Impacts of Perchloric Acid, Nafion, and Alkali Metal Ions on Oxygen Reduction Reaction Kinetics in Acidic and Alkaline Solutions

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of ClO₄^–, Nafion ionomer, and cations on Pt/C, Pd/C, and transition metal, N codoped carbon-based (Me–N–C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO₄^– had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand, showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however, is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The noncovalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This noncovalent interaction, however, was not observed on Pd/C, which showed the same ORR activity in all three electrolytes (LiOH, NaOH, and KOH). The ORR activity of Me–N–C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C. The mechanisms for the material-dependent activity trend in different cation solutions were discussed

Active Sites on Heterogeneous Single-Iron-Atom Electrocatalysts in CO₂ Reduction Reaction

Nitrogen-coordinated single-metal-atom catalysts (Me–N–C) are promising candidates for CO2-to-CO electrocatalytic conversion. The nature of real active sites in this type of electrocatalyst, however, is not clear. In this Letter, we study the specific interactions between the reaction intermediates and a model single-iron-atom catalyst (Fe–N–C) by combining in situ infrared absorption spectroscopy and density functional theory (DFT) calculations. For the first time, we confirm that the Fe centers in Fe–N4 moieties hosted by the complete graphitic layer are poisoned by strongly adsorbed CO and should not be the real active sites for gaseous CO production. Further DFT calculation results suggest that the high CO selectivity and reaction rate may originate from Fe–N4 moieties embedded in a defective graphitic layer that have balanced binding energies of adsorbed COOH and CO species. These findings add significant new insights into the mechanisms of CO2 reduction on carbon-based single-atom electrocatalysts

Palladium–Platinum Core–Shell Electrocatalysts for Oxygen Reduction Reaction Prepared with the Assistance of Citric Acid

Core–shell structure is a promising alternative to solid platinum (Pt) nanoparticles as electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). A simple method of preparing palladium (Pd)–platinum (Pt) core–shell catalysts (Pd@Pt/C) in a gram-batch was developed with the assistance of citric acid. The Pt shell deposition involves three different pathways: galvanic displacement reaction between Pd atoms and Pt cations, chemical reduction by citric acid, and reduction by negative charges on Pd surfaces. The uniform ultrathin (∼0.4 nm) Pt shell was characterized by in situ X-ray diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images combined with electron energy loss spectroscopy (EELS). Compared with state-of-the-art Pt/C, the Pd@Pt/C core–shell catalyst showed 4 times higher Pt mass activity and much better durability upon potential cycling. Furthermore, both the mass activity and durability were comparable to that of Pd@Pt/C synthesized by a Cu-mediated-Pt-displacement method, which is more complicated and difficult for mass production

Solution-Phase Synthesis of PdH_0.706 Nanocubes with Enhanced Stability and Activity toward Formic Acid Oxidation

Palladium is one of the few metals capable of forming hydrides, with the catalytic properties being dependent on the elemental composition and spatial distribution of H atoms in the lattice. Herein, we report a facile method for the complete transformation of Pd nanocubes into a stable phase made of PdH0.706 by treating them with aqueous hydrazine at a concentration as low as 9.2 mM. Using formic acid oxidation (FAO) as a model reaction, we systematically investigated the structure–catalytic property relationship of the resultant nanocubes with different degrees of hydride formation. The current density at 0.4 V was enhanced by four times when the nanocubes were completely converted from Pd to PdH0.706. On the basis of a set of slab models with PdH(100) overlayers on Pd(100), we conducted density functional theory calculations to demonstrate that the degree of hybrid formation could influence both the activity and selectivity toward FAO by modulating the relative stability of formate (HCOO) and carboxyl (COOH) intermediates. This work provides a viable strategy for augmenting the performance of Pd-based catalysts toward various reactions without altering the loading of this scarce metal

<i>In Situ</i> Infrared Spectroscopic Evidence of Enhanced Electrochemical CO₂ Reduction and C–C Coupling on Oxide-Derived Copper

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C–C coupling. First, the strong *COO– redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction

Impact of Heat Treatment on the Electrochemical Properties of Carbon-Supported Octahedral Pt–Ni Nanoparticles

Thermal annealing is commonly used to remove surface contaminants and redistribute elements in alloys. In this study, Pt–Ni alloy nanoparticles supported on carbon black are selected as a model catalyst to understand the relationship between the annealing conditions (temperature and atmosphere) and the electrocatalytic performance for oxygen reduction, hydrogen evolution, and ethanol oxidation reactions. The impacts of thermal treatment temperature and atmosphere on structures, compositions, and in turn electrocatalytic activities are systematically studied. Interestingly, an ultrathin carbon layer can be formed on the nanoparticle surface by heat treatment in Ar atmosphere at temperatures higher than 350 °C, which significantly decreases its activity toward oxygen reduction and ethanol oxidation reactions. This carbon coating, however, is absent in other atmospheres including N2, air, 7% H2/Ar, and vacuum. Aberration-corrected scanning transmission electron microscopic characterizations with atomic-level resolutions confirm the formation of a Ni-enriched surface on Pt–Ni/C after treatment in Ar, which plays a critical role in catalyzing the growth of stable carbon layers from the surrounding carbons. Further density functional theory calculation results suggest that the absence of a carbon layer in N2 may originate from the stable N–C bond formed during heat treatment and passivation effect of adsorbed N2. It illustrates different effects of inert gases on carbon layer formation by combining experimental and computational approaches. These results may shed light on the proper design of postheat treatment protocols for carbon-supported catalysts and may also provide a feasible method to coat carbon layers on nanoparticle surfaces for various energy storage and conversion applications

The Role of Ru in Improving the Activity of Pd toward Hydrogen Evolution and Oxidation Reactions in Alkaline Solutions

Improving the reaction kinetics of hydrogen evolution and oxidation reactions (HER/HOR) in alkaline media is critical to promote the development of alkaline fuel cells and electrolyzers. Here, we prepared Pd3Ru alloy nanocatalysts with Ru segregated on the surfaces, forming adatoms and clusters. This structure dramatically lowered the overpotential of Pd toward HER in 1 M KOH by 104 mV at 10 mA cm–2. The HER activity was even higher than that of Pt (6 mV improvement at 10 mA cm–2). Theoretical simulation results revealed that Ru adatoms/clusters on the surface could weaken the hydrogen-binding energy and promote the OH adsorption, consequently lowering the reaction barrier of the rate-determining step in HER. Our findings are of significance for clarifying the role of Ru in bimetallic catalysts and rational design of more active catalysts for HER/HOR

<i>In Situ</i> Infrared Spectroscopic Evidence of Enhanced Electrochemical CO₂ Reduction and C–C Coupling on Oxide-Derived Copper

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C–C coupling. First, the strong *COO– redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction

The Role of Glyoxal as an Intermediate in the Electrochemical CO₂ Reduction Reaction on Copper

The C2 product formation mechanism in the electrochemical reduction reaction of CO2 (CO2RR) is still poorly understood. This work aims to analyze the copper-catalyzed electroreduction of aqueous glyoxal to understand its role as a potential reaction intermediate during CO2RR. Multiple reaction pathways are observed during glyoxal reduction, including its electroreduction to ethanol and ethylene glycol, disproportionation to glycolate and formate, and further coupling toward the formation of C4 compounds and graphitic carbon. A significantly high ethylene glycol to ethanol ratio indicates that glyoxal may not be the main intermediate toward ethanol production in CO2RR on Cu, contradicting previous hypotheses. Density functional theory calculations show that the hydration of aldehyde functional groups can shift the ethylene glycol vs ethanol selectivity, in which the former is preferred when the carbonyl groups remain unhydrated. A CO2-to-glycolate pathway is also possible as a consequence of the base-catalyzed internal Cannizzaro disproportionation of glyoxal. Finally, C–C coupling during glyoxal reduction may open up a CO2RR pathway toward C4 products such as tetroses and 1,4-butanediol that have not been previously observed in electrochemical CO2RR. The formation of graphitic carbon also suggests that the carbon deposits usually observed during CO2RR may originate from glyoxal-derived C–C coupling. Our findings offer valuable insights onto the glyoxal pathway of CO2RR and the various multicarbon products that result from the further conversion of glyoxal