(83c) Improving Cobalt Stability and Activity Via Strategic Synthesis of Cobalt-Dilute Bimetallic Thin Film Electrocatalysts for Acidic Sustainable Energy Technologies

Cobalt (Co) plays a significant role within acidic sustainable energy technologies, such as hydrogen fuel cells for vehicular transportation, lithium-ion batteries for electric vehicles and portable devices, and water electrolyzers for renewable hydrogen fuel production. In devices that contain a proton exchange membrane (PEM), namely proton exchange membrane fuel cells (PEMFCs) and proton exchange membrane water electrolyzers (PEMWEs), Co-containing electrocatalysts are often employed to perform the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), respectively. However, the PEM in such devices induces an acidic environment (pH = 1) in which Co is thermodynamically unstable under ORR- and HER-relevant potentials. This instability results in Co dissolution and, as a result, substantial decreases in device performances and lifetimes.

One route to improve material performance and stability involves atomically mixing different metals [1,2]. While there is extensive literature on the activity and durability of CoPt electrocatalysts for the ORR, there is limited literature on how Co behaves when combined with other metals during acidic electrocatalysis. Drawing inspiration from computational oxygen volcano plots to identify electrochemically active material combinations [3], we utilize physical vapor deposition (PVD) to synthesize thin films that contain a low atomic percentage of Co combined with a high atomic percentage of Ag, Au, and Pd. To understand how Co stability and electrochemical ORR activity change within these Co-dilute bimetallic materials, we employ on-line inductively coupled plasma mass spectrometry (ICP-MS) to monitor time-resolved relationships between applied potential, current density, and material dissolution.

In O₂-saturated 0.1 M HClO₄, dissolution of a Co thin film commences at ~ 0 V_RHE, with a maximum dissolution rate of 3300 ng s^-1 cm^-2. However, in the AuCo and PdCo bimetallic thin films, Co dissolution does not commence until 1.5 V_RHE and 1.0 V_RHE, respectively, thus providing an opportunity for Co to operate within a wider potential window in acidic media. Additionally, Co dissolution is significantly reduced, with maximum Co dissolution rates suppressed by a factor of ~450 for AuCo and ~100 for PdCo, both of which are observed during oxidation/reduction of the host metal (i.e, Au and Pd). As for the AgCo bimetallic thin film, the maximum Co dissolution rate is suppressed by a factor of ~225 until 0.6 V_RHE, where it undergoes exponential dissolution until there is complete loss of the thin film. Regarding the electrochemical ORR activity, the trend in onset potential at -0.1 mA cm^-2 is as follows: PdCo > Pd > AgCo > Au > Ag > AuCo > Co. In all cases except for AuCo, the ORR activity is improved relative to that of the host metal, allowing for a route to decrease precious metal loading while improving ORR activity and Co stability. Altogether, through strategic synthesis of Co-dilute bimetallic thin films, along with time-resolved activity and stability assessments, this work showcases material combinations that result in significantly improved stability of Co in electrocatalysts that can be leveraged to decrease Co losses in acidic sustainable energy technologies while retaining high activity.

References:

[1] Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. Chem. Rev. 2018, 118 (5), 2302–2312. https://doi.org/10.1021/acs.chemrev.7b00488.

[2] Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1 (7), 552–556. https://doi.org/10.1038/nchem.367.

[3] Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108 (46), 17886–17892. https://doi.org/10.1021/jp047349j.