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pH Effects on the Selectivity of the Electrocatalytic CO2 Reduction on Graphene-Embedded Fe-N-C Motifs: Bridging Concepts between Molecular Homogeneous and Solid-State Heterogeneous Catalysis Ana Varela, Matthias Kroschel, Nathaniel Leonard, Wen Ju, Julian Steinberg, Alexander Bagger, Jan Rossmeisl, and Peter Strasser ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00273 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018
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ACS Energy Letters
pH Effects on the Selectivity of the Electrocatalytic CO2 Reduction on Graphene-Embedded Fe-N-C Motifs: Bridging Concepts between Molecular Homogeneous and Solid-State Heterogeneous Catalysis Ana Sofia Varela,a,b*, Matthias Kroschelb, Nathaniel D. Leonardb, Wen Jub, Julian Steinbergb, Alexander Baggerc, Jan Rossmeislc and Peter Strasserb* a, b
c
Institute of Chemistry, National Autonomous University of Mexico, 04510, Mexico City, Mexico Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 10623 Berlin, Germany
Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen, Denmark
Contacts: Dr. Ana Sofia Varela. Email:
[email protected] Prof. Peter Strasser. Email:
[email protected] 1 ACS Paragon Plus Environment
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Abstract. The electrochemical CO2 reduction reaction (CO2RR) is a promising route for converting CO2 and excess renewable energy into valuable chemicals and synthetic fuels. Recently, carbon-based solid materials containing dopant-levels of transition meta and nitrogen (MN-C) have emerged as a cost-effective, energy-efficient catalyst for the direct co-reduction of CO2 and H2O to CO. Fe-N-C catalysts are particularly interesting as they can reduce CO further to hydrocarbons. Despite these promising reports, the influence of the reaction conditions on the catalytic performance of Fe-N-C catalysts has not been addressed. Here, we study the role of the electrolyte on the CO2RR selectivity. Unlike hydrogen or methane generation, catalytic CO production is independent of the electrolyte pH on the NHE electrode potential scale, suggesting a decoupled elementary proton-electron transfer mechanism for CO formation. The similarity between this heterogeneous charge-transfer reaction mechanism and that of molecular metal-nitrogen porphyrin-type macrocyclic complexes strongly suggests that the carbon-embedded FeNx motifs of the solid state electrocatalyst act as the primary catalytically active center, and illustrates yet another example of unifying concepts between molecular and solid state catalysis.
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ACS Energy Letters
The direct electrochemical CO2 reduction reaction (CO2RR) to fuels and chemicals has attracted attention, given its potential use for the direct conversion of CO2 into valuable carbon based products using renewable energies as a driving force.1 The technological viability of this process, however, relies on finding the optimal catalyst and reaction conditions for the process to be selective and energy efficient.2,3 Studies on different catalysts materials, mainly metals, have shown that the product distribution during CO2RR is greatly affected by the cathode material.4,5 In particular, Cu has been found to be an interesting material on which CO2 is directly reduced to alcohols and hydrocarbons.6,7 Therefore, many studies have been carried out using Cu catalyst showing that the reaction conditions (electrolyte and working potential) as well as the morphology and pretreatment of the surface affect the energy efficiency and the selectivity of the process.8-11 Furthermore, despite copper’s ability to produce hydrocarbons, the reaction takes place at high over potentials producing a complex mixture of products. Most of the energy losses from hydrocarbon production are attributed to the complexity of the reaction that requires multiple proton/electron transfer steps. By contrast, the electroreduction of CO2 to CO requires only the transfer of two electrons and two protons, which makes it a substantially less hindered process. In fact, CO2 can be reduced at low overpotential on different metals, such as Ag,12 Au13,14 and bimetallic catalysts15,16. The formation of CO, however, is accompanied by hydrogen evolution reaction (HER) which makes the selectivity one of the major challenges for producing pure CO streams to be used as chemical precursors. Furthermore, earth-abundant catalyst materials are desired as low cost alternative to these metallic catalysts. Molecular catalyst based on metal porphyrins are known to be active catalyst for CO2RR to CO17, and so are heterogeneous catalyst containing Metal-Nx centers such Metal-Organic Frameworks (MOFs)18 and immobilized Metal-N complexes.19,20 Recently, solid MetalNitrogen doped carbon materials (M-N-C) have been shown to be promising low cost catalysts for the CO2RR, since they can efficiently produce CO at low overpotentials.21-25 The high selectivity towards the CO2RR of these solid catalysts has been attributed to the presence of insolated active sites on which the competitive HER is suppressed. 26,27 Among the different metal centers that can can be incorporated to the carbon strucutre, iron is 3 ACS Paragon Plus Environment
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particularly intersting, as it can reduce CO2 to CO at low overpotentials, reaching 80% efficiency at –0.6 VRHE. 21,23,24 Furthermore, the Fe centers can bind CO strong enough to further reduce it to methane.21,24 In addition to the catalysts material, the pH and local pH are known to be on the selectivity of the reaction28,29. To date, however, no study has investigated the effect of the experimental parameters other than the applied electrode potential on the performance of Fe-N-C catalysts. Here, we fill that gap and study the effect of the pH on CO2RR selectivity to understand the role of proton concentration on the catalytic process. We find that while H2 and CH4 production depend on the pH on the Normal Hydrogen Electrode (NHE) scale, CO production is independent on pH. These results, suggest a decoupled proton electron transfer (DPET) as rate limiting step for CO production. In addition, the different effect of pH in HER and CO2RR to CO offers an opportunity to tune the CO/H2 ration via the electrolyte's pH For this study we synthesized an Iron and nitrogen-containing (
