Quinone-Mediated Electrochemical O2 Reduction Accessing High Power Density with an Off-Electrode Co-N/C Catalyst

Abstract

Dissolved organic redox mediators enable transport of electrons and protons between an electrode and off-electrode heterogeneous catalysts. A new tetrasubstituted quinone has been developed that exhibits excellent solubility and stability under strongly acidic conditions and supports electrochemical reduction of O 2 in an off-electrode packed-bed reactor containing a heterogeneous Co-N/C catalyst. These components are integrated in a ''flow-cathode'' fuel cell system that achieves high current and power densities.

Comparison of Quinone‐Based Catholytes for Aqueous Redox Flow Batteries and Demonstration of Long‐Term Stability with Tetrasubstituted Quinones

“…[ 14,23,24 ] Hydroquinones 6 and 7 were synthesized by addition of sodium mercaptoethanesulfonate to benzoquinone. The tris‐thioether compound 6 is an analogue of the major byproduct formed during the previously reported synthesis of 4 , [ 22 ] and it was produced by attempting the synthesis of 5 at elevated temperature (see Supporting Information). The yield of the byproduct was instead improved, allowing 6 to be isolated for further studies.…”

“…The hydroquinones 1 – 8 were oxidized chemically to the corresponding quinones using a modification of Kochi's previously reported NO x ‐based method (see Supporting Information for details). [ 22,26 ] With the isolated quinones in hand, we examined the stability of the quinone forms of 5 , 6 , and 7 using a 1 H NMR spectroscopic protocol, similar to that previously used with compounds 1 – 4 . [ 22 ] Briefly, samples of each quinone were prepared in 0.5 m D 2 SO 4 with 0.05 m CH 3 SO 3 H as an internal standard, and the fate of each quinone was monitored by 1 H NMR spectroscopy.…”

“…We recently reported the design of tetra‐substituted quinones containing four sulfonated thioether substituents, compounds 4 and 5 , which display substantially enhanced stability relative to quinones 1 – 3 in preliminary chemical stability tests. [ 22,23 ] Herein, we report related quinones (compounds 6 and 7 ) and conduct stability tests of the entire series of quinones in Scheme 2 to elucidate the factors that contribute to improved mediator stability. These solution‐phase tests are complemented by flow battery testing of quinones 1 – 4 .…”

Bioeletrocatálise de etanol utilizando álcool desidrogenase em eletrodos de carbono funcionalizados com quinonas: da eletroquímica molecular para uma abordagem operando em resonância paramagnética de elétrons

“…Several authors have reported that quinones act as a redox mediators. [18][19][20][21][22][23][24] Even though the quinoneelectrolyte interface plays an important role as an electron donor/acceptor, there is no consensus about the mechanism of electron transfer. As far as the experimental approach is concerned, there is an intrinsic problem in solving the reaction mechanism: both the mediated electron transfer mechanism and electrocatalysis can result in similar responses in steady-state electrochemistry because both increase the faradaic current at a low overpotential.…”

“…3 These results suggest that the electrochemically modified electrode and the FCF electrode treated chemically in phosphate buffer solution (0.1 mol L -1 , pH 7.5) showed the best electrochemical activity towards the NADH electrooxidation. Different mechanisms have been proposed in the literature for the electro-oxidation of NADH in the presence of redox mediator, [18][19][20][21][22][23][24] where the redox mediators have been coupled with NAD-dependent enzymes. In our case, the electro-oxidation of NADH catalyzed by ortho-and anthraquinone modified FCF electrodes is followed by a reaction pathway shown in Figure 22.…”