Quasi-1D Oxides as Electrocatalysts for Water-Splitting:\nCase Study of Sr<sub>9</sub>M<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub> (M = Co, Ni, Cu, Zn)

We have demonstrated that multimetal\noxides with a quasi-1D structure\ncan be effective electrocatalysts for water electrolysis. Four materials\nhave been systematically investigated, namely, Sr<sub>9</sub>Co<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub>, Sr<sub>9</sub>Ni<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub>, Sr<sub>9</sub>Cu<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub>, and Sr<sub>9</sub>Zn<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub>, comprising 1D chains of face-sharing MnO<sub>6</sub> octahedra and MO<sub>6</sub> trigonal prisms (M = Co, Ni, Cu or\nZn), which are held together by strontium ions located between the\nchains. These materials show a consistent trend in electrocatalytic\nproperties for both half-reactions of water-splitting, i.e., oxygen\nevolution and hydrogen evolution reactions (OER and HER). In particular,\nSr<sub>9</sub>Ni<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub> has an outstanding\nperformance for OER, with an overpotential of 0.37 V, which is lower\nthan those of many 3D and 2D oxides, and rivals the activity of noble\nmetal catalysts, such as RuO<sub>2</sub>. This is important given\nthat the OER is considered the bottleneck of the water-slitting process.\nChronopotentiometry studies, combined with X-ray photoelectron spectroscopy\n(XPS) and X-ray diffraction experiments pre- and post-reaction, indicate\nthat Sr<sub>9</sub>Ni<sub>2</sub>Mn<sub>5</sub>O<sub>21</sub> is highly\nstable and retains its structural integrity upon electrocatalytic\nreactions. This study highlights the potential of quasi-1D oxides\nas active electrocatalysts for water-splitting.