Accurate Coulometric Quantification of Hydrogen Absorption in Palladium Nanoparticles and Thin Films

Abstract

We report here an electrochemical method for precise and accurate quantification of hydrogen absorption in palladium materials. We demonstrate that conventional chronocoulometry over-reports adsorbed hydrogen due to charge from the accompanying hydrogen oxidation reaction (HOR). We designed and built a bespoke electrochemical flow cell that mitigates the concurrent HOR reaction and consequently provides improved accuracy and reproducibility relative to other existing electrochemical techniques. The efficacy of this technique is demonstrated experimentally for a series of palladium sample types: a 100 nm electron-beam deposited thin film, a 20 μm electrodeposited palladium film, a casting of 21 nm edge-length cubic nanoparticles, and a casting of 27 nm edge-length octahedral nanoparticles. We contend that this method is the most effective for measuring hydrogen uptake in different palladium samples.

Strain Influences the Hydrogen Evolution Activity and Absorption Capacity of Palladium

“…We measured the amount of hydrogen that absorbed into the Pd j substrate electrodes in the TED by adapting ac oulometry method designed for electrochemical flow cells. [15] Implementing this procedure required the installation of fluid inlets on the transparent front cover so electrolyte could be continuously flowed toward the surface of the electrode to remove hydrogen bubbles from the catalyst during electrolysis (complete setup shown in Figure S6). This technique enabled the quantity of hydrogen absorbed into the palladium electrode to be measured using the following three steps:i ) À0.35 V(vs.Ag/AgCl) was applied to the electrode for 120 s to facilitate HER and hydrogen absorption into the palladium lattice ( Figure 2a); ii)next, 0.30 V(vs.Ag/AgCl) was applied, which oxidized hydrogen absorbed in the lattice back into protons ( Figure 2a); and iii)t he total current passed during this oxidation step was integrated to calculate the H:Pd ratio (Figure 2b).…”

“…We next fabricated flexible electrodes by evaporating 50 nm of palladium onto aK apton 300HN polyimide substrate with a1 0nmc hromium adhesion layer and 20 nm gold conductivity layer (denoted herein as "Pd j substrate"; see SI for details). [15] This deposition method yielded ananocrystalline Pd film with % 50 nm diameter crystallites (see Figures S14 and S17). With this sample architecture,s tretching the Kapton substrate mechanically strains the welladhered palladium film.…”

“…When anegative potential (versus the reversible hydrogen electrode;R HE) is applied to ap alladium electrode immersed in protic electrolyte,p rotons are electrochemically reduced at the metal surface to produce adsorbed hydrogen atoms which can either absorb into the palladium bulk (i.e., HAR) or recombine to evolve as H 2 gas (i.e., HER). [15] During sustained electrolysis,H AR induces ap hase change in the palladium lattice to form b-PdH x (where x is % 0.7 in ambient conditions) [16] and the active surface for HER becomes the hydrided phase. [11] Common methods used to measure the activity of electrocatalysts (e.g.,c yclic voltammetry or linear sweep voltammetry) [15,17] can therefore overestimate the HER activity for palladium electrodes because the measured current includes contributions from both HAR and HER.…”

“…[15] During sustained electrolysis,H AR induces ap hase change in the palladium lattice to form b-PdH x (where x is % 0.7 in ambient conditions) [16] and the active surface for HER becomes the hydrided phase. [11] Common methods used to measure the activity of electrocatalysts (e.g.,c yclic voltammetry or linear sweep voltammetry) [15,17] can therefore overestimate the HER activity for palladium electrodes because the measured current includes contributions from both HAR and HER. This situation necessitated the development of methods to characterize how strain affects HAR and HER in isolation.…”

“…This device is capable of applying ab roader range of tensile strain to electrocatalytic films than those previously reported, [17,19,23] and is hermetically sealed to enable electrolyte to be flowed across the surface of the electrode during electrochemical measurements. [15] These design features provided us with the ability to independently measure the amount of hydrogen that absorbed into the palladium and HER activity while mechanically subjecting palladium electrodes to controlled amounts of tensile strain. We validated that the TED provided access to lattice strain in palladium by conducting as eries of X-ray diffraction experiments wherein we used ac ustom sample stage to stretch the flexible palladium samples while monitoring the change in reflection peak position, which is proportional to lattice strain.…”