Evolution of Mn-Bi 2 O 3 from the Mn-doped Bi 3 O 4 Br electro(pre)catalyst during the oxygen evolution reaction.

Mn-doped Bi 3 O 4 Br has been synthesized using a solvothermal route. The undoped Bi 3 O 4 Br and Mn-Bi 3 O 4 Br 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 Bi 3 O 4 Br lattice. Elemental mapping indicated 7% Mn doping in the Bi 3 O 4 Br lattice structure. A core-level X-ray photoelectron study (XPS) indicates the presence of Bi III and Mn II valence-states in Mn-Bi 3 O 4 Br. Doping with a cation (Mn II ) containing a different charge and ionic radius resulted in vacancy/defects in Mn-Bi 3 O 4 Br 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 Bi 3 O 4 Br required 385 mV overpotential to reach the same activity. The pronounced OER activity of the Mn-Bi 3 O 4 Br sample over the pristine Bi 3 O 4 Br highlights the necessity of Mn II doping. The superior activity of the Mn-Bi 3 O 4 Br catalyst over that of Bi 3 O 4 Br 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-Bi 3 O 4 Br 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 Bi 2 O 3 as the electrocatalytically active species. Herein, an ultra-low Mn-doping into Bi 3 O 4 Br leads to an improvement in the electrocatalytic performance of the inactive Bi 3 O 4 Br material.