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

• The influence of leached metal ions on C dl is illustrated. • After HER testing, a significant increase (∼22 %) in C dl of NiMo was observed. • The majority of the increase in C dl correlates with the presence of leached multivalent Mo ions. 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. Electrochemical experiments indicate a correlation between leached multivalent Mo ions residing within surface cavities of the NiMo alloy and an increase in its double layer capacitance after the hydrogen evolution reaction. Consequently, this falsifies the assessment of the electrochemically active surface area of NiMo after stress tests.