Evidence that Crystal Facet Orientation Dictates Oxygen Evolution Intermediates on Rutile Manganese Oxide

Abstract

Elucidating the mechanism that differentiates the oxygen-evolving center of photosystem II with its inorganic counterpart is crucial to develop efficient catalysts for the oxygen evolution reaction (OER). Previous studies have suggested that the larger overpotential for MnO 2 catalysts under neutral conditions may result from the instability of the Mn 3+ intermediate to charge disproportionation. Here, by monitoring the surface intermediates of electrochemical OER on rutile MnO 2 with different facet orientations, a correlation between the stability of the intermediate species and crystal facets is confirmed explicitly for the first time. The coverage of the Mn 3+ intermediate is found to be 11-fold higher on the metastable (101) surfaces compared to (110) surfaces, leading to the superior OER activity of (101) surfaces. The difference in OER activity may result from the difference in surface electronic states of Mn 3+ , where interlayer charge comproportionation of Mn 2+ and Mn 4+ to generate two Mn 3+ species is favored on (101) facets. Considering the fact that the OER enzyme accommodates Mn 3+ stably during the Kok cycle, the enhanced OER activity of the rutile MnO 2 catalyst with a metastable surface highlights the importance of mimicking not only the crystal structure but also the electronic structure of the targeted natural enzyme.

Uniform, Assembled 4 nm Mn3O4 Nanoparticles as Efficient Water Oxidation Electrocatalysts at Neutral pH

“…By forming MnN bonds through the introduction of nitrogen ligands on the MnO 2 surface, Mn(III) species were stabilized by suppressing the CD reaction, thus leading to the OER activity enhancement . Recently, there have been some insightful studies on the Mn based water oxidation catalysts regarding the mechanistic studies including OO bond formation and identification of Mn VII O intermediate during catalytic water oxidation, structural and electronic effects on superior OER activity of activated‐MnO 2 and the correlation between the stability of the intermediate species and crystal facet …”

“…studies including OO bond formation [38] and identification of Mn VII O intermediate [39] during catalytic water oxidation, structural and electronic effects on superior OER activity of activated-MnO 2 [40] and the correlation between the stability of the intermediate species and crystal facet. [41] Based on our understanding of previous studies, our group developed various Mn-based synthetic electrocatalysts. Initially, Mn 3 (PO 4 ) 2 ·3H 2 O and Li 2-x MnP 2 O 7 catalysts were developed which showed high OER performance under neutral conditions.…”

Dual Improvement of β‐MnO2 Oxygen Evolution Electrocatalysts via Combined Substrate Control and Surface Engineering

“…[2,[4][5][6][7][8] Actually, due to the sluggish kinetics and complex four-electron mechanism, the bottleneck limiting the overall process efficiency is the oxygen evolution reaction (OER), that requires the use of highly efficient and durable catalysts. [1,[5][6][7][8][9][10][11][12][13][14] Up to date, the state-of-the-art systems are based on RuO 2 and IrO 2 , [1,[6][7][8]10,11,13,14] but their high cost and low natural abundance have stimulated the search for alternative eco-friendly, cheap and efficient (photo) electrocatalysts. In this scenario, manganese oxides (MnO x ) have emerged as appealing functional platforms for various processes and energy-related applications.…”

“…Nevertheless, the more sluggish OER kinetics at the solid/liquid interface, [10,26] and lower electrical conductivity compared to RuO 2 or IrO 2 , [6,22,25,26,29] render the optimization of the system properties an imperative task in view of practical utilizations. [1,2,9,14] A promising strategy to overcome the first issue consists in MnO 2 surface decoration with highly dispersed nanoparticles (NPs) of suitable materials, that can improve charge separation and transport, and beneficially influence the overall water oxidation efficiency. [22,30,31] So far, the functionalization of MnO 2 with metallic or oxide NPs has in fact enabled to achieve an enhanced electrocatalytic activity.…”

Cr‐Dopant Induced Breaking of Scaling Relations in CoFe Layered Double Hydroxides for Improvement of Oxygen Evolution Reaction

“…Electrochemical water splitting is one of the most promising technologies toward developing sustainable energy storage and conversion systems . Unfortunately, the full water splitting reaction is restricted by the oxygen evolution reaction (OER) process, mainly owing to the slow kinetic process of the oxygen related reactions . Various OER catalysts were thus developed to overcome the specific reaction overpotential so as to accelerate the reaction rate.…”