Directly Grown Aligned Carbon Nanotubes As a Novel Microporous Layer for Proton Exchange Membrane Fuel Cells

Abstract

Proton Exchange Membrane Fuel Cells (PEMFC) convert electrochemical energy into electricity. Nevertheless, the system still needs to be improved in order to be economically relevant. In that purpose, it is needed to produce more current for a reduced use of materials. Working on developing new materials for PEMFCs, managing both high gas diffusion and adequate water transport, is necessary to achieve these goals. One of the most important materials used in the fuel cell is the gas diffusion layer (GDL). A microporous layer (MPL) is generally deposited on top of the gas diffusion layer to improve gas transport and water management, and thus enhance the cell performances and stability.Carbon nanotubes interest as a MPL or as catalyst support has already been demonstrated in several works [Kannan (2009); Tang (2011); Xie (2015)]. In the literature, most of works about CNTs as microporous layers are using bulk carbon nanotubes integrated in a carbon black MPL. A few works are displacing a different microporous layer structure where carbon nanotubes are obtained by direct growth and are forming a foam around the gas diffusion layer fibers. In this work, a layer made of vertically aligned CNTs was grown in-situ on the fibers of a gas diffusion layer by a hot filaments assisted chemical vapor deposition (HFCVD). It has been possible to successfully grow aligned CNTs on different commercial supports. Functional properties of the GDL with CNTs were characterized with SEM imaging, contact angle and electrical conductivity measurements. A variety of growth time, filaments temperature, chamber temperature and gas flows have been tested. These parameters influence the carbon nanotubes length, thickness, and the CNTs forest density. At last, electrochemical characterizations were conducted in a differential single cell. This cell allows a better homogeneity of flows on the surface. Several cell configurations were studied: gas diffusion layer with carbon nanotubes on the anode, on the cathode, and on both electrodes, either in dry conditions (no condensed water) or wet conditions (with condensed water). Polarization curves have been measured to study the electrochemical phenomena leading to the variation of performances of the various cell configurations. Impedance spectroscopy has been done to measure the overall cell resistance and its intrinsic composition such as the protonic resistance or the transport resistance.SEM imaging has shown that the CNTs layers are distributed all along the carbon fibers of the GDL, also inside the GDL pores. The overall structure strictly differs from conventional microporous layers as well as other CNT-made MPLs in the literature. 10-25 µm-long multiwall carbon nanotubes with a diameter ranging between 7-10nm were obtained. CNTs as a microporous layer gives better performances (10% in dry conditions, 25% in wet conditions) than commercial MPLs. Contact angle measurements indicates that the obtained gas diffusion media (GDL+MPL) is hydrophilic, which also differs from commercial hydrophobic GDLs. Although carbon nanotubes are individually expected to be hydrophobic, the CNT forest structure and its combination with the former GDL porosity have a different property. The measurement of electrical resistance of the global fuel cell with a commercial MPL or with CNT layers shows that carbon nanotubes don’t provide a better electrical transport.Electrochemical measurements gave access to the fuel cell performance in operating conditions. The results have been compared to the reference SGL 29BC and to the Department of Energy expectations for the fuel cell performances for the next years. In dry state, GDL with CNTs work as well as the reference gas diffusion media. In wet conditions, a dramatic improvement is obtained, especially at low current density. These results are all the more interesting as the gas diffusion layer with carbon nanotubes is hydrophilic, but can compete with the hydrophobic commercial GDLs. The arrangement of carbon nanotubes might also be important as it is aligned in the direction of gas flows and water flows in the fuel cell.In closing, the results of this work show the need to develop nanostructured materials for energy applications. Carbon nanotubes proved their interest as a material for fuel cells. Even with a hydrophilic and a strictly different structure of microporous layer, the fuel cell performances with carbon nanotubes compete with the best commercial reference results. This could lead to a better understanding of the flows phenomena in fuel cells on one side, and of the predominant factors inhibiting a dramatic improvement of the PEMFCs performances. Figure 1