The Influence of Surface Area and Pore Volume of Relevant Carbon Materials on the Vanadium(IV)/(V) Reaction

Porous carbon materials are widely used as support or active materials in electrocatalytic reactions. The understanding of the porous inner surface and volume of the carbon and its contribution to the overall current is of particular importance to avoid misinterpretation of electrochemical data or electrocatalytic activity. In this work, a comprehensive study of relevant carbon materials (graphite flakes, carbon nanotubes, Vulcan XC-72, and Black Pearls 2000) is presented that quantifies the electrochemical surface area (ECSA) as well as the electrochemical pore volume (ECPV) by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results are compared to those of the specific surface area (SSA) and specific pore volume (SPV) determined by nitrogen adsorption. It was found that the ECSA evaluated from the electrochemical double layer capacity (CDL) correlates very well with the SSA of the respective sample. However, the pore volume that is available during the ferri-/ferrocyanide redox reaction in CV (ECPV) significantly exceeds the SPV due to the essential contribution of the inter-particle volume (IPV) of the electrode. In the case of an electrocatalytic reaction, which is the vanadium(IV)/(V) redox reaction in this work, the peak current correlates with the ECPV. However, a high ECSA-to-ECPV ratio seems to be favorable to the chemical (C) as well as the electrochemical step (E) of a CEC reaction mechanism as in the case of vanadium(V) reduction. The results indicate that, instead of the highest possible ECSA or ECPV, the carbon material must have the most suitable morphology, which must be determined separately for each electrocatalytic reaction.