Alkaline water splitting is an attractive method for sustainable hydrogen production. Owing to the sluggish kinetics of alkaline water reduction and oxidation, it is crucial to understand the ...mechanism of the hydrogen evolution reaction/oxygen evolution reaction (HER/OER) and construct efficient electrocatalysts based on the structure-activity relationship. This review describes the fundamentals of the alkaline HER and OER, the design of noble and nonnoble HER electrocatalysts with low energy barriers, OER electrocatalysts based on binding energy, electronic structure, lattice oxygen, and surface reconstruction as well as the recent developments of bifunctional HER/OER electrocatalysts. Future perspectives towards alkaline water splitting electrocatalysts are also proposed.
Alkaline water splitting is an attractive method for sustainable hydrogen production.
Owing to the considerable publicity that has been given to petroleum related economic, environmental, and political problems, renewed attention has been focused on the development of highly efficient ...and stable catalytic materials for the production of chemical/fuel from renewable resources. Supported nickel nanoclusters are widely used for catalytic reforming reactions, which are key processes for generating synthetic gas and/or hydrogen. New challenges were brought out by the extension of feedstock from hydrocarbons to oxygenates derivable from biomass, which could minimize the environmental impact of carbonaceous fuels and allow a smooth transition from fossil fuels to a sustainable energy economy. This tutorial review describes the recent efforts made toward the development of nickel-based catalysts for the production of hydrogen from oxygenated hydrocarbons
via
steam reforming reactions. In general, three challenges facing the design of Ni catalysts should be addressed. Nickel nanoclusters are apt to sinter under catalytic reforming conditions of high temperatures and in the presence of steam. Severe carbon deposition could also be observed on the catalyst if the surface carbon species adsorbed on metal surface are not removed in time. Additionally, the production of hydrogen rich gas with a low concentration of CO is a challenge using nickel catalysts, which are not so active in the water gas shift reaction. Accordingly, three strategies were presented to address these challenges. First, the methodologies for the preparation of highly dispersed nickel catalysts with strong metal-support interaction were discussed. A second approach-the promotion in the mobility of the surface oxygen-is favored for the yield of desired products while promoting the removal of surface carbon deposition. Finally, the process intensification
via
the
in situ
absorption of CO
2
could produce a hydrogen rich gas with low CO concentration. These approaches could also guide the design of other types of heterogeneous base-metal catalysts for high temperature processes including methanation, dry reforming, and hydrocarbon combustion.
This review describes recent progress in the design and synthesis of nickel catalysts with anti-coke and -sintering properties for reforming processes.
Electrochemical reduction of CO
(CO
ER) has received significant attention due to its potential to sustainably produce valuable fuels and chemicals. However, the reaction mechanism is still not well ...understood. One vital debate is whether the rate-limiting step (RLS) is dominated by the availability of protons, the conversion of water molecules, or the adsorption of CO
. This paper describes insights into the RLS by investigating pH dependency and kinetic isotope effect with respect to the rate expression of CO
ER. Focusing on electrocatalysts geared towards two-electron transfer reactions, we find the generation rates of CO and formate to be invariant with either pH or deuteration of the electrolyte over Au, Ag, Sn, and In. We elucidate the RLS of two-electron transfer CO
ER to be the adsorption of CO
onto the surface of electrocatalysts. We expect this finding to provide guidance for improving CO
ER activity through the enhancement of the CO
adsorption processes by strategies such as surface modification of catalysts as well as careful control of pressure and interfacial electric field within reactors.
Collecting and storing solar energy to hydrogen fuel through a photo‐electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth‐abundance, low ...biotoxicity, robustness, and an ideal n‐type band position, hematite (α‐Fe2O3), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.
Hematite (α‐Fe2O3) is one of the most promising photoanode candidates for solar water splitting. However, photocarrier loss severely restricts its application. From a spatial perspective, recent strategies to improve the surface, bulk, and hematite/substrate interface, respectively, for hematite‐based water‐splitting cells are summarized. Future directions are also proposed on hematite for efficient solar water splitting.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
5.
Advances in solar energy conversion Gong, Jinlong; Li, Can; Wasielewski, Michael R
Chemical Society reviews,
04/2019, Volume:
48, Issue:
7
Journal Article
Peer reviewed
Developing sustainable energy resources is one of the most urgent missions for human beings as increasing energy demand is in drastic conflict with limited global fossil fuels. Among the various ...types of sustainable energy resources, solar energy is considered to be promising due to its inexhaustible supply, universality, high capacity, and environmental friendliness. However, natural solar irradiation is decentralized, intermittent and fluctuates constantly. Therefore, effective utilization of solar energy in a clean, economic, and convenient way remains a grand challenge.
Guest Editors Jinlong Gong, Can Li, and Michael R. Wasielewski introduce this themed issue on solar energy conversion.
The ever‐increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO2 have led to a severe energy crisis and climate change. Photocatalytic reduction of CO2 into fuels ...using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO2 can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO2 on the surface of a semiconductor‐based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO2 reduction over semiconductor‐based photoelectrodes are described, and the basic principles of PEC CO2 reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO2 are also presented.
The recent developments of cocatalysts in photoelectrochemical reduction of CO2 over semiconductor‐based photoelectrodes are described. The basic principles of PEC CO2 reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between photoelectrodes and cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder the charge transfer between them.
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Water splitting for hydrogen production by harvesting sunlight is widely accepted as one of the most promising routes to relieve the energy crisis and environmental issues caused by excessive use of ...fossil fuels. Earth-abundant silicon (Si) is emerging as a suitable candidate for a photoelectrode material for efficient solar water splitting. This review describes the current status and prospects of single-crystal Si-based photoelectrodes in photoelectrochemical (PEC) water splitting for hydrogen production. We start with highlighting the recent achievements in single-crystal Si-based photocathodes and photoanodes for PEC water reduction and oxidation. We then discuss the recent progress in the design and fabrication of unbiased solar water splitting cells with single-crystal Si-based photoelectrodes. Finally, we provide an overview from the optimization of a single-crystal Si-based electrode (both a photocathode and a photoanode) to the integration of a full cell for unassisted overall solar water splitting.
This review describes recent developments of single-crystal silicon (Si) as the photoelectrode material for solar water splitting, including the promising strategies to obtain highly efficient and stable single-crystal Si-based photoelectrodes for hydrogen evolution and water oxidation, as well as the future development of spontaneous solar water splitting with single-crystal Si-based tandem cells.
H2 generation by solar water splitting is one of the most promising solutions to meet the increasing energy demands of the fast developing society. However, the efficiency of solar‐water‐splitting ...systems is still too low for practical applications, which requires further enhancement via different strategies such as doping, construction of heterojunctions, morphology control, and optimization of the crystal structure. Recently, integration of plasmonic metals to semiconductor photocatalysts has been proved to be an effective way to improve their photocatalytic activities. Thus, in‐depth understanding of the enhancement mechanisms is of great importance for better utilization of the plasmonic effect. This review describes the relevant mechanisms from three aspects, including: i) light absorption and scattering; ii) hot‐electron injection and iii) plasmon‐induced resonance energy transfer (PIRET). Perspectives are also proposed to trigger further innovative thinking on plasmonic‐enhanced solar water splitting.
The plasmonic enhancement for solar water splitting and the enhancement mechanisms are reviewed in three aspects including: i) light absorption and scattering; ii) hot‐electron injection, and iii) plasmon‐induced resonance energy transfer (PIRET). Perspectives are also proposed to trigger further innovative thinking on plasmonic‐enhanced solar water splitting.
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Continuous efforts have been devoted to searching for sustainable energy resources to alleviate the upcoming energy crises. Among various types of new energy resources, solar energy has been ...considered as one of the most promising choices, since it is clean, sustainable, and safe. Moreover, solar energy is the most abundant renewable energy, with a total power of 173 000 terawatts striking Earth continuously. Conversion of solar energy into chemical energy, which could potentially provide continuous and flexible energy supplies, has been investigated extensively. However, the conversion efficiency is still relatively low since complicated physical, electrical, and chemical processes are involved. Therefore, carefully designed photocatalysts with a wide absorption range of solar illumination, a high conductivity for charge carriers, a small number of recombination centers, and fast surface reaction kinetics are required to achieve a high activity. This Account describes our recent efforts to enhance the utilization of charge carriers for semiconductor photocatalysts toward efficient solar-to-chemical energy conversion. During photocatalytic reactions, photogenerated electrons and holes are involved in complex processes to convert solar energy into chemical energy. The initial step is the generation of charge carriers in semiconductor photocatalysts, which could be enhanced by extending the light absorption range. Integration of plasmonic materials and introduction of self-dopants have been proved to be effective methods to improve the light absorption ability of photocatalysts to produce larger amounts of photogenerated charge carriers. Subsequently, the photogenerated electrons and holes migrate to the surface. Therefore, acceleration of the transport process can result in enhanced solar energy conversion efficiency. Different strategies such as morphology control and conductivity improvement have been demonstrated to achieve this goal. Fine-tuning of the morphology of nanostructured photocatalysts can reduce the migration distance of charge carriers. Improving the conductivity of photocatalysts by using graphitic materials can also improve the transport of charge carriers. Upon charge carrier migration, electrons and holes also tend to recombine. The suppression of recombination can be achieved by constructing heterojunctions that enhance charge separation in the photocatalysts. Surface states acting as recombination centers should also be removed to improve the photocatalytic efficiency. Moreover, surface reactions, which are the core chemical processes during the solar energy conversion, can be enhanced by applying cocatalysts as well as suppressing side reactions. All of these strategies have been proved to be essential for enhancing the activities of semiconductor photocatalysts. It is hoped that delicate manipulation of photogenerated charge carriers in semiconductor photocatalysts will hold the key to effective solar-to-chemical energy conversion.
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IJS, KILJ, NUK, PNG, UL, UM
Solar energy utilization is one of the most promising solutions for the energy crises. Among all the possible means to make use of solar energy, solar water splitting is remarkable since it can ...accomplish the conversion of solar energy into chemical energy. The produced hydrogen is clean and sustainable which could be used in various areas. For the past decades, numerous efforts have been put into this research area with many important achievements. Improving the overall efficiency and stability of semiconductor photocatalysts are the research focuses for the solar water splitting. Tantalum-based semiconductors, including tantalum oxide, tantalate and tantalum (oxy)nitride, are among the most important photocatalysts. Tantalum oxide has the band gap energy that is suitable for the overall solar water splitting. The more negative conduction band minimum of tantalum oxide provides photogenerated electrons with higher potential for the hydrogen generation reaction. Tantalates, with tunable compositions, show high activities owning to their layered perovskite structure. (Oxy)nitrides, especially TaON and Ta
3
N
5
, have small band gaps to respond to visible-light, whereas they can still realize overall solar water splitting with the proper positions of conduction band minimum and valence band maximum. This review describes recent progress regarding the improvement of photocatalytic activities of tantalum-based semiconductors. Basic concepts and principles of solar water splitting will be discussed in the introduction section, followed by the three main categories regarding to the different types of tantalum-based semiconductors. In each category, synthetic methodologies, influencing factors on the photocatalytic activities, strategies to enhance the efficiencies of photocatalysts and morphology control of tantalum-based materials will be discussed in detail. Future directions to further explore the research area of tantalum-based semiconductors for solar water splitting are also discussed.
This review describes the current status of the design, synthesis, and applications of tantalum-based semiconductors, including tantalum oxides, tantalates and tantalum (oxy)nitrides, for photocatalytic and photoelectrochemical water splitting.
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