Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO₂ Electroreduction

Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO) in aqueous or nonaqueous electrolytes. However, polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high selectivity toward CO₂ reduction, as compared to the side reaction of hydrogen evolution. Here we report a high-surface-area Ag nanocoral catalyst, fabricated by an oxidation–reduction method in the presence of chloride anions in an aqueous medium, for the electro-reduction of CO₂ to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current density of 2 mA cm^–2. A lower limit of TOF of 0.4 s^–1 and TON > 8.8 × 10⁴ (over 72 h) was estimated for the Ag nanocoral catalyst at an overpotential of 0.49 V. The Ag nanocoral catalyst demonstrated a 32-fold enhancement in surface-area-normalized activity, at an overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanomorphology, the adsorbed chloride anions play a critical role in the observed enhanced activity and selectivity of the Ag nanocoral electrocatalyst toward CO₂ reduction. Synchrotron X-ray photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions, can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic performance for CO₂ reduction

Structures and Catalytic Properties of PtRu Electrocatalysts Prepared via the Reduced RuO₂ Nanorods Array

Structures and properties of PtRu electrocatalyts, derived from the aligned RuO2 nanorods (RuO2NR), are investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and cyclic voltammetry toward COads and methanol oxidation. The catalytic activity of methanol oxidation and the CO tolerance are promoted significantly by reducing RuO2 into Ru metal before decorating with Pt. Reduction of RuO2NR was carried out by either thermal decomposition at 650 °C in vacuum or H2-reduction at 130 °C in low-pressure hydrogen. Reduction assisted by hydrogen allows infiltrating decomposition at low temperature and produces an array of nanorods with rugged walls featuring small Ru nuclei and larger surface area. Pt−RuNR, whose surface Pt:Ru ratio = 0.58:0.42 was prepared by decorating with 0.1 mg cm-2 Pt on the H2-reduced array containing 0.39 mg cm-2 Ru, demonstrates a favorable combination of CO tolerance and high methanol oxidation activity superior to other RuO2NR-derived catalysts. When compared with a commercial electrocatalyst of PtRu (1:1) alloy (<4 nm), the activity of Pt−RuNR in methanol oxidation is shown to be somewhat lower at potential <0.48 V and higher at potential ≥0.48 V

Modification of CO₂ Reduction Activity of Nanostructured Silver Electrocatalysts by Surface Halide Anions

This paper describes the effect of halide anions (X = Cl, Br, I) immobilized on the surface of nanostructured silver electrocatalysts on the efficiency and the mechanism of CO2 reduction to CO in aqueous carbonate solutions. A simple oxidation–reduction cycle on Ag foil in the presence of halide anions produces high-surface-area nanostructured catalysts mainly composed of metallic Ag with a small amount of halide anions attached to the electrode surface (X–Ag) as demonstrated by XPS, XRD, and SEM studies. The activity of X–Ag electrocatalysts in 0.1 M NaHCO3 at pH 6.8 is significantly higher than that of Ag foil or Ag nanoparticles with comparable surface area and morphology. The activity enhancement is attributed to the formation of active catalytic sites, presumably Cl––Agn+ clusters on the surface of metallic Ag, as evidenced by XPS analysis. The activity of X–Ag catalysts is in the order Cl > Br > I, which is consistent with the proposed model of an active site. The Tafel analysis of electrochemical CO2 reduction points to the sensitivity of the mechanism of electrocatalysis on the nature of X

High Performance Pt Monolayer Catalysts Produced via Core-Catalyzed Coating in Ethanol

Platinum monolayer core–shell nanocatalysts were shown to have excellent catalytic activities and stabilities. Usually, they are fabricated via electrochemical routes. Here, we report a surfactant-free, ethanol-based, wet chemical approach to coating Pd nanoparticles with uniform Pt atomic layers, inspired by aerobic alcohol oxidation catalyzed by the Pd cores. The as-prepared Pt monolayer electrocatalysts also exhibited high electrocatalytic performance toward the oxygen reduction reaction

DFT Study of Oxygen Reduction Reaction on Os/Pt Core–Shell Catalysts Validated by Electrochemical Experiment

Proton exchange membrane fuel cells (PEMFCs) have attracted much attention as an alternative source of energy with a number of advantages, including high efficiency, sustainability, and environmentally friendly operation. However, the low kinetics of the oxygen reduction reaction (ORR) restricts the performance of PEMFCs. Various types of catalysts have been developed to improve the ORR efficiency, but this problem still needs further investigations and improvements. In this paper, we propose advanced Os/Pt core–shell catalysts based on our previous study on segregation of both bare surfaces and surfaces exposed to ORR adsorbates, and we evaluate the catalytic activity of the proposed materials by density functional theory (DFT). Quantum mechanics was applied to calculate binding energies of ORR species and reaction energy barriers on Os/Pt core–shell catalysts. Our calculations predict a much better catalytic activity of the Os/Pt system than that of pure Pt. We find that the ligand effect of the Os substrate is more important than the lattice compression strain effect. To validate our DFT prediction, we demonstrate the fabrication of Os/Pt core–shell nanoparticles using the underpotential deposition (UPD) technique and succeeding galvanic displacement reaction between the Pt ions and Cu-coated Os nanoparticles. The Os/Pt/C samples were evaluated for electrocatalytic activities toward the ORR in acidic electrolytes. The samples with two consecutive UPD-displacement reaction cycles show 3.5 to 5 times better ORR activities as compared to those of commercially available Pt/C. Our results show good agreement between the computational predictions and electrochemical experimental data for the Os/Pt core–shell ORR catalysts