TY - JOUR

T1 - Cobalt(II) complexes with azole-pyridine type ligands for non-aqueous redox-flow batteries

T2 - tunable electrochemistry via structural modification

AU - Armstrong, Craig G.

AU - Toghill, Kathryn E.

N1 - This is the author’s version of a work that was accepted for publication in Journal of Power Sources. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Power Sources, 349, 2017 DOI: 10.1016/j.jpowsour.2017.03.034

PY - 2017/5/1

Y1 - 2017/5/1

N2 - Abstract A single species redox flow battery employing a new class of cobalt(II) complexes with ‘tunable’ tridentate azole-pyridine type ligands is reported. Four structures were synthesised and their electrochemical, physical and battery characteristics were investigated as a function of successive substitution of the ligand terminal pyridyl donors. The Co(II/I) and Co(III/II) couples are stable and quasi-reversible on gold and glassy carbon electrodes, however redox potentials are tunable allowing the cobalt potential difference to be preferentially increased from 1.07 to 1.91 V via pyridine substitution with weaker σ-donating/π-accepting 3,5-dimethylpyrazole groups. The charge-discharge properties of the system were evaluated using an H-type glass cell and graphite rod electrodes. The complexes delivered high Coulombic efficiencies of 89.7–99.8% and very good voltaic efficiencies of 70.3–81.0%. Consequently, energy efficiencies are high at 63.1–80.8%, marking an improvement on other similar non-aqueous systems. Modification of the ligands also improved solubility from 0.18 M to 0.50 M via pyridyl substitution with 3,5-dimethylpyrazole, though the low solubility of the complexes limits the overall energy capacity to between 2.58 and 12.80 W h L−1. Preliminary flow cell studies in a prototype flow cell are also demonstrated.

AB - Abstract A single species redox flow battery employing a new class of cobalt(II) complexes with ‘tunable’ tridentate azole-pyridine type ligands is reported. Four structures were synthesised and their electrochemical, physical and battery characteristics were investigated as a function of successive substitution of the ligand terminal pyridyl donors. The Co(II/I) and Co(III/II) couples are stable and quasi-reversible on gold and glassy carbon electrodes, however redox potentials are tunable allowing the cobalt potential difference to be preferentially increased from 1.07 to 1.91 V via pyridine substitution with weaker σ-donating/π-accepting 3,5-dimethylpyrazole groups. The charge-discharge properties of the system were evaluated using an H-type glass cell and graphite rod electrodes. The complexes delivered high Coulombic efficiencies of 89.7–99.8% and very good voltaic efficiencies of 70.3–81.0%. Consequently, energy efficiencies are high at 63.1–80.8%, marking an improvement on other similar non-aqueous systems. Modification of the ligands also improved solubility from 0.18 M to 0.50 M via pyridyl substitution with 3,5-dimethylpyrazole, though the low solubility of the complexes limits the overall energy capacity to between 2.58 and 12.80 W h L−1. Preliminary flow cell studies in a prototype flow cell are also demonstrated.

KW - Redox flow battery

KW - Energy storage

KW - Tunable ligands

KW - Non-aqueous

U2 - 10.1016/j.jpowsour.2017.03.034

DO - 10.1016/j.jpowsour.2017.03.034

M3 - Journal article

VL - 349

SP - 121

EP - 129

JO - Journal of Power Sources

JF - Journal of Power Sources

SN - 0378-7753

ER -