Technology to quantify the mechanical strength of fuel cell electrodes for vehicles

Mechanical stability, chemical stability and thermal stability are important considerations for the durability of fuel cell membrane electrode assemblies. Quantifying the mechanical strength of the catalytic layer can provide important support for the development of highly durable CCM.

The key components of the proton exchange membrane fuel cell monomer are CCM, gas diffusion layer, electrode plate and seal. Among them, the catalytic layer of the vehicle fuel cell CCM is composed of a Pt or Pt alloy catalyst, a supporting carbon carrier and an ionomer. The mechanical strength of the proton exchange membrane fuel cell electrode (or catalytic layer) is mainly determined by the ionomer. The ionomer is not only an adhesive connecting the catalyst, but also a proton conducting carrier between the proton membrane and the active points of the catalytic layer. The durability of the electrode is an important part of the durability of the fuel cell. Due to the influence of external stress, the porous structure of the catalytic layer is easily deformed and destroyed. Therefore, understanding the mechanical strength of the catalytic layer in membrane electrodes is of great significance to the development of high-durability electrodes.

Generally, the durability verification test of newly developed electrodes is time-consuming and costly. In particular, fuel cells frequently encounter performance degradation under start-stop, cold start, and dry/wet cycle conditions. Among them, the freezing of water in a low temperature environment below zero will accelerate the attenuation of the electrode. Since the cohesive strength of the catalytic layer and its interfacial adhesive strength with the proton membrane are small, the multiple cycles of freezing/melting during low-temperature startup will cause the interface between the catalytic layer and the proton membrane to delaminate . However, it takes up to 6 months to complete the durability test of a membrane electrode sample icing/melting cycle, which is less acceptable for the development of new electrodes. Therefore, quantifying the cohesive force (or internal stress) and interface adhesion of the catalytic layer is of great reference value for the efficient development of highly durable electrodes. Therefore, in order to quantify the mechanical strength of fuel cell electrodes for vehicles, Hyundai Motor Company has proposed a new technology based on the Surface and Interface Cut Analysis System (SAICAS), which can be used for research on cohesion and adhesion.

The Surface and Interface Cut Analysis System (SAICAS) is a multi-purpose tool for measuring the mechanical properties (cohesion and adhesion) of composite materials with thicknesses ranging from a few μm to several hundred μm. Therefore, SAICAS has been widely used in composite electrodes of lithium-ion batteries. However, the fuel cell membrane electrode is porous and fragile. It is difficult to process CCM with the existing SAICAS technology. The difficulty lies in the control of the blade depth during the cutting and peeling process. In addition, sample preparation is also critical to the reliability of the measurement. Because the SAICAS method fixes the sample through the vacuum work table (negative pressure produces "adsorption force" to adsorb the sample to the table), it is difficult to fix the CCM characteristics of the porous medium, and the CCM sample is easy to slip.

In this study, Hyundai Motor Company placed the CCM sample on a hard PET film coated with an adhesive at room temperature at a pressure of 0.7 MPa for nearly 4 s. The CCM sample can be successfully attached to the sticky PET film. So as to produce SAICAS test and obtain reliable data. It should be noted that, as a mechanical support carrier for CCM, since the proton membrane is soft and flexible, the SAICAS method requires strict consideration of the rake angle.

For composite electrodes of lithium-ion batteries, a 20° rake angle is usually used in the SAICAS test. However, for the fuel cell CCM with flexible proton membrane, the study found that the 20° rake angle is too vertical to guarantee a reliable SAICAS test. Therefore, Hyundai Motor Company increased the rake angle from 20° to 40° in the SAICAS test, so that the incident angle of the micro blade on the CCM surface is smaller, as shown in the figure below. In addition, the research also optimized the moving speed of the micro blade in the vertical/horizontal direction, where the vertical speed was changed from 0.01 to 0.2 μm/s, and the horizontal speed was changed from 0.1 to 2 μm/s. In the end, Hyundai successfully tested the cohesion and interface adhesion of CCM.

References: Byun S, Yu J H, Choi J, et al. Unraveling the cohesive and interfacial adhesive strengths of electrodes for automotive fuel cells[J]. Journal of Power Sources,2020,455.