Photoelectrochemical Conversion Research Articles

Photoelectrochemical Conversion of Methane to Ethane and Hydrogen under Visible Light Using Functionalized Tungsten Trioxide Photoanodes with Proton Exchange Membrane.

Developing methane utilization technologies is desired to convert abundant and renewable carbon resources, such as natural gas and biogas, into value-added chemical products. This study provides insights into emerging photoelectrochemical (PEC) technology for the photocatalytic transformation of methane to C2H6 and H2 using visible light at room temperature. The PEC conversion of methane to oxygenates has been investigated in aqueous electrolytes. Herein, we demonstrate the gas-phase PEC methane conversion using a proton exchange membrane (PEM) as a solid polymer electrolyte and a gas-diffusion photoanode for methane oxidation. Tungsten trioxide (WO3), a semiconductor photocatalyst responsive to visible light, is utilized as the photoanode material. Ultraviolet light (∼365 nm) excitation predominantly results in CO2 production with lower C2H6 selectivity in humidified methane. In contrast, visible light (∼453 nm) effectively promotes C2H6 production over the WO3 photoanode, attributed to preferential hydroxyl radical (•OH) formation compared to UV irradiation. Photogenerated holes formed near the valence band maximum of WO3 contribute to •OH formation through a single-electron water oxidation. The photogenerated •OH activates gaseous methane molecules to methyl radicals, subsequently coupled into C2H6 at the gas-electrolyte-semiconductor boundary. H2 is concurrently formed on the cathode electrocatalyst. Improving the selectivity for the dehydrogenative coupling of methane is pivotal for enhancing the energy efficiency in the PEM-PEC system.

Surface Engineering of BiVO4 Photoanodes for Photoelectrochemical Water Splitting: Recent Advances

Energy demand worldwide demands clean, cheap, and renewable energy. Through the use of photoelectrochemical (PEC) conversion, solar energy can be transformed into chemical energy. Bismuth vanadate (BiVO4), a material exhibiting visible light activity, favourable conduction band edge energies, and ease of synthesis, has become increasingly popular in recent years. In BiVO4, charge carriers recombine rapidly, which adversely affects the PEC performance and stability. There have been several strategies developed to mitigate these deficiencies, including novel heterojunctions, doping with metals, coupling with cocatalysts, interface modification and modifying morphology. To achieve the best results, it is required to develop PEC devices with exceptional cost‐to‐efficiency ratios and long‐term durability. This review also examines novel yet commercially viable applications for BiVO4‐based photoanodes. Lastly, we discuss the challenges and perspectives facing PEC water splitting systems based on BiVO4.

Paired photoelectrochemical conversion of CO2/H2O and glycerol at high rate

AbstractPhotoelectrochemistry holds the promise of directly converting sunlight to valuable chemical products. Photoelectrochemical (PEC) methods, however, lag behind their electrochemical counterparts in terms of current density. In this work, we demonstrate that, by using concentrated sunlight, we can achieve current densities similar to electrochemical methods, but with lower energy input. Specifically, we combined the direct PEC oxidation of glycerol with the dark hydrogen evolution or CO2 reduction in a membrane-separated continuous-flow PEC cell. We achieved over 110 mA cm−2 photocurrent density, which is at least an order of magnitude larger than those typically reported in the literature. We demonstrated that the product distribution of glycerol oxidation is notably different in PEC and electrochemical scenarios at the same current density, and the parasitic oxygen evolution reaction can be suppressed in the PEC case. This approach raises opportunities to drive complex electrochemical reactions in a more selective manner.

Indium sulfide-sensitized 2D stannum indium sulfide nanosheet arrays promoting photoelectrochemical conversion efficiency in cathodic protection

This study reports the fabrication of 2D stannum indium sulfide (SnIn4S8) nanosheet arrays sensitized with indium sulfide (In2S3) and their potential application as photoanodes in photoelectrochemical cathodic protection for 304 stainless steel. The deposition of In2S3 flakes on the surface of the SnIn4S8 nanosheet arrays induces a notable alteration in the optical absorption of the composite samples, showing a maximum redshift from 560 nm to 645 nm. The incorporation of In2S3 regulates the energy band structure of SnIn4S8 by pulling its Fermi level and valence band maximum toward a negative direction. The type II heterojunction formed by sensitization with In2S3 strengthens the separation efficiency of photoinduced carriers in SnIn4S8, inhibiting self-corrosion. The particular 5%-In2S3/SnIn4S8 composite demonstrates a notable shift in the mixed potential toward negative values, reaching a potential of −1.05 V when exposed to visible light. An excessive amount of In2S3 is not conducive to the directional flow of carriers at the heterojunction interface, preventing efficient separation of electron-hole pairs. This research highlights the advantages of the In2S3-sensitized SnIn4S8 composites for photogenerated cathodic protection.

Advancements on metal oxide semiconductor photocatalysts in photo-electrochemical conversion of carbon dioxide into fuels and other useful products

Advancements on metal oxide semiconductor photocatalysts in photo-electrochemical conversion of carbon dioxide into fuels and other useful products

Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon.

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis.

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Engineering Surface Passivation and Hole Transport Layer on Hematite Photoanodes Enabling Robust Photoelectrocatalytic Water Oxidation.

Regulation of charge transport at the molecular level is essential to elucidating the kinetics of junction photoelectrodes across the heterointerface for photoelectrochemical (PEC) water oxidation. Herein, an integrated photoanode as the prototype was constructed by use of a 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin-cobalt molecule (CoTCPP) and ZnO on hematite (α-Fe2O3) photoanode. CoTCPP molecules serve as a typical hole transport layer (HTL), accelerating the transport of the photogenerated holes to oxygen evolution cocatalysts (OECs). Meanwhile, ZnO as the surface passivation layer (SPL) can passivate the interfacial state and reduce the level of electron leakage from hematite into the electrolyte. After the integration of OECs, the state-of-the-art α-Fe2O3/ZnO/CoTCPP/OECs photoanode exhibits a distinguished photocurrent density and excellent stability in comparison with pristine α-Fe2O3. The simultaneous incorporation of a ZnO and CoTCPP dual interlayer can effectively modulate the interfacial photoinduced charge transfer for PEC reaction. This work provides in-depth insights into interfacial charge transfer across junction electrodes and identifies the critical roles of solar PEC conversion.

Tailoring Interfaces for Enhanced Methanol Production from Photoelectrochemical CO2 Reduction.

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm-2, surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

Interfacial engineering of a multijunctional In2O3/WO3@Ti4N3Tx S-scheme photocatalyst with enhanced photoelectrochemical properties.

Achieving high photoelectrochemical conversion efficiency requires the logical layout of a composite photocatalyst with optimal charge separation and transfer with ideal light harvesting capabilities to enhance the photocatalytic performance and the degradation rate towards organic pollutants. Herein, a novel In2O3/WO3@Ti4N3Tx S-scheme heterojunction was successfully synthesized and confirmed through valence band VB-XPS and Mott Schottky combined analysis. The formed MXene-doped In2O3/WO3@Ti4N3Tx S-scheme significantly enhances the charge flow and spatial separation with an improved oxidation and reduction ability. An in-built interfacial electric field at the WO3-In2O3 boundary enhanced the light-harvesting capacity, whereas Ti4N3Tx MXene offers a unique electron trapping effect which effectively lowers high charge carrier recombination rate-related photocatalytic deficit. It preserves the exceptional redox potency of the photocatalyst by providing a directed acceleration and effective separation of the photogenerated charges. A high carrier density (ND = 7.83 × 1021 cm-3) with a lower negative flat band (VFB = -0.064 V vs. Ag/AgCl) was obtained by Mott-Schottky analysis for 3 wt% In2O3/WO3@Ti4N3Tx, an indicator that a low overpotential is needed to activate photocatalytic reactions. This study, therefore, provides a novel thought for the design and fabrication of an S-scheme heterojunction for photocatalytic reactions for mineralization of organic pollutants in water and clean energy production.

Electrogenerated chemiluminescence imaging of plasmon-induced electrochemical reactions at single nanocatalysts.

This study explores plasmon-induced electrochemical reactions on single nanoparticles using electrogenerated chemiluminescence microscopy (ECLM). Under laser irradiation, real-time screening showed lower plasmon-induced reaction efficiency for bimetallic Au@Pt nanoparticles compared to monometallic Au nanoparticles. ECLM offers a high-throughput imaging and precise quantitative approach for analyzing photo-electrochemical conversion at single nanoparticle level, valuable for both theoretical exploration and optimization of plasmonic nanocatalysts.

Extending the Success of Halide Perovskites from Solar Cells to Photoelectrodes and Photocatalysts

Photoelectrochemical and photocatalytic conversion of water and carbon dioxide using solar energy offers a clean and sustainable solution to meet the world’s energy requirements. Achieving their full potential depends on developing inexpensive photoelectrodes and photocatalysts that can efficiently absorb solar light and drive photoinduced charges to react with water and carbon dioxide. In this talk, I will present the recent developments of our group in the preparation of inexpensive photoanodes and photocatalysts made of halide perovskites such as CsPbBr3. These materials are revolutionizing the photovoltaics field due to their high absorption coefficient and excellent optoelectronic properties, and now offer an opportunity to revolutionize photoelectrochemistry and photocatalysis if their instability in aqueous environments is addressed. We have found novel strategies to protect halide perovskites with printed carbon layers and graphite sheets functionalized with NiFeOOH water-oxidation electrocatalyst, achieving photoanodes with low onset potential (+0.4 VRHE) for solar water oxidation. We have also managed to boost charge separation in these photoanodes by engineering the CsPbBr3 phase to a 3D one that achieves enhanced energetics and better band alignment to the TiO2 electron transport layer and printed carbon electrode layer, achieving photocurrents around 8 mA cm-2 at 1.23 VRHE for water oxidation. Moreover, our carbon/graphite protection and NiFeOOH functionalization methods impart days of stability to the photoanodes, easily extended to weeks, compared with seconds in the absence of protection. Regarding photocatalysts made of halide perovskite, we have found novel approaches based on mechanochemical syntheses and antisolvent crystallization to prepare different composites of CsPbBr3 and other perovskites, including Cs2AgBiBr3/bismuthene and Cs3Bi2Br9/g-C3N4, all photocatalytically active in the reduction of CO2. An extended characterization allows us to relate their physical and charge-transfer properties to their performance, providing guidance in their rational design for their optimization and future application.

Nanoscale Photoelectrochemical Mapping of Two-Dimensional Electrodes

Electron transfer plays a significant role in many chemical processes, especially in the catalytic reactions involved in photoelectrochemical energy conversion and storage. Revealing the electron transfer behavior at the interface of electrode-electrolyte is thus of great importance. Here we fabricate atomically thin semiconductor MX2 (M = Mo, W; X = S, Se) by chemical vapor deposition method and directly map the photoelectrochemical behavior using Atomic Force Microscopy-combined with light-induced Scanning Electrochemical Microscopy (AFM-SECM). AFM-SECM feedback mapping shows a layer-dependent photoelectrocatalytic ability of MX2 to oxidize the redox species Ru2+ back to Ru3+. Moreover, AFM-SECM generation and collection mode shows the potential-dependent edge and basal plane activity for the hydrogen evolution reaction of WSe2. Compared with monolayer and bilayer, few-layer WSe2 shows better stability in an electrochemical environment, faster electron transfer, and higher hydrogen production. Furthermore, finite element method based numerical simulations using MATLAB® and COMSOL Multiphysics® is performed to calculate the electron transfer rate constants k0 and simulate the steady-state concentration gradient. Finally, the photoelectrochemistry at MX2 electrode-electrolyte interfaces are spatially resolved and understanding this behavior will be useful for the future photoelectrochemical devices.

Defect Engineering of WO3 by Rapid Flame Reduction for Efficient Photoelectrochemical Conversion of Methane into Liquid Oxygenates.

Photoelectrochemical (PEC) conversion is a promising way to use methane (CH4) as a chemical building block without harsh conditions. However, the PEC conversion of CH4 to value-added chemicals remains challenging due to the thermodynamically favorable overoxidation of CH4. Here, we report WO3 nanotube (NT) photoelectrocatalysts for PEC CH4 conversion with high liquid product selectivity through defect engineering. By tuning the flame reduction treatment, we carefully controlled the oxygen vacancies of WO3 NTs. The optimally reduced WO3 NTs suppressed overoxidation of CH4 showing a high total C1 liquid selectivity of 69.4% and a production rate of 0.174 μmol cm-2 h-1. Scanning electrochemical microscopy revealed that oxygen vacancies can restrain the production of hydroxyl radicals, which, in excess, could further oxidize C1 intermediates to CO2. Additionally, band diagram analysis and computational studies elucidated that oxygen vacancies thermodynamically suppress overoxidation. This work introduces a strategy for understanding and controlling the selectivity of photoelectrocatalysts for direct conversion of CH4 to liquids.

Hydrofluoric acid-free etched MAX on 3D-printed nanocarbon electrode for photoelectrochemical hydrogen production

MXenes have emerged as a promising material for a disparate range of photo-electrochemical conversion and energy storage devices. However, most reported synthesis process involves hydrofluoric acid (HF) and fluoride-based compounds. HF severe toxicity persists impediment to the scalable production and fabrication of MXenes. Thus, fluoride-free protocols are currently being explored. Here, we demonstrate a novel fluoride-free protocol for the etching of the MAX phase (Ti3AlC2) utilizing an acidic mixture of nitric acid, phosphoric acid, and acetic acid. Such acidic etching modulated the electronic structure and chemical composition of the MAX surface converting it into MAX-derived flower-like titanium phosphate and titanium oxide which boosts its photo-electrochemical properties. The photo-electrochemical hydrogen evolution reaction is evaluated using 3D-printed and screen-printed carbon electrodes exposing under visible light illumination with 365 and 460 nm wavelengths. Such HF-free etched Ti3AlC2 MAX and the MAX-derived flower-like titanium phosphate and titanium oxide coated 3D-printed nanocarbon and screen-printed carbon electrodes demonstrate a novel process to be applied for an energy conversion application. This work will pave the way to etch other MAX phases through an environment-friendly and easy-handling technique that is to be used beyond photo-electrochemical applications.

Rational design of nanostructured BiVO4/FeOx photoanode coupling with 2D Co(OH)2 cocatalyst for enhanced photoelectrochemical water splitting

Efficient charge transport and separation is of prime significance to achieve high efficiency of photoelectrochemical (PEC) solar energy conversion. In this study, we designed a novel ternary nanostructured BiVO4/FeOx/Co(OH)2 composite as photoelectrode for enhanced PEC water splitting. Compared with pristine BiVO4, the fabricated BiVO4/FeOx/Co(OH)2 photoelectrode produced a doubled increase in photocurrent (from 0.36 to 0.72 mA cm−2 at 1.23 V versus RHE) with a negative shift of 390 mV in the initial potential (from 0.72 to 0.33 V). In addition, the conversion efficiency and photostability for PEC water oxidation were also improved. The elevated PEC properties of ternary composite photoelectrode were originated from the synergy of heterojunction construction and 2D cocatalyst loading that provides effective pathway for promoting charge separation as well as sufficient sites for photoelectrochemical reaction. These findings could provide an exemplary PEC model for enhanced solar hydrogen evolution of BiVO4 photoanode.

Selective photoelectrocatalytic CO2 reduction to ethanol using nanotubular oxides grown on metastable Ti-Cu alloy

The photoelectrochemical conversion of CO2 to C2 products such as ethanol is a topic of much current interest and relevance for decreasing the amount of greenhouse gases in the atmosphere. Here, the photoelectrocatalytic selectivity for the CO2 reduction reaction on Ti-O-Cu nanotubes grown on bimetallic alloys (Ti-xCu; x = 0.5, 5.5 and 10 at. %) was investigated. The product selectivity in the CO2 reduction reaction was found to depend on the effects of composition and distribution of the oxidant species along the nanotubes. This behavior was coordinated by the substrate phases obtained from Ti-Cu alloys subjected to different heat treatments (annealed followed by rapid cooling – quenched (Q) or annealed with controlled cooling (A)). A nanotubular (Nt) oxide layer was grown on these substrates and the influence of metastable (quenched) or equilibrium (annealed) phases was related to the diffusion of the copper species. The two types of electrodes were compared in terms of their relative proclivity toward CO2 photoreduction. Methanol was produced as the dominant reduction product on Nt/Ti-10Cu (A). On the other hand, Nt/Ti-10Cu (Q) produced a respectable yield and selectivity for ethanol, reaching 2.32 mmol/L of ethanol. In a broader vein, this study demonstrates the importance of surface defects in an active heterogeneous photocatalyst and provides insights into the key parameters for the precursor design, selection, and optimal performance of materials for the CO2 reduction reaction leading to C2 compounds.

Substitutional Cd Dopant as Photohole Transfer Mediator Boosting Photoelectrochemical Solar Energy Conversion of 2D Cd-ZnIn2 S4 Photoanode.

Fast recombination dynamics of photocarriers competing with sluggish surface photohole oxidation kinetics severely restricts the photoelectrochemical (PEC) conversion efficiency of photoanode. Here, a defect engineering strategy is developed to regulate photohole transfer and interfacial injection dynamics of 2DZnIn2 S4 (ZIS). Via selectively introducing substitutional Cd dopant at Zn sites of the ZIS basal plane, energy band structure and surface electrochemical activity are successfully modulated in the Cd-doped ZIS (Cd-ZIS) nanosheet array photoanode. Comprehensive characterizations manifest that a shallow acceptor level induced by Cd doping and superior electrochemical activity make surface Cd dopants simultaneously act as capture centers and active sites to mediate photohole dynamics at the reaction interface. In depth photocarrier dynamics analysis demonstrates that highly efficient photohole capture of Cd dopants brings about effective space separation of photocarriers and acceleration of surface reaction kinetics. Therefore, the optimum 2D Cd-ZIS achieves excellent PEC solar energy conversion efficiency with a photocurrent density of 5.1mA cm-2 at 1.23 VRHE and a record of applied bias photon-to-current efficiency (ABPE) of 3.0%. This work sheds light on a microstructure design strategy to effectively regulate photohole dynamics for the next-generation semiconducting PEC photoanodes.

Tungsten oxide nanosheets photoanode for highly efficient methane conversion

Photoelectrochemical (PEC) conversion of methane (CH4) to high-value-added liquid products has been regarded as an eco-friendly and sustainable strategy for the utilization of natural gas but remains challenging in terms of high yield and selectivity for diatomic carbon (C2) products. Herein, we report a facile oxalic acid reduction strategy to synthesize ultrathin tungsten oxide (WO3) nanosheets (NSs) with controllable oxygen vacancies (Ov) as photoanodes for efficient PEC CH4 conversion to ethanol. As a result, the ultrathin WO3 NSs photoanode shows a superior PEC CH4 conversion efficiency with a high ethanol production rate and Faraday efficiency of 98.9 mmol/g/h and 63.6% under a low applied positive potential of 0.9 V vs. reversible hydrogen electrode (RHE) during 4 h. Incident photon-to-current efficiency of 23.2% is achieved at 365 nm. The reaction path and catalytic mechanism of CH4→CH3→CH3OH→CH2OH→CH3CH2OH are studied by in-situ Fourier-transform infrared spectroscopy (FTIR), electron spin resonance spectroscopy (ESR), and density functional theory (DFT) simulation. The results indicate that in-situ-formed OOH is the main reactive oxygen species during CH4 conversion, and Ov significantly lowers the reaction energy of the rate-determinate elemental step.

Construction of Co9S8/MoS2/Ni2P double S-scheme heterojunction for enhanced photocatalytic hydrogen evolution

Rational design of composite catalysts with efficient charge separation and transfer and good light collection ability is of great significance to achieve high photoelectrochemical conversion efficiency. In this paper, a Co9S8/MoS2/Ni2P dual S-scheme heterojunction photocatalyst was successfully synthesized by a simple sulfidation and hydrothermal method. The optimized Co9S8/MoS2/Ni2P composite with hierarchical structure, large surface area and numerous reactive active sites showed a good photocatalytic hydrogen evolution rate of 5.69 mmol g−1 h−1, which is 2.28, 4.12 and 10.74 times higher than that of pure Co9S8, MoS2 and Ni2P, respectively. A combination of time-resolved photoluminescence (TRPL) spectroscopy, valence band XPS (VB-XPS), ultraviolet photoelectron spectroscopy (UPS), and density-functional theory (DFT) calculations confirms that the formation of the Co9S8/MoS2/Ni2P double S-scheme greatly facilitates charge migration and spatial separation, and improves the oxidation and reduction ability. This work will provide insights into the development of PBA-based dual S-scheme heterojunction catalysts for photocatalytic hydrogen evolution.