Investigating Cobalt Degradation in Acidic Environments for Oxygen and Hydrogen Electrocatalysis with Time-Resolved Mass Spectrometry Techniques
Authors: Aleman Ashton M. , Kamat Gaurav A. , Crago Colin Flynn , Mule Aniket Sandip Sandip , Nielander Adam C. , Burke Stevens Michaela , Jaramillo Thomas F. 
Published: August 2024

Cobalt is an important additive within commercially available energy devices including hydrogen fuel cell vehicles, lithium-ion batteries for electric vehicles and portable devices, and electrolyzers for water splitting. Regarding devices that include a proton exchange membrane (PEM), such as proton exchange membrane fuel cells (PEMFCs) and proton exchange membrane water electrolyzers (PEMWEs), cobalt-based materials are commonly used to perform the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), respectively. However, given the acidic environment induced by the PEM (pH = 1), cobalt is thermodynamically unstable at ORR and HER relevant potentials, which results in cobalt dissolution and consequently decreases in device performance and lifetime. Minimizing electrode degradation of cobalt-based materials in acidic environments therefore requires a holistic understanding of cobalt’s degradation mechanisms, as it would be necessary to simultaneously monitor electrochemical output (current/voltage), cobalt dissolution, and product formation, which is not possible with one technique alone. To gain such an understanding, in this presentation I will discuss our work on developing comparable time-resolved techniques such as on-line inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical mass spectrometry (EC-MS) to investigate the relationship between applied potential, time, cobalt dissolution, and product formation.

With usage of EC-MS, it was observed that cobalt is active for HER in 0.1 M HClO4. and undergoes little to no cobalt dissolution (< 1 ng s-1) at potentials where cobalt is evolving H2 per ICP-MS measurements. This suggests that cobalt stability in acid strongly coincides with hydrogen evolution. In addition, when comparing cobalt’s material stability from the on-line ICP-MS experiments with its thermodynamic stability from its Pourbaix diagram, there is little to no cobalt dissolution at potentials outside its thermodynamic stability window. This is unexpected, as cobalt is projected to be thermodynamically stable only at potentials less than -0.277 VRHE (Co2+/Co = -0.277 VRHE for a dissolved total metal ion concentration of 1 M at pH = 1), whereas our data shows that cobalt dissolution is suppressed until -0.05 VRHE. Through combining findings from both EC-MS and ICP-MS, we have postulated a stability mechanism related to surface hydrogen coverage on cobalt that can be used to explain why cobalt is stable during HER and at potentials it is thermodynamically expected to be unstable.

In addition to cobalt being HER active, we also uncovered its activity for ORR in O2 saturated 0.1 M HClO4. Cobalt dissolution under O2 versus N2 saturated 0.1 M HClO4 was then compared, and it was observed that in the presence of O2, cobalt dissolution occurs faster (~3x more dissolution), likely due to O2 induced chemical dissolution. However, after 3 cycles, the observed cobalt dissolution rates under both gas types are equivalent (~225 ng s-1), indicating an important relationship between time, potential, and electrochemical environment. Altogether, this work showcases how time-resolved techniques such as ICP-MS and EC-MS can be connected to unravel stability mechanisms of materials and motivative future studies to apply a holistic approach when evaluating material stability.