Photoelectrochemical Water Splitting Cells Research Articles

Laser-Printed Photoanode: Femtosecond Laser-Induced Crystalline Phase Transformation of WO3 Nanorods for Space-Efficient and Flexible Thin-Film Solar Water-Splitting Cells.

Despite its potential for clean hydrogen harvesting, photoelectrochemical (PEC) water-splitting cells face challenges in commercialization, particularly related its harvesting performance and productivity at an industrial scale. Herein, a facile fabrication method of flexible thin-film photoanode for PEC water-splitting to overcome these limitations, based on laser processing technologies, is proposed. Laser-induced graphene, a carbon structure produced through direct laser writing carbonization (DLWC), plays a dual role: a flexible and stable current collector and a substrate for the hydrothermal synthesis of tungsten trioxide (WO3) nanorods (NRs). To facilitate water-splitting, a femtosecond-pulsed laser (fs laser) is focused on the WO3 NRs, converting their crystalline phase from pristine orthorhombic to monoclinic structure without thermal damage. With NiFe layered double hydroxide (LDH) catalyst, the flexible thin-film photoanode exhibits good PEC performance (1.46mAcm-2 at 1.23 VRHE) and retains ≈90% of its performance after 3000 bending cycles. With its excellent mechanical properties, the flexible photoanode can be operated in various shapes with different curvatures, enabling space-efficient PEC water-splitting by loading larger photoanode within a given space. This study is expected to contribute to the advancement of large-scale solar water-splitting cells, introducing a new approach to enhance H2/O2 production and expand its application range.

Non-innocent partners: bias distribution between photoanodes and cathodes in photoelectrochemical overall water-splitting cells.

Non-innocent partners: bias distribution between photoanodes and cathodes in photoelectrochemical overall water-splitting cells.

Bias distribution and regulation in photoelectrochemical overall water-splitting cells.

The water oxidation half-reaction at anodes is always considered the rate-limiting step of overall water splitting (OWS), but the actual bias distribution between photoanodes and cathodes of photoelectrochemical (PEC) OWS cells has not been investigated systematically. In this work, we find that, for PEC cells consisting of photoanodes (nickel-modified n-Si [Ni/n-Si] and α-Fe2O3) with low photovoltage (Vph < 1V), a large portion of applied bias is exerted on the Pt cathode for satisfying the hydrogen evolution thermodynamics, showing a thermodynamics-controlled characteristic. In contrast, for photoanodes (TiO2 and BiVO4) with Vph > 1V, the bias required for cathode activation can be significantly reduced, exhibiting a kinetics-controlled characteristic. Further investigations show that the bias distribution can be regulated by tuning the electrolyte pH and using alternative half-reaction couplings. Accordingly, a volcano plot is presented for the rational design of the overall reactions and unbiased PEC cells. Motivated by this, an unbiased PEC cell consisting of a simple Ni/n-Si photoanode and Pt cathode is assembled, delivering a photocurrent density of 5.3±0.2mA cm-2.

Nonstoichiometric In–S group yielding efficient carrier transfer pathway in In2S3 photoanode for solar water oxidation

AbstractThe construction of high‐efficiency photoanodes is essential for developing outstanding photoelectrochemical (PEC) water splitting cells. Furthermore, insufficient carrier transport capabilities and sluggish surface water oxidation kinetics limit its application. Using a solvothermal annealing strategy, we prepared a nonstoichiometric In–S (NS) group on the surface of an In2S3 photoanode in situ and unexpectedly formed a type II transfer path of carrier, thereby reducing the interfacial recombination and promoting the bulk separation. First‐principles calculations and comprehensive characterizations demonstrated NS group as an excellent oxygen evolution cocatalyst (OEC) that effectively facilitated carrier transport, lowered the surface overpotential, increased the surface active site, and accelerated the surface oxygen evolution reaction kinetics by precisely altering the rate‐determining steps of * to *OH and *O to *OOH. These synergistic effects remarkably enhanced the PEC performance, with a high photocurrent density of 5.02 mA cm−2 at 1.23 V versus reversible hydrogen electrode and a negative shift in the onset potential by 310 mV. This work provides a new strategy for the in situ preparation of high‐efficiency OECs and provides ideas for constructing excellent carrier transfer and transport channels.

Comparing Charge Dynamics in Organo-Inorganic Halide Perovskite: Solid-State versus Solid-Liquid Junctions

In this study, we explore the dynamics of a perovskite-electrolyte photoelectrochemical cell, pivotal for advancing electrolyte-gated field effect transistors, water-splitting photoelectrochemical and photocatalytic cells, supercapacitors, and CO2 capture and reduction technologies. The instability of hybrid perovskite materials in aqueous electrolytes presents a significant challenge, yet recent breakthroughs have been achieved in stabilizing organo-inorganic halide perovskite films. This stabilization is facilitated by employing liquid electrolytes, specifically those formed by dissolving tetrabutylammoniumperchlorate in dichloromethane. A critical aspect of this research is the comparative analysis of charge and ion kinetics at the perovskite/liquid electrolyte interface versus the perovskite/solid charge transport layer interface. Employing Intensity Modulated Photocurrent Spectroscopy (IMPS), Open-Circuit Voltage Decay (OCVD), and Capacitance-Frequency (C-F) methods, the study scrutinizes charge dynamics in both perovskite/electrolyte and perovskite/solid interfaces. Furthermore, the investigation extends to contrasting the properties of solid–liquid and solid-state junctions, focusing on mobile ions, electric field impacts, and electron-hole transport. The research also examines variations in recombination resistance and ionic double layer charging in perovskite-based devices, aiming to elucidate the operational mechanisms and kinetic complexities at the hybrid perovskite/electrolyte interface.

Proton exchange membrane photoelectrochemical cell for water splitting under vapor feeding

This review provides an overview of recent advancements in vapor-fed photoelectrochemical (PEC) systems specifically designed for utilizing water vapor as a hydrogen resource. The PEC system under water vapor feeding utilizes a proton exchange membrane as a solid polymer electrolyte. Additionally, it utilizes gas-diffusion photoelectrodes composed of a fibrous conductive substrate with macroporous structures. Herein, the porous photoelectrodes are composed of n-type oxides for oxygen evolution reactions and used with a Pt electrocatalyst cathode for hydrogen evolution reactions. The topics covered include the conceptual framework of vapor-fed PEC hydrogen production, strategic design of gas-phase PEC reaction interfaces, and development of porous photoanodes such as titanium dioxide (TiO2), strontium titanate (SrTiO3), tungsten trioxide (WO3), and bismuth vanadate (BiVO4). A significant enhancement in the PEC efficiency was achieved through the application of a thin proton-conducting ionomer film on these porous photoelectrodes for surface functionalization. The rational design of proton exchange membrane-based PEC cells will play a pivotal role in realizing renewable-energy-driven hydrogen production from atmospheric humidity in the air.

Operando spectroscopic characterization of formamidinium lead iodide perovskite quantum dots for tracking electrochemical reactions

Multidimensional ABX3 hybrid perovskites three-dimensionally confined dot-shaped structure demonstrate versatile potential to photoelectrochemical cells for water splitting, hydrogen generation, solar cells, and light-emitting diodes. To apply perovskite quantum dots (PQDs) to solar-driven chemistry and optoelectronic devices, understanding the photoinduced charge carrier dynamics of PQDs under electrochemical conditions or applied bias are important. In this study, the detailed transformation mechanism of formamidinium lead iodide perovskite quantum dots under electrochemical conditions was studied by tracking the products of the reaction through cyclic voltammetry, X-ray photoemission spectroscopy, in-situ UV–visible spectroelectrochemistry, etc. Through comprehensive characterizations, the mechanism of irreversible oxidative transformation of perovskite quantum dots was presented. This study provides deeper insight into the electrochemical behavior of PQDs for successful solar-driven chemistry and optoelectronic device applications.

Enhancing solar energy conversion with nickel/iron oxyhydroxide-modified hematite photoanodes: The role of Fe species in alkaline electrolyte

Hematite photoanodes have shown potential for use in practical applications of photoelectrochemical (PEC) water splitting cells due to their abundance and stability, yet their efficiency needs to be improved. To optimize photoanode performance, co-catalysts such as amorphous nickel/iron oxyhydroxide layers are often deposited on the surface of hematite to enhance activity by catalyzing oxygen evolution. Recent studies have shown that the role of oxyhydroxide layers in changing PEC activity can vary depending on factors such as film morphology, co-catalyst layer thickness, and electrolyte composition. In this study, we investigate the effect of NiOOH and (Ni,Fe)OOH layers on the PEC activity of electrodeposited nanostructured hematite films. Our analysis of charge transfer and recombination rate constants determined from chronoamperometry and electrochemical impedance data suggests that the coatings' role is purely passivating, which is crucial for discontinuous electrodeposited films. Importantly, we show that the passivating properties of these coatings are enhanced by the presence of ppm concentrations of Fe in the electrolyte. We believe that our findings will be useful for further development of hematite photoanodes.

Impact of Varying the Photoanode/Catalyst Interfacial Composition on Solar Water Oxidation: The Case of BiVO4(010)/FeOOH Photoanodes.

Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron-hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO4(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO4 interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO4 and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO4(010)/FeOOH photoanodes show drastically different Fe2+:Fe3+ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO4 surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO4 and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.

Transferable Highly (001) Oriented Sb2Se3 Nanorod Films on Flexible Substrates for Innovative Optoelectronic Devices

AbstractThin films of antimony chalcogenides, Sb2X3 (X = S, Se, or SxSe1−x), comprised of (001) oriented nanorods are highly desirable for optoelectronic applications owing to their light trapping capacity and highly efficient charge transport along the optimally aligned quasi‐1D (Sb4X6)n ribbons. Here, highly (001) oriented Sb2Se3 nanorod (Sb2Se3NR) layers are obtained via a novel template growth method, and transfer these layers to functional substrates via a polymer‐assisted technique. Transferable layers overcome the current challenges associated with achieving oriented growth directly on the desired substrate. Using a combination of a low‐temperature processable SnO2 nanoparticle transport layer and encapsulation/etching techniques photodetector devices are fabricated with excellent electrical contact. The optimized transferred devices show broad range photoresponse with signal‐to‐noise ratios up to 105 and responsivities reaching 0.96 mA W−1. Importantly, access to low‐temperature processing enables the fabrication of flexible devices. Based on this platform, a proof‐of‐concept self‐powered flexible heart rate monitor is fabricated. The achievement of transferable Sb2Se3NR layers introduced in this work is expected to be readily applicable to generate new (001) Sb2Se3 device architectures that may otherwise not be achievable via direct deposition, with application in photoelectrochemical water splitting, battery electrodes, and solar cells.

Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution

AbstractAmong the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X‐ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat‐band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high‐performance TiO2‐based devices.

A Photoelectrochemical and Spectroscopic Studies of Photoelectrodes with Hematite∣Catalyst∣Electrolyte Interface Toward Oxygen Evolution Reaction

One of the recent issues in the world is climate change caused by global warming. Many countries have set an ultimate goal for carbon neutrality to prevent from further increase of the global temperatures. In this regard, research on development of renewable energy sources without any carbon emissions has been intensively conducted. Among many promising chemical fuels with sustainability, Hydrogen (H2) can be produced by water (H2O) splitting.Especially, water splitting driven by photoelectrochemical reactions has great advantages in H2 generation: photovoltage generated by light absorption in semiconductors may significantly reduce the applied overpotential which should be provided by external circuits. This photoelectrochemical system, called photoelectrochemical cell (PEC) for water splitting is basically composed of photoelectrodes, catalysts, and membranes for generation of photogenerated charge carriers, reducing overpotential by improving charge transfer kinetics, and prevention of H2 and Oxygen (O2) crossover, respectively. When photons from solar light with energies greater than band gap are incident on the photoelectrodes, an electron-hole pair (EHP) is formed in the semiconductor. These EHPs are separated and transported to the surface, driven by band bending formed at the semiconductors. In photoanodes, the most of the charge carriers arrived at the electrode surface are holes (h+s), which oxidizes H2O at the photoelectrode-electrolyte interface. The transfer of holes accompanies the proton transfers, thereby producing O2. The separated electrons (e−s) should travel to the (photo)cathode along the external circuit and generate H2. However, the overall efficiency of water splitting using PEC still confronts many challenges for wide commercialization, including the low solar-to-hydrogen (STH) efficiencies. In order to improve the performance, the semiconductor efficiently utilizes the photons in the visible light region, which takes 45% of solar irradiation. In this perspective, hematite (α-Fe2O3) is one of the most promising candidates for the semiconductor as photoanodes owing to is narrow band gap of 1.9 ~ 2.2 eV, which is in the range between 550 ~ 650 nm. Meticulous cares should be also taken in catalysts localization to prevent from the parasitic photon loss, as well as the selection of catalytic materials. Because catalysts attribute to the improved charge transfer kinetics but also simultaneously become the sites that obstruct the light penetration to the semiconductor, resulting in the lower light absorption. Furthermore, physical and chemical composition of catalysts on the semiconductor surface significantly influence the charge transfer rates for the photoelectrochemical reactions by varying the electrical field gradient formed at the semiconductor∣catalyst∣electrolyte interface. As several procedures are additionally involved in the PEC, the electrochemical reaction pathways become more complicated rather than the one in electrolyzers utilizing conductive electrodes. Therefore, underlying understanding in semiconductor∣catalyst∣electrolyte interface is crucial to achieve the high performance in PECs.Herein, we demonstrate the hematite∣catalyst∣electrolyte interface as the photoanodes to investigate the photoelectrochemical reaction pathways in water oxidation. Hematite is the perfect material as the semiconductor model, not only due to the narrow band gap in the region of visible light but also the excellent stability even when exposed to the electrolytes and electrochemical environments. The hematite is fabricated by simple hydrothermal reactions, followed by step-wise heat treatment. The structure of hematite is optimized by varying hydrothermal temperatures, affecting the crystallinity and exposed area of the conductive substrate, fluorine-doped tin oxide (FTO). The recombination by back reduction is occurred at the exposed FTO surface by pinholes in hematite, which directly contact the electrolyte. Moreover, the localization of Co-based catalysts is also easily controlled by light-guided electrodeposition. In this hematite∣catalyst∣electrolyte interface, the overall water oxidation pathways are investigated by photoelectrochemical and Raman spectroscopic studies. It is expected that our study in here contributes to the in-depth understanding on the complicated photoelectrochemical reaction mechanism, might leading to the improved performance for wide commercialization of PEC.

(Digital Presentation) 2D Molybdenum Disulfide Nanosheets As Cocatalyst and Passivation Layer over CuInS2 Based Photocathode for Solar Hydrogen Evolution

An excellent strategy to improve photoelectrochemical (PEC) water splitting is to develop cost-effective co-catalysts that do not use noble metals such as Platinum (Pt), Iridium (Ir), Ruthenium (Ru). Transition metal cocatalysts have shown excellent catalytic property towards hydrogen evolution (HER) and oxygen evolution reaction (OER). The surface active sites and lower free energy of H* adsorption (GH adsorption) on the surface are the factors ameliorating and boosting their catalytic activity. Molybdenum Disulfide (MoS2), one the transition metal dichalcogenide with 2D-sheet like structure with high surface area, active sites, and excellent property to catalyze HER comparable to Pt. The property of higher stability towards the photocorrosion as compared to other cocatalysts provides MoS2 with an additional feature of passivation layer. The sheet-like structure also enhances the light absorption minimizing the effect of parasitic light absorption.Copper based oxides and chalcogenides are well known p-type semiconductors being investigated for photocathode materials to be used in unassisted PEC water splitting cell. CuInS2, a ternary copper chalcogenide material with tunable bandgap (Eg = 1.5-1.8 eV), high absorption coefficient (> 104), suitable band positions for hydrogen evolution reaction. Despite all the necessary features and properties, it suffers with the problem of high rate of recombination between photogenerated charge carriers, lower diffusional lengths and poor photostability. Several approaches have been developed to improve the PEC performance of CuInS2 that include decoration of metal NPs, heterojunction fabrication, deposition of electron (ETLs) and hole transporting layers (HTLs). CdS forms a proper heterojunction with CuInS2 owing to its conduction (CB) and valence (VB) offsets.The integration of CuInS2 with CdS could form a facile heterojunction for the effective charge separation and reduce the rate of recombination and decoration of MoS2 over the CuInS2/CdS can be beneficial in terms of improved reaction kinetics at the surface and photostability by avoiding direct contact of CuInS2 and CdS with electrolyte. A CuInS2 nanosheet array-based photocathode that is modified with CdS and the co-catalyst MoS2. This green approach enhances water splitting under solar irradiation. By introducing CdS and MoS2, we significantly improved the visible light absorption of the modified hybrid photocathode (CIS/CdS/MoS2). Photoluminescence, impedance spectroscopy, and Mott–Schottky analysis confirmed that excited electron-hole pairs were better separated, the resistance of charge transfer was minimized, and the excited-state charge carrier concentration increased, leading to increased photocurrent.Typical results demonstrated that CIS/CdS/MoS2 photocathode delivered higher photocurrent (-1.75 mA/cm2 at 0 VRHE) and HC-STH conversion efficiency (0.42% at 0.49 VRHE) than those of CIS and CIS/CdS photoelectrodes. We attribute this improved PEC performance to the synergetic impact of CdS in charge generation and transfer and MoS2 as a cocatalyst with active surface sites for proton reduction. The hydrogen evolution experiment showed CIS/CdS/MoS2 (56.9 μmol/h) showed high activity and rate of hydrogen evolution compared to CIS (22.6 μmol/h). This study not only demonstrates the promising nature of CuInS2-based light absorber photocathodes for solar energy utilization but also recommends the use of MoS2 as a cocatalyst for the proton reduction reactions for widespread applications in solar-to-hydrogen conversion. Reference (1) Kumar, M.; Meena, B.; Subramanyam, P.; Ummethala, G.; Malladi, S. R. K.; Dutta-Gupta, S.; Subrahmanyam, C. CuInS 2 Nanosheet Arrays with a MoS 2 Heterojunction as a Photocathode for PEC Water Splitting. Energy & Fuels 2023, 37 (3), 2340–2349. https://doi.org/10.1021/acs.energyfuels.2c03502. Figure 1

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting.

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9mA cm-2 at 1.23V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3V.

Fully inkjet-printed large-scale photoelectrodes

Small-area photoelectrodes are used to study fundamental science and material development for photoelectrochemical (PEC) water splitting cells at the laboratory scale. For practical applications, however, one needs to develop scalable geometrical designs and architectures of large photoelectrodes as well as their fabrication using low-cost, solution-processed, scalable methods. In this perspective, we first discuss the device physics concepts for developing large photoelectrodes using dimensional engineering (size, geometry, shape, and structures) and scalable architectures (such as symmetric and asymmetric designs with gridlines as well as monolithically integrated modules with interconnections), similar to the earlier development of large thin-film photovoltaic cells. Finally, we propose a novel and strategic protocol to fabricate these designs for the development of large-photoelectrode modules via commercially deployable, fully inkjet-printing as a solution processed thin-film deposition method. Small-area photoelectrodes are used to study fundamental science and material development for photoelectrochemical (PEC) water splitting cells at the laboratory scale. For practical applications, however, one needs to develop scalable geometrical designs and architectures of large photoelectrodes as well as their fabrication using low-cost, solution-processed, scalable methods. In this perspective, we first discuss the device physics concepts for developing large photoelectrodes using dimensional engineering (size, geometry, shape, and structures) and scalable architectures (such as symmetric and asymmetric designs with gridlines as well as monolithically integrated modules with interconnections), similar to the earlier development of large thin-film photovoltaic cells. Finally, we propose a novel and strategic protocol to fabricate these designs for the development of large-photoelectrode modules via commercially deployable, fully inkjet-printing as a solution processed thin-film deposition method.

The effects of potential and solar input on Z-scheme C3N4-TiO2 nanotubes @ Ti electrode in a broad potential window

Construction of Z-scheme graphitic carbon nitride-titanium dioxide nanotubes (C3N4-TNT) has been known useful to optimize the band structure for improving photon capture and for accelerating charge carrier separation and transfer rate in photoelectrochemical water splitting (PECWS) cells. However, the reported operating potential window in a PECWS cell, often in 0 – 1.23 VRHE (volt versus reversible hydrogen electrode) plus its overpotential, is too narrow to understand the C3N4-TNT electrode. Herein, a broad potential window of −0.5 − 2.5 VRHE is applied to C3N4-TNT@Ti and recorded via the polarization test under chopped sunlight to analyze the effect of both electrons from external electrical circuit and photons from simulated sunlight. In 0 – 2.5 VRHE, the potential enhances the photocurrent density. For example, at 1.6 VRHE, the C3N4-TNT sample exhibits 1.8-time higher photocurrent density than that of pure TNT. In −0.5 − 0 VRHE, i.e., both samples do not give photo-current response. In addition, for advanced water oxidation/reduction beyond WS to oxygen/hydrogen, a large potential window will be expected. Further, the light capture ability, the charge carrier recombination rate, and the electron flow path through the C3N4-TNT junction without and with reverse/forward potentials are discussed to elucidate the effect of the applied potential.

Thermoelectric voltage triggered self-biased photoelectrochemical water splitting utilizing visible light active Ag/NaNbO3 nanocomposite photoanode

Nanostructure semiconductor photoactive materials are often used as working photoelectrodes to absorb solar light in conventional photoelectrochemical (PEC) cells for water splitting to demonstrate enhanced PEC performance. However, an external bias is required to start PEC water splitting reactions for the generation of hydrogen. Here, we demonstrate a novel way of designing a self-biased hybrid system that combines a darkened thermoelectric device (TE) and a photoelectrochemical cell with a visible light-active n-type Ag/NaNbO3 photoanode to produce hydrogen via water splitting by utilizing the entire solar spectrum. This method can combine utilization of a visible light photon energy from the Ag/NaNbO3 photoanode with thermal energy from the darkened TE device to overcome the overpotential requirements. Upon light illumination, the photoactive behaviour of the Ag/NaNbO3 photoanode and the voltage generated from the darkened TE device in our hybrid system modifies the band positions of an Ag/NaNbO3 photoanode and spontaneously provide the external overpotential requirement for the PEC process and enhances the H2 production. Compared to the stand-alone PEC device, the TE-coupled PEC hybrid system exhibited ∼17 times enhancement in the photocurrent density and H2 production at zero bias. The electrochemical impedance spectroscopy analysis also shows that the charge transfer process gets significantly enhanced in the hybrid PEC system compared to the PEC cell alone. This finding indicates the synergistic coupling between TE and PEC cells, where a single light energy source can increase H2 production in the hybrid systems without needing an external bias.

Engineering the Interfacial Structure of Heavy Metal‐Free Colloidal Heterostructured Quantum Dots for High‐Efficiency Photoelectrochemical Water Oxidation without Co‐Catalyst

Solar‐driven photoelectrochemical (PEC) water splitting cell fabricated using colloidal quantum dots (QDs) is deemed as low‐cost and high‐efficiency solar‐to‐fuel conversion systems for future carbon neutrality. However, current QDs‐based PEC technologies are still hindered by several critical restrictions including the sluggish water oxidation kinetics and the frequent use of highly toxic QDs (e.g., Pb, Cd‐chalcogenides) as well as co‐catalysts, thus limiting their prospective commercial developments. Herein, the optoelectronic properties of heavy metal‐free InP/ZnSe core/shell QDs are tailored by introducing the interfacial GaP layers with variable thicknesses. As‐prepared InP/GaP/ZnSe core/shell QDs are used to sensitize TiO2 film as photoanodes for PEC water oxidation, showing an unprecedented photocurrent density of 4.1 mA cm−2 at 1.23 V versus reversible hydrogen electrode with excellent durability under one sun AM 1.5 G illumination. It is demonstrated that engineering the thickness of the interfacial GaP layer enables optimized optical properties and can introduce appropriate intermediate energy levels to promote the charge extraction from the InP core to the ZnSe shell for enhanced PEC water oxidation efficiency. This study paves the way for interfacial engineering of “green” core/shell QDs to realize cost‐effective, environment‐friendly, high‐performance, and co‐catalyst‐free QDs‐based PEC water splitting system.

Cobalt as a sacrificial metal to increase the photoelectrochemical stability of CuBi2O4 films for water splitting

CuBi2O4 is an excellent photocathode candidate in water-splitting photoelectrochemical cells. However, its poor photoelectrochemical stability caused by the reduction of Cu2+ to Cu metal limits its use. Here, we show a strategy to decrease the reduction of Cu2+ to Cu using cobalt as a sacrificial metal. Co-doped CuBi2O4 films were prepared by spray pyrolysis using Co2+ salt as the precursor. Co2+ ions replace Cu2+ in the CuBi2O4 structure, and subsequent heat treatment at 500 °C leads to partial oxidation of Co2+ to Co3+. As the reduction potential of Co3+/Co2+ is higher than that of Cu2+/Cu, Cu2+ reduction can be minimized. Comparatively, about 72% of the photocurrent produced by the CuBi2O4 film is lost in the first few minutes of illumination. In the Co-doped CuBi2O4 film, the photocurrent drops by less than 7%. Thus, the Co-doping can increase the CuBi2O4 photostability and be helpful for the fabrication of more stable photocathodes.

(Invited) Photochemical Energy Conversion with Integrated Plasmonics

Plasmonic nanostructures can dramatically increase the chemical reaction rates by providing more efficient solar-to-chemical energy conversion. Upon light excitation, metallic nanostructures sustain the collective oscillation of surface electrons, i.e. surface plasmons, producing local field enhancement and after one hundred femtoseconds decaying by generating a non-thermal distribution of hot carriers. Their consequent thermalization results in intense local heating of the nanostructures and their surrounding. All these effects can alter the reaction pathways and boost the reaction kinetics. Such discoveries have boosted the field of plasmonic photocatalysis, which aims at driving industrially-relevant chemical reactions using solar illumination. In this talk, I will present our recent progress in the fabrication of large-area plasmonic metasurfaces exhibiting a rich set of electromagnetic resonances and their integration in photoelectrochemical water splitting cells, direct alcohol fuel cells, and photochemical devices for hydrogen evolution. I will discuss the complex relationship between the optical resonances of the plasmonic metasurfaces with their chemical reactivity by distinguishing the various effects enhancing their photochemical performance. Finally, a new scanning photoelectrochemical method that provide the two-dimensional mapping of the chemical reactivity of plasmonic nanostructures on planar substrates with sub-micron resolution will be introduced.