Controlling the selectivity of CO 2 electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH

•The buffer capacity of the electrolyte determines the activity and selectivity of Cu.•Our observations are attributed to a different local pH in different electrolytes.•H2 and CH4 formation rate is improved when the local pH close to neutral.•Ethylene's formation rate shows Tafel relation independently of local pH.