Cathode contamination side reaction

Eykholt GR, Daniel DE. (1994). Impact of system chemistry on electroosmosis in contaminated soil. Journal of Geotechnical and Geoenvironmental Engineering 120(5) 797-815. Franz AJ, Rucker JW, Flora JRV. (2002). Electrolytic oxygen generation for subsurface delivery effects of precipitation at the cathode and an assessment of side reactions. Water Research 36 2243-2254. 

EOF (in this example consisting only of water) is enhanced by diffusion in the same direction due to the existing concentration gradient and enters the cathode chamber at the cathode surface. This may generate problems with contamination and may hinder the desired cathode reaction. Moreover, this may enable or enhance undesired side reactions (a well-known example is the crossover of methanol from anode to... 

Currently, several air-side contamination models have been published in the literature, ranging from simple empirical and adsorption models to general kinetic models. These models have been applied to simulate and predict SO2, NO2, NH3, and toluene contamination. The kinetic model is a very general one based on the associative oxygen reduction mechanism. It takes into account contaminant reactions, such as surface adsorption, competitive adsorption, and electrochemical oxidation, and has the capability of simulating and predicting both transient and steady state cell performance. The model can be applied to other cathode contaminants, e.g., SO2 and NO2. 

St-Pierre (2009) developed a zero-dimensional model that considers competitive adsorption for a contaminant with O2 or H2 at the cathode or anode side, respectively. This model assumes that contaminant transport through the gas flow channels, GDLs and ionomer in the catalyst layers is much faster compared to surface kinetics. The rate determining step is considered to be due to contaminant reaction or desorption of reaction product from the platinum surface. Other model assumptions include the absence of lateral interaction between adsorbates, first-order reaction kinetics, constant pressure, and constant temperature at the cathode/anode sides. Using a set of parameters, St-Pierre (2009) successfully used his model in order to describe experimental transient data obtained in the presence of SOj, NOj, and HjS in the cathode airstreams. 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

A kinetic model that accounts for adsorption and oxidation of contaminants at the cathode side of a PEMFC was developed by Shi et al. (2009). This model is based on five reactions adopted from the literature to describe the mechanisms of the ORR, with the limiting step given below ... 

In the cathode side, continuous acid-base reactions, ionic exchange, and possible dissolution of the transition metal ions also lead to an increase in the impedance (in addition to capacity-fading due to the structural changes [201]). Highly critical in this respect is the contamination level in solutions, especially the HF concentration. 

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