Investigating the role of electrolyte acidity on hydrogen uptake in mesoporous activated carbons

The relationship between electrolyte solution pH and the local retention of electrochemically-generated hydrogen under extreme bias conditions for highly receptive activated carbons was studied for the first time. Hydrogen was generated through electrolysis on the surface of custom-made activated carbon electrodes in aqueous solutions with a range of pH (approximately 2-10). The carbon's ability to retain this generated hydrogen was analyzed via a galvanostatic test protocol, while the pH was monitored in situ. A rise in the basicity and corresponding Pourbaix shift was observed for tests performed in intermediate pH solutions, in contrast to that observed for strongly acidic or alkaline solutions. For the three intermediate pH solutions, an increased hydrogen evolution overvoltage was observed and was correlated to an increased hydrogen storage efficiency. These results clearly demonstrate the importance of pH control to extract maximum degree of hydrogen storage in idealized carbons, and show that the optimal pH for this effect is in the range of approximately 4-8. Furthermore, these high surface area carbons were found to have over 300 mAh g^-1 of energy storage capacity when charged with hydrogen in the proper pH range.