The subsequent materials were bought from Sigma Aldrich: carbon black (CB), niobium nitrate (Nb(NO3)5), KOH pellets, silver nitrate (AgNO3), hydrochloric acid (HCl), activated carbon, sodium sulfide hydrate (Na2S·9H2O) and niobium nitrate (Nb(NO3)5). The chemicals were bought in their purest form and weren't further purified before use. Deionized water (DIW) was used to make solutions, wash samples, and prepare electrolytes. The reference electrodes (Hg/HgO), nickel foam, and platinum wire were imported from Japan.
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