(617et) Reducing Greenhouse Gasses Using Low Temperature AEM Electrochemical Devices

Climate change is potentially the largest area of concern in the scientific community today. The global carbon dioxide emissions are 35 giga tons per year and climbing, and account for the largest contribution to the greenhouse gas problem.¹ In addition, global anthropogenic methane emissions total 350 million tons annually and have 25 times the greenhouse gas effect of carbon dioxide.² This results in wasteful flaring of the energy rich methane produced as a byproduct of the fossil fuel industry. This research has utilized a two pronged approach to the greenhouse gas problem applying electrochemical techniques utilizing anion exchange membranes (AEM) to decrease greenhouse gasses release into the atmosphere. Methane emission is decreased by converting it into methanol, the worldâ??s 5^th largest commodity by volume. Methanol can also be used to synthesize numerous products, and has be touted as an important energy carrier of the future. It has a high energy density and is a liquid at atmospheric conditions, which make it ideal for stable transportation and storage that is compatible with existing petroleum infrastructure, which is quite the opposite of methane gas.³ Methane activation and conversion is typically accomplish through the energy intensive syngas pathway, along with many intermediate reaction steps that result in an inelegant and energetically circuitous process. Allowing for control of the catalyst surface free energy, electrochemical methods have the potential to reduce the thermal and overall energy barrier to convert methane to oxygenates, and more specifically methanol or formaldehyde production may be reduced to a single step.⁴ In this research methane activation is shown on a nickel oxide anode catalyst in AEM devices utilizing both hydroxide and carbonate ions and the oxygen donor.

Similar AEM electrochemical devices can be used for carbon dioxide separation, specifically with (bi)carbonate as the mobile ion.^{5, 6} In a typical carbonate-based device, oxygen is reduced with carbon dioxide at the cathode, generally through an indirect pathway first to hydroxide, then (bi)carbonate. This pathway results in a mixed anion composition throughout the AEM where three very different anions can carry the charge across the membrane. Bicarbonate and carbonate (especially bicarbonate) have lower conductivities than hydroxide, but this cannot alone explain the 2 to 3 order of magnitude performance loss from a carbonate-free cell. In a carbon dioxide pump, the oxygen evolution reaction at the anode results in evolved and separated carbon dioxide. The cell potential and resulting half reactions influence the mixed transport and the amount of carbonate, bicarbonate, and hydroxide anions transported. In the carbon dioxide pump a relationship is observed between the evolution of carbon dioxide at the anode and the flow of electrons in the cell. This relationship between current and evolved CO₂ is indicative of the anion ratio and the exchange mechanisms occurring at the anode the cathode. In this study, a peak current density of 100 mA/cm² was reached in CO₂ pumping mode. This performance was achieved with radiation grafted anion exchange membranes and ionomers.⁷It was observed that performance was very sensitive to AEM humidification; that there is a delicate balance between maintaining a high water content in the membrane to facilitate anion transport and preventing catalyst and GDL flooding. The cell anion balance was examined in these high performing cells. The order of magnitude increase in the carbonate/bicarbonate transport observed in this work significantly increases the viability of membrane based carbon dioxide pumps and fuel conversion devices. However, our results also point to advances that are needed in both catalysts and membranes to increase the faradaic efficiency in high current density carbonate devices.

1. IPCC, Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)], p. 1132, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA (2014).

2. S. Menon, K. L. Denman, G. Brasseur, A. Chidthaisong, P. Ciais, P. M. Cox, R. E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S. Ramachandran, Leite da Silva Dias, Pedro, S. C. Wofsy and X. Zhang, in Climate Change 2007 â?? The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the IPCC, S. Solomon, Editor, p. 498, Cambridge University Press, 2007 (2007).

3. G. A. Olah, A. Goeppert and S. K. Surya-Prakash, Beyond oil and gas: the methanol economy, Wiley-VCH (2009).

4. N. Spinner and W. E. Mustain, J. Electrochem. Soc.,159, 12 (2012).

5. W. A. Rigdon, T. J. Omasta, C. Lewis, M. A. Hickner, J. R. Varcoe, J. N. Renner, K. E. Ayers and W. E. Mustain, Journal of Electrochemical Energy Conversion and Storage.,February SI (2017). DOI: 10.1115/1.4033411.

6. J. Landon and J. R. Kitchin, J.Electrochem.Soc.,157, 8 (2010).

7. S. D. Poynton, R. C. Slade, T. J. Omasta, W. E. Mustain, R. Escudero-Cid, P. Ocón and J. R. Varcoe, Journal of Materials Chemistry A., 2, 14 (2014).