Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction

Abstract

Alloying is a powerful tool that can improve the electrocatalytic performance and viability of diverse electrochemical renewable energy technologies. Herein, we enhance the activity of Pd-based electrocatalysts via Ag-Pd alloying while simultaneously lowering precious metal content in a broad-range compositional study focusing on highly comparable Ag-Pd thin films synthesized systematically via electron-beam physical vapor co-deposition. Cyclic voltammetry in 0.1 M KOH shows enhancements across a wide range of alloys; even slight alloying with Ag (e.g. Ag0.1Pd0.9) leads to intrinsic activity enhancements up to 5-fold at 0.9 V vs. RHE compared to pure Pd. Based on density functional theory and x-ray absorption, we hypothesize that these enhancements arise mainly from ligand effects that optimize adsorbate–metal binding energies with enhanced Ag-Pd hybridization. This work shows the versatility of coupled experimental-theoretical methods in designing materials with specific and tunable properties and aids the development of highly active electrocatalysts with decreased precious-metal content.

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

“…Similar features are seen across all acids tested with the distinctive redox features being related to hydrogen underpotential deposition (HUPD) processes from 0.4 to 0 V vs. RHE (Figure 2e). [42,43,45,46] While the HUPD features in the acids are not exactly the same, such differences are commonly observed in literature. [13,43,45,46] Anion adsorption/ desorption redox features may influence the overall CV profile in the HUPD region but the large magnitude and common differences of HUPD features obscure the presence of any anion-related redox features despite Pd characteristic CVs in nitric acid and perchloric acid having very small oxidation peaks at~0.6 V vs. RHE that could be associated with the anions.…”

“…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.…”

“…Developing this understanding of the role of the anion on the ORR performance in acidic media on different catalysts will help enable the design and development of non-Pt electrocatalysts and electrochemical microenvironments optimal for the ORR. Therefore, we investigate the ORR performance of 70 nm thick, smooth/flat, preferentially h111i orientated out of plane, polycrystalline (face-centered cubic (fcc)) Ag and Pd thin film electrodes (made by e-beam PVD and characterized previously), [42] by cyclic voltammetry (CV) using a rotating (ring) Aiming to understand the effects of acid electrolyte identity, we investigated the ORR performance of weak oxygen-binding polycrystalline Ag thin films in various electrolytes at pH 1. The films were cycled up to 0.4 V vs. RHE to prevent instability problems seen by us (see potential-dependent study of Ag dissolution measured by ICP-MS in 0.1 M HClO 4 Supporting Figure S1) and others at potentials � 0.6 V vs. RHE.…”

“…AFM is one of the most reliable techniques to measure the true exposed surface area of continuous material surfaces such as thin films or disks (see further discussion in Supporting Note S1). [42,56] Minor variations in oxygen solubility within the different electrolytes leading to slightly different mass transport are accounted for in this analysis, and thus the specific activity trends are indeed representative of intrinsic activity (Figure 1d). [55] Furthermore, the specific activity here is normalized by AFM surface area of the Ag both pre and post electrochemical testing to account for any surface changes related to ORR (Figure 1d).…”

“…[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.…”