Electrodeposition of a cobalt phosphide film for the enhanced photoelectrochemical water oxidation with α-Fe2O3 photoanode

Gold-Promoted Electrodeposition of Metal Sulfides on Silicon Nanowire Photocathodes To Enhance Solar-Driven Hydrogen Evolution

Constructing composite structures is the key to breaking the dilemma of slow reaction kinetics and easy oxidation on the surface of lightly doped p-type silicon nanowire (SiNW) array photocathodes. Electrodeposition is a convenient and fast technique to prepare composite photocathodes. However, the low conductivity of SiNWs limits the application of the electrodeposition technique in constructing composite structures. Herein, SiNWs were loaded with Au nanoparticles by chemical deposition to decrease the interfacial charge transfer resistance and increase active sites for the electrodeposition. Subsequently, co-catalysts CoS, MoS₂, and Ni₃S₂ with excellent hydrogen evolution activity were successfully composited by electrodeposition on the surface of SiNWs/Au. The obtained core-shell structures exhibited excellent photoelectrochemical hydrogen evolution activity, which was contributed by the plasma property of Au and the abundant hydrogen evolution active sites of the co-catalysts. This work provided a simple and efficient solution for the preparation of lightly doped SiNW-based composite structures by electrodeposition.

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

A rational regulation of the solar water splitting reaction pathway by adjusting the surface composition and phase structure of catalysts is a substantial approach to ameliorate the sluggish reaction kinetics and improve the energy conversion efficiency. In this study, we demonstrate a nanocrystalline iron pyrophosphate (Fe4(P2O7)3, FePy)-regulated hybrid overlayer with amorphous iron phosphate (FePO4, FePi) on the surface of metal oxide nanostructure with boosted photoelectrochemical (PEC) water oxidation. By manipulating the facile electrochemical surface treatment followed by the phosphating process, nanocrystalline FePy is localized in the FePi amorphous overlayer to form a heterogeneous hybrid structure. The FePy-regulated hybrid overlayer (FePy@FePi) results in significantly enhanced PEC performance with long-term durability. Compared with the homogeneous FePi amorphous overlayer, FePy@FePi can improve the charge transfer efficiency more significantly, from 60% of FePi to 79% of FePy@FePi. Our density-functional theory calculations reveal that the coexistence of FePi and FePy phases on the surface of metal oxide results in much better oxygen evolution reaction kinetics, where the FePi was found to have a typical down-hill reaction for the conversion from OH* to O2, while FePy has a low free energy for the formation of OH*.

MOF-on-MOF Strategy to Construct a Nitrogen-Doped Carbon-Incorporated CoP@Fe-CoP Core-Shelled Heterostructure for High-Performance Overall Water Splitting

The design and preparation of efficient and low-cost catalysts for water electrolysis are crucial and highly desirable to produce eco-friendly and sustainable hydrogen fuel. Herein, we prepared nitrogen-doped carbon-incorporated CoP@Fe-CoP core-shelled nanorod arrays grown on Ni foam (CoP@Fe-CoP/NC/NF) through phosphorization of ZIF-67@Co-Fe Prussian blue analogue (ZIF-67@CoFe-PBA). The hierarchical nanorod arrays combined with the core-shelled structure offer favorable mass/electron transport capacity and maximize the active sites, thus enhancing the electrochemically active surface area. The synergistic effect of the bimetallic components and the nitrogen-doped carbon matrix endow the composite with an optimized electronic structure. Benefiting from the above superiorities of morphological and chemical compositions, this self-supported CoP@Fe-CoP/NC/NF heterostructure can drive alkaline hydrogen evolution reaction and oxygen evolution reaction with overpotentials of 97 and 270 mV to yield 100 mA cm^-2, respectively. The two-electrode alkaline electrolyzer constructed by this heterostructure shows a low cell voltage of 1.58 V to yield 10 mA cm^-2, superior to the precious-metal-based electrocatalyst apparatus (IrO₂∥Pt/C). This study offers a feasible and facile approach to develop efficient electrocatalysts for water electrolysis, which applies to other electrochemical energy conversion and storage applications.

Constructed Interfacial Oxygen-Bridge Chemical Bonding in Core-Shell Transition Metal Phosphides/Carbon Hybrid Boosting Oxygen Evolution Reaction

A designed structure which CoP nanoparticles (NPs) ingeniously connected with graphene-like carbon layer via in-situ generated interfacial oxygen-bridge chemical bonding was achieved by a mild phosphorization treatment. The results proved that the presence of phosphorus vacancies is a crucial factor enabling formation of Co-O-C bonds. The direct coupling of edge Co of CoP with the oxygen from functional groups on the carbon layer was proposed. As a catalyst for electrocatalytic water splitting, the manufactured Fe₂ O₃ @C@CoP core-shell structure manifested a low overpotential of 230 mV, a low Tafel slope of 55 mV dec^-1 , and long-term stability. Density functional theory calculations verified that the Co-O-C bond played a critical role in decreasing the thermodynamic energy barrier of reaction rate-determining step for the oxygen evolution reaction (OER). This synthetic route might be extended to construct metal-O-C bonds in other transition metal phosphides (or selenides, sulfides)/carbon composites for highly efficient OER catalysts.

Improving light absorption and photoelectrochemical performance of thin-film photoelectrode with a reflective substrate

The charge separation/transport efficiency is relatively high in thin-film hematite photoanodes in which the distance for charge transport is short, but simultaneously the high loss of light absorption due to transmission is confronted. To increase light absorption in thin-film Fe₂O₃:Ti, commercial substrates such as Cu foil, Ag foil, and a mirror are adopted acting as back-reflectors and individually integrated with the Fe₂O₃:Ti electrode. The promotion effect of the commercial back-reflectors on the light absorption efficiency and photoelectrochemical (PEC) performance of the hydrothermally prepared Fe₂O₃:Ti electrodes with a variety of film thicknesses is investigated. As a result, Ag foil and the mirror show favorable and equal efficacy while the promoting effect of Cu foil is limited. In addition, the photocurrent increment achieved by the Ag back-reflector decreases linearly along with the logarithmic of the film thickness and the optimized film thickness of the Fe₂O₃:Ti electrode is decreased from 520 to 290 nm. The high durability of Ag foil in the alkaline electrolyte during solar light irradiation is demonstrated. Furthermore, the reflective substrate also shows a promotion effect on the BiVO₄ photoanode and CuBi₂O₄ photocathode, as well as the unbiased photocurrent from a tandem cell constituted by TiO₂ and CuBi₂O₄.

The charge separation/transport efficiency is relatively high in thin-film hematite photoanodes in which the distance for charge transport is short, but simultaneously the high loss of light absorption due to transmission is confronted.