Ni-doped covalent triazine frameworks (Ni-CTF) has been known as an e ffi cient electrocatalysts that achieve conversion of carbon dioxide (CO 2 ) to carbon monoxide (CO) with the faradaic e ffi ciency (FE) exceeding 90% in neutral electrolytes. Here we report that the FE can reach 60% even in acidic electrolytes, where the hydrogen evolution can proceed competitively, by loading Ni-CTF on gas di ff usion electrode (GDE). In the presence of Zn(II) ions, zinc hydroxide analogues that can be formed only in alkaline conditions were deposited on the GDE during the CO 2 reduction in an electrolyte with a bulk pH of 2. On the other hand, when a normal plate electrode (PE) was used, the FE was only 1.2% in an electrolyte with the same pH, and the zinc hydroxide analogues were not formed. These results indicate that the local pH around GDE increased during the cathodic process, which led to an increasing FE of CO 2 reduction even in an acidic electrolyte.