Ti-doped hematite photoanode with surface phosphate ions functionalization for synergistic enhanced photoelectrochemical water oxidation

A tungsten phosphide cocatalyst enhanced bismuth tungstate photoanode with the robust built-in electric field towards highly efficient photoelectrochemical water splitting

The use of low-cost and effective cocatalyst is a potential strategy to optimize the effectiveness of photoelectrochemical (PEC) water splitting. In this study, tungsten phosphide (WP) is introduced as a remarkably active cocatalyst to enhance the PEC efficiency of a Bi2WO6 photoanode. The onset potential of Bi2WO6/WP demonstrates a negative shift, while the photocurrent density demonstrates a significant 5.5-fold increase compared to that of unmodified Bi2WO6 at 1.23 VRHE (reversible hydrogen electrode). The loading of WP cocatalyst facilitates the rapid transfer of holes, increasing the range of visible light absorption, the water adsorption ability as well as promoting the separation of photogenerated electrons and holes via the built-in electric field between Bi2WO6 and WP. This study proposes a strategy to hinder the recombination of electron-hole pairs by using WP cocatalyst as a hole capture agent, improve the photoelectric conversion efficiency, and enhance the overall photoelectrochemical properties of Bi2WO6 photoanode.

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*.

A New Photoelectrocatalyst for Water Oxidation: A Polyoxometalate-Graphitic Carbon Nitride Hybrid Nanomaterial

A new photoelectrocatalyst for the water oxidation process is designed by immobilizing [CoW₁₂O₄₀]^6- (CoW₁₂) heteropolyanions on the surface of covalently modified graphitic carbon nitride nanosheets (g-C₃N₄). For this purpose, g-C₃N₄ is first modified with cysteamine hydrochloride through the well-known thiol-ene click reaction. Afterward, [CoW₁₂O₄₀]^6- heteropolyanions are immobilized on the surface of modified g-C₃N₄ nanosheets with electrostatic interaction with ammonium groups. After confirming the preparation of CoW₁₂/clicked g-C₃N₄ with various physicochemical methods, its photoelectrocatalytic activity is evaluated in the oxygen evolution reaction. The CoW₁₂/clicked g-C₃N₄ exhibits a low onset potential of 1.32 V versus NHE and a low overpotential of 230 mV at 10 mA cm^-2 with a low Tafel slope of 67 mV dec^-1 under visible light illumination. Moreover, the stability of the catalyst is investigated through the chronoamperometric measurements.

Performance Enhancement of Actual Wastewater Treatment and Electricity Generation Through Surface Modified TiO₂ Nanotube Arrays Based Photoanode Photocatalytic Fuel Cell

Herein, we report a surface modified TiO₂ nanowire arrays (NAs) photoanode based photocatalytic fuel cell (PFC) towards simultaneous enhancement of actual wastewater treatment and electricity generation under visible light irradiation. TiO₂ NAs were facile fabricated via two-step anodization process in ethylene glycol and glycerin solution, respectively. Actual wastewater samples were directly applied to evaluate the PFC performance in terms of wastewater degradation and electricity generation through the as-prepared TiO₂ NAs photoanode without loading noble-metals or semiconductors. TiO₂ NAs photoanode prepared from ethylene glycol solution demonstrated a highly ordered surface network, exhibiting short-circuit current density and fill factor nearly 4.3 times and 1.4 times higher than pristine TiO₂ NAs photoanode prepared according to previous reports. The experimental results revealed that the fabrication of TiO₂ NAs by a facile surface modification in ethylene glycol solution can be considered a low-cost and scalable routine for enhancing performance of PFC photoanode towards efficient actual wastewater treatment and electricity generation.

A highly activated iron phosphate over-layer for enhancing photoelectrochemical ammonia decomposition

Environmentally friendly ammonia (NH₃) decomposition has attracted a lot of interests in recent years to resolve the issue of water eutrophication from a wastewater and achieve a clean H₂ storage. Here, we report a novel strategy for solar-driven ammonia decomposition by introducing a highly-activated iron phosphate (FePi) over-layer on the surface of α-Fe₂O₃ nanorods photoanode (FePi/Fe₂O₃), and innovatively propose a photoelectrochemical (PEC) ammonia degradation system with enhanced performance. After a facile electrochemical (EC) activation, the FePi over-layer is converted into FeOOH. The EC-activated over-layer provides the efficient active sites for the ammonia adsorption process, which promotes the high catalytic kinetics for ammonia oxidation reaction (AOR). Due to the synergistic effect of the electrocatalytic and the photocatalytic process, the FePi/Fe₂O₃ exhibits the enhanced PEC AOR performance, which competes with water oxidation reaction (WOR). Comparing to the initial concentration of ammonia, the FePi/Fe₂O₃ achieves a 54.4% ammonia degradation rate within 3 h at 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, which demonstrates the reliable ammonia decomposition performance. This study confirms that it is feasible to achieve PEC ammonia decomposition in an aqueous solution without chloride mediators and provides a promising strategy for the harmless treatment of ammonia wastewater.

Copper-doped ZrO2 nanoparticles as high-performance catalysts for efficient removal of toxic organic pollutants and stable solar water oxidation

Doping effect on the photoelectrochemical (PEC) water splitting efficiency and photocatalytic activities of ZrO₂ under visible light are reported. The XRD analysis revealed that pure, 0.1 and 0.3 mol% doped samples showed mixed crystal phases (tetragonal and monoclinic) and 0.5 mol% doped sample showed a pure tetragonal phase. Under visible light, 90% of methyl orange dye degradation was achieved with in 100 min. Moreover, the optimal doped sample showed a significant degradation rate constant over other samples. The doped photoelectrodes display a better PEC water oxidation performance over pure photoelectrode. Furthermore, the optimal doped (0.3 mol %) electrode shows 0.644 mAcm^-2 photocurrent density, corresponding to an approximate 50-fold enhancement over pure electrode (0.013 mAcm^-2). The optimized doped sample achieved 98% degradation of methyl orange within 100 min of light irradiation. The superior PEC water oxidation and photocatalytic activity of optimal doped samples under visible light are credited to suitable doping content, crystalline size, greater surface area, suitable bandgap, a lower charge carrying resistance, surface properties and the ability for decreasing the charge carrier's recombination rate.

Activating a hematite nanorod photoanode via fluorine-doping and surface fluorination for enhanced oxygen evolution reaction

Poor charge separation and sluggish oxygen evolution reaction (OER) kinetics are two typical factors that hinder the photoelectrochemical (PEC) applications of hematite. Dual modification via heteroatom doping and surface treatment is an attractive strategy to overcome the above problems. Herein, for the first time, a hematite nanorod photoanode was ameliorated via the fluorine treatment (F-treatment) of both bulk and surface, enabling simultaneous charge separation from the interior to the interface. Accordingly, the novel photoanode (FeF_x/F-Fe₂O₃) exhibited an outstanding PEC water oxidation activity, with a 3-fold improved photocurrent density than that obtained using unmodified α-Fe₂O₃. More specifically, fluorine doping (F-doping) in the hematite bulk remarkably increased the concentration of charge carriers and endowed it with favorable electrical conductivity for rapid charge transfer. Further surface F-treatment on F-doped α-Fe₂O₃ (F-Fe₂O₃) enriched the F-Fe bonds on the surface, which significantly boosted the OER kinetics and thereby inhibited the detrimental charge recombination. As a consequence, the efficiencies of bulk electron-hole pair separation and surface hole injection increased by 2.8 and 1.7 times, respectively. This study points to fluorine modulation as an attractive avenue to advance the PEC performance of metal oxide-based photoelectrode materials.