MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

Abstract

The oxygen evolution reaction (OER) and chlorine evolution reaction (CER) are electrochemical processes with high relevance to water splitting for (solar) energy conversion and industrial production of commodity chemicals, respectively. Carrying out the two reactions separately is challenging, since the catalytic intermediates are linked by scaling relations. Optimizing the efficiency of OER over CER in acidic media has proven especially difficult. In this regard, we have investigated the OER versus CER selectivity of manganese oxide (MnOx), a known OER catalyst. Thin films (∼5–20 nm) of MnOx were electrodeposited on glassy carbon-supported hydrous iridium oxide (IrOx/GC) in aqueous chloride solutions of pH ∼0.9. Using rotating ring–disk electrode voltammetry and online electrochemical mass spectrometry, it was found that deposition of MnOx onto IrOx decreases the CER selectivity of the system in the presence of 30 mM Cl– from 86% to less than 7%, making it a highly OER-selective catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnOx film is in fact not a catalytically active phase, but functions as a permeable overlayer that disfavors the transport of chloride ions.

Controlling Stability and Selectivity in the Competing Chlorine and Oxygen Evolution Reaction over Transition Metal Oxide Electrodes

“…Furthermore, an increase in the pH enhances the OER overpotential at a constant electrode potential on the SHE scale and hence shifts the selectivity of the competing CER and OER towards the side of the OER. Yet, there are also electrode materials that catalyze the OER with superior selectivity in an acidic electrolyte, e. g., a MnO x /IrO x layered electrode as reported by Koper and co‐workers only recently . The proposed methodology herein might be a first starting point to understand the reverse selectivity by simple DFT calculations.…”

“…I would like to point out that the presented strategy of exploring CER electrocatalysts with improved selectivity and stability can be equally employed to search for selective OER electrocatalysts, such as for electrocatalytic seawater splitting . In this regard, the opposite free energy difference, i. e., Δ G (OCl)–Δ G (OOH), needs to be maximized to obtain a selective OER electrocatalyst based on transition metal oxides, where the CER is sufficiently suppressed . However, in this case the stability issue might be problematic, since the OOH adsorbate limits the stability window of the transition metal oxide in acidic medium (cf.…”

“…[88,89] In this regard, the opposite free energy difference, i. e., ΔG(OCl)-ΔG(OOH), needs to be maximized to obtain a selective OER electrocatalyst based on transition metal oxides, where the CER is sufficiently suppressed. [13] However, in this case the stability issue might be problematic, since the OOH adsorbate limits the stability window of the transition metal oxide in acidic medium (cf. Figure 6 b).…”

“…Yet, there are also electrode materials that catalyze the OER with superior selectivity in an acidic electrolyte, e. g., a MnO x /IrO x layered electrode as reported by Koper and co-workers only recently. [13] The proposed methodology herein might be a first starting point to understand the reverse selectivity by simple DFT calculations.…”

“…[1][2][3][4] The anode material consists of Ti plates coated with TiO 2 À RuO 2 mixed oxides, commonly denoted as dimensionally stable anodes (DSA). [5][6][7] RuO 2 has been identified as active component of the DSA for electrochemical chlorine evolution (CER), [8] which is impaired by a selectivity problem: the evolution of gaseous oxygen (OER) is a competing side reaction in the same potential window of CER, [9][10][11][12][13] which from a thermodynamic point of view is even preferred over CER according to the smaller equilibrium potential.…”