Yingfei Hu

Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation

Active, stable, and cost-effective electrocatalysts are attractive alternatives to the noble metal oxides that have been used in water splitting. The direct nucleation and growth of electrochemically active LDH materials on chemically modified MWCNTs exhibit considerable electrocatalytic activity toward oxygen evolution from water oxidation. CoMn-based and NiMn-based hybrids were synthesized using a facile chemical bath deposition method and the as-synthesized materials exhibited three-dimensional hierarchical configurations with tunable Co/Mn and Ni/Mn ratio. Benefiting from enhanced electrical conductivity with MWCNT backbones and LDH lamellar structure, the Co5Mn-LDH/MWCNT and Ni5Mn-LDH/MWCNT could generated a current density of 10 mA cm(-2) at overpotentials of ∼300 and ∼350 mV, respectively, in 1 M KOH. In addition, the materials also exhibited outstanding long-term electrocatalytic stability.

Enhanced Performance of Photoelectrochemical Water Splitting with ITO@α-Fe2O3 Core–Shell Nanowire Array as Photoanode

Hematite (α-Fe2O3) is one of the most promising candidates for photoelectrodes in photoelectrochemical water splitting system. However, the low visible light absorption coefficient and short hole diffusion length of pure α-Fe2O3 limits the performance of α-Fe2O3 photoelectrodes in water splitting. Herein, to overcome these drawbacks, single-crystalline tin-doped indium oxide (ITO) nanowire core and α-Fe2O3 nanocrystal shell (ITO@α-Fe2O3) electrodes were fabricated by covering the chemical vapor deposited ITO nanowire array with compact thin α-Fe2O3 nanocrystal film using chemical bath deposition (CBD) method. The J-V curves and IPCE of ITO@α-Fe2O3 core-shell nanowire array electrode showed nearly twice as high performance as those of the α-Fe2O3 on planar Pt-coated silicon wafers (Pt/Si) and on planar ITO substrates, which was considered to be attributed to more efficient hole collection and more loading of α-Fe2O3 nanocrystals in the core-shell structure than planar structure. Electrochemical impedance spectra (EIS) characterization demonstrated a low interface resistance between α-Fe2O3 and ITO nanowire arrays, which benefits from the well contact between the core and shell. The stability test indicated that the prepared ITO@α-Fe2O3 core-shell nanowire array electrode was stable under AM1.5 illumination during the test period of 40,000 s.

Two-dimensional nanomaterials for photocatalytic CO2 reduction to solar fuels

As a “kill two birds with one stone” approach, photocatalytic CO2 reduction to solar fuels can save supplying energy and simultaneously protect our environment. Specifically, the use of CO2 as the starting carbon source can help with the required emission cuts. Meanwhile, it directly generates short-chain hydrocarbon products such as CH4, CH3OH, C2H6 and so on, which can serve as a renewable energy source (solar fuels) to alleviate the increasingly tense energy crisis. Two-dimensional (2D) nanomaterials possess several extraordinary advantages, including large surface-to-volume ratio, abundant active sites, atomic thickness, and a high fraction of coordinated unsaturated surface sites, making them promising candidates with high photocatalytic activity for CO2 reduction. This review summarizes a series of typical 2D nanomaterials for photocatalytic CO2 conversion, such as graphene-based photocatalysts, graphitic carbon nitride-based photocatalysts, 2D metal oxide-based photocatalysts, 2D metal chalcogenide-based photocatalysts, 2D metal oxyhalide-based photocatalysts, and layered double hydroxide-based photocatalysts. Furthermore, based on the characteristics of 2D materials and the current status of research on photocatalytic CO2 reduction, the challenges and opportunities of 2D materials as prospective photocatalysts for CO2 reduction will also be discussed.

Enhancement of Photoelectrochemical Performance in Water Oxidation over Bismuth Vanadate Photoanodes by Incorporation with Reduced Graphene Oxide

Bismuth vanadate photoanodes often suffer from poor electron transport, which restricts their photoelectrochemical performance in water oxidation. Here, reduced graphene oxide sheets with excellent electronic conductivity are introduced to bismuth vanadate photoanode films to provide a channel for electron transport. As a result, the charge separation efficiency of the photoanode films increases from 59 to 78 % at 420 nm and 1.23 V (vs. the reversible hydrogen electrode) under back irradiation. In particular, the onset potential for photoelectrochemical water oxidation over reduced graphene oxide–bismuth vanadate photoanode films has a cathodic shift of approximately 120 mV in potassium borate electrolyte compared to bare bismuth vanadate photoanode films.

Rational design of electrocatalysts for simultaneously promoting bulk charge separation and surface charge transfer in solar water splitting photoelectrodes

For a photoanode, a large overpotential for water oxidation, which limits the solar-to-hydrogen efficiency, may be caused by slow water oxidation kinetics and the recombination of photo-induced electrons and holes. Herein, taking flat and thin BiVO4 photoanodes as an example, the composite AgOx/NiOx electrocatalyst was found to promote not only the water oxidation kinetics, but also the bulk charge separation. As a result, the surface charge injection efficiency (ηinj) and the bulk charge separation efficiency (ηsep) of BiVO4 photoanodes were improved by the composite AgOx/NiOx electrocatalyst. Photo-assisted electrochemical impedance spectroscopy (EIS) was employed to illustrate the significantly reduced surface charge transfer resistance of the BiVO4/AgOx/NiOx sample at the interface between the photoanode surface and the electrolyte. Analysis of the surface potential changes obtained from photo-assisted Kelvin probe force microscopy (KPFM) revealed that the surface photovoltage (SPV) of the BiVO4/AgOx/NiOx photoanode is higher than those of BiVO4/AgOx and BiVO4/NiOx, representing its large band bending region for bulk charge separation. The open circuit photovoltage (OCP) measurements also demonstrated the superior charge separation ability of the BiVO4/AgOx/NiOx photoanode. The possible working mechanism is that one component of the composite AgOx/NiOx electrocatalyst may stabilize the high valence states of the other metal ions, which is beneficial for the formation of water oxidation active sites and the extension of the band bending region.

Improved water-splitting performances of CuW1−xMoxO4 photoanodes synthesized by spray pyrolysis

CuW1−xMoxO4 solid solution of CuWO4 and CuMoO4, which is a copper-based multi-component oxide semiconductor, possesses much narrower band gap than CuWO4. In theory, it can absorb a larger part of the visible spectrum, widening the use of solar spectroscopy and obtaining a higher photo-to-chemical conversion efficiency. In this study, CuW1−xMoxO4 thin-film photoanodes on conducting glass were prepared using a simple and low-cost spray pyrolysis method. The resulting CuW1−xMoxO4 photoanodes perform higher photocurrent than CuWO4 photoanodes under AM 1.5G simulated sunlight illumination (100 mW cm−2) in 0.1 mol L−1 phosphate buffer at pH 7. Combined with IPCE and Mott-Schottky analysis, the enhancement of the photocurrent is due to the improvement of photon utilization and the increase of carrier concentration with the incorporation of Mo atoms. Moreover, with the optimal Mo/W atomic ratio, the photocurrent density increases obviously from 0.07 to 0.46 mA cm−2 at 1.23 VRHE bias. In addition, compared with particle-assembled thin-film photoanodes prepared by solidphase reaction and drop-necking treatment, the photoanodes prepared by spray pyrolysis have obvious advantages in terms of reducing resistance and facilitating charge transport.摘要CuWO4和CuMoO4的固溶体CuW1−xMoxO4是一种铜基多组分氧化物半导体, 拥有比CuWO4更窄的带隙. 理论上, 它可以拓宽对太阳光谱的响应范围, 吸收更大部分的可见光, 获得更高的太阳能-化学能转换效率. 本研究通过简单、 低成本的喷雾热裂解法在导电玻璃上制备了CuW1−xMoxO4薄膜光阳极. 在AM 1.5G模拟太阳光(100 mW cm−2)照射下, 制备出的CuW1−xMoxO4光阳极在pH 7的0.1 mol L−1磷酸缓冲液中产生了比CuWO4光阳极更高的光电流. 结合IPCE和莫特-肖特基分析可知, 光电流的增长来源于Mo原子的加入所造成的光子利用率的提高以及载流子浓度的增加. 而且, 在最优的Mo/W原子比例下, 1.23 VRHE偏压时的光电流从0.07 mA cm−2显著地增加到 0.46 mA cm−2. 与对应的颗粒组装薄膜光阳极(通过固相反应结合涂覆-粘结后处理得以制备)相比较, 喷雾热裂解法所制备的光阳极有利于降低电阻和促进电荷传输.

Improved charge separation efficiency of hematite photoanodes by coating an ultrathin p-type LaFeO3 overlayer

Many metal-oxide candidates for photoelectrochemical water splitting exhibit localized small polaron carrier conduction. Especially hematite (α-Fe2O3) photoanodes often suffer from low carrier mobility, which causes the serious bulk electron-hole recombination and greatly limits their PEC performances. In this study, the charge separation efficiency of hematite was enhanced greatly by coating an ultrathin p-type LaFeO3 overlayer. Compared to the hematite photoanodes, the solar water splitting photocurrent of the Fe2O3/LaFeO3 n-p junction exhibits a 90% increase at 1.23 V versus the reversible hydrogen electrode, due to enlarging the band bending and expanding the depletion layer.