Faradaic Transistor for Investigating Electrochemical Interface

Electrocatalysis plays an essential role in various clean energy technologies, including fuel cells and electrolyzers. A fundamental understanding of the electrochemical processes on the electrode and electrolyte interface is of paramount importance for rational design of advanced catalysts. Direct monitoring the interface under operando condition is the most effective way to investigate interfacial reactions, however, the interface is buried between catalyst material and electrolyte and is thus notoriously difficult to probe. Here in, I designed a faradaic transistor based on surface electron scattering effect, which provided surface specific information on the electrochemical interface. Reaction intermediates that adsorbed on catalyst surface can be directly monitored by detecting the conductance change of catalyst during a reaction. Faradaic transistor was firstly applied to investigate hydrogen adsorption process on platinum nanowires (PtNWs) surface in electrochemical environment. The hydrogen adsorption in the entire potential range, including hydrogen underpotential deposition (Hupd) and hydrogen evolution reaction (HER) region, was successfully resolved. An overpotentially deposited hydrogen (Hopd) adsorption peak near equilibrium potential was observed for the first time. DFT calculation on nanowire with same surface structure revealed the Hopd is the hydrogen that adsorbed on the edge sites, which is considered as kinetically active reaction intermediate. We later used faradaic transistor to investigate hydrogen adsorption behavior on PtNWs in solutions of different pH. A pH-dependent Hopd peak intensity was identified, which is correlated to the pH-dependent HER activity. DFT calculation also exhibited consistent results. Therefore, we proposed the HER reaction rate is governed by Hopd coverage. Further Tafel analysis demonstrated the reaction mechanism of HER is also determined by Hopd coverage. Combining theoretical and experimental results, we concluded that Hopd coverage is the unified descriptor for HER. Hydrogen adsorption on Pt/Ni(OH)2 nanowire in alkaline solution was also investigated. Compared with pure PtNWs, much higher Hopd coverage was observed on Pt/Ni(OH)2 surface in alkaline solution, which revealed the molecular-level origin of the improved HER activity on Pt/Ni(OH)2 surface. Therefore, our findings not only offered new insight into understanding the relationship between adsorbed state of reaction intermediate and reaction kinetics, but also provide a governing principle for practical catalyst design