Unassisted Photoelectrochemical H2O2 Production with In Situ Glycerol Valorization Using α-Fe2O3.

Photoelectrochemical (PEC) H2O2 production via two-electron O2 reduction is promising for H2O2 production without emitting CO2. For PEC H2O2 production, α-Fe2O3 is an ideal semiconductor owing to its earth abundance, superior stability in water, and an appropriate band gap for efficient solar light utilization. Moreover, its conduction band is suitable for O2 reduction to produce H2O2. However, a significant overpotential for water oxidation is required due to the poor surface properties of α-Fe2O3. Thus, unassisted solar H2O2 production is not yet possible. Herein, we demonstrate unassisted PEC H2O2 production using α-Fe2O3 for the first time by applying glycerol oxidation, which requires less bias compared with water oxidation. We obtain maximum Faradaic efficiencies of 96.89 ± 0.6% and 100% for glycerol oxidation and H2O2 production, respectively, with high stability for 25 h. Our results indicate that unassisted and stable PEC H2O2 production is feasible with in situ glycerol valorization using the α-Fe2O3 photoanode.

High-Performance Electrochemical and Photoelectrochemical Water Splitting at Neutral pH by Ir Nanocluster-Anchored CoFe-Layered Double Hydroxide Nanosheets.

Highly efficient electrocatalysts for the oxygen evolution reaction (OER) in neutral electrolytes are indispensable for practical electrochemical and photoelectrochemical water splitting technologies. However, there is a lack of good, neutral OER electrocatalysts because of the poor stability when H+ accumulates during the OER and slow OER kinetics at neutral pH. Herein, we report Ir species nanocluster-anchored, Co/Fe-layered double hydroxide (LDH) nanostructures in which the crystalline nature of LDH-restrained corrosion associated with H+ and the Ir species dramatically enhanced the OEC kinetics at neutral pH. The optimized OER electrocatalyst demonstrated a low overpotential of 323 mV (at 10 mA cm-2) and a record low Tafel slope of 42.8 mV dec-1. When it was integrated with an organic semiconductor-based photoanode, we obtained a photocurrent density of 15.2 mA cm-2 at 1.23 V versus reversible hydrogen in neutral electrolyte, which is the highest among all reported photoanodes to our knowledge.

Bias-free solar hydrogen production at 19.8 mA cm-2 using perovskite photocathode and lignocellulosic biomass.

Solar hydrogen production is one of the ultimate technologies needed to realize a carbon-neutral, sustainable society. However, an energy-intensive water oxidation half-reaction together with the poor performance of conventional inorganic photocatalysts have been big hurdles for practical solar hydrogen production. Here we present a photoelectrochemical cell with a record high photocurrent density of 19.8 mA cm-2 for hydrogen production by utilizing a high-performance organic-inorganic halide perovskite as a panchromatic absorber and lignocellulosic biomass as an alternative source of electrons working at lower potentials. In addition, value-added chemicals such as vanillin and acetovanillone are produced via the selective depolymerization of lignin in lignocellulosic biomass while cellulose remains close to intact for further utilization. This study paves the way to improve solar hydrogen productivity and simultaneously realize the effective use of lignocellulosic biomass.

Unassisted selective solar hydrogen peroxide production by an oxidised buckypaper-integrated perovskite photocathode.

Hydrogen peroxide (H2O2) is an eco-friendly oxidant and a promising energy source possessing comparable energy density to that of compressed H2. The current H2O2 production strategies mostly depend on the anthraquinone oxidation process, which requires significant energy and numerous organic chemicals. Photocatalyst-based solar H2O2 production comprises single-step O2 reduction to H2O2, which is a simple and eco-friendly method. However, the solar-to-H2O2 conversion efficiency is limited by the low performance of the inorganic semiconductor-based photoelectrodes and low selectivity and stability of the H2O2 production electrocatalyst. Herein, we demonstrate unassisted solar H2O2 production using an oxidised buckypaper as the H2O2 electrocatalyst combined with a high-performance inorganic-organic hybrid (perovskite) photocathode, without the need for additional bias or sacrificial agents. This integrated photoelectrode system shows 100% selectivity toward H2O2 and a solar-to-chemical conversion efficiency of ~1.463%.

Alkali-Metal-Mediated Reversible Chemical Hydrogen Storage Using Seawater.

The economic viability and systemic sustainability of a green hydrogen economy are primarily dependent on its storage. However, none of the current hydrogen storage methods meet all the targets set by the US Department of Energy (DoE) for mobile hydrogen storage. One of the most promising routes is through the chemical reaction of alkali metals with water; however, this method has not received much attention owing to its irreversible nature. Herein, we present a reconditioned seawater battery-assisted hydrogen storage system that can provide a solution to the irreversible nature of alkali-metal-based hydrogen storage. We show that this system can also be applied to relatively lighter alkali metals such as lithium as well as sodium, which increases the possibility of fulfilling the DoE target. Furthermore, we found that small (1.75 cm2) and scaled-up (70 cm2) systems showed high Faradaic efficiencies of over 94%, even in the presence of oxygen, which enhances their viability.

High-performance and stable photoelectrochemical water splitting cell with organic-photoactive-layer-based photoanode.

Considering their superior charge-transfer characteristics, easy tenability of energy levels, and low production cost, organic semiconductors are ideal for photoelectrochemical (PEC) hydrogen production. However, organic-semiconductor-based photoelectrodes have not been extensively explored for PEC water-splitting because of their low stability in water. Herein, we report high-performance and stable organic-semiconductors photoanodes consisting of p-type polymers and n-type non-fullerene materials, which is passivated using nickel foils, GaIn eutectic, and layered double hydroxides as model materials. We achieve a photocurrent density of 15.1 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE) with an onset potential of 0.55 V vs. RHE and a record high half-cell solar-to-hydrogen conversion efficiency of 4.33% under AM 1.5 G solar simulated light. After conducting the stability test at 1.3 V vs. RHE for 10 h, 90% of the initial photocurrent density are retained, whereas the photoactive layer without passivation lost its activity within a few minutes.

Superaerophobic hydrogels for enhanced electrochemical and photoelectrochemical hydrogen production.

The efficient removal of gas bubbles in (photo)electrochemical gas evolution reactions is an important but underexplored issue. Conventionally, researchers have attempted to impart bubble-repellent properties (so-called superaerophobicity) to electrodes by controlling their microstructures. However, conventional approaches have limitations, as they are material specific, difficult to scale up, possibly detrimental to the electrodes' catalytic activity and stability, and incompatible with photoelectrochemical applications. To address these issues, we report a simple strategy for the realization of superaerophobic (photo)electrodes via the deposition of hydrogels on a desired electrode surface. For a proof-of-concept demonstration, we deposited a transparent hydrogel assembled from M13 virus onto (photo)electrodes for a hydrogen evolution reaction. The hydrogel overlayer facilitated the elimination of hydrogen bubbles and substantially improved the (photo)electrodes' performances by maintaining high catalytic activity and minimizing the concentration overpotential. This study can contribute to the practical application of various types of (photo)electrochemical gas evolution reactions.

Unassisted solar lignin valorisation using a compartmented photo-electro-biochemical cell.

Lignin is a major component of lignocellulosic biomass. Although it is highly recalcitrant to break down, it is a very abundant natural source of valuable aromatic carbons. Thus, the effective valorisation of lignin is crucial for realising a sustainable biorefinery chain. Here, we report a compartmented photo-electro-biochemical system for unassisted, selective, and stable lignin valorisation, in which a TiO2 photocatalyst, an atomically dispersed Co-based electrocatalyst, and a biocatalyst (lignin peroxidase isozyme H8, horseradish peroxidase) are integrated, such that each system is separated using Nafion and cellulose membranes. This cell design enables lignin valorisation upon irradiation with sunlight without the need for any additional bias or sacrificial agent and allows the protection of the biocatalyst from enzyme-damaging elements, such as reactive radicals, gas bubbles, and light. The photo-electro-biochemical system is able to catalyse lignin depolymerisation with a 98.7% selectivity and polymerisation with a 73.3% yield using coniferyl alcohol, a lignin monomer.

Toward practical solar hydrogen production - an artificial photosynthetic leaf-to-farm challenge.

Solar water splitting is a promising approach to transform sunlight into renewable, sustainable and green hydrogen energy. There are three representative ways of transforming solar radiation into molecular hydrogen, which are the photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-electrolysis (PV-EC) routes. Having the future perspective of green hydrogen economy in mind, this review article discusses devices and systems for solar-to-hydrogen production including comparison of the above solar water splitting systems. The focus is placed on a critical assessment of the key components needed to scale up PEC water splitting systems such as materials efficiency, cost, elemental abundancy, stability, fuel separation, device operability, cell architecture, and techno-economic aspects of the systems. The review follows a stepwise approach and provides (i) a summary of the basic principles and photocatalytic materials employed for PEC water splitting, (ii) an extensive discussion of technologies, procedures, and system designs, and (iii) an introduction to international demonstration projects, and the development of benchmarked devices and large-scale prototype systems. The task of scaling up of laboratory overall water splitting devices to practical systems may be called "an artificial photosynthetic leaf-to-farm challenge".

Tailorable Au Nanoparticles Embedded in Epitaxial TiO2 Thin Films for Tunable Optical Properties.

The unique property of plasmonic materials to localize light into deep sub-wavelength regime has greatly driven various applications in the field of photovoltaics, sensors, and photocatalysis. Here, we demonstrate the one-step growth of an oxide-metal hybrid thin film incorporating well-dispersed gold (Au) nanoparticles (NPs) with tailorable particle shape and diameters (ranging from 2 to 20 nm) embedded in highly epitaxial TiO2 matrix, deposited using pulsed laser deposition. Incorporation of Au NPs reduces the band gap of TiO2 and enhances light absorption in the visible regime owing to the excitation of localized surface plasmons. Optical properties, including the plasmonic response and permittivity, and photocatalytic activities of the Au-TiO2 hybrid materials are effectively tuned as a function of the Au NP sizes. Such optical property tuning is well captured using full-field simulations and the effective medium theory for better understanding of the physical phenomena. The tailorable shape and size of Au NPs embedded in TiO2 matrix present a novel oxide-metal hybrid material platform for optical property tuning and highly efficient plasmonic properties for future oxide-based photocatalytic sensors and devices.

Decellularized sciatic nerve matrix as a biodegradable conduit for peripheral nerve regeneration.

The use of autologous nerve grafts remains the gold standard for treating nerve defects, but current nerve repair techniques are limited by donor tissue availability and morbidity associated with tissue loss. Recently, the use of conduits in nerve injury repair, made possible by tissue engineering, has shown therapeutic potential. We manufactured a biodegradable, collagen-based nerve conduit containing decellularized sciatic nerve matrix and compared this with a silicone conduit for peripheral nerve regeneration using a rat model. The collagen-based conduit contains nerve growth factor, brain-derived neurotrophic factor, and laminin, as demonstrated by enzyme-linked immunosorbent assay. Scanning electron microscopy images showed that the collagen-based conduit had an outer wall to prevent scar tissue infiltration and a porous inner structure to allow axonal growth. Rats that were implanted with the collagen-based conduit to bridge a sciatic nerve defect experienced significantly improved motor and sensory nerve functions and greatly enhanced nerve regeneration compared with rats in the sham control group and the silicone conduit group. Our results suggest that the biodegradable collagen-based nerve conduit is more effective for peripheral nerve regeneration than the silicone conduit.

Strong O 2p-Fe 3d Hybridization Observed in Solution-Grown Hematite Films by Soft X-ray Spectroscopies.

Photoelectrochemical (PEC) water splitting holds the potential as a direct route for solar energy conversation and storage. The performance of a PEC device is strongly influenced by the electronic properties of the photonanode surface. It has been shown that the synthesis methods can have a profound impact on the electronic properties and PEC performance of various photoelectrode materials such as hematite. Soft X-ray spectroscopic techniques, including O K-edge XAS and Fe L-edge XAS/XES, have been employed to investigate how the synthesis methods impact the electronic structure of resulting hematite materials. It is found that the hematite samples via solution regrowth methods show dramatically increased 3d-4sp band ratios in O K-edge XAS spectra and decreased relative elastic peak intensities in Fe L-edge RIXS spectra compared with samples synthesized via ALD or solution grown. The difference observed in O-K and Fe-L spectra indicated that solution regrowth strategy alters the O 2p-Fe 3d hybridization and hence the electronic structure of the hematite films, which proves to be beneficial for PEC performance of the hematite photoanode. Our findings provided new insights and potentially useful strategies for enhancing the PEC performance of photoanode materials.

Hetero-type dual photoanodes for unbiased solar water splitting with extended light harvesting.

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO4 and α-Fe2O3 as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm-2 at 1.23 VRHE under 1 sun irradiation. A tandem cell composed with the dual photoanodes-silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forward en route to the goal of >10% efficiency required for practical solar hydrogen production.

Self-Assembled Heteroepitaxial Oxide Nanocomposite for Photoelectrochemical Solar Water Oxidation.

We report on spontaneously phase ordered heteroepitaxial SrTiO3 (STO):ZnFe2O4 (ZFO) nanocomposite films that give rise to strongly enhanced photoelectrochemical solar water oxidation, consistent with enhanced photoinduced charge separation. The STO:ZFO nanocomposite yielded an enhanced photocurrent density of 0.188 mA/cm2 at 1.23 V vs a reversible hydrogen electrode, which was 7.9- and 2.6-fold higher than that of the plain STO film and ZFO film cases under 1-sun illumination, respectively. The photoelectrode also produced stable photocurrent and Faradaic efficiencies of H2 and O2 formation that were more than 90%. Incident-photon-to-current-conversion efficiency measurements, Tauc plots, Mott-Schottky plots, and electrochemical impedance spectroscopy measurements proved that the strongly enhanced photogenerated charge separation resulted from vertically aligned pseudosingle crystalline components, epitaxial heterojunctions, and a staggered band alignment of the components of the nanocomposite films. This study presents a completely new avenue for efficient solar energy conversion applications.

Modified Surgical Intervention for Extensive Mitral Valve Endocarditis and Posterior Mitral Annular Calcification.

The concomitant presence of posterior mitral annular calcification and infectious mitral valve lesions poses a technical challenge with considerable perioperative risk when using previously proposed techniques for mitral valve surgery. Herein, we report a case of the use of a modified surgical technique to successfully treat a patient with mitral infective endocarditis complicated by a subendocardial abscess and extensive posterior mitral annular calcification.

Understanding the origin of photoelectrode performance enhancement by probing surface kinetics.

Photoelectrochemical (PEC) water splitting holds the potential to meet the challenges associated with the intermittent nature of sunlight. Catalysts have often been shown to improve the performance of PEC water splitting, but their working mechanisms are not well understood. Using intensity modulated photocurrent spectroscopy (IMPS), we determined the rate constants of water oxidation and recombination at the surface of three different hematite-based photoanodes. It was found that the best performing electrodes, in terms of photocurrent onset potential, exhibited the slowest water oxidation rate constants, which was a surprise. The performance of these photoelectrodes was enabled by the slow surface recombination. When amorphous NiFeO x , a water oxidation catalyst, was present, the rate of surface hole transfer actually slowed down; what was slowed more was the recombination rate at the hematite surface, resulting in better water oxidation performance. As such, NiFeO x primarily serves as a passivation layer rather than a catalytic layer. Together a better understanding of the role of catalytic overlayers for water oxidation has been achieved.

Defective ZnFe2O4 nanorods with oxygen vacancy for photoelectrochemical water splitting.

A one-dimensional zinc ferrite (ZnFe2O4) nanorod photoanode was prepared by a simple solution method on the F-doped tin oxide glass substrate. Thermal treatment under a hydrogen or vacuum atmosphere improved the photoelectrochemical water oxidation activity up to 20 times. The various physical characterization techniques used revealed that oxygen vacancies were created by the treatments in the near surface region, which increased the donor density and passivated the surface states. Hydrogen treatment was more effective and it was important to find optimum treatment conditions to take advantage of the positive role of oxygen vacancy as a source of electron donors and avoid its negative effect as electron trap sites.

Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf.

A stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem a CH3NH3PbI3 perovskite single junction solar cell with a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm(2) at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performance to best known cobalt phosphate (Co-Pi) (5.0 mA/cm(2) at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but with significantly better stability. A tandem artificial-leaf-type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3% (wired), 3.0% (wireless) under simulated 1 sun illumination. Hence, our device based on a D4 tandem photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.

Enabling unassisted solar water splitting by iron oxide and silicon.

Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

Tree branch-shaped cupric oxide for highly effective photoelectrochemical water reduction.

Highly efficient tree branch-shaped CuO photocathodes are fabricated using the hybrid microwave annealing process with a silicon susceptor within 10 minutes. The unique hierarchical, one-dimensional structure provides more facile charge transport, larger surface areas, and increased crystallinity and crystal ordering with less defects compared to irregular-shaped CuO prepared by conventional thermal annealing. As a result, the photocathode fabricated with the tree branch-shaped CuO produces an unprecedently high photocurrent density of -4.4 mA cm(-2) at 0 VRHE under AM 1.5 G simulated sunlight compared to -1.44 mA cm(-2) observed for a photocathode fabricated by thermal annealing. It is also confirmed that stoichiometric hydrogen and oxygen are produced from photoelectrochemical water splitting on the tree branch-shaped CuO photocathode and a platinum anode.