Oxygen Electroreduction on Non-Pt Group Metal Electrocatalysts in Acid and Alkaline Media

The oxygen reduction reaction (ORR) is a significant challenge for proton-and anion-exchange polymer electrolyte membrane fuel cells (PEMFC and AEMFC, respectively). Present state-of-the-art electrocatalysts for fuel cells contain Pt; however, the cost of Pt and Pt-group metals (PGM) is high and imposes limitations for commercialization. It is essential that non-PGM electrocatalysts be investigated for future fuel cell research to alleviate cost concerns. However, non-PGM electrocatalysts pose their own challenges. The low pH environment in PEMFCs creates harsh conditions that effect non-PGM longevity, making it necessary to develop electrocatalysts and supports with better durability. Alternatively, a shift to a high pH environment in AEMFCs provides a media suitable to a broader range of non-PGM electrocatalysts. The present work investigates the ORR on different non-PGM electrocatalysts in both acid and alkaline media. The non-PGM electrocatalysts were prepared by three different synthesis techniques, and their ORR activity and selectivity evaluated.;For the first approach, nitrogen-doped carbons devoid of any metal species were investigated. A plasma-enhanced chemical vapor deposition (PECVD) process created carbon nanowalls (CNWs) on glassy carbon electrodes. Moreover, altering the PECVD reaction conditions changed the morphology and thickness of the resulting CNW layer. Using a post-processing plasma treatment, the CNWs were doped with nitrogen (N-CNWs). X-ray photoelectron spectroscopy (XPS) measurements and Raman spectroscopy quantified the surface functional groups and defect nature of the CNWs, respectively. The post-processing plasma treatment created different CNx species, and the relative quantities of each CN x species depended on the post-processing reaction conditions. The ORR selectivity and activity were studied on the CNWs and N-CNWs in alkaline media and were quantified according to different processing conditions. The results indicate that nitrogen-doped CNWs have higher electrochemical reactivity than the non-doped CNWs. However, the ORR activity on N-CNWs is too low for practical consideration in fuel cells and additional approaches were explored.;For the second approach, electrospun Fe- and Co-containing fibers were prepared and pyrolyzed at different temperatures. The carbon, nitrogen and metal surface species were probed using XPS. The resulting XPS spectra showed that the pyrolysis temperature affects the total carbon and nitrogen contents, as well as the resulting carbon-nitrogen functional species present. The samples containing Fe exhibited kinetic activities for oxygen reduction similar to Pt supported on high surface area carbon, however the volumetric activity was ~6 times lower. The Fe-containing carbon fibers exhibited a near 4e - reduction, but the selectivity for Co-containing carbon fibers was less than 4e-. Moreover, the Co-containing carbon fibers exhibited lower ORR kinetic activities compared to Fe-containing fibers. Complimentary density functional theory calculations probed O2 adsorption on hypothetical Co- and Fe-containing clusters. The Fe-containing cluster showed an increased O-O bond distance after adsorption compared to the Co-containing cluster. The increased O-O bond distance for the Fe-containing clusters is consistent with the increased selectivity measured in the electrochemistry experiments.;We studied the ORR in acidic media on Fe-containing carbon fibers and found that the ORR activity decreased with potential cycling. For our third approach, and to increase the durability, we used atomic layer deposition (ALD) and coated the Fe-containing carbon fibers with TiOy deposits. Heat-treating Fe-containing carbon fibers, with and without TiOy deposits, resulted in a slight decrease in the Fe content but increased the ORR activity in acidic media compared to the samples not twice heat treated. After potential cycling, the presence of Ti species on the Fe-containing carbon fibers decreased the production of H2O2 and retained a larger mass activity for the ORR compared to samples without Ti species.