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.
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.
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.
Window of opportunity: Spectroscopic measurements allowed the identification of a stable potential window in which γ‐MnO2 is able to catalyze the oxygen evolution reaction under acidic conditions for more than 8000 hours. This shows how the limitations on the stability of 3d‐metal materials acting as electrocatalysts can be overcome.
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•The plasmon-induced water oxidation of Au/TiO2 was achieved (λ>480nm).•The plasmon-driven solid Z-scheme system for overall water splitting was achieved.•The enhanced charge ...separation in solid Z-scheme system was demonstrated by KPFM.•The bi-functional role of Au NPs in Z-scheme photocatalyst was proposed.
Plasmonic photocatalysis is one of the potential approaches for solar hydrogen production, but successful construction of plasmonic photocatalysts for overall water splitting (OWS) is still a challenge. In this work, we achieved OWS by developing a plasmon-based solid Z-scheme photocatalytic system. Firstly, visible-light responsive plasmonic photocatalysts (Au/TiO2) was demonstrated asa stable and efficient photocatalyst for water oxidation half reaction. When Au/TiO2 was combined with a H2-evolution photocatalyst, i.e. Rh doped SrTiO3 (SrTiO3:Rh), hydrogen and oxygen evolution in a stoichiometric ratio (2:1) were achieved under visible light irradiation without using any redox mediator. In addition, the charge separation process in the plasmonic Z-scheme system is studied through employing photoassisted Kelvin probe force microscopy (KPFM) technology and we demonstrated that the plasmon-induced hot electrons can be transferred to SrTiO3:Rh through a solid interface. At last, the mechanism of plasmon induced solid Z-scheme system was proposed, in which Au nanoparticles play both as visible light absorber and electron conductor. These results give us an alternative way to efficiently utilize both the hot electrons and holes from plasmon effects in an integrated plasmonic solid photocatalyst.
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•The noble metals could be preferentially photodeposited on the (110) facets of rutile TiO2, but not on the (111) facets.•The rutile TiO2 samples with cocatalyst selectively deposited ...on the (110) facets have much higher photocatalytic H2 production activities than those with cocatalysts randomly deposited on the entire surface.•The photocatalytic activity is highly correlated with the (110)/(111) ratios—higher (110)/(111) ratios result in higher photocatalytic activity.
The spatial loading of redox cocatalysts on the different facets of a semiconductor is a novel and ideal approach to constructing an efficient photocatalytic system. Here, taking the series of rutile TiO2 as the research model, the origin of the preferential deposition of noble metal cocatalysts on the particular facet (110) was carefully studied under various experimental conditions, including the effect of fluoride species on the surface, as well as different noble metal deposition methods. Our results clearly show that the noble metals could be photodeposited exclusively on the (110) facet. This implies that the origin of noble metal preferential deposition on the (110) facet of rutile TiO2 is due to the reduction of the noble metal precursors by the photoexcited electrons enriched on the particular facet of (110). Rutile TiO2 samples with cocatalyst selectively deposited on the (110) facets have much higher photocatalytic H2 production activity than rutile samples with noble metal cocatalysts randomly deposited on the entire surface. This demonstrates that facet charge separation upon photoexcitation may be a unique route for efficient photocatalytic systems.
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.
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.
Can you pump and deliver? Converting sunlight to solar fuels is an innovative science and technology for renewable energy. This Review introduces not only the fundamental strategies for efficient charge separation and transfer, the key process of artificial photosynthesis, but also how to apply these “energy pump and delivery” strategies in a combinatory manner to achieve high efficiency in artificial photosynthesis of solar fuels.
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.
Electrode cooperation: A dual‐photoelectrode photo fuel cell (PFC) is constructed by coupling a semiconducting polyterthiophene photocathode with a W‐doped BiVO4 photoanode, which is capable of converting solar and biomass energy simultaneously into electricity through the photoelectrochemcial oxygen reduction and biomass oxidation at the cathodic and anodic sides, respectively. The device yields a power density of 82 μW cm−2 that outperforms previously reported dual‐photoelectrode PFCs.
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.
The power of tungsten doping: Photo fuel cells with BiVO4 and tungsten‐doped BiVO4 as the photoanode exhibited excellent performance toward conversion of various biomass derivatives into electricity under AM 1.5 G simulated sunlight. The tungsten‐doped photoanode led to increases in both open‐ circuit voltage and short‐circuit current density compared to undoped BiVO4, yielding a 0.33 mW cm−2 increase in the maximum power density of the cell.
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.
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