Carbon Shell on Active Nanocatalyst for Stable Electrocatalysis

Conspectus Electrocatalysis is a key process for renewable energy conversion and fuel production in future energy systems. Various nanostructures have been investigated to optimize the electrocatalytic activity and realize efficient energy use. However, the long-term stability of electrocatalysts is also crucial for the sustainable and reliable operation of energy devices. Nanocatalysts are degraded by various processes during electrocatalysis, which causes critical performance loss. Recent operando analyses have revealed the mechanisms of electrocatalyst failure, and specific structures have been identified as robust against degradation. Nevertheless, achieving both high activity and robust stability with the same nanostructure is challenging because the structure–property relationships that affect activity and stability are different. The optimization of electrocatalysis is often limited by a large trade-off between activity and stability in catalyst structures. Therefore, it is essential to introduce functional structural units into catalyst design to achieve electrochemical stability while preserving high activity. In this Account, we highlight the strategic use of carbon shells on catalyst surfaces to improve the stability during electrocatalysis. For this purpose, we cover three issues in the use of carbon-shell-encapsulated nanoparticles (CSENPs) as robust and active electrocatalysts: the origin of the improved stability, the identification of active sites, and synthetic routes. Carbon shells can shield catalyst surfaces from both (electro)chemical oxidation and physical agglomeration. By limiting the exposure of the catalyst surface to an oxidizing (electro)chemical environment, carbon shells can preserve the initial active site structure during electrocatalysis. In addition, by providing a physical barrier between nanoparticles, carbon shells can maintain the high surface area of CSENPs by reducing particle agglomeration during electrocatalysis. This barrier effect is also useful for constructing more active or durable structures by annealing without surface area loss. Compared to the clear stabilizing effect, however, the effect of the shell on active sites on the CSENP surface can be puzzling. Even when they are covered by a carbon shell that can block molecular adsorption on active sites, CSENP catalysts remain active and even exhibit unique catalytic behavior. Thus, we briefly cover recent efforts to identify major active sites on CSENPs using molecular probes. Furthermore, considering the membranelike role of the carbon shell, we suggest several remaining issues that should be resolved to obtain a fundamental understanding of CSENP design. Finally, we describe two synthetic approaches for the successful carbon shell encapsulation of nanoparticles: two-step and one-step syntheses. Both the postmortem coating of nanocatalysts (two-step) and the in situ formation via precursor ligands (one step) are shown to produce a durable carbon layer on nanocatalysts in a controlled manner. The strengths and limitations of each approach are also presented to promote the further investigation of advanced synthesis methods. The hybrid structure of CSENPs, that is, the active catalyst surface and the durable carbon shell, provides an interesting opportunity in electrocatalysis. However, our understanding of CSENPs is still highly limited, and further investigation is needed to answer fundamental questions regarding both active site identification and the mechanisms of stability improvement. Only when we start to comprehend the fundamental mechanisms underlying electrocatalysis on CSENPs will electrocatalysts be further improved for sustainable long-term device operation.