Mn-Based Non-Precious Catalyst for the Polymer Electrolyte Fuel Cell Cathode
Authors: Wu Gang , Zelenay Piotr 
Published: February 2020

The development of non-precious metal catalysts (NPMCs) with high oxygen reduction reaction (ORR) activity and practical durability has become a major focus area for the polymer electrolyte fuel cell (PEFC) research as a way of potentially reducing the currently prohibitive PEFC cost. During several decades of research effort in non-precious metal electrocatalysis, significant progress has been achieved in the synthesis, performance improvement, and understanding of the ORR mechanism.1-3 Among many attempted catalyst formulations, the compounds based on heat-treated precursors of transition metals, nitrogen and carbon have attracted more interest than other NPMCs.4

While some metal-free nitrogen-doped carbon materials are at least to some degree capable of catalyzing the ORR,5 an addition of transition metal(s) appears necessary for achieving high catalytic activity and improved durability of heat-treated catalysts. The influence of different transition metals on ORR activity was studied in both acid and alkaline solutions. The results indicate that the nature of the metallic center plays a critical role in the ORR catalysis and can be tied to the activity increase after the heat-treatment conditions. By now, there is convincing experimental evidence that iron and cobalt result in catalysts with the highest ORR activity regardless of the solution pH. However, the nature of the active ORR site may be different for the two metals. Sites generated in the presence of Co appear to have the onset ORR potential similar to that observed in metal-free nitrogen-doped carbon (N-C)formulations that are abundant in CN
x
groups (pyridinic, graphitic nitrogens).6 There is strong evidence that, unlike Co-derived species, Fe directly participates in the ORR, likely via the formation of Fe-N
x
type sites.7 Such sites have generally higher ORR activity than the metal-free sites formed in Co-based catalysts1
,
8
,
9.

Figure 1. RRE plots for PANI-derived non-precious catalysts in 0.5 M H2SO4solution at 25°C and 900 rpm.

In this work, a new transition metal, Mn, was found to efficiently induce high activity and four-electron selectivity for the ORR in acidic media (Figure 1). Similarly to Co and Fe, Mn is capable of catalyzing carbonization of nitrogen/carbon precursor (e.g., polyaniline-PANI) and formation of highly graphitized carbon nanostructures. Apart from the obvious advantages, such as high electronic conductivity and corrosion-resistance, these highly graphitized carbon nanostructures may also serve as a matrix for hosting ORR active nitrogen-metal moieties. One specific hypothesis for explaining the role of nanostructures in ORR catalysis is that nitrogen-containing carbon samples with more plane-edge exposure (pyridinic nitrogen) could be more active for the ORR.10
,
11Unlike in Co- and Fe-derived catalysts, various manganese oxides were found in Mn-derived catalyst (cf. the inset in Figure 2), even after an intensely acid leaching treatment. This makes the determination of active ORR species even more challenging in this case. So far, it is unclear whether such Mn oxides species is active for the ORR or not. More data focusing on that very issue will be presented at the meeting.

Acknowledgments

Financial support for this work has been provided by DOE-EERE through Fuel Cell Technologies Office and by Los Alamos National Laboratory through Laboratory Directed Research and Development (LDRD) program.

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