Recent Insights into Manganese Oxides in Catalyzing Oxygen Reduction Kinetics

Abstract

The sluggish kinetics of the oxygen reduction reaction (ORR) limit the efficiency of numerous oxygen-based energy conversion devices such as fuel cells and metal-air batteries. Among earth abundant catalysts, manganese-based oxides have the highest activities approaching that of precious metals. In this Review, we summarize and analyze literature findings to highlight key parameters that influence the catalysis of the ORR on manganese-based oxides, including the number of electrons transferred, specific and mass activities. These insights can help develop design guides for highly active ORR catalysts, and shape future fundamental research to gain new knowledge regarding the molecular mechanism of ORR catalysis.2

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

“…The perovskite structure ABO3−δ has been chosen for many systematic studies of catalytic activity [25,[46][47][48][49][50], including oxygen reduction and evolution (i.e., oxygen electrocatalysis) [14,22,[25][26][27][28][51][52][53][54][55]. The key idea of this section is a demonstration of both intentional and unintentional effects of chemical substitution or changes in stoichiometry on the activity for oxygen reduction ( Figure 2).…”

“…In practical terms, a 100-fold increase in loading of the oxides is currently necessary to match the activity of Pt with state-of-the-art perovskites [125]. In terms of cost, it was estimated composite electrodes of micron-sized LaMnO 3+δ [22] and Pt have about equal activity per cost [26], so that the most active nanoparticle LaMnO 3 composite in Table 3 would be about four times cheaper as compared to the commercial Pt/C benchmark in Figure 3b, albeit at significantly increased catalyst weight. Thus, perovskite oxides have been employed in many prototype devices of fuel cells [66,144,145] and Zn-air batteries [101,102,108], which is reviewed in detail elsewhere [11,89,90].…”

“…Mn oxides containing Mn 3+ in addition to Mn 2+ or Mn 4+ were identified as the most active for ORR [26], where the 3+/4+ redox is particularly important for Mn-based perovskites [163]. Direct evidence of the active valence by X-ray based methods is rare for the ORR in alkaline media.…”

“…Systematic empirical studies of the ORR on oxides have been performed since the 1970s (Table 1), where highly active oxides were characterized by amphoteric metallic conduction [17,21], sigma bands [17] with an occupation near unity of e g orbitals [22], high covalence [22], and paramagnetism [23,24] fostering a specific ('side-on') configuration of the likewise paramagnetic oxygen molecule on the surface. Oxides with perovskite or closely related structures played an important role in the discovery of these insights because their physicochemical properties can be systematically tuned [14,22,[25][26][27][28].…”

A Review of Model-Based Design Tools for Metal-Air Batteries

“…The BAE consists of a porous substrate and a bi-functional air catalyst (e.g., MnO 2 ) to facilitate the oxygen-reduction and oxygen-evolution reactions (ORR, OER) [51][52][53]. The design of the BAE is similar to gas diffusion layers (GDL) from fuel cell applications.…”