Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2_2O4_4

Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LiMn$_x$O$_4$ system in aqueous alkaline electrolyte. The investigated LiMn$_2$O$_4$ particles exhibit two distinct Mn dissolution processes; one independent of OER and the other associated to OER. Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive X-ray photoelectron spectroscopy and electrochemical detection of Mn dissolution using rotating ring-disk electrode, we explore the less understood Mn dissolution mechanism during OER. We correlate near-surface oxidation with the charge attributed to dissolved Mn, which demonstrates increasing Mn dissolution with the formation of surface Mn4+ species under anodic potential. The observed stronger dissolution during the OER is attributed to the formation of additional Mn$^{4+}$ from Mn$^{3+}$ during OER. We show that control over the amount of Mn4+ in Li$_x$Mn2O$_4$ before the onset of the OER can partially mitigate the OER-triggered dissolution. Overall, our atomistic insights into the Mn dissolution processes are crucial for knowledge-guided mitigation of electrocatalyst degradation, which can be broadly extended to manganese-based oxide systems

Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2O4

Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LixMn2O4 system in aqueous alkaline electrolyte. The investigated LiMn2O4 particles exhibit two distinct Mn dissolution processes; one independent of OER and the other associated to OER. Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive X-ray photoelectron spectroscopy and electrochemical detection of Mn dissolution using rotating ring-disk electrode, we explore the less understood Mn dissolution mechanism during OER. We correlate near-surface oxidation with the charge attributed to dissolved Mn, which demonstrates increasing Mn dissolution with the formation of surface Mn4+ species under anodic potential. The observed stronger dissolution during the OER is attributed to the formation of additional Mn4+ from Mn3+ during OER. We show that control over the amount of Mn4+ in LixMn2O4 before the onset of the OER can partially mitigate the OER-triggered dissolution. Overall, our atomistic insights into the Mn dissolution processes are crucial for knowledge-guided mitigation of electrocatalyst degradation, which can be broadly extended to manganese-based oxide systems

Spatially dispersed one-dimensional carbon architecture on oxide framework for oxygen electrochemistry

The rational design of bifunctional electrocatalyst is important for sustainable energy storage and conversion devices such as metal-air batteries and fuel cells. Herein, we have designed a unique architecture where carbon nanotubes (CNTs) are supported on an oxide template. NiCo encapsulated N-doped carbon nanotubes were grown vertically outward from the nickel–cobalt oxide flowers for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The hybrid shows enhanced water oxidation performance (321 mV at 10 mA/cm2) and decent ORR activity (E1/2 at 0.75 V vs RHE) due to better conductivity and a large electrochemical surface area. Enhanced OER activity can be ascribed to high Ni and Ni3+ content whereas improved ORR activity results from enhanced active nitrogen species (pyridinic, M−Nx and graphitic) and higher water contact angle (due to unique architecture). Further, reversible oxygen electrochemistry with Δ E = 0.80 V indicates its potential as a bifunctional electrocatalyst. The hybrid electrocatalyst has shown good operational stability and durability for OER and ORR. Finally, the practical feasibility as cathode catalyst for metal-air battery has been demonstrated by powering a light emitting diode.Ministry of Education (MOE)Submitted/Accepted versionThe authors gratefully acknowledge DST-FIST (SR/FST/PSII-009/2010), India and Ministry of Education, Singapore (RG15/16, RG16/18) for the financial support

Transition metal doped NiOx faceted nanosheets for electrocatalytic water oxidation

In this study, NiO nanosheets mainly oriented in the (111) crystallographic planes were synthesized by either hydrothermal (HT) or microwave-assisted (MW) routes, to investigate the influences of faceting as well as the enhancements by doping with transition metals (Fe, Mn, Co). Furthermore, the synthesis was adopted to produce Fe, Mn and Co doped NiO (111) nanosheets with various dopant concentrations. The material was studied with XRD, TEM, XAS and RDE