The H2SO4-PVA gel electrolyte was prepared as follows: 1 g of H2SO4 was added into 10mL of deionized water, and then 1 g of PVA (molecular weight: 89 000-98 000, Sigma-Aldrich) was added. The whole mixture was then heated to 85°C under stirring until the solution became clear. The electrolyte was drop-casted and left to dry overnight under ambient conditions to ensure that the electrolyte completely wetted the electrode and to allow for evaporation of any excess water.