Transition metal carbides (TMCs) and transition metal nitrides (TMNs) have attracted attention as promising electrocatalysts that could replace noble metals of high price and limited supply. Relative to parent metals, TMC and TMN behave like noble metals for electrochemical reactions such as oxidation of hydrogen, CO and alcohols, and reduction of oxygen. When TMC and TMN are combined with other metals, the electrocatalytic synergy is often observed in electrochemical reactions. Thus, combinations with a minute amount of Pt or even non-Pt metals give performance comparable to heavily loaded Pt-based electrocatalysts for low temperature fuel cells. It appears that TMC based electrocatalysts are more active as anode catalysts for oxidation of fuels, whereas TMN based catalysts are more active for cathode catalysts for oxygen reduction and more stable.

Hexagonal WO 3 (hex-WO 3 ) nanowires with high aspect ratio and crystallinity have been prepared for the first time by a microwave-assisted hydrothermal method. By using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy, the phase and morphology of the products were identified, which were controlled by reaction temperature, holding time and added salts. Uniform hex-WO 3 nanowires with a diameter of 5-10 nm and lengths of up to several micrometres were synthesized by a microwave-assisted hydrothermal process at 150 C for 3 h in a solution containing (NH 4 ) 2 SO 4 as a capping reagent and Na 2 WO 4 as a starting material. The aspect ratio and specific surface area of hex-WO 3 nanowires were 625 and 139 m 2 g À1 , respectively, which represented one of the highest values reported for WO 3 . The electrocatalytic activity for hydrogen evolution reaction of hex-WO 3 nanowires was also investigated by cyclic voltammetry and linear sweep voltammetry. The results demonstrated that hex-WO 3 nanowires were a promising electrocatalyst for the hydrogen evolution reaction (HER) from water. Experimental Synthesis and characterizationTo synthesize 1D WO 3 , 2 g analytical grade sodium tungstate dihydrate (Na 2 WO 4 $2H 2 O) were dissolved in 45 ml distilled water under stirring at room temperature and 5 ml 3 M HCl was

1 Solar conversion of carbon dioxide and water to value-added chemicals remains a challenge. A 2 number of solar-active catalysts have been reported but still suffer from low selectivity, poor 3 energy efficiency, and instability, and fail to drive simultaneous water oxidation. Herein, we 4 report CuFeO 2 and CuO mixed p-type catalysts fabricated via a widely employed electroplating 5 of earth-abundant cupric and ferric ions followed by annealing under atmospheric air. The 6 composite electrodes exhibited onset potentials at +0.9 V vs. RHE in CO 2 -purged bicarbonate 7 solution and converted CO 2 to formate with over 90% selectivity under simulated solar light (Air 8 Mass 1.5, 100 mW⋅cm −2 ). Wired CuFeO 2 /CuO photocathode and Pt anode couples produced 9 formate over 1 week at a solar-to-formate energy conversion efficiency of ~1% (selectivity 10 >90%) without any external bias while O 2 was evolved from water. Isotope and nuclear magnetic 11 resonance analyses confirmed the simultaneous production of formate and O 2 at the stand-alone 12 couples. Solar CO 2 recycling has received wide attention primarily to address global CO 2 emission and to 1 convert CO 2 and water to value-added chemicals. 1-3 Despite a long research history over the past 2 four decades, 4,5 the technology remains in an early stage, with low CO 2 conversion efficiency 3 and selectivity. CO 2 is highly stable and has limited solubility in water, and its reduction requires 4 multiple proton-coupled electron transfers, resulting in a range of carbon intermediates (C1 -5 C3) 2,6 as well as a larger amount of H 2 over CO 2 conversion products. 7-9 6 For the realization of solar CO 2 recycling, the system of interest should be operated 7 sustainably, which requires the development of not only energy-efficient and cost-effective 8 materials but also stand-alone, complete reaction processes (CO 2 reduction and water oxidation) 9 operating for long periods without any external bias. 10-12 A range of semiconductors (mostly p-10 types) have been studied for CO 2 conversion, including GaP, 4 InP, 5 GaAs, 13 Si, 8,14 Cu 2 O, 15-18 and 11 CuFeO 2 , 19,20 all of which have narrow bandgaps (E g ) and sufficient Fermi levels (E F ) capable of 12 reducing CO 2 . Although promising, these materials inherently require potential biases to drive 13 the CO 2 reduction reaction and compete with other metallic electrodes, 21 whereas complete 14 reactions (CO 2 reduction and water oxidation) have been rarely demonstrated due to large 15 overpotentials. Photocathode-photoanode couples have been demonstrated to operate, 11 yet the 16 syntheses of materials are complicated and the energy conversion efficiency is low (max. 0.14%). 17 We have searched for high-efficiency, low-cost, and scalable p-type materials and found 18 that CuFeO 2 and CuO mixed materials meet all requirements. To our surprise, this material 19 converted CO 2 to formate with selectivity greater than 90% over 1 week and simultaneously 20 produced molecular oxygen via water oxidation when simply ...

display highly effi cient charge carrier separation. In addition, the charge carriers must be injected effi ciently into solution and drive the desired reactions. [ 10 ] Silicon (Si) has a great potential as a photoelectrode because it is an earth-abundant element with several desirable properties, including a narrow energy band gap of ≈1.2 eV, high carrier mobility, stability over a wide pH range, non-toxicity, and commercial availability. [ 11 ] Si is a key material in the solid-state photovoltaic industry, while modifi ed Si has been used increasingly in solid/liquid photoelectrochemistry. For example, the surface of a p-Si was doped heavily with donor (n + p-Si) to acquire a larger open circuit voltage in photoelectrochemical (PEC) H 2 production. [ 12,13 ] Metal oxides were deposited on the surface of the n-Si photoanodes as a protective layer in PEC water oxidation. [ 14 ] Although planar p-Si is promising, [ 15 ] charge carrier recombination can occur due to the low diffusion length of the minority carriers in the same absorber thickness. [ 16 ] However, a wire-array geometry possesses long optical paths for effi cient photon absorption and increased collection effi ciency for the minority carrier. A comparison of planar p-Si and p-Si wire arrays indicated that the latter exhibits a signifi cantly lower refl ectance [ 17 ] and 0.1-0.3 V higher anodic onset potentials in PEC water splitting processes. [ 13,18 ] With this in mind, this study attempted, for the fi rst time, to fabricate Sn-coupled p-Si nanowire arrays for application to solar CO 2 conversion ( Scheme 1 ). Vertically aligned, freestanding p-Si nanowire arrays of varying lengths were grown on p-Si wafers using an electroless chemical etching technique. The wire arrays prepared using this method exhibited a >0.5 V higher anodic onset potential compared to planar p-Si and an approximately two-fold increase in photocurrent generation and formate production. However, the Faradaic effi ciencies for formate formation of the planar and wire electrodes were similar at <10%, presumably due to the same surface characteristics.In an attempt to catalyze formate production, Sn nanoparticles were strategically photo-electrodeposited onto the p-Si electrodes because of its high electrocatalytic effect for converting CO 2 to formate. [ 19 ] These heterojunction wire/Sn arrays displayed a signifi cant increase (>5 fold) in the production of formate with Faradaic effi ciencies of ≈40% in a single cell and 88% in an H-type cell. In addition, H 2 production in a single cell was suppressed by 50% compared to that observed with bare wire arrays.The Ag-catalyzed electroless chemical etching of the p-Si wafer produced vertically aligned, free-standing Si wires on the substrate ( Figure 1 A). The wires were tightly packed with interwire pores in the submicrometer size range (≈100 nm) formed by the metal catalyst. The sizes of the interwire pores were Solar CO 2 conversion has recently attracted increasing attention in an attempt to reduce anthropogenic carbon emissio...

Mesoporous tungsten carbides displayed an excellent solar conversion efficiency (7.01%) as a counter electrode for dye sensitized solar cells under 100 mW cm(-2), AM 1.5G illumination, which corresponded to ca. 85% of the efficiency of the conventional platinum electrode.