Design and Performance of Rh Nanocatalysts for Boosted H2 Generation in Alkaline Media

Hydrogen generation from electrocatalytic water splitting is widely regarded as an important energy conversion process based on its high compatibility with clean and renewable energy sources. It thus has the potential to help society achieve cleanness and sustainability. For the long-term production of highly purified hydrogen for industrial applications, it is preferred for the hydrogen evolution reaction (HER) to occur under mild working conditions or in alkaline media. However, such practical reaction efficiencies are hindered by additional water dissociation and multistep electron transfer. This stems from the sluggish kinetics of the HER in alkaline media, where the reaction rate is 2 to 3 orders of magnitude lower than in acidic media. Up to now, Pt has been extensively accepted as the symbolic catalyst for HER thanks to its appropriate hydrogen adsorption capability. Unfortunately, its activity is not acceptable due to its poor efficiency for water dissociation in alkaline solutions. In this regard, advanced catalysts that have both high catalytic activity for water dissociation and an appropriate hydrogen adsorption capacity are highly desired. Among the reported Pt-free catalysts for alkaline HER, Rh-based electrocatalysts exhibit obviously enhanced performances. It has been experimentally and theoretically confirmed that the Rh catalyst is located near the top of the Trasatti volcano plot for the HER, indicating its suitable adsorption energy of H* to facilitate the HER. In addition, the Rh catalyst possesses stronger competence for water dissociation than the Pt catalyst, which is a prerequisite for highly effective alkaline HER. In this context, a series of experiments with Rh-based electrocatalysts toward the alkaline HER have been actualized over the past decades. These proposed Rh-based catalysts have displayed impressive performances and promising prospects for hydrogen production. To realize the industrial application of hydrogen generation, Rh-based catalysts still require development, such as with the conceptual design of the catalysts and corresponding advanced synthesis methods. In short, further improvement of the catalytic performances of Rh-based catalysts depends on the development of various regulation strategies. In this Account, we highlight the recent progress and achievements of the regulation strategies for Rh-based catalysts and the corresponding enhancement principle for alkaline HER. We start with an introduction and comparison of the different mechanisms in acidic versus alkaline HER processes, emphasizing the principles of designing highly efficient catalysts toward alkaline HER. On the basis of previously reported studies, we conduct an in-depth discussion about the regulation strategies for Rh-based electrocatalysts and the performance optimization toward alkaline HER. This part is divided into three sections: structural engineering (e.g., size, alloy, and morphology regulation), surface engineering (e.g., element doping, facet controlling, and compound constructing), and interface engineering (e.g., Rh/compound and Rh/carbon interfaces). The effective electronic modulation of Rh-based catalysts in these strategies is also outlined. At the end of this Account, some insights regarding the current challenges of Rh-based catalysts for alkaline HER and future research directions in this exciting field are provided.