Abstract
AbstractElectrolyzers combining CO2 reduction (CO2R) with organic substrate oxidation can produce fuel and chemical feedstocks with a relatively low energy requirement when compared to systems that source electrons from water oxidation. Here, we report an anodic hybrid assembly based on a (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) electrocatalyst modified with a silatrane‐anchor (STEMPO), which is covalently immobilized on a mesoporous indium tin oxide (mesoITO) scaffold for efficient alcohol oxidation (AlcOx). This molecular anode was subsequently combined with a cathode consisting of a polymeric cobalt phthalocyanine on carbon nanotubes to construct a hybrid, precious‐metal‐free coupled AlcOx–CO2R electrolyzer. After three‐hour electrolysis, glycerol is selectively oxidized to glyceraldehyde with a turnover number (TON) of ≈1000 and Faradaic efficiency (FE) of 83 %. The cathode generated a stoichiometric amount of syngas with a CO:H2 ratio of 1.25±0.25 and an overall cobalt‐based TON of 894 with a FE of 82 %. This prototype device inspires the design and implementation of nonconventional strategies for coupling CO2R to less energy demanding, and value‐added, oxidative chemistry.
Highlights
The electrosynthesis of fuels is being pursued as a potential solution to intermittent electricity production via renewable wind and solar technologies.[1]
Several mechanistic studies have highlighted the effect of pH on the TEMPO catalytic cycle, with enhanced oxidation rates recorded under more basic conditions.[22,23,24]
We designed STEMPO, where a silatrane unit offers increased linker stability under more alkaline conditions,[26,27] a result of the strong siloxane bonds being formed upon hydrolysis of the caged moiety on the Metal oxide (MOx) surface (Figure 1)
Summary
The electrosynthesis of fuels is being pursued as a potential solution to intermittent electricity production via renewable wind and solar technologies.[1]. The current density increases with increasing pH, accompanied by a lower onset potential for catalysis (from 0.75 V at pH 7.3, to 0.68 V at pH 10, vs NHE), which is comparable to previous reports for immobilized TEMPO on carbon-based electrodes.[40,41] This observation is in-line with the established TEMPO-mediated oxidation mechanism, whereby alcohol deprotonation leads to formation of a pre-oxidation complex via nucleophilic attack of the alkoxide on the electrophilic nitrogen of the oxidized TEMPO moiety (the oxoammonium cation), prior to aldehyde formation.[41,42,43,44] the enhancement starts to plateau between pH 9 and pH 10, contrary to what is observed for TEMPO, and related nitroxyl derivatives, in solution.[44] The plateau shown in Figure 3a for the mesoITO|STEMPO system could be due to a combination of factors, and we rationalize this behavior to stem from the relatively slow electron transfer between the ITO electrode and immobilized STEMPO, as well as from mass transport limitations of the substrate in the mesoporous network.
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