(139d) Electrochemical Activation of Methane at Low Temperatures for Applications in Polymer Electrolyte Membrane Energy Conversion Devices

Authors: 
Herring, A. M., Colorado School of Mines
Joglekar, M., Colorado School of Mines
Gunnoe, T. B., University of Virginia
Trewyn, B., Colorado School of Mines

Methane, a primary constituent of natural gas, has attracted widespread attention as a fuel due its higher energy content per mass unit (55.7 kJg-1) compared to other hydrocarbons. In the last decade, advances in drilling technologies have expanded the access and reduced the expense of natural gas. Currently, the predominant use of methane in the energy sector is via combustion. An alternative approach is to utilize methane in fuel cells. Fuel cells are dramatically more efficient than heat engines at lower temperatures, and their exhaust is composed of only CO2 and water making them attractive clean energy conversion devices. Methane is an excellent fuel for a high temperature solid oxide fuel cell (SOFC), but to realize the efficiency benefits of these systems their waste heat must be utilized making them only practical as large stationary devices.  Furthermore SOFC technology is challenged to run in transient node and is still expensive. Proton exchange membrane fuel cells (PEMFCs) have the advantage of much higher power densities, faster start up and shut down, good cyclability, and the potential for scalability from micro to large-scale distributed power generation; however, their lower temperature of operation makes the activation of methane extremely challenging under the operating conditions of the fuel cell.

 The use of natural gas as a fuel in PEM fuel cell would be extremely attractive.

 There have been attempts to use methane in low temperature proton exchange membrane fuel cells from at least the 1960s.1 However, non of these attempts have produced practical current densities due to the resistance of methane to activation on conventional Pt based catalysts.2  It is therefore necessary to develop new catalytic strategies if methane is to be utilized in a low temperature PEM based fuel cell.  Joglekar et al. have recently demonstrated a method to covalently anchor molecular complexes onto a conductive mesoporous carbon support.3 This has been achieved through a lithiation strategy to selectively deprotonate defect sites in the graphitic structure of mesoporous carbons, thereby allowing covalent functionalization of the surface at the defect sites. We have developed a new selective surface functionalization strategy for covalently tethering molecular complexes on the surface of ordered mesoporous carbon (OMC) materials. This enables us to introduce unique molecular systems into the fuel cell configurations. Previously, such materials were difficult to prepare due to the limitations of carbon functionalization techniques. The new OMC-support Pt catalysts afford the direct oxidation of methane without poisoning by carbon monoxide adsorption at substantially low temperatures (< 150 °C), leading to the design, fabrication and testing of new low temperature DMEFC.

In this presentation we will show data from attempts to use tethered molecular catalyst in PEM fuel cells and discuss the challenges to integrating such functionalized materials into practical fuel cell electrodes from both from an engineering perspective and the development of new molecular catalysts.

            (1)       Niedrach, L. W. J. Electrochem. Soc., 1962, 109, 1092.

            (2)       Ferrell III, J. R.; Sachdeva, S.; Pez, G.; Gopalakrishan, G.; Koh, C. A.; Herring, A. M. j. Electrochem. Soc. 2012, 159, B371.

            (3)       Joglekar, M.; Pylypenko, S.; Otting, M. M.; Valenstein, J. S.; Trewyn, B. G. Chem. Mater. 2014, 26, 2873.