Strategies for enhancing the photocurrent, photovoltage, and stability of photoelectrodes for photoelectrochemical water splitting

To accelerate the deployment of hydrogen produced by renewable solar energy, several technologies have been competitively developed, including photoelectrochemical (PEC), photocatalytic, and photovoltaic-electrolysis routes. In this review, we place PEC in context with these competing technologies and highlight key advantages of PEC systems. After defining the unique performance metrics of the PEC water splitting system, recently developed strategies for enhancing each performance metric, such as the photocurrent density, photovoltage, fill factor, and stability are surveyed in conjunction with the relevant theoretical aspects. In addition, various advanced characterization methods are discussed, including recently developed in situ techniques, allowing us to understand not only the basic properties of materials but also diverse photophysical phenomena underlying the PEC system. Based on the insights gained from these advanced characterization techniques, we not only provide a resource for researchers in the field as well as those who want to join the field, but also offer an outlook of how thin film-based PEC studies could lead to commercially viable water splitting systems

Molecular ink-derived chalcogenide thin films: Solution-phase mechanisms and solar energy conversion applications

The use of solution processing to fabricate metal chalcogenide thin films has gained considerable interest because of its low cost and scalability compared to vacuum deposition. Among the various types of solution processing, homogeneous molecular ink-based processes can lead to high-quality metal chalcogenide thin films, a prerequisite for high-efficiency solar energy conversion devices. In this review, we summarise studies on the fundamental understanding of the solution-phase mechanism of various types of molecular inks classified into three categories: hydrazine-, thiol-amine-, and organochalcogen-complex-based systems. The unique chemistry of each system is presented in conjunction with appropriate characterisation techniques to understand molecular interactions. Additionally, we also survey the recent solar energy conversion applications, such as solar cells and photoelectrodes for water splitting, with chalcogenide thin-film light absorbers prepared using each molecular ink technique. The strengths, limitations, and future possibilities of each system are discussed to provide insight into future advancements in molecular ink-derived chalcogenide thin films

Direct methane solid oxide fuel cells based on catalytic partial oxidation enabling complete coking tolerance of Ni-based anodes

Solid oxide fuel cells (SOFCs) can oxidize diverse fuels by harnessing oxygen ions. Benefited by this feature, direct utilization of hydrocarbon fuels without external reformers allows for cost-effective realization of SOFC systems. Superior hydrocarbon reforming catalysts such as nickel are required for this application. However, carbon coking on nickel-based anodes and the low efficiency associated with hydrocarbon fueling relegate these systems to immature technologies. Herein, we present methane-fueled SOFCs operated under conditions of catalytic partial oxidation (CPOX). Utilizing CPOX eliminates carbon coking on Ni and facilitates the oxidation of methane. Ni-gadolinium-doped ceria (GDC) anode-based cells exhibit exceptional power densities of 1.35 W cm−2 at 650 °C and 0.74 W cm−2 at 550 °C, with stable operation over 500 h, while the similarly prepared Ni-yttria stabilized zirconia anode-based cells exhibit a power density of 0.27 W cm−2 at 650 °C, showing gradual degradation. Chemical analyses suggest that combining GDC with the Ni anode prevents the oxidation of Ni due to the oxygen exchange ability of GDC. In addition, CPOX operation allows the usage of stainless steel current collectors. Our results demonstrate that high-performance SOFCs utilizing methane CPOX can be realized without deterioration of Ni-based anodes using cost-effective current collectors

Unraveling chirality transfer mechanism by structural isomer-derived hydrogen bonding interaction in 2D chiral perovskite

Abstract In principle, the induced chirality of hybrid perovskites results from symmetry-breaking within inorganic frameworks. However, the detailed mechanism behind the chirality transfer remains unknown due to the lack of systematic studies. Here, using the structural isomer with different functional group location, we deduce the effect of hydrogen-bonding interaction between two building blocks on the degree of chirality transfer in inorganic frameworks. The effect of asymmetric hydrogen-bonding interaction on chirality transfer was clearly demonstrated by thorough experimental analysis. Systematic studies of crystallography parameters confirm that the different asymmetric hydrogen-bonding interactions derived from different functional group location play a key role in chirality transfer phenomena and the resulting spin-related properties of chiral perovskites. The methodology to control the asymmetry of hydrogen-bonding interaction through the small structural difference of structure isomer cation can provide rational design paradigm for unprecedented spin-related properties of chiral perovskite

Solar water splitting exceeding 10% efficiency via low-cost Sb2Se3 photocathodes coupled with semitransparent perovskite photovoltaics

Solar water splitting directly converts solar energy into H2 fuel that is suitable for storage and transport. To achieve a high solar-to-hydrogen (STH) conversion efficiency, elaborate strategies yielding a high photocurrent in a tandem configuration along with sufficient photovoltage should be developed. We demonstrated highly efficient solar water splitting devices based on emerging low-cost Sb2Se3 photocathodes coupled with semitransparent perovskite photovoltaics. A state-of-the-art Sb2Se3 photocathode exhibiting efficient long-wavelength photon harvesting enabled by judicious selection of junction layers was employed as a bottom absorber component. The top semitransparent photovoltaic cells, i.e., parallelized nanopillar perovskites using an anodized aluminum oxide scaffold, allowed the transmittance, photocurrent, and photovoltage to be precisely adjusted by changing the filling level of the perovskite layer in the scaffold. The optimum tandem device, in which similar current values were allocated to the top and bottom cells, achieved an STH conversion efficiency exceeding 10% by efficiently utilizing a broad range of photons at wavelength over 1000 nm

Solar water splitting exceeding 10% efficiency via low-cost Sb2Se3 photocathodes coupled with semitransparent perovskite photovoltaics

Solar water splitting directly converts solar energy into H2 fuel that is suitable for storage and transport. To achieve a high solar-to-hydrogen (STH) conversion efficiency, elaborate strategies yielding a high photocurrent in a tandem configuration along with sufficient photovoltage should be developed. We demonstrated highly efficient solar water splitting devices based on emerging low-cost Sb2Se3 photocathodes coupled with semitransparent perovskite photovoltaics. A state-of-the-art Sb2Se3 photocathode exhibiting efficient long-wavelength photon harvesting enabled by judicious selection of junction layers was employed as a bottom absorber component. The top semitransparent photovoltaic cells, i.e., parallelized nanopillar perovskites using an anodized aluminum oxide scaffold, allowed the transmittance, photocurrent, and photovoltage to be precisely adjusted by changing the filling level of the perovskite layer in the scaffold. The optimum tandem device, in which similar current values were allocated to the top and bottom cells, achieved an STH conversion efficiency exceeding 10% by efficiently utilizing a broad range of photons at wavelength over 1000 nm

Surface restoration of polycrystalline Sb2Se3 thin films by conjugated molecules enabling high-performance photocathodes for photoelectrochemical water splitting

Achieving both high onset potential and photocurrent in photoelectrodes is a key challenge while performing unassisted overall water splitting using tandem devices. We propose a simple interface modification strategy to maximize the performance of polycrystalline Sb2Se3 photocathodes for photoelectrochemical (PEC) water splitting. The para-aminobenzoic acid (PABA) modification at Sb2Se3/TiO2 interface enhanced both the onset potential and photocurrent of the Sb2Se3 photocathodes. The surface defects in the polycrystalline Sb2Se3 limited the photovoltage production, lowering the onset potential of the photocathode. Surface restoration using the conjugated PABA molecules efficiently passivated the surface defects on the Sb2Se3 and enabled the rapid photoelectron transport from the Sb2Se3 to the TiO2 layer. The PABA treated Sb2Se3 photocathode exhibited substantially improved PEC performance; the onset potential increased from 0.35 to 0.50 V compared to the reversible hydrogen electrode (VRHE), and the photocurrent density increased from 24 to 35 mA cm−2 at 0 VRHE

Chemically Driven Enhancement of Oxygen Reduction Electrocatalysis in Supported Perovskite Oxides

Perovskite oxides have the capacity to efficiently catalyze the oxygen reduction reaction (ORR), which is of fundamental importance for electrochemical energy conversion. While the perovskite catalysts have been generally utilized with a support, the role of the supports, regarded as inert toward the ORR, has been emphasized mostly in terms of the thermal stability of the catalyst system and as an ancillary transport channel for oxygen ions during the ORR. We demonstrate a novel approach to improving the catalytic activity of perovskite oxides for solid oxide fuel cells by controlling the oxygen-ion conducting oxide supports. Catalytic activities of (La_0.8Sr_0.2)_0.95MnO₃ perovskite thin-film placed on different oxide supports are characterized by electrochemical impedance spectroscopy and X-ray absorption spectroscopy. These analyses confirm that the strong atomic orbital interactions between the support and the perovskite catalyst enhance the surface exchange kinetics by ∼2.4 times, in turn, improving the overall ORR activity

Stable water splitting using photoelectrodes with a cryogelated overlayer

Abstract Hydrogen production techniques based on solar-water splitting have emerged as carbon-free energy systems. Many researchers have developed highly efficient thin-film photoelectrochemical (PEC) devices made of low-cost and earth-abundant materials. However, solar water splitting systems suffer from short lifetimes due to catalyst instability that is attributed to both chemical dissolution and mechanical stress produced by hydrogen bubbles. A recent study found that the nanoporous hydrogel could prevent the structural degradation of the PEC devices. In this study, we investigate the protection mechanism of the hydrogel-based overlayer by engineering its porous structure using the cryogelation technique. Tests for cryogel overlayers with varied pore structures, such as disconnected micropores, interconnected micropores, and surface macropores, reveal that the hydrogen gas trapped in the cryogel protector reduce shear stress at the catalyst surface by providing bubble nucleation sites. The cryogelated overlayer effectively preserves the uniformly distributed platinum catalyst particles on the device surface for over 200 h. Our finding can help establish semi-permanent photoelectrochemical devices to realize a carbon-free society

Interfacial Dipole Layer Enables High-Performance Heterojunctions for Photoelectrochemical Water Splitting

TiO2 has been widely used as an n-type overlayer, simultaneously serving as a protective layer for photocathodes. However, the photovoltage generated from a TiO2 junction with p-type absorbers, such as p-Si, Sb2Se3, SnS, and Cu2O, is insufficient. We report a dipole reorientation strategy to overcome this limitation by inserting a polyethylenimine ethoxylated (PEIE) layer between a p-type absorber and TiO2. Furthermore, we demonstrate that the PEIE dipole orientation can be rearranged by increasing the layer thickness, leading to an upward shift of the TiO2 band edge. The magnitude of band shift induced by the dipole effect depends on the TiO2 layer thickness. Using this approach, the onset potential was significantly improved to 0.5 V versus the reversible hydrogen electrode (VRHE) in a p-Si/PEIE/TiO2/Pt device. The versatility of the effective dipole reorientation strategy was demonstrated by application to a range of TiO2-protected heterojunction photocathodes based on Sb2Se3, Cu2O, and SnS