Higher Efficiency with Lower Platinum Loading: A Systematic Study of Nano Porous Solid Acid Fuel Cell Electrodes
Authors: Wagner Maximilian 
Published: February 2020

The performance limiting and cost-driving component of solid acid fuel cells (SAFCs) is, similar to other low and intermediate temperature fuel cells, the electrode. 1 Much effort has been spent on optimizing the electrode structure to obtain acceptable levels of mass normalized activity of the state-of-the-art catalyst, namely platinum.2-4 Effectively scalable techniques, such as spraydrying and electrospray deposition, have been used to create porous nanostructures with co-deposition of catalyst particles or subsequent deposition of catalyst thin films via sputtering or chemical vapor deposition.  

For this work, we use spraydrying to fabricate nanostructured, CsH2PO4 based, SAFC electrodes, directly on pre-pressed electrolyte (CsH2PO4) and stabilized with the polymer PVP. Magnetron sputtering was used to deposit thin-film Pt as the electrocatalyst, thus creating a symmetric electrochemical cell (Pt thin-film + CsH2PO4
porous ½CsH2PO4½CsH2PO4
porous
+ Pt thin-film), figure 1.

The layer size of the porous structure was varied systematically between 0 and 16 µm while the Pt film thickness was kept constant at 30 nm in order to determine the optimal amount of deposited nanostructure. AC impedance spectroscopy is used to evaluate the electrode performance in a humidified hydrogen environment. Analyzing the time dependent electrode impedance gives us an insight to the effect of the used stabilizing polymer polyvinylpyrrolidone (PVP) as well as to long term stability.

The optimal electrolyte layer size was determined to be ~5 µm with a Pt loading of 0.064 mg/cm2 and electrode impedance of ~0.8 Ω cm2, resulting in a Pt mass normalized activity of 20 S/mgPt. This represents a five-fold increase of the mass normalized activity compared to a flat platinum thin film of the same thickness. A simple equivalent circuit model consisting of two “parallel R-Q circuits” connected in series was used to describe the a) electrode reaction and b) the ion transport through the stabilizing PVP skin formed around the electrolyte nanoparticles. Remarkably, the latter R-Q circuit shows an apparent decreasing size with time, indicating a removal of the stabilizing polymer from the boundary of the electrolyte nanoparticles even though overall structural stability remains largely unaffected.

The results of this work allow significant improvements of the synthesis and geometry of solid acid fuel cell electrodes and shed light on the long-term stability as well as the challenges arising from using a stabilizing polymer.

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Figure 1