Surprisingly Low Reactivity of Layered Manganese Oxide toward Water Oxidation in Fe/Ni-Free Electrolyte under Alkaline Conditions

Abstract

So far, many studies on the oxygen-evolution reaction (OER) by Mn oxides have been focused on activity; however, the identification of the best performing active site and corresponding catalytic cycles is also of critical importance. Herein, the real intrinsic activity of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the first time. At pH ≈ 14, the onset of OER for layered Mn oxide in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease in the overpotential of OER was recorded for layered Mn oxide as well as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel slope. Furthermore, we find that both Ni and Fe ions increase OER remarkably in the presence of layered Mn oxide, but that pure layered Mn oxide is not an efficient catalyst for OER without Ni and Fe under alkaline conditions. Thus, pure layered Mn oxide and electrolytes are critical factors in finding the real intrinsic activity of layered Mn oxide for OER. Our results call into question the high efficiency of layered Mn oxides toward OER under alkaline conditions and also elucidate the significant role of Ni and Fe impurities in the electrolyte in the presence of layered Mn oxide toward OER under alkaline conditions. Overall, a computational model supports the conclusions from the experimental structural and electrochemical characterizations. In particular, substitutional doping with Fe decreases the thermodynamic OER overpotential up to 310 mV. Besides, the thermodynamic OER onset potential calculated for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus, not in the stability range of Mn oxides.

High-Density NiMnFe Hydroxide Hierarchical Nano-arrays for Industrial-level Electro-chemical Oxygen Evolution Reaction

“…Homogenous distribution of Mn element on the surface of M-Ni(OH) x indicates the successful doping of Mn into the hydroxide film, which can modulate the electronic structure of catalyst for enhancing OER performance (Figure S3d ). 29 The rough surface is conducive to growing high-density nanosheet arrays, which can provide more active sites and accelerate the desorption of bubble under industrial-level operation condition. 30 The treated Ni foam was then transferred into 0.5 M FeCl 3 solution and maintained for 10 min at room temperature to further grow nanosheets on porous nanoparticle layer.…”

Candidate for Catalyst during Water-Oxidation Reaction in the Presence of Manganese Compounds, from Nanosized Particles to Impurities: Sleep with One Eye Open

“…Surprisingly, a computational model shows that substitutional doping of Fe in the structure of Mn oxide decreases the WOR overpotential to 310 mV. 1 Although we investigated the effect of impurities in the structure of birnessite but not all Mn oxides, as previously discussed, different phases of Mn oxides transformed to a layered Mn oxide during WOR. Indeed, the adsorption of Ni or Fe occurs on birnessite.…”

“…50 We investigated the WOR activity of layered Mn oxides in Fe/Ni-free electrolytes. 1 Indeed, in the absence of Fe/Ni ions, layered Mn oxide showed low activity under alkaline conditions. In KOH (1.0 M) and in the presence of Fe salt, a 190 mV decrease in the overpotential of WOR was observed for layered Mn oxide, as was a remarkable decrease (from 173 to 49 mV/decade) in the Tafel slope.…”

“…In KOH (1.0 M) and in the presence of Fe salt, a 190 mV decrease in the overpotential of WOR was observed for layered Mn oxide, as was a remarkable decrease (from 173 to 49 mV/decade) in the Tafel slope. 1 The significant difference in WOR activity and Tafel slopes shows that the mechanism of WOR changes after Fe is added to the structure of Mn oxide. Surprisingly, a computational model shows that substitutional doping of Fe in the structure of Mn oxide decreases the WOR overpotential to 310 mV.…”

A copper(ii) coordination compound under water-oxidation reaction at neutral conditions: decomposition on the counter electrode

“…Operando visible spectroscopy is a promising method for tracking solids formed on an electrode surface. [47][48][49][50][51] In the next step, operando visible spectroscopy is performed at different potentials to detect the solid during the OER (Fig. 6).…”