Potentiostatic regeneration of oxygen reduction activity in MnOx @ graphene hybrid nanostructures

High-Shell Sulfur Doping Enhances Mn-N4 Spin States and Boosts Oxygen Reduction Reaction Performance in both Acidic and Alkaline Media

The development of platinum group metal-free catalysts for the oxygen reduction reaction (ORR) is critical to advancing sustainable energy conversion technologies. Manganese (Mn)-based catalysts, known for their reduced toxicity and promising durability, have traditionally exhibited lower ORR activity compared to state-of-the-art iron-nitrogen-carbon (Fe-N-C) catalysts. In this study, a highly efficient Mn-N-C-S catalyst is presented, engineered through a sulfur-mediated high-shell coordinated doping strategy, that markedly enhances ORR activity and stability. The Mn-N-C-S catalyst achieves a record-high half-wave potential of 0.94 V in alkaline media, among the highest values reported for Mn-based catalysts. Additionally, in acidic media, it exhibits a half-wave potential of 0.80 V, placing it among the top-performing M-N-C catalysts. The catalyst also demonstrates a high peak power density of 0.82 W cm-2 in H2-O2 fuel cells and 0.264 W cm-2 in Zn-air batteries, outperforming previously reported Mn-based catalysts. Both experimental findings and theoretical computations suggest that the high-shell S-doping can increase the spin density of Mn sites, strengthen Mn-N bonds, and thereby improve the durability of Mn-N4 sites. This work underscores the effectiveness of high-shell sulfur doping and paving the way for their deployment in the cathodes of fuel cells and metal-air batteries.

Morphology-Driven Bifunctional Activity of Layered Birnessite-Based Materials toward Oxygen Electrocatalysis

The chemical, structural, and morphological diversity of birnessite, a 2D layered MnO2, has opened avenues for its application as an electrocatalyst toward both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Among pristine birnessites prepared by different methods, the freestanding flakes (primary structure) obtained from molten salt (MS-KMnO) showed remarkable bifunctional activity as compared to samples with thicker plates or a hierarchical honeycomb-like (type-I secondary structure) morphology. While the ORR onset potential (E onset) and halfwave potential (E 1/2) for MS-KMnO were recorded at 0.89 and 0.81 V vs RHE, respectively, the OER overpotential (η) was found to be 300 mV. We demonstrated heat-induced secondary structure evolution by modification of the molten salt method, which led to a decrease in activity. In contrast to previous studies, the Co-doped birnessite (Co-KMnO) prepared in molten salt showed lower bifunctional activity (ORR, E 1/2 = 0.72 V; OER, η= 460 mV) as compared to MS-KMnO. Co-KMnO showed an interwoven wrinkled sheet-like (type-II secondary structure) morphology, with Co3+ present in both the in-layer and the interlayer. However, in Co-KMnO/360 prepared at a lower reaction temperature, the areal coverage of the type-II structure reduces, leading to an increase in ORR (E 1/2 = 0.76 V) and OER (η = 440 mV) activity. The chronopotentiometry for 100 h at a constant OER current of 50 mA cm-2 showed an increase in potential from 1.62 to 1.89 V and the characterization of the sample post-treatment showed degradation of the layered structure in MS-KMnO. The samples obtained after 1000 CV cycles in both the ORR and the OER regions showed the formation of secondary structures with a substantial decrease in the Mn3+/Mn4+ ratio. This study demonstrates that morphology tuning within the 2D birnessite system has a marked effect on its bifunctional activity.

The synergistically enhanced activity and stability of layered manganese oxide via engineering of defects and K+ ions for oxygen electrocatalysis

Structural defects and interlayered ions are two classic architectures that regulate the electrochemical activity of layered manganese oxides. However, the synergistic effect of defects and interlayered ions and how it...

Advanced electrocatalysts based on two-dimensional transition metal hydroxides and their composites for alkaline oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is a crucial part in developing high-efficiency fuel cells and metal-air batteries, which have been cherished as clean and sustainable energy conversion devices/systems to meet the ever-increasing energy demand. ORR electrocatalysts currently employed in the cathodes of fuel cells and metal-air batteries are mainly based on high-cost and scarce noble metal elements. It is thus of great importance to develop cheap and earth-abundant ORR electrocatalysts. In this aspect, redox-active transition metal hydroxides, a class of multifunctional inorganic layered materials, have been proposed as prospective candidates on account of their abundance and high ORR activities. In this article, the preparation and structural evolution of transition metal hydroxides, in particular their exfoliation into two-dimensional (2D) nanosheets, as well as compositing/integrating with catalytic active and/or conductive components to overcome the insulating nature of hydroxides in alkaline ORR, are summarized. Recent advances have demonstrated that 2D transition metal hydroxides with carefully tuned compositions and elaborately designed nanoarchitectures can achieve both high activity and high pathway selectivity, as well as excellent stability comparable to those of commercial Pt/C electrocatalysts. To realize the dream of renewable electrochemical energy conversion, new strategies and insights into rational designing of 2D hydroxide-based nanostructures with further enhanced electrocatalytic performance are still to be vigorously pursued.