Electrochemical energy storage can enable decarbonization of the electric grid by stabilizing power delivery from intermittent and unpredictable renewable resources and by extending the operating lifetime of aging grid infrastructure. While many redox chemistries and cell formats have been considered for such purposes, high system cost and emerging application needs motivate the search for new, unconventional redox couples. One such class of molecules are room temperature liquid organic species, which are relatively abundant, inexpensive, chemically stable, energy dense, and, under the right conditions, do not fully oxidize to CO₂.¹ Due to the inherent stability of these redox couples, in both the oxidized and reduced states, there is a need for selective, durable, and active electrocatalysts to drive the bidirectional electrochemical reactions. Certain electrocatalysts suggest that reversible operation of such couples on a single electrode is possible (e.g., platinum ruthenium (PtRu) alloys facilitate the interconversion of isopropanol and acetone).² Fundamental studies of the isopropanol/acetone redox couple have demonstrated the selectivity of PtRu alloys and have partially described the reaction mechanism, whereas flow cell studies have primarily focused on improving the galvanic power density (either isopropanol oxidation coupled to oxygen reduction or acetone reduction coupled to hydrogen oxidation).^3–5 Yet, to date, studies underemphasize the long-term and dynamic performance of catalysts in these reversible systems, especially under recharging conditions which could significantly alter the catalyst state. Moreover, the electrolyte conditions in ex-situ studies (dilute active species in aqueous acidic electrolyte solutions) do not match those of the fuel cell studies (concentrated active species and ionomer-based solid electrolytes). These discrepancies in the electrocatalyst microenvironment and dynamic operation leave critical questions about catalyst efficacy unaddressed.

In this presentation, we explore the isopropanol-acetone redox couple in the context of electrochemically rechargeable liquid organic fuel cells. We focus on electrochemical analyses that assess performance-defining processes on platinic catalysts under acidic conditions. First, we quantify faradaic efficiencies via bulk electrolysis to validate the selectivity and reversibility of the isopropanol/acetone couple, demonstrating the need to separate catalytic and electrocatalytic effects in the presence of oxygen. Then, we employ rotating disk voltammetry in conjunction with surface area quantification methods to estimate intrinsic kinetic reaction rates, which are complicated by dynamic poisoning effects. Thereafter, we focus on characterizing the time-dependent electrocatalyst activity by quantifying decay rates and comparing the poisoning behavior to simple adsorption models. Measurements of these transient changes are anticipated to help predict the performance of electrochemical reactors utilizing aqueous acidic electrolytes and immobilized electrocatalysts, helping bridge fundamental and device studies using techniques that are likely applicable to other water-soluble organic redox couples.

Acknowledgements

A.H.Q. gratefully acknowledges the National Science Foundation Graduate Research Fellowship Program under Grant Number 1745302. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. A.H.Q also acknowledges the Alfred P. Sloan Foundation’s Minority Ph.D. (MPHD) Program.

References

(1) Perry, M. L. ECS Trans. 2021, 104 (1), 23.

(2) Perry, M. L.; Yang, Z. J. Electrochem. Soc. 2019, 166 (7), F3268–F3276.

(3) Brodt, M.; et al. Energy Technology 2021, n/a (n/a), 2100164.

(4) Li, C.; et al. Energies 2018, 11 (10), 2691.

(5) Mangoufis-Giasin, I.; et al. Journal of Catalysis 2021, 400, 166–172.