Cathode/electrolyte interface cell voltage

The reduction of the current flowing along the cell is furtllcr shown in Fig. ure 4.27, where the current distribution along the cathode-electrolyte interface is depicted, when the cell voltage is 0.7. 

Lithium bis(oxalato) borate was investigated as an additive for the stabilization of a high-voltage cathode electrolyte interface (57). It has been found that the electrochemical performance of Li/LiNio.5Mni.504 cells with a hthium bis(oxalato) borate additive was improved at 60°C. 

For simple calculations of the electric current, in the simplest case, a ladder circuit model is sufficient for the simulation. A discrete anode/electrolyte/cathode unit shown in Figure 10.4 is employed for the modeling. The direction of the electric current path in the electrolyte is vertical to the interfaces between the electrolyte and electrodes, and the in-plane path is treated as negligible. The electric current I, in the /th discrete cell is determined by the operating cell voltage Voeii as follows ... 

Electron Transfer in Electrochemistry. In electrochemical cells electron transfer occurs within the electrode-solution interface, with electron removal (oxidation) at the anode, and with electron introduction (reduction) at the cathode. The current through the solution is carried by the ions of the electrolyte, and the voltage limits are those for electron removal from and electron insertion into the solvent-electrolyte [e.g., H20/(H30+)(C10j ) (Na )(-OH) ... 

To develop any electrochemical process, a voltage should be applied between anodes and cathodes of the cell. This voltage is the addition of several contributions, such as the reversible cell voltage, the overvoltages, and the ohmic drops, that are related to the current in different ways. One of these contributions, the overvoltage, controls the rate of the transfer of electrons to the electrochemically active species through the electrode-electrolyte interface when there is no limitation in the availability of these active species on the interface (no mass-transfer control and no control by a preceding reaction). In this case, the relationship between the current that flows between the anodes and the cathodes of a cell and the overpotential is... 

Figure 3.4. Photographs of the STR PEM fuel cell. The anode and cathode gas plenums with the pillars are easily seen. There are no horizontal surfaces for water to accumulate in the gas plenums, so the water drains and does not hinder gas diffusion to the electrode/electrolyte interface. The photograph to the right shows the overall fuel cell including the temperature and mass flow controllers. Everything is connected to a computer that continuously records the cell temperature, the gas flows to the anode and cathode, the relative humidities in the anode and cathode effluents, the current through the circuit and the voltage across the external load.

The cell voltage Ucell is defined as the potential difference between the cathode and the anode. It is usually measured during fuel-ceU operation. The potential difference between the electrode and the electrolyte, which is caUed the anode or cathode potential in the following, is responsible for the electrochemical reaction occurring within the catalyst layers but cannot be measured directly. In the further text, we use electrolyte and membrane as equivalent expressions. While the electrode potential can be sensed from the bipolar plates, it is not feasible to sense the membrane potential directly, since each measurement equipment forms an interface between the membrane and the metal contact Two methods for the installation of a reference electrode within the ceU have been discussed in the Hterature, namely the reverse hydrogen electrode (RHE) [18] and the dynamic hydrogen electrode (DHE). In addition to ceU internal methods, a conventional... 

Each of the two electrode reactions creates a characteristic potential difference across the interface solid electrode/electrolyte, which is different for the two reactions according to the different reactants. The overall cell voltage between the two electrodes, which are joined by the same electrolyte, allows the electrons generated at the anode (HOR) and consumed at the cathode (ORR) to create work in the external circuit. Hence, chemical energy released by the individual electrode reactions at the locally separated electrodes is directly transferred into electrical energy. This pathway is different from the combustion step in the classical thermomechanical power generation, where the oxidation of fuel and reduction of oxidant occur in the same volume element, thereby generating heat only. 

Moreover, when a current density j is provided by the cell its voltage U(j) decreases greatly. In the first approximation, these effects result mainly from three limiting factors the charge transfer overpotentials rja and T c at the anode and at the cathode, respectively, due to reaction rates of the electrochemical processes, the ohmic drop Rej in the electrolyte and interfaces, and mass transfer limitations for reactants and products. The cell voltage can then be expressed as follows ... 

The performances of the Nafion and the hydroxide-ion-conduction membrane cells were compared for two cathode solutions (1) D1 water (18 Mil cm) saturated with carbon dioxide by continuous bubbling of the pure gas at a pressure of 1 atm and (2) 1 M sodium bicarbonate solution. As shown in Figure 10.13, the cell voltages with the hydroxide-ion membrane were found to decrease substantially upon changing the electrolyte from C02-saturated D1 water to 1 M sodium bicarbonate. However, the performance of the Nalion-based cell did not alter significantly for the same change in the cathode solution. This phenomenon can be ascribed to differences in ionic contact at the electrode/manhrane interface, as hydroxide-ion membrane is cross-linked and could not be hot pressed to improve the ionic contact. Thus, the use of a liquid electrolyte could substantially improve the ionic contact between the catalyst layer and the hydroxide-ion-conduction manbrane. 

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