Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems, when combined with suitable semiconductor photocatalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms’ 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance. Downsizing platinum based nanocatalysts has the twin advantages of lower platinum usage and increased activity per platinum atom. Here, the authors report an atomic layer deposition technique for single platinum atom catalyst fabrication and assess their hydrogen evolution activity.

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Platinum single-atom and cluster catalysis of the hydrogen evolution reaction

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Herein, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ca. 218 mV at 10 mA cm−2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyzer delivers a current density of 10 mA cm−2 at a very low cell voltage of ca. 1.56 V. In combination with DFT calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygen-containing intermediates, thus accelerating the overall electrochemical water splitting.

/pdf/interface-engineering-of-mos2-ni3-s2-heterostructures-for-44umgy6hhp.pdf

Interface Engineering of MoS2 /Ni3 S2 Heterostructures for Highly Enhanced Electrochemical Overall-Water-Splitting Activity.

Low-cost hydrogen-evolution catalysts based on monolayer platinum on tungsten monocarbide substrates.

Catalytic conversion of cellulose into ethylene glycol over supported carbide catalysts

Polycrystalline tungsten monocarbide (WC) surfaces are synthesized and evaluated as a potential replacement of platinum (Pt) electrocatalysts. A combined approach utilizing ultrahigh vacuum (UHV) and electrochemical techniques demonstrates that WC possesses the following prerequisites for alternative electrocatalysts:  high activity toward methanol decomposition, improved resistance to poisoning by CO, and promising electrochemical stability.

https://pubs.acs.org/doi/pdf/10.1021/jp075504z

Tungsten Monocarbide as Potential Replacement of Platinum for Methanol Electrooxidation

Reactions of oxygen-containing molecules on transition metal carbides: Surface science insight into potential applications in catalysis and electrocatalysis

In the current paper we will provide a review of our recent efforts in experimental studies of tungsten carbides as alternative electrocatalysts for methanol electro-oxidation We will first discuss ultrahigh vacuum (UHV) studies on single crystal surfaces to demonstrate the feasibility of using tungsten carbides for methanol decomposition We then discuss UHV studies on polycrystalline thin films and foils to approximate commercially relevant catalysts, thus bridging the “materials gap” and demonstrating that the fundamental chemistry observed in UHV over single crystal surfaces is applicable to morphologically complex surfaces Electrochemical studies of thin films will be discussed to bridge the “pressure gap” and to verify that tungsten carbides are both active and stable in an electrochemical environment Finally, we will provide performance data from direct methanol fuel cell (DMFC) testing that incorporates tungsten carbides as the anode electrocatalysts

Alan L. Stottlemyer

Papers

Low-cost hydrogen-evolution catalysts based on monolayer platinum on tungsten monocarbide substrates.

Catalytic conversion of cellulose into ethylene glycol over supported carbide catalysts

Tungsten Monocarbide as Potential Replacement of Platinum for Methanol Electrooxidation

Reactions of oxygen-containing molecules on transition metal carbides: Surface science insight into potential applications in catalysis and electrocatalysis

Tungsten Carbides as Alternative Electrocatalysts: From Surface Science Studies to Fuel Cell Evaluation