Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO₂ Electrocatalysts

Abstract

Manganese oxides function as efficient electrocatalysts for water oxidation to molecular oxygen in strongly alkaline conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidation reaction, we performed UV-vis spectroelectrochemical detection of the intermediate species for water oxidation by a manganese oxide electrode. Layered manganese oxide nanoparticles, δ-MnO(2) (K(0.17)[Mn(4+)(0.90)Mn(3+)(0.07)□(0.03)]O(2)·0.53H(2)O) deposited on fluorine-doped tin oxide electrodes were shown to catalyze water oxidation at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was observed with a concomitant increase of anodic current for O(2) evolution. Using pyrophosphate as a probe molecule, the 510 nm absorption was attributable to Mn(3+) on the surface of δ-MnO(2). The onset potential of the water oxidation current was constant at approximately 1.5 V vs SHE from pH 4 to pH 8, but sharply shifted to negative at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn(3+) acts as the precursor of water oxidation. Mn(3+) is unstable at pH < 9 due to the disproportionation reaction resulting in the formation of Mn(2+) and Mn(4+), whereas it is effectively stabilized by the comproportionation of Mn(2+) and Mn(4+) in alkaline conditions. Thus, the low activity of manganese oxides for water oxidation under neutral conditions is most likely due to the inherent instability of Mn(3+), whose accumulation at the surface of catalysts requires the electrochemical oxidation of Mn(2+) at a potential of approximately 1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn(3+) is essential for the development of Mn catalysts that afford water oxidation with a small overpotential at neutral pH.

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

“…A systematic investigation was also reported for Mn electrodes 165, 166, 167. When an unbuffered solution was used as a supporting electrolyte (0.5 mol L −1 Na 2 SO 4 ), the onset potential for the OER was found to be independent of the solution pH on the SHE scale (Figure 13 a),166 which interestingly coincides with the observation for the HER (see Section 3.2).…”

“…When an unbuffered solution was used as a supporting electrolyte (0.5 mol L −1 Na 2 SO 4 ), the onset potential for the OER was found to be independent of the solution pH on the SHE scale (Figure 13 a),166 which interestingly coincides with the observation for the HER (see Section 3.2). The observed pH independence in their study was ascribed to the formation of Mn 3+ from Mn 2+ 165. The OER–pH relationship was drastically altered when buffering action was introduced into the system 166, 167.…”

“…The solid line represents the standard potential for oxygen evolution. Reproduced from 165 with permission from the American Chemical Society (copyright 2012). b) Plot of pH versus potential showing the pH dependence of the onset potential ( U on, j ) defined at 130 μA cm −2 for water oxidation in the presence of the indicated bases at pH ranging from 5 to 9.…”

Reconstruction of Mini‐Hollow Polyhedron Mn2O3 Derived from MOFs as a High‐Performance Lithium Anode Material

“…Manganese oxides materials have shown great potential application as energy materials, such as water oxidation and oxygen reduction,1, 2, 3 electrochemical capacitors,4, 5 lithium–O 2 batteries,6 and lithium‐ion batteries (LIBs) 7, 8, 9, 10, 11, 12, 13, 14, 15. Owing to their high specific theoretical capacity, low cost, and environmentally benign nature, manganese oxides (MnO x , 1 ≤ x ≤ 2) are believed to be the most promising alternative anode materials for next generation LIBs 11, 13, 16, 17.…”

Electrocatalytic Water Oxidation by MnOx/C: In Situ Catalyst Formation, Carbon Substrate Variations, and Direct O2/CO2 Monitoring by Membrane‐Inlet Mass Spectrometry

“…It is so far not fully understood why exactly birnessites excel for Mn-based OER catalysis, but an umber of studies indicate that they combine two features that are most likely required for a well-working MnO x catalyst: 1) Mn ions connected by af lexible network of m-oxido ligandsw ithin al ow-ordered oxide to allow for substrate binding and turnover; [9,14,[17][18][19][20] and 2) the stabilization of as ignificanta mount of Mn 3 + ions to be available for oxidation state changes at OER potentials. [21,22] Metal oxide based OER electrodes can be prepared by using an umber of methods such as electrodeposition, direct carbonization methods,o rc oating-techniques (screen-printing, spincoating,s pray-coating,e tc.). [23] Thel atter include at ransfer of synthetic oxide powders onto conductive substrates such as fluorine-o ri ndium-doped tin oxides( FTO/ITO) by mixing them with macromolecular binders (e.g.,N afion or polyethylene oxide, PEO) or also by using ionic liquidst oa chieveastable coating.…”