The rate of hydrogen evolution in the all-vanadium redox flow battery (VRFB) is quantified. The method for determining the electrochemical surface area of the VRFB electrode is proposed. Higher surface area electrode leads to a higher hydrogen evolution rate. a b s t r a c t This work demonstrates a quantitative method to determine the hydrogen evolution rate occurring at the negative carbon electrode of the all vanadium redox flow battery (VRFB). Two carbon papers examined by buoyancy measurements yield distinct hydrogen formation rates (0.170 and 0.005 mmol min À1 g À1). The carbon papers have been characterized using electron microscopy, nitrogen gas adsorption, capacitance measurement by electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). We find that the specific electrochemical surface area (ECSA) of the carbon material has a strong influence on the hydrogen generation rate. This is discussed in light of the use of high surface area material to obtain high reaction rates in the VRFB. Published by Elsevier B.V.