Investigation of components for fuel cells: GDL, polymer membranes and bipolar plates on a nanometric scale with the aid of AFM
Fuel cell stacks consist of the so-called membrane-electrode assembly (MEA), the gas diffusion layers flanking both sides, with so-called micro-porous boundary layers (MPL) where necessary, and the metal bipolar plates to connect the individual cells to the stack. The MEA, the heart of the fuel cell, consists of the solid electrolyte, e. g. Nafion, and the two electrodes, in which the electrochemical reactions take place, thus generating the usable potential difference.
Apart from measuring the conductivity in contact mode, two new techniques are used, both in tapping mode, for the analysis of the surface and its chemical-physical properties:
1. With the “HarmoniX” mode, not only the surface structure but also the local adhesion, the energy loss, the phase damping of the oscillation, the maximum forces and the stiffness are measured. The method is based on the calculation of the local force-distance curve with the aid of a Fourier synthesis of the higher-harmonic oscillations when the tip is excited for each measurement spot of the image.
2. In “QNM” mode, the sample holder vibrates at 2 kHz and thus determines the mechanical properties. Topography, adhesion, deformation and stiffness can be measured with high resolution. The use of a conducting AFM tip additionally allows the local conductivity to be measured.
Surface, phase, adhesion, dissipation and stiffness of a GDL produced as part of a student project, measured with HarmoniX-mode, size: (1 x 1) µm²
One of the most important components of PEM fuel cells is the ion-conducting solid-state electrolyte membrane, which is typically made of perfluoro-sulfonic acid (usually Nafion (Dupont)). High ion conductivity and homogeneous electrochemical activity across the whole area of the membrane is desirable for an optimum performance. The local, unequal distribution of voltage, current density, temperature and gas composition leads not only to reduced efficiency, but also to a strong thermo-mechanical loading of the materials and thus also has a negative impact on the reliability and ageing resistance of the cells.
Figure 2 illustrates surface and conductive areas, measured in QMN mode with conducting tip, on a membrane of a commercial half-MEA manufactured by Ion Power (Nafion NR 112).
Surface and current image of a Nafion NR 212 membrane (Ion Power, measured with QNM at 30°C and RH=75%, max. current I=200 nA at U=1.2 V, size of original: (3 x 3) µm²
Bipolar plates must be electrically conductive and are often made of rustproof stainless steel. The corrosion behaviour in long-term operation is likewise very important for the service life. Evaluating the surface structure with the AFM makes it possible to calculate RMS values to determine the roughness. Corroded bipolar plates exhibit much greater roughness than new samples before they are put into operation.
Bipolar plate, left surface, right phase image, size (5 x 5) µm²
Gas diffusion layer (GDL)
The gas diffusion layers flank the electrodes, and coat the electrolyte membrane. Their task is to distribute the gas to the electrodes, conduct the current, transport water to and from the electrodes and the membrane, drain the heat of reaction, and protect the electrolyte membrane from mechanical damage. They consist mainly of carbon and Teflon (PTFE) and have a porous structure. The addition of PTFE serves to make the inner channels hydrophobic and is important for the water budget and thus the efficiency of the cells. A reduction in the PTFE content while the fuel cell is in operation worsens its performance and can be used as a measure for the degradation of the cell. HarmoniX measurements were used to perform statistical evaluations of the post-operation change in adhesion and dissipation as a measure for the reduction of the PTFE content on fuel cells operated in different modes.
Comparison of two micro-porous layers (MPL) of gas diffusion layers before and after being used in operation on anode and cathode and an artificially aged MPL, measured under dry as well as under wet conditions. The points are mean values from the histograms of the relevant measurands of 1 µm images. The green dot represents the reference measurement on pure (100%) PTFE.