Electrochemical charge storage properties of porous carbon electrodes in salt-in-water and water-in-salt potassium acetate electrolytes

• Different carbons with various porous textures and electrochemical charge storage • Electrochemical stability window of SIW and WIS KOAc electrolytes using carbon • Pseudocapacitive hydrogen sorption/desorption redox reaction in KOAc electrolytes Electrochemical energy storage technologies using water-in-salt electrolytes offer alternatives to existing systems. Such electrolytes could be useful in electrochemical capacitors, yet the generally much larger specific surface area of the electrode materials, compared to those used in rechargeable batteries, may lead to different electrochemical reactivity between the cell components. Here, aqueous potassium acetate electrolytes (1, 10 and 27 molal KOAc) and porous carbon electrodes, characterized with various features like pore size distribution and specific surface area, are used to investigate the effect of salt concentration on the electrochemical stability window and charge storage properties as well as hydrogen sorption/desorption processes. The electrochemical stability window for all porous carbons in the presence of aqueous KOAc electrolytes is not significantly affected by an increase in KOAc concentration. The 27 m KOAc electrolyte could provide a potential window of 1.4 V using porous carbon composite electrodes. A correlation between the specific capacitance and the porous texture parameters of carbons revealed that both higher total and micropore volumes can lead to higher specific capacitance, and that the effect of micropores is more pronounced. A small mesopore volume has a detrimental effect on the rate capability in the 27 m KOAc solution, which contains bulkier ionic species. A notable contribution of the pseudocapacitive hydrogen sorption/desorption reaction and hydrogen evolution is observed for porous carbon electrodes. Interestingly, at a given overpotential, no significant effect of the KOAc concentration is observed on the hydrogen evolution reaction, despite the smaller relative water content in 27 m KOAc solution.