The charge storage mechanism of birnessite-type MnO2 nanosheet electrode with ca. 100 mm in thickness in 0.5M Na2SO4 electrolytes with different pH values was investigated by an in situ electrochemical X-ray absorption spectroscopy (XAS) technique. It is found that the charge storage capacity of MnO2 is controlled by both surface adsorption and redox reaction. At pH 1 and 2, the rate of surface adsorption is faster than that of the surface redox reaction. Whilst, at pH 3, 4, and 5.9, the major capacitance originates from the redox reaction or the insertion/de-insertion of solvated cations into the layered MnO2 nano-structures. As a result, the MnO2-based supercapacitor electrode in 0.5M Na2SO4 with diluted H2SO4 (pH 4) exhibits higher specific capacitance than that in electrolytes with pH 1-2. The decrease of capacitance at low pH is due to the partial dissolution of MnO2. This finding may lead to better understanding on the charge storage mechanism of the MnO2 materials. (c) 2018 Elsevier Ltd. All rights reserved.