Ultrathin Co–Fe hydroxide nanosheet arrays for improved oxygen evolution during water splitting

The Fe-doping of hierarchical Co hydroxide nanosheet arrays (CoyFe1−y(OH)x NSAs) integrated on a three-dimensional electrode is shown to contribute to both increasing the available surface area and number of active sites. Ultrathin secondary nanosheets with different Co to Fe ratios that are subsequently grown on these primary nanoarrays are found to exhibit high oxygen evolution reaction (OER) activity. The optimal composition of CoyFe1−y(OH)x NSAs turns out to be Co0.7Fe0.3(OH)x NSAs, which allows for an OER onset overpotential as low as 220 mV and a small Tafel slope at 62.4 mV dec−1, while also providing excellent long-term durability (>100 h) and a high turnover frequency (TOF) of 0.172 s−1 at an overpotential of 380 mV. The specific activity of Fe-doped Co0.7Fe0.3(OH)x NSAs at an overpotential of 350 mV (0.37 mA cmBET−2) is also twice as high as that of undoped Co(OH)2 NSAs.