Reactant Crossover and Internal Currents

In the previous sections, we looked into the losses in the fuel cell potential contributed by the resistance to the reaction kinetics at the cathode and anode (activation losses), resistance to ion or electron transport (ohmic losses), and the mass concentration variation near the electrode (mass transfer losses). In addition to these losses, fuel cells show significant potential losses as a result of a short circuit in the electrolyte and crossover of reactants through the electrolyte. 

Although the electrolyte of a fuel cell conducts mainly ions, it is not completely insulated from electrons. It will always be able to support very small amounts of electron conduction. This electron conduction in electrolyte or internal current is a net loss of current to external load. In a practical fuel cell, some reactants will diffuse from one electrode to another through the electrolyte where it will react without external electron transfer. 

Consider a fuel cell with anodic and cathodic reactions, 

For example, in PEMFC, hydrogen from the anode can diffuse through its electrolyte to the cathode and will undergo oxidation on the Ft catalyst with oxygen electrochemically. The crossing over of one hydrogen molecule from 

Schematic of oxidation and reduction reactions and the resulting external current and internal loss currents. Note that the internal loss current is the sum of the current caused by anode crossover, cathode crossover, and electrical short. 

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The fuels crossover and internal currents are equivalent that is, they both contribute voltage loss owing to a small equivalent cell current. However, fuel crossover and the internal cmrents have a different physical effect on fuel cell. In the internal current, the oxidation reaction has already taken place and the electrons are short-circuited through electrolyte. In case of fuel crossover such as hydrogen permeation from the anode to the cathode, first the fuel crosses over from the anode to the cathode and then oxidation and reduction reactions occur near the cathode. With reactant crossover and internal currents, a small amount of current is lost. In both cases, the current losses are similar to activation losses, and hence as an approximation, the current and potential behavior can be represented by the Tafel law. 

The response of the fuel cell is determined by the electrochemical processes and associated kinetics at the electrode and electrode interface. The electrochemical processes depend on the mass and charge transfer between the bulk electrolyte solution and electrode surface. The rates at which these transfers occur are determined by the number of localized phenomena and largely depend on the materials involved. These processes are presented in this chapter and the relations between the fuel cell potential and current density are given in terms of BV and Tafel equations. The key losses in the fuel cell include the activation losses, ohmic losses, mass transport losses, and losses owing to reactant crossover and internal currents that are discussed in this chapter. The fuel cell polarization curve is presented and is discussed for low-temperature and high-temperature fuel cells such as PEMFC and SOFC, respectively. 

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