Platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) often exhibit a complex functionalized graphitic structure. Because of this complex structure, limited understanding exists about the design factors for the synthesis of high-performing materials. Graphene, a two-dimensional hexagonal structure of carbon, is amenable to structural and functional group modifications, making it an ideal analogue to study crucial properties of more complex graphitic materials utilized as electrocatalysts. In this paper, we report the synthesis of active nitrogen-doped graphene oxide catalysts for the ORR in which their activity and four-electron selectivity are enhanced using simple solvent and electrochemical treatments. The solvents, chosen based on Hansen’s solubility parameters, drive a substantial change in the morphology of the functionalized graphene materials by (i) forming microporous holes in the graphitic sheets that lead to edge defects and (ii) inducing 3D structure in the graphitic sheets that promotes ORR. Additionally, the cycling of these catalysts has highlighted the multiplicity of the active sites, with different durability, leading to a highly selective catalyst over time, with a minimal loss in performance. High ORR activity was demonstrated in an alkaline electrolyte with an onset potential of ∼1.1 V and half-wave potential of 0.84 V vs RHE. Furthermore, long-term stability potential cycling showed minimal loss in half-wave potential (<3%) in both N2- and O2-saturated solutions with improved selectivity toward the four-electron reduction after 10000 cycles. The results described in this work provide additional understanding about graphitic electrocatalysts in alkaline media that may be utilized to further enhance the performance of PGM-free ORR electrocatalysts.

Material interactions at the polymer electrolytes–catalyst interface play a significant role in the catalytic efficiency of alkaline anion-exchange membrane fuel cells (AEMFCs). In this work, the surface adsorption behaviors of the cation–hydroxide–water and phenyl groups of polymer electrolytes on Pd- and Pt-based catalysts are investigated using two Pd-based hydrogen oxidation catalystsPd/C and Pd/C-CeO2and two Pt-based catalystsPt/C and Pt-Ru/C. The rotating disk electrode study and complementary density functional theory calculations indicate that relatively low coadsorption of cation–hydroxide–water of the Pd-based catalysts enhances the hydrogen oxidation activity, yet substantial hydrogenation of the surface adsorbed phenyl groups reduces the hydrogen oxidation activity. The adsorption-driven interfacial behaviors of the Pd- and Pt-based catalysts correlate well with the AEMFC performance and short-term stability. This study gives insight into the potential use of non-Pt hydrogen oxidation reaction catalysts that have different surface adsorption characteristics in advanced AEMFCs.

Effect of organic cations on hydrogen oxidation reaction (HOR) of carbon supported platinum (Pt/C) is investigated using three 0.1 M alkaline electrolytes, tetramethylammonium hydroxide (TMAOH), tetrabutylammonium hydroxide (TBAOH) and tetrabutylphosphonium hydroxide (TBPOH). Rotating disk electrode experiments indicate that the HOR of Pt/C is adversely impacted by time-dependent and potential-driven chemisorption of organic cations. In-situ infrared reflection adsorption spectroscopy experiments indicated that the specific chemisorption of organic cations drives the hydroxide co-adsorption on Pt surface. The co-adsorption of TMA + and hydroxide at 0.1 V vs. reversible hydrogen electrode is the strongest; consequently, complete removal of the co-adsorbed layer from Pt surface is difficult even after exposure the Pt surface to 1.2 V. Conversely, the chemisorption of TBP + is the weakest, yet notable decrease of HOR current density is still observed. The adsorption energies, E, for TMA + , TBA + , and TBP + on Pt (111) surface from density functional theory are computed to be −2.79, −2.42 and −2.00 eV, respectively. The relatively low adsorption energy of TBP + is explained by the steric hindrance and electronic effect. This study emphasizes the importance of cationic group on HOR activity of alkaline anion exchange membrane fuel cells. Alkaline anion exchange membrane fuel cells (AAEMFCs) are drawing increasing interest due to the fact that non-precious group metal (non-PGM) catalysts perform well in alkaline environment. 1Over the last decade, numerous non-PGM catalysts have demonstrated excellent oxygen reduction reaction (ORR) activity.2,3 However, poor activity for the hydrogen oxidation reaction (HOR) is still a challenge under alkaline conditions even for Pt catalysts. Therefore, development of highly active HOR catalysts remain one of the major technical challenges, hampering the success of AAEMFCs. 4 The reason for the poor HOR activity in alkaline environments is unclear. Markovic et al. suggested that an optimal balance between the adsorption/dissociation of H 2 and the adsorption of hydroxyl species is critical for improving HOR activity.5 Conversely, Gasteiger et al. suggested that the hydrogen binding energy is the relevant descriptor for the HOR in alkaline electrolytes.6 Yan et al. further elucidated that the hydroxide species do not directly participate in the reaction through adsorption, while the alkalinity change induced by the hydroxide ion affects the hydrogen binding energy and in turn influences the HOR activity. 7While aforementioned studies primarily consider the fundamental Heyrovsky and Volmer steps to explain the slow HOR kinetics, recently Janik et al. have proposed that a co-adsorbed alkali metal cation-hydroxide-water layer on the Pt surface may impact the HOR activity as observed from density functional theory (DFT) calculations and electrochemical experiments.8 Substantial Pt HOR inhibition was observed with organic cation solutions, in addition to the alkali metal cation in aqueo...

Removal of intercalated water within graphitic sheets is critical to achieving high-performing oxygen reduction reaction catalysts.