Microkinetic Model to Rationalize the Lifetime of Electrocatalysis: Trade-off between Activity and Stability.

Electrocatalysts which can operate for several years are required to produce hydrogen and commodity chemicals in an environmentally friendly manner. However, designing materials with long operational lifetimes is challenging, due to the lack of a conceptual framework to predict catalytic lifetimes quantitatively. Here, we report a microkinetic equation which quantifies the lifetime of an electrocatalyst undergoing dissolution. This equation was obtained by taking advantage of the fact that catalysis is much faster than deactivation, which allows the ordinary differential equations to be solved via the quasi steady-state approximation. All chemical reactions were modeled as irreversible, first-order elementary reactions. Under this assumption, the catalytic rate correlates linearly with the deactivation rate, leading to a trade-off relationship between activity and stability. Our model was supported by the correlation between theoretical and experimental lifetimes (r2 = 0.86) of a manganese oxide electrocatalyst during the oxygen evolution reaction.

Atomically dispersed hexavalent iridium oxide from MnO2 reduction for oxygen evolution catalysis.

Hexavalent iridium (IrVI) oxide is predicted to be more active and stable than any other iridium oxide for the oxygen evolution reaction in acid; however, its experimental realization remains challenging. In this work, we report the synthesis, characterization, and application of atomically dispersed IrVI oxide (IrVI-ado) for proton exchange membrane (PEM) water electrolysis. The IrVI-ado was synthesized by oxidatively substituting the ligands of potassium hexachloroiridate(IV) (K2IrCl6) with manganese oxide (MnO2). The mass-specific activity (1.7 × 105 amperes per gram of iridium) and turnover number (1.5 × 108) exceeded those of benchmark iridium oxides, and in situ x-ray analysis during PEM operations manifested the durability of IrVI at current densities up to 2.3 amperes per square centimeter. The high activity and stability of IrVI-ado showcase its promise as an anode material for PEM electrolysis.

Tracing and regulating redox homeostasis of model benthic ecosystems for sustainable aquaculture in coastal environments.

Aquaculture in coastal environments has an increasingly important role in the world's food supply; however, the accumulation of organic compounds on seafloors due to overfeeding adversely affects benthic ecosystems. To assess the ecological resilience of aquafarms to nutrient influx, we investigated the redox homeostasis of benthic ecosystems using a marine oligochaete as a model benthic organism in aquaculture fields. Real-time monitoring of the redox potential of a model benthic ecosystem constructed in an electrochemical reactor allowed evaluation of the homeostatic response of the system to nutrient addition. Although the detrimental effects of overfeeding were confirmed by irreversible potential changes in the sediment, redox homeostasis was reinforced through a cooperative relationship between oligochaetes and sediment microorganisms. Specifically, the oligochaetes exhibited reversible changes in metabolism and body position in response to dynamic changes in the sediment potential between -300 and 500 mV, thereby promoting the decomposition of organic compounds. The potential-dependent changes in metabolism and body position were reproduced by artificially manipulating the sediment potential in electrochemical reactors. Given the importance of benthic animals in sustaining coastal ecosystems, the electrochemical monitoring and physiologic regulation of marine oligochaetes could offer an intriguing approach toward sustainable aquaculture.

Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides Under Acidic Conditions.

Efficient, earth-abundant, and acid-stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the production of hydrogen via water electrolysis. Here, we report how the limitations on the stability of 3d-metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ-MnO2 ), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid-stable 3d-metal OER catalysts, but cannot be realized if there is a deviation as small as 50-mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer-electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment.

Earth-Abundant Transition-Metal-Based Electrocatalysts for Water Electrolysis to Produce Renewable Hydrogen.

Fundamentals of water electrolysis, and recent research progress and trends in the development of earth-abundant first-row transition-metal (Mn, Fe, Co, Ni, Cu)-based oxygen evolution reaction (OER) and hydrogen evolution (HER) electrocatalysts working in acidic, alkaline, or neutral conditions are reviewed. The HER catalysts include mainly metal chalcogenides, metal phosphides, metal nitrides, and metal carbides. As for the OER catalysts, the basic principles of the OER catalysts in alkaline, acidic, and neutral media are introduced, followed by the review and discussion of the Ni, Co, Fe, Mn, and perovskite-type OER catalysts developed so far. The different design principles of the OER catalysts in photoelectrocatalysis and photocatalysis systems are also presented. Finally, the future research directions of electrocatalysts for water splitting, and coupling of photovoltaic (PV) panel with a water electrolyzer, so called PV-E, are given as perspectives.

Strategies for Efficient Charge Separation and Transfer in Artificial Photosynthesis of Solar Fuels.

Converting sunlight to solar fuels by artificial photosynthesis is an innovative science and technology for renewable energy. Light harvesting, photogenerated charge separation and transfer (CST), and catalytic reactions are the three primary steps in the processes involved in the conversion of solar energy to chemical energy (SE-CE). Among the processes, CST is the key "energy pump and delivery" step in determining the overall solar-energy conversion efficiency. Efficient CST is always high priority in designing and assembling artificial photosynthesis systems for solar-fuel production. This Review not only introduces the fundamental strategies for CST but also the combinatory application of these strategies to five types of the most-investigated semiconductor-based artificial photosynthesis systems: particulate, Z-scheme, hybrid, photoelectrochemical, and photovoltaics-assisted systems. We show that artificial photosynthesis systems with high SE-CE efficiency can be rationally designed and constructed through combinatory application of these strategies, setting a promising blueprint for the future of solar fuels.

Influence of the Electrostatic Interaction between a Molecular Catalyst and Semiconductor on Photocatalytic Hydrogen Evolution Activity in Cobaloxime/CdS Hybrid Systems.

The influence of the electrostatic interaction on photocatalytic H2 evolution activity in cobaloxime/cadmium sulfide (CdS) hybrid systems was studied by measuring the charges of the cobaloximes and the zeta potentials of CdS under different pH conditions (pHs 4-7). Cobaloxime/CdS hybrid systems may have potential as a valid model for the investigation of the electrostatic interaction between a molecular catalyst and semiconductor because the kinetics of methanol oxidation and the driving force of electron transfer from photoirradiated CdS to cobaloxime have little effect on the pH-dependent photocatalytic H2 evolution activity. Our experimental results suggest that electrostatic repulsion between cobaloxime and CdS disfavors the electron transfer from CdS to cobaloxime and hence lowers the photocatalytic H2 evolution activity. Whereas, electrostatic attraction favors the electron transfer process and enhances the photocatalytic H2 evolution activity. However, an electrostatic attraction interaction that is too strong may accelerate both forward and backward electron transfer processes, which would reduce charge separation efficiency and lower photocatalytic H2 evolution activity.

Design and Fabrication of a Dual-Photoelectrode Fuel Cell towards Cost-Effective Electricity Production from Biomass.

A photo fuel cell (PFC) offers an attractive way to simultaneously convert solar and biomass energy into electricity. Photocatalytic biomass oxidation on a semiconductor photoanode combined with dark electrochemical reduction of oxygen molecules on a metal cathode (usually Pt) in separated compartments is the common configuration for a PFC. Herein, we report a membrane-free PFC based on a dual electrode, including a W-doped BiVO4 photoanode and polyterthiophene photocathode for solar-stimulated biomass-to-electricity conversion. Air- and water-soluble biomass derivatives can be directly used as reagents. The optimal device yields an open-circuit voltage (VOC ) of 0.62 V, a short-circuit current density (JSC ) of 775 µA cm-2 , and a maximum power density (Pmax ) of 82 µW cm-2 with glucose as the feedstock under tandem illumination, which outperforms dual-photoelectrode PFCs previously reported. Neither costly separating membranes nor Pt-based catalysts are required in the proposed PFC architecture. Our work may inspire rational device designs for cost-effective electricity generation from renewable resources.

Substrate-Electrode Interface Engineering by an Electron-Transport Layer in Hematite Photoanode.

The photoelectrochemical water oxidation efficiency of photoanodes is largely limited by interfacial charge-transfer processes. Herein, a metal oxide electron-transport layer (ETL) was introduced at the substrate-electrode interface. Hematite photoanodes prepared on Li(+)- or WO3-modified substrates deliver higher photocurrent. It is inferred that a Li-doped Fe2O3 (Li:Fe2O3) layer with lower flat band potential than the bulk is formed. Li:Fe2O3 and WO3 are proved to function as an expressway for electron extraction. Via introducing ETL, both the charge separation and injection efficiencies are improved. The lifetime of photogenerated electrons is prolonged by 3 times, and the ratio of surface charge transfer and recombination rate is enhanced by 5 times with Li:Fe2O3 and 125 times with WO3 ETL at 1.23 V versus reversible hydrogen electrode. This result indicates the expedited electron extraction from photoanode to the substrate can suppress not only the recombination at the back contact interface but also those at the surface, which results in higher water oxidation efficiency.

Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting.

New insight into junction-based designs for efficient charge separation is vitally important for current solar energy conversion research. Herein, an anatase-rutile phase junction is elaborately introduced into TiO2 films by rapid thermal annealing treatment and the roles of phase junction on charge separation and transfer are studied in detail. A combined study of transient absorption spectroscopy, electrochemical and photoelectrochemical (PEC) measurements reveals that appropriate phase alignment is essential for unidirectional charge transfer, and a junction interface with minimized trap states is crucial to liberate the charge separation potential of the phase junction. By tailored control of phase alignment and interface structure, an optimized TiO2 film with an appropriately introduced phase junction shows superior performance in charge separation and transfer, hence achieving ca. 3 and 9 times photocurrent density enhancement compared to pristine anatase and rutile phase TiO2 electrodes, respectively. This work demonstrates the great potential of phase junctions for efficient charge separation and transfer in solar energy conversion applications.

Conversion of Biomass Derivatives to Electricity in Photo Fuel Cells using Undoped and Tungsten-doped Bismuth Vanadate Photoanodes.

The photo fuel cell (PFC) is a promising technology for simultaneously converting solar energy and bioenergy into electricity. Here, we present a miniature air-breathing PFC that uses either BiVO4 or W-doped BiVO4 as the photoanode and a Pt/C catalyst as the air-breathing cathode. The PFC exhibited excellent performance under solar illumination and when fed with several types of biomaterial. We found the PFC performance could be significantly enhanced using W-doping into the BiVO4 photoanode. With glucose as the fuel and simulated sunlight (AM 1.5 G) as the light source, the open-circuit voltage increased from 0.74 to 0.92 V, the short-circuit current density rose from 0.46 to 1.62 mA cm(-2) , and the maximum power density was boosted from 0.05 to 0.38 mW cm(-2) , compared to a PFC using undoped BiVO4 as the anode.