Computational and Experimental Study of Fluorine Doped (Mn_1–xNb_x)O₂ Nanorod Electrocatalysts for Acid-Mediated Oxygen Evolution Reaction

Abstract

Identification, development, and engineering of high-performance, earth-abundant, and cost-effective precious group metal (PGM)-free electrocatalysts for catalyzing oxygen evolution reaction (OER) in acidic electrolytes are vital for the commercialization of proton exchange membrane based water electrolysis (PEMWE) technology. Utilizing the density functional theory (DFT) calculations to rationalize the thermodynamics and kinetics of adsorption of OER, juxtaposed with cohesive energy and electronic structure, we report the generation of 10 wt % fluorine (F)-doped (Mn1–x Nb x )­O2:10F nanorods (NRs) as active and durable PGM-free solid solution electrocatalysts for acid-mediated OER. The DFT calculations reveal an optimal solid solution composition of (Mn0.8Nb0.2)­O2:10F containing Nb and F in α-MnO2 structure, exhibiting the optimized surface electronic structure (ΔG for the OER rate-determining step ∼ 1.72 eV) and cohesive energy (E coh ∼ −16.30 eV/(formula unit)) for OER, contributing to its higher catalytic performance in comparison to α-MnO2. Consequently, (Mn1–x Nb x )­O2:10F compositions with well-defined one-dimensional (1D) nanorod architectures are synthesized with the optimal composition of (Mn0.8Nb0.2)­O2:10F, demonstrating improved electrocatalytic performance for acidic OER in good agreement with the DFT calculations. The superior electrochemical performance of (Mn0.8Nb0.2)­O2:10F NRs includes significantly lower charge transfer resistance (∼11.8 Ω cm2), lower Tafel slope (∼371.17 mV dec–1), lower overpotential to deliver a current density of 10 mA cm–2 geo (∼0.68 V), higher mass activity (∼29 A g–1), large electrochemically active surface area (ECSA ∼ 26.28 m2g–1), and turnover frequency (TOF ∼ 0.0065 s–1) with higher BET and ECSA normalized activity (∼0.5 mA cm–2 BET and 0.11 mA cm–2 ECSA) contrasted with (Mn1–x Nb x )­O2:10F (x = 0, 0.1, and 0.3) compositions, at an overpotential of 0.67 mV. Further, (Mn0.8Nb0.2)­O2:10F NRs exhibit good electrochemical stability in acidic OER regimes, with no substantial catalytic activity degradation, validating its structural robustness for prolonged OER and making it a promising PGM-free OER electrocatalyst for acid-mediated PEMWE.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

“…For this purpose, numerous research groups have been working on the development of transition metal oxides for OER in acid. [192][193][194][195][196][197][198][199][200][201][202][203][204] Mn-based oxides and Cobased oxides are the two most widely investigated transition metal oxides. First, Mn-based oxides are considered for OER in acid due to the nature of self-healing to compensate their dissolution in acid.…”

“…Several groups have demonstrated higher performance towards OER in perchloric acid [187] 0.5 m H 2 SO 4 290 mV @10 mA cm −2 36.9 mV dec −1 8 h@1 mA cm −2 Other noble-metalcontaining catalysts Ir/g-C 3 N 4 /NG (5.9 wt%) [142] 0.5 m H 2 SO 4 287 mV @10 mA cm −2 1.15 s −1 @1.53V 2.31 A mg Ir −1 @1.53V 72.8 mV dec −1 2000 CV cycles between 0.0 and 1.5 V RHE Single-atomic ruthenium [157] (RuNC) 0.5 m H 2 SO 4 267 mV @10 mA cm −2 3348 O 2 h −1 @1.497 V 3571 A g metal −1 @1.497 V 52.6 mV dec −1 30 h@1.5 V RHE TiN/IrO 2 -31 [190] 0.5 m H 2 SO 4 313 mV @10 mA cm −2 874 A g IrO2 [201] 0.5 m H 2 SO 4 672 ± 9 mV @10 mA cm −2 80 mV dec −1 12 h@10 mA cm −2 γ-MnO 2 /FTO; γ-MnO 2 /carbon paper [200] 1.0 m H 2 SO 4 489 ± 5 mV @10 mA cm −2 ; 428 ± 5 mV @10 mA cm −2 79 mV dec −1 ; 80 mV dec −1 8000 h@10 mA cm −2 Ag-doped Co 3 O 4 (400) [202] 0. [214] 0.5 m H 2 SO 4 432 mV @10 mA cm −2 73 mV dec −1 Co 2 TiO 4 /CC [203] 0.5 m H 2 SO 4 513 mV @10 mA cm −2 160 mA mg −1 @1.83 V 240 mV dec −1 10 h@1.79 V RHE (Mn 0.9 Nb 0.1 )O 2 -10F [198] 1 n H 2 SO 4 4 mA cm −2 @1.9 V; 820 mV @mA cm −2 0.0029 s −1 @ 1.90 V 13.33 A g −1 @ 1.90 V 470.29 mA cm −2 More than 8 000 s@1.9 V RHE First-row transition metal chalcogenides/ pnictides/borides CoMoNiS-NF-31 [206] 0.5 m H 2 SO 4 228 mV @10 mA cm −2 255 mV dec −1 80 min@1.53 V RHE P-NSC/Ni 4 Fe 5 S 8 -1000 [208] 0.5 m H 2 SO 4 300 mV @ onset overpotential 0.013 s −1 @ 1.8 V 72.1 mV dec −1 1000 CV cycles between 1.2 and 1.9 V RHE Mo-Co 9 S 8 @C [54] 0.5 m H 2 SO 4 370 mV @10 mA cm −2 90.3 mV dec −1 24 h@1.6 V RHE…”

“…First, Mn‐based oxides are considered for OER in acid due to the nature of self‐healing to compensate their dissolution in acid. [ 192 ] Up to now, numerous Mn‐based oxides have been fabricated and optimized, such as TiO 2 ‐incorporated MnO 2 , [ 138 ] hausmannite‐like intermediate (α‐Mn 3 O 4 ), [ 193 ] MnO 2 with metastable Mn 3+ , [ 194 ] Co/Mo‐doped MnO 2 (MnMoCoO), [ 195 ] Cu 1.5 Mn 1.5 O 4 :10F, [ 196 ] Ni x Mn 1− x Sb 1.6–1.8 O y , [ 197 ] (Mn 0.9 Nb 0.1 )O 2 :10F, [ 198 ] NiPbO x , [ 199 ] etc. Among these Mn‐based oxides, the well‐designed gamma manganese oxides (γ‐MnO 2 ) showed a long‐term stability of 8000 h between 1.6 and 1.75 V versus RHE (at 10 mA cm −2 ) in a pH = 2 electrolyte.…”

Mixed Metal-Antimony Oxide Nanocomposites: Low pH Water Oxidation Electrocatalysts with Outstanding Durability at Ambient and Elevated Temperatures

“…First, through the examination of the calculated spin polarised total density of states, we note that Sb substitution into RuO2 maintains its metallic character and even further improves the electrical conductivity as concluded from the shift of the conduction band (Figure S31), which is a positive finding from the perspective of the electrocatalytic activity. Further, and most importantly, improved stability of Ru 4+ within the Sb:RuO2 materials was confirmed by the positive difference in the dissolution potential [71][72][73] Ed = 0.08 V and by the negative increase in the cohesive energies when compared to undoped RuO2. Specifically, the cohesive energy changes from -3.28 (RuO2) to -3.51 (Sb0.0625:Ru0.9375O2) and -3.62 eV per unit formula (Ru0.813Sb0.187O2).…”

“…Extending the analysis to the Mn-Sb combination could not be realised due to a well-documented complex ground state magnetic structure of Mn2O3, which exhibits noncollinear magnetic ordering and introduces significant uncertainties to the modelling of the electronic structure. [69][70] Assessment of the electrochemical and structural stability was undertaken through the computation of cohesive energies of the materials of interest and also differences in the dissolution potentials Ed) [71][72][73] for the Ru-Sb system. Both approaches have been previously validated through comparisons of the theoretical predictions and experimental electrochemical stability data for a range of systems, in particular metal oxides.…”