Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction

Abstract

Manganese oxides have received much attention over the years among the wide range of electrocatalysts for the oxygen evolution reaction (OER) due to their low toxicity, high abundance and rich redox chemistry. While many previous studies focused on the activity of these materials, a better understanding of the material transformations relating to activation or degradation is highly desirable, both from a scientific perspective and for applications. We electrodeposited Na-containing MnO x without long-range order from an alkaline solution to investigate these aspects by cyclic voltammetry (CV), scanning electron microscopy (SEM) and X-ray absorption spectroscopy (XAS) at the Mn-K and Mn-L edges. The pristine film was assigned to a layered edge-sharing Mn 3+/4+ oxide with Mn-O bond lengths of mainly 1.87 Å and some at 2.30 Å as well as Mn-Mn bond lengths of 2.87 Å based on fits to the extended X-ray fine structure (EXAFS). The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186±48 to 114±18 mV dec-1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edge showed no change of the microstructure, bulk local structure or bulk Mn valence. Yet, the surface region of MnO x oxidized toward Mn 4+ , which explains the reduction of the currents in agreement with literature. Surprisingly, we find that MnO x reactivates after 30 minutes at open-circuit (OC), where the currents and also the Tafel slope increase. Reactivation processes during OC are crucial because OC is unavoidable when coupling the electrocatalysts to intermittent power sources such as solar energy for sustainable energy production.

Requirements for Beneficial Electrochemical Restructuring: A Model Study on a Cobalt Oxide in Selected Electrolytes

“…We used a high number of cycles (here 800 cycles), a high upper potential limit (2.1 V vs reversible hydrogen electrode (RHE)), and a high sweep speed (100 mV s ‐1 ) as typical for stability, restructuring, or activation studies. [ 14,44–47 ] The maximum current density, j max (at 2.1 V vs RHE), was comparable among all different electrolytes during the first cycles, i.e., it is mainly that of the as‐synthesized Ery. Yet, j max continuously increased in carbonate electrolyte, whereas, in phosphate and borate electrolytes, it rose during the first 200–300 cycles, after which it started decreasing.…”

“…All spectra were normalized by subtracting a straight line obtained by fitting the data before the K edge and division by a polynomial function obtained by fitting the data after the K edge, as illustrated elsewhere. [ 14 ] The FT of the EXAFS was calculated between 15 and 800 eV above the Co‐ K edge ( E 0 = 7709 eV). A cosine window covering 10% on the left side and 10% on the right side of the EXAFS spectra was used to suppress the side lobes in the FTs.…”

“…[ 6–8 ] Thanks to their unique atomic arrangement, these materials own structural flexibility and distinctively coordinated metal centers. [ 9–11 ] Electrochemically, amorphous oxides can be obtained by electrodeposition [ 12–14 ] or electrochemical restructuring [ 9,15–19 ] of crystalline materials. The latter approach has improved catalytic activity compared to their crystalline variant in some cases, [ 9,10,12,20 ] while this was not the case for other combinations of pristine materials and electrolytes.…”

Requirements for Beneficial Electrochemical Restructuring: A Model Study on a Cobalt Oxide in Selected Electrolytes

“…A high number of cycles (here 800 cycles), a high upper potential limit (2.1 V vs. RHE), and a high sweep speed (100 mVs -1 ) were used as typical for stability, restructuring, or activation studies. [14,[41][42][43][44] The maximum current, imax (at 2.1 V vs. RHE), was comparable among all different electrolytes during the first cycles, i.e., it is mainly that of the as-synthesized Ery. Yet, imax continuously increased in carbonate electrolyte, whereas, in phosphate and borate electrolyte, it rises during the first 200-300 cycles, after which it started decreasing.…”

“…[6][7][8] Thanks to their unique atomic arrangement, these materials own structural flexibility and distinctive coordinated metal center. [9][10][11] Electrochemically, amorphous oxides can be obtained by electrodeposition [12][13][14] or electrochemical restructuring [9,[15][16][17][18][19] of crystalline materials. The latter approach has improved catalytic activity compared to their crystalline variant in some cases, [9,10,12,20] while this was not the case for other combinations of pristine materials and electrolytes.…”