C nanocrystal cores, concurrently accompanying a structural transformation from chitosan whiskers to mesoporous graphitic carbon shells with natural nitrogen (N) doping properties, resulting in the formation of a core–shell structure Fe3C@NGC nanocomposite. The resulting samples were evaluated as electrocatalysts for oxygen reduction reaction (ORR). In comparison with sole N-doped graphitic carbon without Fe3C nanocrystals obtained by direct pyrolysis of chitosan whisker microspheres at 900 °C (CWMs-900), Fe3C@NGC showed significantly improved ORR catalytic activity. The tolerance to fuel cell molecules (e.g., methanol) and the durability of Fe3C@NGC are obviously superior to commercial Pt/C catalysts in alkaline media. The high ORR performance of Fe3C@NGC could be due to its large surface area (313.7 m2 g−1), a synergistic role of Fe3C nanocrystals, N doping in graphitic carbon creating more catalytic active sites, and a porous structure of the nanocomposite facilitating mass transfer to efficiently improve the utilization of these catalytic active sites.

In this work, vertically aligned anatase TiO2 single crystal nanosheets with laterally exposed {001} facets onto a conducting FTO substrate (VATN) were successfully synthesised using hydrofluoric acid (40 wt%) as a crystal facet controlling agent by a simple hydrothermal method. The as-synthesised VATN without calcination exhibited a good crystalline structure, and was used as a photoanode showing superior photoelectrocatalytic activity toward water oxidation under UV irradiation. After thermal treatment at 550 °C for 2 h, the photoelectrocatalytic activity of the VATN photoanode was almost 2.6 times that for unsintered VATN under the same experimental conditions, which could be mainly due to the surface passivation role of surface fluorine in unsintered VATN to decrease the photoelectrocatalytic activity. A photoelectrochemical method was used to manifest the photoelectron transport properties inside VATN photoanodes and concurrently quantify the inherent resistances (R0) of UV illuminated photoanodes before and after calcination. The results demonstrated that the determined R0 values were respectively 155 Ω and 66 Ω for VATN photoanodes before and after calcination, inversely proportional to their photoelectrocatalytic activities. Compared to VATN before calcination, a significantly decreased R0 value of VATN after calcination further confirmed the presence of surface fluorine in VATN unfavorable for photoelectron transport inside a photocatalyst film. This work provided direct evidence to prove the intrinsic photoelectron transport properties of {001} faceted anatase TiO2 nanosheet array film photoanodes in the presence and absence of surface fluorine.

•N-doped activated carbon supported Ni catalyst was simply synthesized by a two-step calcination method.•N doping content and type in activated carbon have important influence on catalytic hydrogenation activity of Ni/NAC.•Highly active and selective H2-hydrogenation/transfer hydrogenation of furfural was studied under mild conditions.•The catalyst can be reutilized during successive catalytic cycles.•The performance enhancement mechanism has been discussed on the basis of the experimental results.In this work, N-doped activated carbon supported metallic nickel (Ni/NAC) catalysts were fabricated by two-step calcination method in N2 atmosphere for liquid-phase hydrogenation of furfural (FAL). It was found that the pyrolysis temperature and amount of melamine as N doping source have important influence on N doping content and type in activated carbon (AC) support, resulting in the subsequently formed Ni nanoparticles on N-doped AC with different sizes and thus affording different catalytic hydrogenation activities. The results demonstrated that using N-doped AC with 1.0 g melamine at 1073 K in N2 atmosphere as support, the obtained Ni/NAC at 873 K in N2 atmosphere with Ni nanoparticle sizes of ∼13.1 nm (denoted as Ni/NAC-1-1073) exhibits a N doping content of 3.65 at.% and a surface area of 561.2 m2 g−1 with a microporous structure. As catalyst for FAL hydrogenation, Ni/NAC-1-1073 demonstrated the best catalytic performance among all investigated catalysts, achieving almost 100% selectivity of tetrahydrofurfuryl alcohol (THFOL) with a complete FAL conversion at 353 K after 3 h reaction, while only 76.7% selectivity of THFOL with a FAL conversion of 86.4% was obtained using Ni/AC catalyst without N doping under the identical experimental conditions. Furthermore, it was found that almost 100% conversion of FAL to furfural alcohol (FOL) can be reached by transfer hydrogenation pathway in 2-proponal solvent using Ni/NAC-1-1073 at 413 K after 5 h reaction, whereas Ni/AC without N doping can only afford 30.2% conversion of FAL to FOL under the same conditions. The superior catalytic performance of Ni/NAC-1-1073 could be ascribed to a synergistic effect of nanosized Ni providing catalytic active sites, suitable N doping content and type in AC to promote catalytic performance, and advantageous structure characteristics of high surface area and porous structure favourable for the exposure of catalytic active sites and mass transport.Download full-size image

The facet-dependent photocatalytic performance of TiO2 nanocrystals has been extensively investigated due to their promising applications in renewable energy and environmental fields. However, the intrinsic distinction in the photocatalytic oxidation activities between the {001} and {101} facets of anatase TiO2 nanocrystals is still unclear and under debate. In this work, a simple photoelectrochemical method was employed to meaningfully quantify the intrinsic photocatalytic activities of {001} and {101} faceted TiO2 nanocrystal photoanodes. The effective surface areas of photoanodes with different facets were measured based on the monolayer adsorption of phthalic acid on TiO2 photoanode surface by an ex situ photoelectrochemical method, which were used to normalize the photocurrents obtained from different faceted photoanodes for meaningful comparison of their photocatalytic activities. The results demonstrated that the {001} facets of anatase TiO2 nanocrystals exhibited much better photocatalytic activity than that of {101} facets of anatase TiO2 nanocrystals toward photocatalytic oxidation of water and organic compounds with different functional groups (e.g., –OH, –CHO, –COOH). Furthermore, the instantaneous kinetic constants of photocatalytic oxidation of pre-adsorbates on {001} faceted anatase TiO2 photoanode are obviously greater than those obtained at {101} faceted anatase TiO2 photoanode, further verifying the higher photocatalytic activity of {001} facets of anatase TiO2. This work provided a facile photoelectrochemical method to quantitatively determine the photocatalytic oxidation activity of specific exposed crystal facets of a photocatalyst, which would be helpful to uncover and meaningfully compare the intrinsic photocatalytic activities of different exposed crystal facets of a photocatalyst.

Development of cheap, abundant and metal-free N-doped carbon materials as high efficiency oxygen reduction electrocatalysts is crucial for their practical applications in future fuel cell devices. Here, three-dimensional (3D) N-doped porous carbon (NPC) materials have been successfully developed by a simple template-assisted (e.g., SiO2 spheres) high temperature pyrolysis approach using shrimp-shell derived N-doped carbon nanodots (N-CNs) as carbon and nitrogen sources obtained through a facile hydrothermal method. The shrimp-shell derived N-CNs with a product yield of ∼5% possess rich surface O- and N-containing functional groups and small nanodot sizes of 1.5–5.0 nm, which are mixed with surface acidification treated SiO2 spheres with an average diameter of ∼200 nm in aqueous solution to form a N-CNs@SiO2 composite subjected to a thermal evaporation treatment. The resultant N-CNs@SiO2 composite is further thermally treated in a N2 atmosphere at different pyrolysis temperatures, followed by acid etching, to obtain 3D N-doped porous carbon (NPC) materials. As electrocatalysts for oxygen reduction reaction (ORR) in alkaline media, the experimental results demonstrate that 3D NPC obtained at 800 °C (NPC-800) with a surface area of 360.2 m2 g−1 exhibits the best ORR catalytic activity with an onset potential of −0.06 V, a half wave potential of −0.21 V and a large limiting current density of 5.3 mA cm−2 (at −0.4 V, vs. Ag/AgCl) among all NPC materials investigated, comparable to that of the commercial Pt/C catalyst with an onset potential of −0.03 V, a half wave potential of −0.17 V and a limiting current density of 5.5 mA cm−2 at −0.4 V. Such a 3D porous carbon ORR electrocatalyst also displays superior durability and high methanol tolerance in alkaline media, apparently better than the commercial Pt/C catalyst. The findings of this work would be valuable for the development of low-cost and abundant N-doped carbon materials from biomass as high performance metal-free electrocatalysts.

In this work, chitosan whiskers (CWs) were first extracted using low-cost and earth-abundant crab shells as materials by a series of chemical processes, and then assembled into chitosan whisker microspheres (CWMs) via a simple photochemical polymerization approach. Subsequently, a cementite (Fe3C) nanocrystal@N-doped graphitic carbon (Fe3C@NGC) nanocomposite was successfully fabricated by high temperature pyrolysis of CWMs adsorbed with ferric acetylacetonate (Fe(acac)3) at 900 °C. It was found that a suitable growth atmosphere generated inside CWMs during high temperature pyrolysis is critically important to form Fe3C nanocrystal cores, concurrently accompanying a structural transformation from chitosan whiskers to mesoporous graphitic carbon shells with natural nitrogen (N) doping properties, resulting in the formation of a core–shell structure Fe3C@NGC nanocomposite. The resulting samples were evaluated as electrocatalysts for oxygen reduction reaction (ORR). In comparison with sole N-doped graphitic carbon without Fe3C nanocrystals obtained by direct pyrolysis of chitosan whisker microspheres at 900 °C (CWMs-900), Fe3C@NGC showed significantly improved ORR catalytic activity. The tolerance to fuel cell molecules (e.g., methanol) and the durability of Fe3C@NGC are obviously superior to commercial Pt/C catalysts in alkaline media. The high ORR performance of Fe3C@NGC could be due to its large surface area (313.7 m2 g−1), a synergistic role of Fe3C nanocrystals, N doping in graphitic carbon creating more catalytic active sites, and a porous structure of the nanocomposite facilitating mass transfer to efficiently improve the utilization of these catalytic active sites.

Due to their controllable morphologies, tunable porous structures, diverse compositions and easy fabrication, metal–organic frameworks (MOFs) are an ideal class of precursor material to develop high performance carbon-based materials for energy applications. In this work, two-dimensional (2D) Co/Ni MOFs nanosheets with a molar ratio of Co2+ to Ni2+ of 1:1 were first synthesized at room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (4,4′-Bpy) as organic linkers. As a precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 °C in N2 atmosphere to incorporate 2D CoNi alloy nanoparticles into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m2 g−1, a porous structure, and good conductivity. Interestingly, it was found that the 2D Co/Ni MOFs nanosheets can be directly used as electrode materials for supercapacitors, delivering a specific capacitance of 312 F g−1 at 1 A g−1, whereas CoNi@SNC derived from its MOFs precursor as an electrode material for supercapacitors exhibits a much higher specific capacitance (1970, 1897 and 1730 F g−1 at 1, 2, 5 A g−1, respectively) with long cycling life (retaining 95.1% of the value at 10 A g−1 after 3000 cycles) and excellent rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 W h kg−1 at a power density of 0.8 kW kg−1 with lifetime stability up to 4000 charge–discharge cycles (capacitance retention of ∼90.6%). The results demonstrate that electrochemical activation-generated CoNi oxides/oxyhydroxides on the surface of the CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements are the electrochemical active species of the CoNi@SNC-constructed supercapacitor. Additionally, the high performance of the CoNi@SNC-constructed supercapacitor can be collectively attributed to its relatively high surface area, which is favourable for the exposure of electrochemical active sites; its porous structure, which promotes redox-related mass transport; and the combination of CoNi alloy nanoparticles with graphitic carbon, which functions as an electron collector to improve electron transfer.

Co/CoO nanoparticles immobilized on Co–N-doped carbon were successfully developed using shrimp-shell derived N-doped carbon nanodots as precursors by a combined approach of polymerization and pyrolysis, as electrocatalysts exhibiting trifunctional catalytic activities toward oxygen reduction, oxygen evolution and hydrogen evolution reactions and high performance in rechargeable zinc–air batteries.

Controllable synthesis of metal–organic framework (MOF) materials with tunable morphologies, sizes, compositions and pore structures is critically important for MOF materials and their pyrolysis derivatives’ applications in environmental and energy fields. Here we report the synthesis of Co-MOF crystals with controllable morphologies, sizes and S/N ratios in water/NaOH and ethylene glycol/NaOH systems using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (Bpy) as S, N dual organic ligands by a “pillar-layer” assembly method. Water and ethylene glycol with different viscosities result in various crystallization processes of [Co(Tdc)(Bpy)]n crystals in the corresponding reaction system, thus respectively obtaining three-dimensional (3D) Co-MOF ([Co(Tdc)(Bpy)]n) bulk and cuboid structures in water/NaOH and ethylene glycol/NaOH reaction systems. The as-prepared Co-MOF crystals in two different reaction systems were further pyrolytically treated at 800 °C in a N2 atmosphere to obtain Co9S8@S,N-doped carbon materials with different surface areas, pore distributions and S/N doping ratios. As electrocatalysts, the Co9S8@S,N-doped carbon cuboid (Co9S8@SNCC) obtained in the ethylene glycol/NaOH system demonstrates superior bifunctional electrocatalytic activities toward both oxygen reduction and evolution reactions resulting from Co9S8 and S, N doping in the carbon structure providing catalytic active sites, better than that of Co9S8@S,N-doped carbon bulk (Co9S8@SNCB) obtained in the water/NaOH system and comparable to commercial Pt/C and RuO2 catalysts. Owing to its high surface area and porous structure, Co9S8@SNCC also exhibits great potential as the electrode material for application in supercapacitors, with high performance and recycling stability.

Metal oxides are a class of promising electrode materials for supercapacitors because of their high theoretical energy density; however, the low electrical conductivity and instability of metal oxides limit their large-scale practical applications. Here we report a facile and scalable method to synthesize ultrafine nickel–cobalt alloy nanoparticles (5–10 nm) embedded into three-dimensional porous graphitic carbon (3D Ni–Co@PGC) using NaCl as the template to create a porous structure and glucose as the carbon source by pyrolysis treatment at 800 °C under N2 atmosphere. As an electrode material for supercapacitors, the ultrafine Ni–Co alloy nanoparticles of 3D Ni–Co@PGC not only serve as current collectors, but also their surfaces convert to corresponding metal oxides when exposed to an alkaline electrolyte, responsible for redox reactions in pseudocapacitors, exhibiting high supercapacitor performance. The results demonstrate that the supercapacitor assembled with 3D Ni–Co@PGC electrodes shows high energy density (1091, 1064 and 1041 F g−1 at 1, 2 and 4 A g−1, respectively), long cycling life and excellent rate capability at a high charge/discharge current. Furthermore, an asymmetric supercapacitor assembled by using 3D Ni–Co@PGC as the positive electrode and active carbon as the negative electrode shows a high energy density of 33.7 W h kg−1 and remarkable cycling stability (98% capacitance retention over 4000 cycles). The superior performance of the 3D Ni–Co@PGC constructed supercapacitor can be ascribed to its high surface area (265 m2 g−1), porous structure and excellent electrical conductivity, favourable for the exposure of reaction active sites, redox-related mass transport and electron transfer, respectively.

In this work, shrimp-shell derived N-doped carbon nanodots (N-CNs) as carbon and nitrogen sources are assembled into particle-like aggregates by a simple polymerization reaction of pyrrole in the presence of Fe3+ to form Fe containing N-CN/polypyrrole (PPY) composites (Fe–N-CN/PPy). The resulting composites are thermally treated by a facile pyrolysis approach under a N2 atmosphere to obtain an Fe,N-doped porous graphitic carbon (Fe-N-PGC) material. The results demonstrate that the pyrolytically converted carbon material at 800 °C (Fe-N-PGC-800) exhibits an approximately mesoporous structure with a pore size distribution centered at ∼1.97 nm and ∼2.8 nm and a surface area of 806.7 m2 g−1. As an electrocatalyst for oxygen reduction reaction (ORR) in alkaline media, Fe-N-PGC-800 shows superior ORR catalytic activity with an onset potential of −0.017 V and a limiting current density of 5.42 mA cm−2 (at −0.4 V, vs. Ag/AgCl), which is superior to that of commercial Pt/C catalysts (onset potential of −0.018 V and a limiting current density of 5.21 mA cm−2 at −0.4 V, vs. Ag/AgCl). Additionally, Fe-N-PGC-800 also exhibits good ORR activity in acidic media with an onset potential of 0.53 V and a limiting current density of 5.58 mA cm−2 (at 0.1 V, vs. Ag/AgCl), comparable to that of most reported Fe-based N-doped carbon electrocatalysts. An air cathode made from Fe-N-PGC-800 shows high performance and superior cycling durability in zinc–air batteries (gravimetric energy density of 752 Wh kg−1), comparable to that of commercial Pt/C-based batteries (gravimetric energy density of 774 Wh kg−1). This work demonstrates the feasibility of utilizing biomass as a starting material to fabricate Fe,N-doped carbon materials as high performance ORR electrocatalysts for practical application in ORR-relevant energy devices.