AMn2O4 spinels (A - Li, Mg, Mn, Cd) as ORR catalysts: The role of Mn coordination and oxidation state in the catalytic activity and their propensity to degradation

Altering Oxygen Binding by Redox‐Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System**

“…[44] However, for Mn 3 O 4 :M materials studied here, XPS analysis of the samples deposited from the Nafion-containing inks did not evidence any noticeable differences in the envelope of the Mn 3p core level region compared to the pristine samples (Figure S9) indicating low reactivity of Mn 3 O 4 :M towards Nafion. Interestingly, Mn 2 + / Mn 3 + /Mn 4 + ratio (oxidized Mn 4 + species are present on the surface of all as-synthesized Mn 3 O 4 :M materials, in line with other reports on XPS analysis of Mn 3 O 4 oxides [43,45] ) did not vary over 20 CV cycles (0.4 V RHE -1.0 V RHE , 100 mV s À 1 , 0.1 M KOH) (Figure S9, Table S4), providing evidence for the high stability of the spinel structure over the period of the catalytic measurements. Selected area electron diffraction (SAED) patterns of the Mn 3 O 4 :M materials after potential cycling (Figure S11) confirm the presence of a nanocrystalline spinel structure and the absence of any detectable amounts of secondary crystalline phases, whereas TEM analysis (Figure S12) does not reveal any noticeable changes in particles morphology, size, or crystallinity indicating that surface changes (if any) can only be confined to surface atomic layer(s).…”

“…Despite the pronounced stability of the bulk Mn 3 O 4 structure, evidenced by Mn K‐edge XAS measurements, [30] some studies report on the instability of Mn 3 O 4 surface during electrochemical cycling at ORR potentials in harsh alkaline environment, [43] as well as effect of Nafion binder used for the preparation of the catalyst inks on the Mn electronic states [44] . However, for Mn 3 O 4 :M materials studied here, XPS analysis of the samples deposited from the Nafion‐containing inks did not evidence any noticeable differences in the envelope of the Mn 3p core level region compared to the pristine samples (Figure S9) indicating low reactivity of Mn 3 O 4 :M towards Nafion.…”

“…Although many studies have explored the effect of metal substitution in oxides (e.g., perovskites [56] ) on their catalytic performance, changes in the catalytic activity have been attributed typically to changes in the valence state of the active site (for aliovalent substitution), [57–59] changes in the crystal structure of the oxide, [60] or the introduction of crystallographic strain, [61] i.e., effects originating from the mismatch of the size or the formal charge of the substituent and the substituted site (note that in this context, we do not consider systems for which the substitution results in a change of the reaction site, e.g., from Fe to Mn in Mn x Fe 3− x O 4 [62] ). (Specifically for Mn 3 O 4 , metal substitution was used as a tool to tailor electrocatalytic activity [43, 63, 64] [65–68] ).…”

Altering Oxygen Binding by Redox‐Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System**

“…(Specifically for Mn 3 O 4 , metal substitution was used as a tool to tailor electrocatalytic activity. [ 43 , 63 , 64 ]) On the other hand, there are very few experimental works that aim to leverage electronic, or inductive, effects arising from the incorporation of substituents to rationally tailor the activity of electrocatalysts (examples can be found in references[ 65 , 66 , 67 , 68 ]). Hence, establishing a unified description of the effect conferred by the introduction of redox‐inactive metals into a host oxide on their catalytic ORR/OER activity (irrespective of their structure) would provide a new perspective (and experimental tool) for the design of materials for targeted electrochemical transformations.…”

What X-ray absorption spectroscopy can tell us about the active state of earth-abundant electrocatalysts for the oxygen evolution reaction

“…88 Finally, elements may dissolve from the electrocatalyst into the electrolyte changing the composition and thereby other properties, such as redox potentials or the catalytic reaction's overpotential. [89][90][91] Dosaev et al 92 recently studied Mn-bases spinels as synthesized, in the ink suspension and after soaking in hydroxide electrolyte, which oxidized Mn 3 O 4 but not MgMn 2 O 4 . We recommend similar control experiments of the electrode soaked for an extended time (at least for the 10/18 same duration of the intended experiment duration, but ideally much longer) to elucidate possible changes prior to electrochemical experiments.…”