Copper Silver Thin Films with Metastable Miscibility for Oxygen Reduction Electrocatalysis in Alkaline Electrolytes

Abstract

Increasing the activity of Ag-based catalysts for the oxygen reduction reaction (ORR) is important for improving the performance and economic outlook of alkaline-based fuel cell and metal–air battery technologies. In this work, we prepare CuAg thin films with controllable compositions using electron beam physical vapor deposition. X-ray diffraction analysis indicates that this fabrication route yields metastable miscibility between these two thermodynamically immiscible metals, with the thin films consisting of a Ag-rich and a Cu-rich phase. Electrochemical testing in 0.1 M potassium hydroxide showed significant ORR activity improvements for the CuAg films. On a geometric basis, the most active thin film (Cu70Ag30) demonstrated a 4-fold activity improvement vs pure Ag at 0.8 V vs the reversible hydrogen electrode. Furthermore, enhanced ORR kinetics for Cu-rich (>50 at. % Cu) thin films was demonstrated by a decrease in Tafel slope from 90 mV/dec, a commonly observed value for Ag catalysts, to 45 mV/dec. Surface enrichment of the Ag-rich phase after ORR testing was indicated by X-ray photoelectron spectroscopy and grazing incidence synchrotron X-ray diffraction measurements. By correlating density functional theory with experimental measurements, we postulate that the activity enhancement of the Cu-rich CuAg thin films arises due to the non-equilibrium miscibility of Cu atoms in the Ag-rich phase, which favorably tunes the surface electronic structure and binding energies of reaction species.

Probing the Effects of Acid Electrolyte Anions on Electrocatalyst Activity and Selectivity for the Oxygen Reduction Reaction

“…Moreover, the characteristic Pd CV profiles in all acids tested lack Pd‐oxide reduction and metal oxidation peaks previously seen in HClO 4 [45,46,48] and H 2 SO 4 , [13,43,48] which suggests that Pd remained metallic during our electrochemical testing. This tendency of metal thin films like Ag and Pd made by e ‐beam PVD to be mainly metallic has been shown before for ORR in alkaline conditions [36,42,61] . Moreover, characterization of the films via AFM, XPS, and ICP‐OES suggest negligible surface roughening (RFs of 1.01 to 1.02), no significant material contamination and a constantly metallic surface, and no significant dissolution after electrochemical testing, respectively (see details in Supporting Figures S3, S5, S6, Supporting Table S1, and Supporting Note S1).…”

“…This tendency of metal thin films like Ag and Pd made by e-beam PVD to be mainly metallic has been shown before for ORR in alkaline conditions. [36,42,61] Moreover, characterization of the films via AFM, XPS, and ICP-OES suggest negligible surface roughening (RFs of 1.01 to 1.02), no significant material contamination and a constantly metallic surface, and no significant dissolution after electrochemical testing, respectively (see details in Supporting Figures S3, S5, S6, Supporting Table S1, and Supporting Note S1). The lack of Pd surface roughening and/or dissolution is in agreement with previous work indicating that Pd is electrochemically stable up to~1 V vs. RHE.…”

“…[35] ORR on Ag materials has largely been studied in alkaline environments, where, Ag has been observed to be highly active and selective to 4e À ORR; and significant activity enhancements have been achieved via alloying. [36][37][38][39][40][41][42] Previous studies of Ag in perchloric acid have shown that compared to in alkaline media, corrosion is more prevalent, and the catalyst exhibits lower ORR activity by approximately~0.4 V, with lower selectivity for the 4e À product, H 2 O. [8] Pd ORR performance, on the other hand, has been better investigated in acidic media, although performance results have been seen to vary depending on crystal structure, catalyst morphology, and conductive support.…”

“…Silver (Ag) and palladium (Pd) are known to be weak and strong oxygen‐binding electrocatalysts, respectively [35] . ORR on Ag materials has largely been studied in alkaline environments, where, Ag has been observed to be highly active and selective to 4 e − ORR; and significant activity enhancements have been achieved via alloying [36–42] . Previous studies of Ag in perchloric acid have shown that compared to in alkaline media, corrosion is more prevalent, and the catalyst exhibits lower ORR activity by approximately ∼0.4 V, with lower selectivity for the 4 e − product, H 2 O [8] .…”

A Review on Challenges and Successes in Atomic-Scale Design of Catalysts for Electrochemical Synthesis of Hydrogen Peroxide

“…Performance bottlenecks in zero-emission fuel-cell technologies have motivated efforts in the past three decades to discover novel catalysts and optimize fuel-cell device design. [42][43][44][45][46][47] The four-electron ORR (4e-ORR) that occurs in a fuel cell is out of the scope of current contribution. Rather, our focus is on the 2e-ORR (eq.…”