The charge storage mechanism of birnessite-type MnO₂ nanosheet electrode with ca. 100 μm in thickness in 0.5 M 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 nanostructures. As a result, the MnO₂-based supercapacitor electrode in 0.5 M 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.

钙铝矾石型MnO ₂纳米片电极的电荷存储机理约为。通过原位电化学X射线吸收光谱（XAS）技术研究了在具有不同pH值的0.5 M Na ₂ SO ₄电解质中厚度为100μm的情况。发现MnO ₂的电荷存储容量通过表面吸附和氧化还原反应两者来控制。在pH 1和2下，表面吸附速率比表面氧化还原反应快。同时，在pH值为3、4和5.9时，主要电容来自氧化还原反应或溶剂化阳离子向层状MnO ₂纳米结构中的插入/去插入。结果，MnO ₂0.5 M Na ₂ SO ₄和稀释的H ₂ SO ₄（pH 4）中的锂基超级电容器电极比pH 1-2的电解质具有更高的比电容。低pH值下电容的降低是由于MnO ₂的部分溶解。该发现可以导致对MnO ₂材料的电荷存储机理的更好的理解。