The increase of the world demand for energy and the depletion of fossil fuels in conjunction with the global warming crisis, has forced many research aiming at obtaining a new source of energy free of environmental damage, sustainable and with low cost. Researchers have studied the electrochemical reduction of the CO2 molecule into carbon-containing useful fuels, for different application, including fuel cells [1,2]. Copper-based electrocatalysts has shown to catalyze the CO2 electroreduction into the hydrocarbons, such as methane (CH4) and ethylene (C2H4), in aqueous media. Copper show quite unique properties because it is the only metal capable of producing hydrocarbons from CO2, with appropriate Faradaic efficiencies (FE), in aqueous solutions, and at mild conditions of temperature and pressure [3]. However, on-line investigations of the reaction products during the CO2 electrocatalytic reduction using nanoparticles are very scarce in the literature. In the present study, the CO2 electroreduction, catalyzed by carbon-supported Cu nanoparticles (Cu/C) was investigated by using rotating disk electrodes (RDE) on-line coupled to a Differential Electrochemical Mass Spectrometry (DEMS) equipment.
Experimental
The Cu/C electrocatalysts, with 33 wt.% and 50 wt.% of metal copper on Vulcan carbon (XC-72R), were prepared according to a previously published method, using sodium citrate/citric acid as stabilizer and buffering media, and borohydride as the reducer [4]. The structural features were investigated by X-ray diffraction (XRD) measurements. The electrochemical experiments were conducted using an Autolab potentiostat (PGSTAT 30).
The working electrode was prepared depositing 300 μL of a suspension composed by 10 mg of catalyst, 2.5 mL of isopropanol and 250 μL of Nafion (5% of water), on an disk-shaped Toray carbon paper disk (TGPH-030 E-TEK, 30wt.%, 1.3 cm diameter). This was placed onto the top of a regular RDE, with an Au disk, and the electric contact was made by using an Au wire. A ring-shaped Au foil was used as the counter Ag/AgCl/Cl-(sat.) as the reference electrode. The electrolyte was N2 or CO2-saturated 0.1 mol L-1 KHCO3 solution, depending on the experiment (CO2 bubbling was maintained during the measurements). For the product distribution analysis, the RDE was positioned close to probe (ca.1 mm), which was composed by a Teflon membrane (Gore-Tex -pore size 0.02 µm) placed onto the top a glass frit, inserted onto a glass tube [5]. This was connected to the DEMS equipment using a below tube.
Results and Discussion
Fig. 1 presents the ionic currents of the mass signals m/z = 2 (H2), 15 (methane), 26 (ethylene) and 44 (CO2) obtained during DEMS measurements of potential step from OCP to -2.5 V and linear sweep experiments for the CO2 electroreduction catalyzed by 50 wt.% Cu/C (a) and by a Cu RDE rod (b) (included for comparison). One can be observed the signals for CH4, C2H4 and H2 for the Cu rod (bulk Cu) during both experiment protocols. However, and interestingly, for the 50 wt.% Cu/C electrocatalyt, only the signals for C2H4 and H2 are evident (similar result was obtained for 33 wt.t% Cu/C, but with lower ionic current intensities due its lower total surface area – not shown for brevity).
These results revealed that C2H4 is selectively formed in relation to CH4. This may indicate that the Cu/C nanoparticle active sites stabilize a reaction intermediate for the C2H4 formation. Accordingly, from bulk to nanosized Cu particles, the (100) facets may increase an this may stabilized the C-C coupling or the 2-carbon atoms intermediate that will receive additional proton and electron transfer, producing ethylene [6].
Additional experiments will be addressed to investigate the effect of the Cu/C nanoparticle size, and of the additional of ad-atoms, on the reaction product distribution, and this will be included in the present study.
Acknowledgements
The authors thank FAPESP (2014/26699-3 and 2013/16930-7) for financial support.
References
[1] Aresta, M. Carbon Dioxide as Chemical Feedstock. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 394 p., 2010.
[2] Windle, C. D.; Perutz, R. N. Coord. Chem. Rev., 2012, 256 (21-22), 2562–2570.
[3] Shibata, H., Moulijn, J. A. and Mul, G. Catal. Lett., 2008, 123,186–192.
[4] Sarkar, A. and Manthiram, A. J. Phys. Chem. C, 2010, 114, 4725–4732.
[5] Wasmus, S., Cattaneo, E. and Vielstich, W. Electrochimica Acta, 1990, 35 (4), 771-775.
[6] Kortlever, R., Shen, J., Schouten, K. J. P., Calle-Vallejo, F. and Koper, M. T. M. J, Phys. Chem. Lett., 2015, 6, 4073-4082.