Oxygen Reduction Reaction on Copper Oxide Electrodes Doped with First-Row Transition Metals

Abstract

The oxygen reduction reaction (ORR) is critical in electrochemical applications such as fuel cells, metal-air batteries, and industrial processes like chlor-alkali and H2O2 synthesis. Its universality lies in the fact that it can serve as the cathodic reaction for many oxidation reactions. Despite extensive research, ORR catalysts face challenges like high overpotentials and losses due to H2O2 formation. Platinum, the benchmark ORR catalyst, is expensive, scarce, and prone to degradation, driving the need for cost-effective, earth-abundant alternatives. Transition metal oxides (TMOs) are promising candidates, particularly in alkaline media, due to their abundance, multiple oxidation states, tuneable redox properties, structural stability, and electronic conductivity. However, TMOs often exhibit lower ORR activity than Pt due to weaker oxygen atom binding, which limits the O–O bond dissociation step. Strategies to enhance oxygen-TMO bonding include introducing oxygen defect sites that can bind O2 more strongly. In this thesis, we have explored a strategy for creating such structures by mixing metals with significant differences in the inherent preference for O coordination. Cu is unique among the first-row TMO due to its preference for four-coordinate square planar coordination under the oxidation state of Cu2+. Even when Cu is reduced to Cu+1, it prefers linear oxygen coordination. In contrast, other first-row transition metals such as Ni, Co, Fe, or Mn generally prefer a six-coordinate octahedral environment of oxygen. Mixing Cu with these metals would result in a significant coordination mismatch. Specifically, in Cu-rich structures, the transition metals would be left with undercoordinated sites, creating coordinative unsaturation (resulting in stress in these systems). Such atomic-level mismatches would induce defects improving O2 binding and, subsequently, O–O bond cleavage. My thesis explores this basic idea and studies ORR activity, selectivity, and durability of first-row transition metal-doped copper oxide electrodes. The idea was generally effective across all metals shown here, with Cu[Co]Ox resulting in maximum ORR currents and the best desired H2O2 selectivity. Notably, all the doped materials Cu[M]Ox/Au (M = Mn, Fe, Co, or Ni) performed well with significantly improved ORR currents and a significant reduction in H2O2 formation, essentially reflecting the preference for the 4e pathway for ORR. Besides this, these materials were tested for potentiodynamic and chronoamperometric stability and demonstrated stability in both activity and selectivity. Chapters 3-6 sequentially explored Cu[M]Ox/Au type materials (M = Mn, Fe, Co, or Ni). Besides electrochemical and materials characterization, we explored material structure-property relationships using in situ Raman spectroscopy. Cu-O-M-type frameworks were integral to the performance of these materials. The cyclic voltammograms revealed that Cu-based redox processes were dominant. The doped metals resulted in some electronic modulation of Cu sites, reflected in the easier oxidation of Cu centers. Interestingly, all the doped catalysts showed stark tuning upon reduction of Cu2+ to Cu1+. Synthesis temperature can be an important parameter in creating these materials. Although we started with atomically mixed solutions of Cu and other transition metals, in principle, the annealing temperature can influence the eventual structure since it can affect diffusion and phase stability. In Chapter 7, the effect of synthesis temperature has been explored for Cu0.8Co0.2Ox/Au catalyst (which were the best performing) by annealing at various temperatures (250–600 °C). While the synthesis temperature did influence certain structural and morphological parameters and improved the crystallinity, its effect on catalytic performance was relatively minor. Comparing various results, the catalysts synthesized at 300 °C showed the best overall performance. In Chapter 8, we examined the influence of temperature on the performance of Cu0.8M0.2Ox/Au catalysts (M = Mn, Fe, Co, or Ni), recognizing its critical role in electrochemical systems like fuel cells, which often operate at elevated temperatures for various reasons. There were two fundamental parameters, one intrinsic to the catalyst and the other extrinsic to the materials, specifically, mass transport-based limitation. The intrinsic effects were explored through variations in kinetic currents vs. temperature. Apparent (at a fixed bias) activation energies were also calculated. Besides this, temperature was found to affect mass transport significantly. As the temperature increases, O2 solubility decreases, but diffusion increases, and kinematic viscosity decreases; the latter affects counterbalancing low oxygen solubility. The competing effects result in optimal performance around 50 °C. Beyond this temperature, the low O2 solubility issue outweighs the positive effect of improved diffusion and lower kinematic viscosity. The H2O2 yield remained relatively stable across the temperature range for all electrodes, with minimal variations. Tafel slope values remained consistent across the temperature range, indicating minimal impact on the reaction mechanism. Activation energy values for H2O and H2O2 formation on Cu0.8Fe0.2Ox/Au, Cu0.8Co0.2Ox/Au, and Cu0.8Ni0.2Ox/Au differed by less than ±4 kJ/mol, suggesting that both processes likely go through the same active site. Chapter 9 represents a study in contrast. Here, we explored the effects of combining cobalt (Co) with other first-row transition metals, such as iron (Fe) and nickel (Ni), to form mixed oxide catalysts (Co[M]Ox/Au, where M = Fe or Ni). Unlike Cu-based mixed oxides, where coordination environment mismatches can create strained sites that enhance O–O bond cleavage, the Co[M]Ox/Au system exhibited low ORR activity and higher H₂O₂ yield than their pure oxide counterparts (CoOx/Au, FeOx/Au, and NiOx/Au). This fundamental difference arises because Co, Fe, and Ni all favor similar six-coordinate oxygen environments, resulting in minimally stressed sites. Their similar ionic radii likely enable good mixing, possibly leading to a generic entropic stabilization of the lattice. This may contribute to the system's increased structural stability and consequently reduced catalytic efficiency for O–O bond cleavage.