Evolution of Mn-Bi₂O₃ from the Mn-doped Bi₃O₄Br electro(pre)catalyst during the oxygen evolution reaction

Mn-doped Bi₃O₄Br has been synthesized using a solvothermal route. The undoped Bi₃O₄Br and Mn-Bi₃O₄Br materials possess orthorhombic unit cells with two distinct Bi sites forming a layered atomic arrangement. The shift in the (020) plane in the powder X-ray diffraction (PXRD) pattern confirms Mn-doping in the Bi₃O₄Br lattice. Elemental mapping indicated 7% Mn doping in the Bi₃O₄Br lattice structure. A core-level X-ray photoelectron study (XPS) indicates the presence of Bi^III and Mn^II valence-states in Mn-Bi₃O₄Br. Doping with a cation (Mn^II) containing a different charge and ionic radius resulted in vacancy/defects in Mn-Bi₃O₄Br which further altered its electronic structure by reducing the indirect band gap, beneficial for electron conduction and electrocatalysis. The irreversible Mn^II to Mn^III transformation at a potential of 1.48 V (vs. RHE) precedes the electrochemical oxygen evolution reaction (OER). The Mn-doped electrocatalyst achieved 10 mA cm^-2 current density at 337 mV overpotential, while the pristine Bi₃O₄Br required 385 mV overpotential to reach the same activity. The pronounced OER activity of the Mn-Bi₃O₄Br sample over the pristine Bi₃O₄Br highlights the necessity of Mn^II doping. The superior activity of the Mn-Bi₃O₄Br catalyst over that of Bi₃O₄Br is due to a low Tafel slope, better double-layer capacitance (C_dl), and small charge-transfer resistance (R_ct). The chronoamperometry (CA) study depicts long-term stability for 12 h at 20 mA cm^-2. An electrolyzer fabricated as Pt(-)/(+)Mn-Bi₃O₄Br can deliver 10 mA cm^-2 at a cell potential of 2.05 V. The post-CA-OER analyses of the anode confirmed the leaching of [Br^-] followed by in situ formation of Mn-doped Bi₂O₃ as the electrocatalytically active species. Herein, an ultra-low Mn-doping into Bi₃O₄Br leads to an improvement in the electrocatalytic performance of the inactive Bi₃O₄Br material.