Surface microstructural controls on electrochemical hydrogen absorption at polycrystalline palladium

Lewis C Yule,Enrico Daviddi,Geoff West,Cameron L Bentley,Patrick R Unwin

doi:10.1016/j.jelechem.2020.114047

Abstract

The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered to be a model system for the study of hydrogen absorption). Correlating spatially-resolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promote higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which are thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process. As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.

Highlights

Palladium (Pd) is considered to be a model metal to study hydrogen absorption, due to high intrinsic hydrogen solubility and rapid entry kinetics [10, 11]
Note that due to the high intrinsic hydrogen solubility and rapid entry kinetics in Pd [10, 11], as well as the fact that the applied potentials are positive of the expected onset of the hydrogen evolution reaction (HER) on Pd (i.e., 0 V vs the reversible hydrogen electrode, RHE, corresponding to ca. −0.47 V vs Ag/AgCl, ) [41], the measured current must arise solely from net hydrogen adsorption-absorption
Our results indicate that electrochemical hydrogen absorption is a strongly grain-dependent process, occurring most readily on grains of high-index orientation compared to those of low-index orientation

Summary

Introduction

Palladium (Pd) is considered to be a model metal to study hydrogen absorption, due to high intrinsic hydrogen solubility and rapid entry kinetics [10, 11]. When applied to polycrystalline electrodes, SECCM electrochemically interrogates the individual grains and grain boundaries that constitute the surface, which is correlated to co-located structural information from electron backscatter diffraction (EBSD) to resolve nanoscale structure−activity directly and unambiguously This pseudo single-crystal approach [25] has previously been employed to investigate the grain (and grain boundary) dependent electrochemistry of polycrystalline platinum [2528], gold [29], boron-doped diamond [30] and low carbon steel [31,32,33]. Building on this body of work, we employ SECCM in tandem with co-located EBSD to investigate grain-dependent electrochemical proton reduction coupled to hydrogen adsorption-absorption into polycrystalline Pd in aqueous sulfuric acid. Enhanced electrochemical kinetics are identified at grains of highindex orientation (e.g., {411}) and grain boundaries relative to the grains of low-index orientation

Methods

Results

Conclusion