Visualization of the Ionomer Distribution in Catalyst Layers by Atomic Force Microscopy

Kurzfassung

The structural composition of the catalyst layer in fuel cells and electrolyzer has a crucial impact on the performance and stability during operation. Made of the catalyst material itself and an ionomeric binder combined to a porous assembling, the catalyst layer is the place where the electrochemical reaction takes place. This requires the conductance of electrons and ions as well as mass transport of educts and products to and from the reactive catalytic centers, respectively. The ionomer enables the conductance of cations (usually H+ in proton exchange membrane technologies) or anions (usually OH- in anion exchange membrane technologies) where a homogenous distribution of the ionomer around the catalytic centers is expected to result in an effective process. In contrast, either too thin or too thick coverages might cause high ionic conductance or mass transport resistances [1, 2]. Thus, structural investigation of the ionomer distribution complements the evaluation between preparation of the catalyst layer and electrochemical characterization and thus allows performance optimization of fuel cells and electrolyzers. In general, visualization of the ionomer in the catalyst layer is technically challenging, but recently the feasibility of materials-sensitive atomic force microscopy (AFM) was demonstrated to analyze the ionomer distribution in fuel cell and electrolyzer [3,4]. AFM analyzes the catalyst layer under ambient conditions and do not require vacuum conditions or high radiation that might harm the ionomer, but also close to fuel cell and electrolyzer conditions can be resembled. The different nanomechanical properties of the ionomer and the catalyst result in material contrast in adhesion force images enabling the visualization of the ionomer distribution (c.f. Figure). In this contribution, we give an overview how the AFM technique can be used for different applications in fuel cell and electrolyzer research. Thus, the average ionomer layer thickness is analyzed depending on the ionomer loading in the catalyst layer and measurement conditions like elevated temperature and defined relative humidity are varied. Later allows the insights to occurring ionomer swelling which might have an impact on the performance and stability during operation.