Due to high power density and superior efficiency, polymer electrolyte membrane fuel cells (PEMFC) are believed to play a significant role for carbon dioxide emissions free electrical energy systems in the future. Unlike in Carnot processes, chemical energy in the form of hydrogen and oxygen is converted directly into electrical energy without a further process step. One issue in the development of PEMFCs for mobile or stationary applications is the utilization of rare and expensive catalyst material like platinum within the membrane electrode assembly (MEA) see figure 1. In addition, the objective is to reduce production costs and to increase the lifetime of PEMFC. One approach to improve PEMFCs is the development of intelligent electrode architectures. However, cost effective high performance materials are necessary to reach the development targets.
A large electrochemical active surface area (ECSA) for catalyst particles is necessary, in order to achieve high power density in PEMFCs. Therefore, in state of the art PEMFC electrodes carbon supported catalyst nano particles are structured at the interface between the electrolyte membrane and the porous electrode material. Typical carbon support materials involve among others carbon blacks, carbon nano tubes, carbon nano fibers and graphene. Material stability is primordially needed for long-term fuel cell operation. Therefore, the corrosion resistance of the graphitic material plays an important role, which is strongly dependent on graphitization degree of carbon support material. Furthermore, a large specific surface area of the carbon support helps to achieve high ECSA.
The Westphalian Energy Institute (WEI) in Gelsenkirchen, Germany develops low-cost PEMFC electrodes by utilizing graphene related materials (GRM) which are produced in an industrial scale by Advanced Carbon Materials (ACM), which is a department of Grupo Antolin Ingeniería SA in Burgos, Spain. Grupo Antolin Carbon Nano Fibers (GANF) from ACM have small diameter of about 50 nm with excellent aspect ratio and highly graphitic structure (graphitization degree ranging from 70 to almost 100 %, depending on the production process). Due to the near ideal parallelism of graphene layers on the surface of GANF, electrical and thermal conductivity is excellent in comparison to typically used carbon blacks. Furthermore, the high graphitization degree improves material stability in fuel cell environment, which makes the utilized material ideally suitable for application in PEMFCs.
Figure 1 depicts the MEA of a PEMFC with its three main components anode, polymer electrolyte membrane and cathode. The electrodes consist of at least to layers, a macro porous gas diffusion layer (GDL) and a catalytic layer, which contains the noble metal catalyst.
Figure 1. Schematic cross section of a MEA for PEMFCs
At the WEI a novel architecture for catalytic layers has been investigated based on GRM from ACM as support material for platinum nano particles. Electrodes have been manufactured by spray coating GANF dispersions on a GDL, followed by a pulsed electro deposition of catalyst material (compare to figure 1 (enlargement)). Due to the subsequent deposition of platinum only on top of this GANF layer (less than 5 µm thickness), catalyst localization is optimized in comparison to typical electrode structures. In figure 2 a microscopy image of the achieved GANF based catalytic layer is presented.
Figure 2. Micrograph of Pt nano particles supported on GANFs
PEMFC in-situ tests of the novel electrodes result in an increase of platinum utilization. Furthermore, higher material stability in acidic fuel cell environment was confirmed. From laboratory analysis it may be concluded, that according to the worked out manufacturing route, noble metal content in PEMFC electrodes can be significantly decreased whereby lifetime can be extended. Further optimization is expected by the deposition of platinum alloys, which have higher catalytic activity than pure platinum. Based on the laboratory preparation route, the Westphalian Energy Institute targets to develop an industrial scale production for low-cost PEMFC electrodes. A roll-based production line is imaginable, which supports a consequent assembling of membrane with the electrodes by a roll-to-roll process. Additional cost-savings are expected, which may help to establish competitive components for prospective PEMFC applications.
By Prof. Dr. Michael Brodmann, Vice President for Research and Development Director of the Westphalian Energy Institute, and Dr. Ulrich Rost, Team Leader Hydrogen Energy Systems of the Westphalian Energy Institute