Abstract
The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.
Highlights
The hydrogen evolution reaction (HER) is one of the simplest but at the same time most important reactions in chemistry, 2H+ + 2e− → H2
Before we present the new kinetic model for HER on single atom catalysts (SACs), we briefly summarize the basic features of the original model
In catalytic processes this is true only in principle, since an extra potential, the overpotential η, is required for the reaction to occur; in the original model, it is assumed that η = 0, an approximation that has been the subject of intense work in the past few years, which leads to a small correction.[25−27] When a proton is reduced on a metal catalyst, the reaction is
Summary
The hydrogen evolution reaction (HER) is one of the simplest but at the same time most important reactions in chemistry, 2H+ + 2e− → H2. A great interest is devoted to the HER using renewable sources of energy (solar, wind, geothermal, etc.), as this can open the way toward a sustainable source of hydrogen for transportation or industrial processes.[7−9] To achieve a high energetic efficiency for water splitting, specific catalysts that minimize the overpotential necessary for the HER are needed.[10] Among the best catalysts is platinum, which has the disadvantage of being a critical and costly raw material.[11,12] For this reason, an intense activity has been dedicated to the search for new materials with high catalytic efficiency for the HER. SACs have the additional advantage of using tiny quantities of precious metals, making them interesting for the design of novel classes of industrial catalysts
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