Sunlight-driven Water Splitting Research Articles

Photocatalytic water splitting for large-scale solar-to-chemical energy conversion and storage

Sunlight-driven water splitting allows renewable hydrogen to be produced from abundant and environmentally benign water. Large-scale societal implementation of this green fuel production technology within energy generation systems is essential for the establishment of sustainable future societies. Among various technologies, photocatalytic water splitting using particulate semiconductors has attracted increasing attention as a method to produce large amounts of green fuels at low cost. The key to making this technology practical is the development of photocatalysts capable of splitting water with high solar-to-fuel energy conversion efficiency. Furthermore, advances that enable the deployment of water-splitting photocatalysts over large areas are necessary, as is the ability to recover hydrogen safely and efficiently from the produced oxyhydrogen gas. This lead article describes the key discoveries and recent research trends in photosynthesis using particulate semiconductors and photocatalyst sheets for overall water splitting, via one-step excitation and two-step excitation (Z-scheme reactions), as well as for direct conversion of carbon dioxide into renewable fuels using water as an electron donor. We describe the latest advances in solar water-splitting and carbon dioxide reduction systems and pathways to improve their future performance, together with challenges and solutions in their practical application and scalability, including the fixation of particulate photocatalysts, hydrogen recovery, safety design of reactor systems, and approaches to separately generate hydrogen and oxygen from water.

Enhanced Charge Separation and Hydrogen Evolution from Water over ZnTe-CdTe Solid Solution Based Photocathode with a Composition Gradient

Introduction The evident climate change associated with the greenhouse emissions from fossil fuels urgently requires the transition towards sustainable and reliable carbon-neutral resources. Photoelectrochemical (PEC) generation of hydrogen through sunlight driven water splitting is a promising alternative to be used as an energy source and as a raw material for chemical feedstock. Achieving efficient PEC cells requires the fabrication of high-quality photocatalytic materials capable of absorbing sunlight efficiently while minimizing carrier recombination. Cadmium telluride (CdTe) has shown promise as photocathode because of its narrow band gap of 1.5 eV with an expected high photocurrent density of 29 mAcm-2. Nevertheless, challenges persist, particularly concerning the limited onset potential (OP) of 0.6 VRHE for cathodic photocurrent, crucial for hydrogen evolution from water.1 In the present study, the introduction of a multilayer structure with copper (Cu) and zinc telluride (ZnTe) as back contact onto the CdTe based photocathode is investigated2. Enhancement on photocurrent at higher potential was attributed to the interdiffusion during film preparation by close space sublimation (CSS) method, and the spontaneously formed functional structure with composition gradient. Consequently, the novel band alignment of the ZnxCd1-xTe (0≤x≤1), generates a gradient on the conduction band, enhancing charge separation and harnessing of electrons outside the depletion layer while reducing electron-hole recombination. Experimental Prior to CdTe and ZnTe deposition, a 3x3 cm glass slide coated with indium tin oxide (ITO) was coated with 1 nm of Cu deposited through physical vacuum evaporation. 3 µm-thick ZnTe and 3 µm-thick CdTe were deposited using a CSS method, with a source temperature of 790ºC, and 700ºC for 3.5, and 2.5 min, respectively. During the deposition process, substrate temperature, Ar flow rate and pressure were maintained to be 500ºC, 50 sccm, and 6.0 Pa, respectively. Then a rapid thermal processing (RTP) was performed in the samples at 400°C for 2 minutes. The surface was modified with a cadmium sulfide layer (CdS), deposited over treated samples through the chemical bath deposition (CBD) method using aqueous chemical bath containing cadmium acetate, thiourea, and ammonia of 25 mM, 375 mM, and 14 wt%, respectively. Finally, nominally 2 nm thick platinum (Pt) was deposited using vacuum evaporation as a cocatalyst. PEC properties of the photocathodes were characterized by using a gas tight glass cell purged with Argon with three electrode configurations. Sample, Ag/AgCl in saturated aqueous KCl solution, and Pt wire were connected to a potentiostat as the working, reference, and counter electrodes, respectively. 1 M KPi with pH 6.5 was employed as an electrolyte. As a light source, AM 1.5G light from a solar simulator was employed. Results and discussions Figure 1 presents the current-potential curves under chopped simulated sunlight for CdS and Pt modified photocathodes prepared using CdTe monolayer (ITO/CdTe/CdS/Pt), CdTe layer with ZnTe back contact layer (ITO/ZnTe/CdTe/CdS/Pt), ITO/ZnTe/CdTe/CdS/Pt with Cu layer (ITO/Cu/ZnTe/CdTe/CdS/Pt), and ITO/Cu/ZnTe/CdTe/CdS/Pt with RTP. While the ITO/CdTe/CdS/Pt showed a typical photocurrent of -8 mA cm-2 at 0 VRHE, with an OP of 0.6 VRHE, the introduction of ZnTe severely decreased the photocurrent to -1.5 mA cm-2 at 0 VRHE. SEM structural analysis on cross-sectional reveals the presence of small ZnTe crystals and grain boundaries between the ZnTe and CdTe layers, favoring charge recombination. On the other hand, ITO/Cu/ZnTe/CdTe/CdS/Pt showed significantly enhanced photocurrent of -15 mA cm-2 at 0 VRHE with onset potential of >0.7 VRHE. Electron backscatter diffraction reveals the epitaxial growth columnar grains composed CdTe layer on the ZnTe layer on ITO/Cu/ZnTe/CdTe/CdS/Pt, while SEM-EDS analysis suggests the formation of a solid solution ZnxCd1-xTe (0≤x≤1) with composition gradient. It can be concluded that introduction of Cu enhances PEC hydrogen evolution from water through improvement of structural properties, larger grains, band alignment with composition gradient, and decrease of series resistance for the photocathodes3. Finally, RTP on ITO/Cu/ZnTe/CdTe/CdS/Pt was accessed, increasing the photocurrent at all potential ≥ 0 VRHE. In particular, the photocurrent was enhanced from −1.2 to −4 mA cm-2 at 0.5 VRHE which is the highest among reported CdTe-based photocathodes, and, consequently, a half-cell solar-to-hydrogen conversion efficiency (HC-STH) reached 3.7% at 0.4 VRHE.1- Su, T. Minegishi, and K. Domen, J. Mater. Chem. A. 5, 13154 (2017).2- Veiga, H. Kumagai, M. Sugiyama, T. Minegishi. Sustain. Energy Fuels (2024) DOI: (10.1039/D4SE00067F)3- Minegishi, S. Yamaguchi, and M. Sugiyama, Appl. Phys. Lett.. 119, 123905 (2021). Figure 1

(Invited) Photocatalyst Sheets for Scalable Solar Fuels Production from Water and CO2

The development of sunlight-driven water splitting or CO2 reduction systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of fuels and energy-rich chemicals.1 Z-scheme photocatalytic systems, employing the two-step photoexcitation of photocatalyst I (PC I) and photocatalyst II (PC II), have garnered significant attention. These systems can utilize narrow bandgap semiconductors capable of either water/CO2 reduction or oxidation potential, enabling the conversion of solar energy. The challenge in developing a Z-scheme system lies in ensuring efficient electron transfer between PC I and PC II.2 To address these challenges, we have developed all-solid-state devices—photocatalyst sheets—for photocatalytic water splitting.3,4 These sheets establish a direct connection between PC I and PC II particles via a conductive layer or conductive nanoparticles, effectively facilitating electron transfer between them. Consequently, these photocatalyst sheets achieve a high solar-to-H2 conversion efficiency exceeding 1%. They outperform analogous powder suspension and photoelectrochemical systems in their optimal reaction solutions due to the effective electron transfer enabled by the conductive layer or nanoparticles bridging PC I and PC II. Compared to traditional photoelectrochemical systems, the photocatalyst sheet design significantly reduces the effects of H+/OH− concentration overpotentials and the IR drop due to the proximity of PC I and PC II in the sheet. Hence, the design of these photocatalyst sheets proves highly suitable for efficient large-scale applications.Additionally, efforts to demonstrate fuel production from CO2 powered by sunlight are currently hampered by the requirements of sacrificial electron donors or external bias, and lack of efficiency, selectivity and scalability. To overcome those barriers, we integrate a selective molecular catalyst on semiconductor light absorbers to form a wireless and monolithic photocatalyst sheet.5 For instance, by incorporating a phosphonated cobalt(II) bis(terpyridine) catalyst (CotpyP) onto a modified SrTiO3:La,Rh|Au|BiVO4:Mo sheet, we have achieved a solar-to-formate conversion efficiency of 0.08%. This configuration harnesses the high selectivity of molecular catalysts for CO2 reduction and the strong water oxidation power of semiconductors, yielding a formate selectivity of 97±3% through the coupled process of CO2 reduction and water oxidation. We have also developed a bio-abiotic hybrid system by combining a photocatalyst sheet, capable of scalable photocatalytic water splitting, with nonphotosynthetic bacteria acting as biocatalysts.6 When pairing CO2-fixing acetogenic bacteria, Sporomusa ovata, with a SrTiO3:La,Rh|ITO|BiVO4:Mo sheet, a solar-to-acetate conversion efficiency of ~0.7% was achieved (as shown in the figure). This coupling exhibited an impressive selectivity for acetate production in CO2 reduction reactions, reaching ~100%.Our study offers novel and versatile strategies toward sustainable solar fuel production.References Q. Wang, C. Pornrungroj, S. Linley, E. Reisner. Nat. Energy 7 , 13–24 (2022)Y. Sasaki, H. Nemoto, K. Saito, A. Kudo. J. Phys. Chem. C 113, 17536–17542 (2009)Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I. D. Sharp, A. Kudo, T. Yamada, K. Domen. Nat. Mater. 15,611–615 (2016)Q. Wang, T. Hisatomi, Y. Suzuki, Z. Pan, J. Seo, M. Katayama, T. Minegishi, H. Nishiyama, T. Takata, K. Seki, A. Kudo, T. Yamada, K. Domen. J. Am. Chem. Soc. 139, 1675–1683 (2017)Q. Wang, J. Warnan, S. Rodríguez-Jiménez, J. J. Leung, S. Kalathil, V. Andrei, K. Domen, E, Reisner. Nat. Energy 5, 703–710 (2020)Q. Wang, S. Kalathil, C. Pornrungroj, D. C. Sahm, E. Reisner. Nat. Catal. 5, 633–641 (2022) Figure 1

Structural and optical properties of composition-graded InGaN nanowires

At the moment, InGaN ternary compounds are of a great interest for the development of devices for sunlight driven water splitting. However, the synthesis of such materials is hindered by the fact that InxGa1–xN layers are susceptible to phase decomposition at x from 20 to 80%. Nanowires can be a promising solution to this problem. The purpose of our study was to analyze the structural and optical properties of InxGa1–xN nanowires with a gradient x content being inside the miscibility gap. InxGa1–xN nanowires were grown on silicon substrates using plasma-assisted molecular beam epitaxy. The structural properties of nanowires were studied using scanning and transmission electron microscopy. The chemical composition and optical properties of nanowires were analyzed using energy-dispersive microanalysis and photoluminescence spectroscopy. It was shown for the first time that the composition-graded InxGa1–xN nanowires with x from 40 to 60% can be grown using plasma-assisted molecular beam epitaxy. The grown samples exhibit photoluminescence at room temperature with a maximum at about 890 nm, which corresponds to an In content of about 62% according to the modified Vegard’s rule and the transmission electron microscopy data. The obtained results can be of practical interest for the development of devices for water splitting induced by sunlight or sources of near IR radiation

Novel two-dimensional square-AlAs: Direct semiconductor coupled with high electron-hole mobility ratio toward sunlight-driven water-splitting

The design of new monolayered atomic motifs always attracts enormous attention due to their interesting properties induced by unique crystal phases and promising catalytic applications in low dimensional devices. Herein, we propose a novel square-AlAs monolayer based on ab initio density functional theory calculations, whose good stability is confirmed from the perspectives of energy, dynamics, thermodynamics, and mechanics. Electronic structure calculations reveal that the newly predicted square-AlAs monolayer is a direct-gap semiconductor with a bandgap of 1.39 eV, which is sensitive to in-plane strain, meanwhile the strain induced changes in bandgap allow advantageous modulations of optical absorption in visible-light region. Unfortunately, the unfavorable hydrogen evolution reaction (HER) band edge levels make square-AlAs not suitable for photocatalytic water-splitting in spite of the appropriate bandgap, high electron-hole mobility ratio and strong visible light absorption. However, a favorable effect caused by strain engineering is revealed by the fact that the square-AlAs can meet the requirement of photocatalytic water-splitting under the suitable biaxial loading. Particularly, compared to pure AlAs, the Gibbs free energy of hydrogen adsorption indicate that Boron-doping can effectively decrease the HER overpotential. Moreover, the external potential provided by photoexcited electrons for oxygen evolution reaction (OER) is quite positive, which can serve as a free and pollution-free driving force to drive OER to proceed spontaneously for pure AlAs or reduce the OER overpotential to a very low value (0.15 V) for Boron-doped AlAs, overmatching widely used IrO2 catalyst.

Investigation of p-CuNb2O6 for use as photocathodes for photoelectrochemical water splitting

P-type copper niobate (CuNb2O6) was prepared for the first time by a two-step spray pyrolysis method on FTO glass substrates. Its feasibility in photoelectrochemical (PEC) decomposition water was systematically evaluated. The physical and photophysical properties of the thin film, including band edge position, bandgap energy, carrier lifetime, and incident photo-to-current efficiency (IPCE), were characterized and analyzed. The optical band gap of the prepared gray CuNb2O6 film is ∼1.41 eV, which indicates that it can absorb visible light and achieve sunlight-driven water splitting. The photocurrent density under simulated sunlight at AM1.5 was 0.25 mA/cm2, and the IPCE value could reach up to 3% at 400 nm in an aqueous solution (pH = 6.8) when applying a bias of 0.4 V vs. RHE. Despite exhibiting a good optical band gap and photocurrent density, the fabricated p-CuNb2O6 thin films remained unstable under illumination in aqueous solutions due to the reduction reaction of copper. Based on these findings, this study provides a new opinion on the applicability of CuNb2O6 as a photocathode material for PEC water splitting.

Plasma-enhanced atomic layer deposition of nickel and nickel oxide on silicon for photoelectrochemical applications

The production of hydrogen fuel through sunlight-driven water splitting has the potential to harness and store large quantities of solar energy in a clean and scalable chemical state, suitable for later use in a range of energy applications. Silicon (Si) possesses many of the required properties to be used effectively as a photoelectrochemical (PEC) water-splitting photoanode. However, its sensitivity to corrosion during the oxygen evolution reaction limits its performance in photoanode applications, thus requiring additional overlayer materials to protect the underlying Si substrate. Nickel oxide (NiO) is one material that acts as an effective protective layer, being transparent, suitably conductive and stable. In this work, we present NiO deposition via state-of-the-art atomic layer deposition and photoemission studies to grow and characterize NiO and Ni–metal protective films. Early-stage nucleation of deposited thin films is illustrated along with the effects of post-deposition annealing and argon milling on depth profile information. Previous reports on the effects of slow argon milling are explored and counter arguments are proposed. Protective films are subjected to PEC testing, which shows enhancement of stability and photocurrent output as a result of the deposited films and plasma annealing on these thin films.

Design and construction of a novel hierarchical Ag/{1 1 1}Ag3PO4/PANI/Pt photoanode with boosted interfacial charge transfer rate and high photocurrent density > 16 mA/cm2 for sunlight-driven water splitting

In this work, a novel hierarchical photoanode structure of Ag/{111}Ag3PO4/PANI/Pt was synthesized, demonstrating a novel and facile approach to design a high-performing Ag3PO4-based photoanode for visible-light-driven solar green hydrogen (H2) generation. The Ag3PO4 photoanode with carefully curated active facets, Ag/{111}Ag3PO4, was interfaced with a conductive polyaniline (PANI) thin film and surface decorated with platinum nanoparticles (Pt NPs) to realise the combined advantages of integrating both PANI and Pt NPs as a hole-transporting layer and co-catalysts, respectively. This novel photoanode achieved a record photocurrent density of 16.34 mA/cm2 at 1.4 V vs Ag/AgCl under AM 1.5 G solar irradiation (100 mW/cm2), which outperformed that of the bare Ag/{111}Ag3PO4 photoanode and is also the highest photocurrent density reported to date for Ag3PO4-based photoanodes. The PEC enhancement is attributed to the new exploitation of the PANI-{111}Ag3PO4p-n heterojunction, the hole-transporting property of PANI and the electron-capturing capability of Pt NPs, in which their synergetic interactions effectively led to improved light absorption, enhanced charge separation and reduced charge recombination. It is foreseen that this current work can provide a base for further nanoarchitectural design strategies to construct efficient photoanodes used in PEC water splitting application.

(Invited) Solar Hydrogen Production with Particulate Photocatalysts

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen on a large scale. Both the efficiency and scalability of water splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of the solar energy. In this context, overall water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over wide areas by inexpensive processes potentially [1], although it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented.The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting [2]. SrTiO3 is an oxide photocatalyst that has been known to be active in overall water splitting under ultraviolet irradiation since 1980 [3]. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [4]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY has been improved to more than 90% at 365 nm [5], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.The author's group has also been developing panel reactors for large-scale applications. Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultraviolet irradiation even when the water depth is merely 1 mm [6]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [7]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. More recently, a solar hydrogen production system based on 100-m2 arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues was reported [8].For practical solar energy harvesting, it is essential to develop photocatalysts that are active under visible light irradiation. Ta3N5 and Y2Ti2O5S2 photocatalysts are active in overall water splitting via one-step excitation under visible light irradiation [9,10]. Particulate photocatalyst sheets efficiently split water into hydrogen and oxygen via two-step excitation, referred to as Z-scheme, regardless of size. In particular, a photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 and Mo-doped BiVO4 splits water into hydrogen and oxygen via the Z-scheme, showing a STH exceeding 1.0% [11,12]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths can also be applied to Z-scheme photocatalyst sheets. Hisatomi et al., Nat. Catal. 2019, 2, 387.Chen et al., Nat. Rev. Mater. 2017, 1, 17050.Domen et al., J. Chem. Soc., Chem. Commun. 1980, 543Ham et al., J. Mater. Chem. A 2016, 4, 3027.Takata et al., Nature 2020, 581, 411.Goto et al., Joule 2018, 2, 509.Lyu et al., Chem. Sci. 2019, 10, 3196.Nishiyama et al., Nature 2021, 598, 304.Wang et al., Nat. Catal. 2018, 1, 756.Wang et al., Nat. Mater. 2019, 18, 827.Wang et al., Nat. Mater. 2016, 15, 611.Wang et al., J. Am. Chem. Soc. 2017, 139, 1675.

NiO Modified CN Film As Photoanodes for Photoelectrochemical Water Oxidation

The utilization of solar energy, by far the most promising renewable energy resource, remains one of the hottest topics in the 21st century. Metal-free carbon nitride (CN) material emerged as a promising water splitting photocatalyst in 2009 due to its appropriate visible light absorption, suitable optical band gap energy (2.7 eV), and facile synthesis. Though CN is one of the leading materials for solar energy conversion, this material is limited by light absorption, rapid charge recombination, and low charge carrier mobility. Metal oxide are thus investigated for counterbalancing CN’s inherent drawbacks. Interfacial CN/metal oxide heterostructures facilitate charge transportation, resulting in improved sunlight-driven water splitting process. In this work, low-cost NiO transition-metal oxide material was applied to speed up the sluggish kinetics of the oxygen evolution reaction (OER). Typically, CN modification is achieved by traditional hydrothermal approach, while its disadvantages such as uneven coating particle size and heterogenous distribution are becoming increasingly apparent. Aiming at a conformal morphology, atomic layer deposition (ALD) is developed as the state-of-the-art technique. It has boosted the depositing accuracy by achieving precisely depositing thickness and extremely homogenous surface. In our process, a uniform CN film was deposited on FTO substrates using a dipping-drying technique with a hot saturated thiourea aqueous solution followed by a thermal treatment. Then plasma-enhanced atomic layer deposition (PEALD) was used to modify the CN film with a thin layer of NiO. PEALD controls the NiO modification on a fine-scale, allowing to with deposit a nanoscale NiO layer on the CN surface while exposing adequate CN photoreactive sites. According to our results, the modified NiO/CN heterostructure has the potential to improve photoelectrochemical water oxidation as a photoanode in alkaline solution. From the morphology side, the NiO loaded flake-like CN creates relatively high specific area for OER. Then we investigated the photo-process kinetic using a series of optical and spectroelectrochemcial techniques. Optical absorption is enhanced, showing stronger visible light absorption up to 700 nm. Fast charge separation is suggested in photoluminescence (PL) characterization. Superior (photo)electrochemical (PEC) activity is foreseen through PEC and EC measurements. To conclude, this is the first time we succeed to modify the CN film with the ALD technique for solar-driven water oxidation, and has the highly possibility of reaching the photo(electro)chemical performance new level.

Size dependence of perovskite-type BaNbO2N particles on sunlight-driven photoelectrochemical water splitting

The size of particulate semiconductors is a critical factor in photoelectrochemical (PEC) water splitting activity because the surface area determined by the particle size establishes the electrochemical reaction sites as well as the resulting photoactivity. Herein we demonstrate the size dependence of perovskite-type BaNbO2N particles, responsive to intensive-visible-light up to 740 nm, on the PEC water splitting activity. The layered perovskite Ba5Nb4O15 particles in different sizes, as a starting oxide for synthesis of BaNbO2N, were synthesized by polymerized complex method, followed by calcination at different temperatures (1073, 1173, 1273, and 1373 K). The size of Ba5Nb4O15 particles was dependent on the calcination temperature so that low-temperature calcination led to smaller BaNbO2N particles. Although the different Ba5Nb4O15 particles experienced nitridation at 1123 K for 13 h, the sizes of the resulting BaNbO2N particles were nearly unchanged from those of the starting oxides. The sizes and Brunauer-Emmett-Teller surface areas of porous BaNbO2N particles largely influenced the PEC water splitting activity. As a result, photoelectrode employing the small BaNbO2N particles, prepared from the calcination at 1173 K, produced the highest photocurrent of 2.20 mA cm−2 at 1.23 VRHE during the sunlight-driven water splitting, showing the size dependence of oxynitride particles on photoactivity. Therefore, findings from this study support that the size reduction of oxynitride particles largely improved the PEC water splitting activity.

Utilizing Cyanobacteria in Biophotovoltaics: An Emerging Field in Bioelectrochemistry.

Anthropogenic global warming is driven by the increasing energy demand and the still dominant use of fossil energy carriers to meet these needs. New carbon-neutral energy sources are urgently needed to solve this problem. Biophotovoltaics, a member of the so-called bioelectrochemical systems family, will provide an important piece of the energy puzzle. It aims to harvest the electrons from sunlight-driven water splitting using the natural oxygenic photosystem (e.g., of cyanobacteria) and utilize them in the form of, e.g., electricity or hydrogen. Several key aspects of biophotovoltaics have been intensively studied in recent years like physicochemical properties of electrodes or efficient wiring of microorganisms to electrodes. Yet, the exact mechanisms of electron transfer between the biocatalyst and the electrode remain unresolved today. Most research is conducted on microscale reactors generating small currents over short time-scales, but multiple experiments have shown biophotovoltaics great potential with lab-scale reactors producing currents over weeks to months. Although biophotovoltaics is still in its infancy with many open research questions to be addressed, new promising results from various labs around the world suggest an important opportunity for biophotovoltaics in the decades to come. In this chapter, we will introduce the concept of biophotovoltaics, summarize its recent key progress, and finally critically discuss the potentials and challenges for future rational development of biophotovoltaics.

Bismuth Oxide Decorated Silicon Photocathodes for CO2 to Formate Solar Driven Conversion

In the field of CO2 electrochemical conversion, [1] the use of semiconducting photocathodes instead of non-photoactive traditionally used electrodes (e.g., metals and carbon) could provide a real benefit in terms of energy gain because it allows the electrochemical process to be activated by photogenerated electrons. [2] In that context, silicon appears as one of the most promising semiconducting materials owing to its small bandgap (1.1 eV) able not only to harvest photons from a large portion of the solar spectrum but also to encompass the different proton-assisted multielectron reduction potentials for CO2. [3],[4],[5] Herein, we report that silicon photocathodes modified with electrodeposited Bi nanostructures are highly active for the photoelectrocatalytic conversion of CO2 to formate in aqueous electrolytic medium. p-type Si photocathodes functionalized with Bi2O3 catalyst were easily fabricated without an additive polymer layer using a one-step and fast electrodeposition method. Both composition and morphology of the deposited Bi cocatalyst could be tuned by controlling the electrodeposition time. The photocathode prepared from a 5 s electrodeposition exhibited the highest photocurrent density (-24.1 mA cm-2) with partial formate photocurrent density j formate = -17.4 mA cm-2 at -1.03 V vs Reversible Hydrogen Electrode (corresponding to a 0.84 V overpotential for CO2 to formate conversion).Such values highlight the excellent catalytic activity for CO2 electroreduction of our photocathodes, [6] outperforming that of recently reported Bi-based catalysts deposited on Si. [7] Moreover, under our conditions, the formate production rate reached 14.9 mg h-1 cm-2 with a Faradaic efficiency for formate of 72%. We anticipate that the conversion efficiency of CO2 could be further improved by using structured silicon surfaces, as recently demonstrated by ourselves for sunlight-driven water splitting, [8] and/or Bi nanostructures. [1] a) J. Qiao, Y. Liu, F. Hong, J. A. Zhang, J. Chem. Soc. Rev. 2014, 43, 631-675. b) C. Costentin, M. Robert, J. M. Saveant, Chem. Soc. Rev. 2013, 42, 2423−2436. [2] a) J. L. White, M. F. Baruch, J. E. Pander III, Y. Hu, I. C. Fortmeyer, J. E. Park, T. Zhang, K. Liao, J. Gu, Y. Yan, T. W. Shaw, E. Abelev, A. B. Bocarsly, Chem. Rev. 2015, 115, 12888–12935. b) K. Maeda, Adv. Mater. 2019, 31, 1808205. [3] K. Sun, S. Shen, Y. Liang, P. E. Burrows, S. S. Mao, D. Wang, Chem. Rev. 2014, 114, 8662−8719. [4] a) D. He, T. Jin, W. Li, S. Pantovich, D. W. Wang, G. H. Li, Chem. Eur. J. 2016, 22, 13064. b) B. Kumar, J. M. Smieja, C. P. Kubiak, J. Phys. Chem. C 2010, 114, 14220. c) B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C. P. Kubiak, Annu. Rev. Phys. Chem. 2012, 63, 541. [5] E. Torralba-Penalver, Y. Luo, J.-D. Compain, S. Chardon-Noblat, B. Fabre, ACS Catal. 2015, 5, 6138. [6] D. Fu, J. Tourneur, B. Fabre, G. Loget, Y. Lou, F. Geneste, S. Ababou-Girard, C. Mériadec, ChemCatChem. 2020, 12, 5819. [7] a) Q. Gong, P. Ding, M. Xu, X. Zhu, M. Wang, J. Deng, Q. Ma, N. Han, Y. Zhu, J. Lu, Z. Feng, Y. Li, W. Zhou, Y. Li, Nat. Commun. 2019, 10:2807. b) P. Ding, Y. Hu, J. Deng, J. Chen, C. Zha, H. Yang, N. Han, Q. Gong, L. Li, T. Wang, X. Zhao, Y. Li, Mater. Today Chem. 2019, 11, 80-85. [8] a) J. Tourneur, B. Fabre, G. Loget, et al. J. Am. Chem. Soc. 2019, 141, 11954. b) K. Oh, V. Dorcet, B. Fabre, G. Loget, Adv. Energy Mater. 2019, 1902963. Figure 1

Harvesting Hot Holes in Plasmon-Coupled Ultrathin Photoanodes for High-Performance Photoelectrochemical Water Splitting.

The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO2 (Au@a-TiO2) generated 2.5 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO2 photoelectrodes produced a small positive photocurrent of 0.1 mA cm-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.

Effect of morphology on the photoelectrochemical performance of nanostructured Cu2O photocathodes

Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph ), onset potential (E onset), band edges, carrier density (NA ), and interfacial charge transfer resistance (R ct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and R ct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (E fb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. E onset of 0.48 ± 0.04 eV obtained for these systems is close to the E fb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.

(Invited) Particulate Photocatalysts with Perovskite-Type Structures and Their Application in Water Splitting

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier. Both the efficiency and scalability of water- splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of solar energy [1]. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because such systems can be spread over wide areas by inexpensive processes potentially. The author’s group has studied various oxide and oxynitride semiconductors as photocatalysts for water splitting. In my talk, recent progress in photocatalytic materials and reactors will be presented with focus on perovskite-type materials.SrTiO3 loaded with proper cocatalysts has been known to be active in overall water splitting under ultraviolet light irradiation since 1980s. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically [2]. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [3]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY of overall water splitting has reached more than 90% at 365 nm [2], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultrviolet illumination even when the water depth is merely 1 mm [4]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [5]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A solar hydrogen production system with a greater size (100 m2) was recently built, and its performance and system characteristics are under investigation.SrTiO3:Al harvests only the ultraviolet light of the sunlight. It is essential to develop photocatalysts active under visible light irradiation. BaTaO2N is a visible light that can evolve hydrogen and oxygen from aqueous solutions containing sacrificial electron donors and acceptors, respectively. Recently, the AQYs of BaTaO2N in these sacrificial reactions have been improved significantly. Highly-crystalline BaTaO2N can be synthesized by directly nitriding a mixture of BaCO3 in the presence of RbCl flux [6]. The resultant BaTaO2N shows decent hydrogen evolution activity when a platinum cocatalyst is loaded by impregnation. By optimizing the cocatalyst loading conditions, the electron transfer from the BaTaO2N photocatalyst to the platinum cocatalyst, and thus the hydrogen evolution activity, was enhanced. The AQY of the resultant Pt-loaded BaTaO2N reached 6.8% at 420 nm in the sacrificial hydrogen evolution reaction. This is exceptionally high among nitride-type photocatalysts with visible light absorption of up to 650 nm.In conventional synthesis, BaTaO2N is nitrided from a mixture of BaCO3, Ta2O5, Ba- and Ta-containing complex oxides, often in the presence of flux. However, these starting materials have different crystal structures from the product BaTaO2N, and so the crystal structures have to be change during the nitridation. This is thought to induce defects in the product material. Therefore, a perovskite-type oxide with a Ba/Ta molar ratio of unity was designed, namely a complex oxide nominally denoted as Na1/4Ba3/4Zn1/4Ta3/4O3 [7]. This oxide can be nitrided into BaTaO2N because Na and Zn components are volatile at high temperature. The resultant BaTaO2N exhibited an AQY of 11.9% at 420 nm in the sacrificial oxygen evolution reaction after loading of a cobalt oxide cocatalyst. Hisatomi et al. Catal. 2019, 2, 387.Takata et al., Nature 2020, 581, 411.Ham et al., Mater. Chem. A 2016, 4, 3027.Goto et al., Joule 2018, 2, 509.Lyu et al. Sci. 2019, 10, 3196.Luo et al., Growth Des. 2020, 20, 255.Jadhav et al. J. Mater. Chem. A. 2020, 8, 1127.

Challenges and prospects about the graphene role in the design of photoelectrodes for sunlight-driven water splitting.

Graphene and its derivatives have emerged as potential materials for several technological applications including sunlight-driven water splitting reactions. This review critically addresses the latest achievements concerning the use of graphene as a player in the design of hybrid-photoelectrodes for photoelectrochemical cells. Insights about the charge carrier dynamics of graphene-based photocatalysts which include metal oxides and non-metal oxide semiconductors are also discussed. The concepts underpinning the continued progress in the field of graphene/photoelectrodes, including different graphene structures, architecture as well as the possible mechanisms for hydrogen and oxygen reactions are also presented. Despite several reports having demonstrated the potential of graphene-based photocatalysts, the achieved performance remains far from the targeted benchmark efficiency for commercial application. This review also highlights the challenges and opportunities related to graphene application in photoelectrochemical cells for future directions in the field.

Z-Scheme Water Splitting under Near-Ambient Pressure using a Zirconium Oxide Coating on Printable Photocatalyst Sheets.

Sunlight-driven water splitting systems operating under ambient pressure are essential for practical renewable hydrogen production. Printable photocatalyst sheets, composed of a hydrogen evolution photocatalyst (HEP), an oxygen evolution photocatalyst (OEP), and conductive metal nanoparticles, are cost-effective and scalable systems. However, the decrease in water splitting activity under ambient pressure due to reverse reactions hampers their practical application. In this study, coating zirconium oxide (ZrOx ) by facile drop-casting onto a printed photocatalyst sheet, which consists of SrTiO3 : Rh, BiVO4 : Mo, and Au nanocolloids as the HEP, OEP, and electron mediator, respectively, effectively maintains the water splitting activity at elevated pressure. The ZrOx -coated photocatalyst sheet retains 90 % and 84 % of the base performance (the pristine sheet at 10 kPa) at 50 and 90 kPa, respectively. Achieving efficient water splitting at the ambient pressure by inexpensive and extensible processes is an important step toward solar hydrogen production.

(Keynote) Large Scale Solar Hydrogen Production with Water Splitting Panel

Sunlight-driven water splitting has been studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier [1]. Both the efficiency and the scalability of water-splitting systems are essential factors for practical utilization of renewable solar hydrogen because of the low areal density of solar energy. Particulate photocatalyst systems do not involve any secure electric circuit and can be spread over wide areas by inexpensive processes potentially. Therefore, it is highly impactful to develop particulate photocatalysts and their reaction systems that efficiently split water. In my talk, the latest progress in photocatalytic materials and reactors and concepts toward large-scale operation will be presented.A semiconductor photocatalyst can split water into hydrogen and oxygen thermodynamically when the band gap straddles the potentials of the hydrogen evolution reaction (0 V vs. RHE) and the oxygen evolution reaction (+1.23 V vs. RHE). SrTiO3 is a photocatalyst known to be active in overall water splitting under UV light irradiation after loading of proper cocatalysts since 1980s. The author's group has found that doping Al into SrTiO3 boosts the water splitting activity by two orders of magnitude [2]. Through tuning of the preparation and modification methods, the apparent quantum yield of photocatalytic water splitting using Al-doped SrTiO3 has reached 95% at wavelengths in the range of 350–360 nm, equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported, and confirms that a particulate photocatalyst can drive the greatly uphill overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively. Nevertheless, it is essential to develop photocatalysts active under visible light irradiation.Various oxides, (oxy)nitrides, and (oxy)chalcogenides have been investigated [3]. Ta3N5 has a band structure suitable for water splitting under visible irradiation of up to 600 nm; however, overall water splitting had not been achieved because of high densities of trap states until recently. The author’s group has developed a Ta3N5 photocatalyst active in overall water splitting via one-step excitation under visible light [4]. Ta3N5 single crystal nanorods are directly evolved on KTaO3 particles by short-time nitridation accompanied by gradual evaporation of K contents. Ta3N5 single crystal nanorods have low defect densities and can utilize photoexcited carriers in the water splitting reaction when being loaded with appropriate cocatalysts. It was also found that Y2Ti2O5S2, an oxysulfide photocatalyst, could be activated in overall water splitting by coloading cocatalysts for hydrogen evolution and oxygen evolution and finely tuning the reaction conditions [5].Two different photocatalysts can also be combined so that hydrogen and oxygen are generated on the respective photocatalysts. The author’s group has developed particulate photocatalyst sheets consisting of the hydrogen evolution photocatalyst (HEP) and the oxygen evolution photocatalyst (OEP) embedded into conductive layers by particle transfer [6,7]. A photocatalyst sheet consisting of La- and Rh-codoped SrTiO3 as a HEP and Mo-doped BiVO4 as an OEP embedded into a carbon conductor exhibits a solar-to-hydrogen energy conversion efficiency (STH) of 1.0% at ambient pressure. Some (oxy)chalcogenides and (oxy)nitrides with longer absorption edge wavelengths have also been shown to be applicable as the HEP and the OEP of particulate photocatalyst sheets recently.The author's group has been developing panel reactors that accommodate photocatalyst sheets in view of large-scale application [2]. A prototype panel reactor containing Al-doped SrTiO3 photocatalyst sheets splits water and releases product hydrogen and oxygen gas bubbles at a rate corresponding to a STH of 10% under intense UV illumination even when the water depth is merely 1 mm, and it can maintain the activity over several months under sunlight illumination [8]. A 1-m2-sized photocatalyst panel reactor splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. Panel reactors can accommodate various kinds of photocatalyst sheets and are expected to be built using light and inexpensive materials, being suitable for large-scale solar hydrogen production from water. A prototype solar hydrogen production system with a greater size is currently under construction.[1] Hisatomi et al. Nat. Catal. 2019, 2, 387.[2] Goto et al. Joule 2018, 2, 509.[3] Chen et al. Nat. Rev. Mater. 2017, 2, 17050.[4] Wang et al. Nat. Catal. 2018, 1, 756.[5] Wang et al. Nat. Mater. 2019, 18, 827.[6] Wang et al. Nat. Mater. 2016, 15, 611.[7] Wang et al. J. Am. Chem. Soc. 2017, 139, 1675.[8] Lyu et al. Chem. Sci., 2019, 10, 3196.

Accelerating charge transfer for highly efficient visible-light-driven photocatalytic H2 production: In-situ constructing Schottky junction via anchoring Ni-P alloy onto defect-rich ZnS

The development of efficient and stable photocatalysts is fundamentally required for sunlight-driven water splitting. Herein, Ni-P alloy is controllably anchored onto the surface of defect-rich ZnS (DR-ZnS) nanospheres through in-situ photodeposition strategy. The optimized Ni-P/DR-ZnS nanocomposite displays superior visible-light photocatalytic H2 production rate (69.92 μmol h−1), which is about 29 times and 3.6 times higher than that of the bare DR-ZnS and 1 wt% Pt/DR-ZnS, respectively, and a notable apparent quantum yield of 2.4 % at 420 nm. The existence of rich defects endows the DR-ZnS with visible-light absorption capability. The constructed Schottky junction between the Ni-P alloy and DR-ZnS accelerates the transfer of photogenerated electrons from DR-ZnS to Ni-P, thus boosting the photocatalytic performance. Moreover, the charge transfer process and underlying photocatalytic mechanism are unravelled by multiple in-depth characterizations. This research sheds novel light on rationally fabricating high-efficient and low-cost photocatalysts through integrating defect engineering and Schottky junction design.