Effect of pH on the Activity of Platinum Group Metal-Free Catalysts in Oxygen Reduction Reaction

Abstract

The impact of the electrolyte's pH on the catalytic activity of platinum group metal-free (PGM-free) catalysts toward the oxygen reduction reaction (ORR) was studied. The results indicate that the ORR mechanism is determined by the affinity of protons and hydroxyls toward multiple functional groups present on the surface of the PGMfree catalyst. It was shown that the ORR is limited by the proton-coupled electron transfer at pH values below 10.5. At higher pH values (>10.5), the reaction occurs in the outer Helmholtz plane (OHP), favoring hydrogen peroxide production. Using a novel approach, the changes in the surface chemistry of PGM-free catalyst in a full pH range were studied by X-ray photoelectron spectroscopy (XPS). The variations in the surface concentration of nitrogen and carbon species are correlated with the electron transfer process and overall kinetics. This study establishes the critical role of the multitude of surface functional groups, presented as moieties or defects in the carbonaceous "backbone" of the catalyst, in mechanism of oxygen reduction reaction. Understanding the pH-dependent mechanism of ORR provides the basis for rational design of PGM-free catalysts for operation across pH ranges or at a specific pH of interest. This investigation also provides the guidelines for developing and selecting ionomers used as "locally-confined electrolytes", by taking into account affinities and possible interactions of specific functional groups of the PGM-free catalysts with protons or hydroxyls facilitating the overall ORR kinetics.

Oxygen Reduction Reaction Electrocatalysts Derived from Iron Salt and Benzimidazole and Aminobenzimidazole Precursors and Their Application in Microbial Fuel Cell Cathodes

“…This observation is consistent with the previous RRDE studies done under similar operating conditions using M–N–C type of PGM-free catalysts for oxygen reduction reaction. 85 , 86 , 90 − 92 It was clearly shown in RRDE experiments that ORR on AC followed a 2 e – transfer mechanism which resulted in the highest amount of H 2 O 2 produced ( Figure 3 c), while the number of electrons transferred is significantly lower than 3.5 ( Figure 3 d). The increase in loading decreased the hydrogen peroxide production and also increased significantly the number of electron transferred unambiguously supporting the explanation related to thicker layer trap of peroxide inside followed by its disproportionation before being oxidized on the ring.…”

“…AC, used as a control, showed to have a direct 2e – transfer mechanism and high peroxide production in agreement with previously presented works. 85 , 86 , 90 − 92 …”

Nanorods of cerium oxide as an improved electrocatalyst for enhanced oxygen reduction in single-chambered microbial biofuel cells

“…Again, a lower exchange current density has also been observed at neutral pH, as compared to the acidic or the basic pH. This is because of the limitations of the transfer of electrons to the protons and the hydroxyls; since, these two species are present in much lower concentrations at neutral pH, compared to the acidic or the basic pH [57].…”

Urine in Bioelectrochemical Systems: An Overall Review

“…[82][83] It was shown recently that the shift between the acidic ORR pathway and alkaline ORR pathway occurs around pH 11. [84][85] When protons or cations move through the membrane, water molecules accompany them or are actually "dragged". This phenomenon (well-known as electroosmotic drag) was first described as charge induced flow by F.F.…”

“…[89] In air breathing cathodes, the electroosmotic drag was first observed in MFCs by Kim et al, as the net water loss through the membrane that varied according to external resistance, [90] that later led to the demonstration of the catholyte being accumulated as a result of the electrical current in wastewater operated MFCs. [82][83][84][85][86][87][88][89][90][91] The composition of the catholyte primarily depends on the type of anolyte (feedstock). [92][93][94] Therefore, in urine operated MFCs, this opens new opportunities for nutrient recovery and recycling of elements suitable for fertilisers or irrigation.…”