Pitfall on the interpretation of double layer capacitance increase after accelerated stress test of hydrogen evolution reaction on NiMo catalysts

Critical investigation of methods used to screen the catalytic activity of different electrocatalysts is crucial for the development of water electrolysis technology. One of such protocols to justify the catalytic activity trend among different catalysts involves determining their electrochemically active surface area (ECSA) which is usually estimated from non-Faradic double layered capacitance (C_dl) of the catalyst material. Furthermore, catalytic current normalized with ECSA is frequently used to gain insight into the intrinsic activity of a catalyst. Since not all the metallic catalysts are highly stable under the electrolysis conditions, it is highly important, though rarely explored, to investigate whether or not the adsorption/desorption of leached/dissolved multivalent metal ions influences the measurement of non-Faradic C_dl. Here, we explore the possible influence of Mo leached from NiMo alloy on the measured C_dl of NiMo. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), C_dl of NiMo alloy was measured before and after carrying out long term (chronopotentiometric and CV cycling) hydrogen evolution reaction (HER) in alkaline conditions. After extended HER testing, we observe a notable rise (∼22% averaging all our experiments) in the C_dl of NiMo alloy when measured with traditional methods. Interestingly, only 8% of this increase can be attributed to an expansion in surface area. We hypothesize that the majority of the increase in C_dl stems from the higher amount of charges stored through leached multivalent ions, which accumulate within surface cavities.