Probing the Electrode-Electrolyte Dynamics of Metal Electrocatalysts for Oxygen Reduction with Multimodal X-Ray Scattering and Mass Spectrometry Techniques
Authors: Kamat Gaurav A. , Aleman Ashton M. , Mule Aniket Sandip S. , Hannagan Ryan Travis , Schröder Johanna , Perryman Joseph T. , Wei Lingze , Marin Daniela , Vargas Alfred , Benedek Peter , Rémy Charline , Crago Colin Flynn , Rojas Mendoza Brith Milenia , Burke Stevens Michaela , Jaramillo Thomas F. 
Published: December 2024

The electric double layer is a highly dynamic and complex region with a consequential effect on electrocatalyst performance. The plethora of processes in this near-electrode microenvironment are a function of the catalyst material, electrolyte composition, potential range, and temperature among others. For the electrochemical oxygen reduction reaction (ORR) in acidic media, perchloric acid is commonly used as the electrolyte in fundamental studies due to the perchlorate anion’s minimal interaction with most metal catalysts compared to other anions. However, such anions classically thought to be spectator species influence several electrode processes including, but not limited to, roughening, competitive adsorption, interfacial electric field attenuation, and water structure modulation. Some of these phenomena have been shown to occur over millisecond timescales while others occur over tens of minutes, and with dependence on the ionic species in solution. Moreover, industrial devices operate with ionomer materials on the catalyst such as Nafion that contain sulfonate groups which are chemically dissimilar to perchlorate anions. Given this discrepancy between how catalysts are evaluated in fundamental studies and how they are practically applied, there is motivation to understand the effect of anions in acidic environments during catalyst operation. This line of investigation will help bridge the fundamental-to-device gap and uncover details about catalyst dynamics as a function of microenvironment.
In this work, we resolve key electrolyte-dependent and potential-dependent electrode processes contributing to changes in the catalyst surface structure by utilizing x-ray scattering techniques and on-line mass spectrometry. Taking palladium (Pd) and silver (Ag) thin films as a model ORR system based on their high material stability in acid, we have previously shown the effects of anions on ORR onset potential and selectivity in a variety of pH 1 acids. For the ORR onset potential, the elucidated trend for Ag was HClO4 > HNO3 > H2SO4 > HCl ≫ HBr, whereas for Pd, it was HClO4 ∼ H2SO4 > HNO3 > HCl > HBr. Since the previous study, we have constructed an electrochemical flow cell coupled to an on-line inductively coupled plasma-mass spectrometry (ICP-MS) that can quantify metal dissolution with part-per-billion sensitivity in each of these five electrolytes. In these five oxygen-saturated electrolytes, Pd is tested in the range of 0 to 0.9 V vs Reversible Hydrogen Electrode (RHE) and elevated rates of dissolution are measured at potentials above 0.55 V vs RHE for all electrolytes. Notably, Pd in HNO3 and H2SO4 is more stable than in HClO4 around the onset potential region which breaks the trend of onset potential. For Ag in the same electrolytes in potentials between 0.5 V and -0.5 V vs RHE, the evolution of nanoparticles from the surface is detected along with ionic dissolution near the ORR onset potential. Ag also experiences unexpectedly high dissolution in HNO3 possibly due to competition from electrochemical nitrate reduction that may destabilize the surface. These findings from ICP-MS contribute to greater understanding of electrode instability but do not reveal surface and double layer structure changes.
To investigate the surface structure specifically, a combination of synchrotron grazing incidence x-ray diffraction (GI-XRD) and x-ray reflectivity (XRR) are employed on Ag and Pd thin films with the same electrolytes. GI-XRD reveals the near-surface crystal structure and demonstrated slight electrolyte-dependent and potential-dependent shifts in diffraction peaks, indicating that surface strain can be affected by anions in the double layer. On a two-dimensional detector, non-specular diffraction patterns also enable the determination of preferential grain orientation during catalysis as a function of electrolyte. Furthermore, XRR data clearly indicates the transient roughening of the metal surface during catalysis as evidenced by changes in the amplitude and spacing of Kiessig fringes. Fitting these XRR datasets has previously been shown to successfully elucidate anion distributions in the first few nanometers away from the electrode surface, key information for understanding how anions behave as a function of potential. With this information about dynamic surfaces in various acidic environments under operating conditions, we develop hypotheses on the relation of the near-surface anion distribution, anion physisorption, and anion chemisorption to interfacial processes. Alongside other insights from ICP-MS, XRR, and GI-XRD datasets, an even clearer picture of dynamic electrode processes emerges with regard to the effect of anions and potential. This experimental framework enhances present understanding of the double layer region and its crucial role in processes that lead to both electrode dissolution and reorganization. Understanding the double layer could enable higher efficiency for key electrochemical devices and speed up the adoption of these technologies towards a more sustainable energy future.