Fuel cell operating conditions humidity

Removing CO by virtue of a catalyst outside the CL [92] or fuel cell itself, for example, adopting a CO oxidation reactor, to remove CO is practical and may bring good result [93,94]. He et al. [95] studied the possibility of integrating a CO removal reactor into a fuel cell stack, for which six typical catalysts were screened under fuel cell operation conditions. Nearly 100% CO conversion was achieved with an Ir/CoO,-Al203/carbon catalyst with good selective oxidation (O2/CO = 1.5) at 76°C and 100% relative humidity. 

The function of the polymeric membrane electrolyte is to permit the transfer of protons produced in anodic semi-reaction (3.11) from anode to cathode, where they react with reduced oxygen to give water. This process is of course essential for fuel cell operation, as it allows the electric circuit to be closed inside the cell. On the other hand, the membrane must also hinder the mixing between fuel and oxidant, and exhibit chemical and mechanical properties compatible with operative conditions of the fuel cell (temperature, pressures, and humidity). 

There are unresolved issues in device models. For example sorption isotherms, the waters per acid group measured at steady state under zero flux conditions as a function of ambient relative humidity and temperature, play a central role in modeling hydration levels in ionomer membranes. However, the sorption isotherms depend sensitively upon pretreatment of the membrane, see [3], and moreover there is no reliable data for hydration levels under the conditions typical of fuel cell operation in which significant water and ion flux pass through the membrane which is under mechanical constraints which impact its ability to swell. 

The electrodes must contain catalytically active materials to catalyze the electrochemical oxidation of hydrogen at the anode and the electrochemical reduction of oxygen at the cathode. These catalysts must be stable under potentials, humidity conditions and pH values encountered during all steady state and transient fuel cell operation phases. 

Tsushima et al. (2010) developed an MRI system to investigate the effects of relative humidity (RH) and current density on the transverse water content profile in a membrane under fuel cell operation at a practical PEMFC operating temperature. The MRI visualization revealed that in dry conditions (40% RH), the membrane hydration X number was 3, and the water content profile in the membrane was fiat because the diffusion process in the membrane was dominant in the water transport. In a standard condition (80% RH) the water content in the membrane was 8, and a partial dehydration at the anode was observed at a current density of 0.2 A/cm, indicating that electroosmosis was influential. At the higher RH level of 92%, the water content X within the membrane at 0.2 A/cm was around 22, corresponding to the eqnilibrium state of the membrane in liquid water, and the water content profile with the increase in current density became fiat. This indicates that the liquid water generated in the cathode catalyst layer permeated the membrane, where water transport plays a more dominant role. 

Abstract The world s first highly durable perfluorinated polymer-based membrane electrode assembly (MEA) for polymer electrolyte membrane fuel cells, under conditions of high temperature and low humidity, has been developed. The newly developed MEA, which is composed of a new perfluorinated polymer composite membrane, reduces the degradation rate to 1/lOOth to l/l,000th of that of the conventional MEA. The new perfluorinated polymer composite MEA can be operated for more than 6,000h at 120°C and 50% relative humidity. 

A 10-kW, hydrogen-air fuel cell operates at 0.7V/cell at 70°C and ambient pressure, with oxygen stoichiometry of 2.25. Liquid water is separated from the cathode exhaust. Ambient conditions are 23°C, 101.3kPa, and 75% relative humidity. Calculate how much water would have to be stored for 7 days operation. Propose a solution to make this system water neutral (support your proposal with calculation showing the resulting water balance). 

Fig. 14.11 Polarization curves of Pt/C (BASF-fuel cell, 20 wt.%) or Pt/SC-CNFs (homemade, 20 wt.%) based MEAs with varying Nafion amount in the cathode. Operating conditions 70 °C cell temperature, 100% relative humidity (RH) anode/cathode H2/02, 200 mLmin-1 flow rate (Reprinted from [140] with permission from Elsevier).

Various research groups have been able to demonstrate that the best PTFE loading in the MPL is around 20 wt% when a fuel cell is operated at fairly high humidity conditions [109,136,137,155,157]. It is important to note that in most cases, at low current densities (<0.2 A cm ), differences due to PTFE... 

One final example of multiple layer MPL was presented by Karman, Cindrella, and Munukutla [172]. A four-layer MPL was fabricated by using nanofibrous carbon, nanochain Pureblack carbon, PIPE, and a hydrophilic inorganic oxide (fumed silica). The first three layers were made out of mixtures of the nanofibrous carbon, Pureblack, carbon, and PTFE. Each of these three layers had different quantities from the three particles used. The fourth layer consisted of Pureblack carbon, PTPE, and fumed silica to retain moisture content to keep the membrane humidified. Therefore, by using these four layers, a porosity gradient was created that significantly improved the gas diffusion through the MEA. In addition, a fuel cell with this novel MPL showed little performance differences when operated at various humidity conditions. 

As briefly mentioned in Section 4.3.S.2, Atiyeh et al. [152] performed water balance measurements and calculations to determine the effect of using DLs with MPLs (on either or both cathode and anode sides). In their fuel cell test station, water collection systems were added in order to be able to collect and measure accurately the water leaving both anode and cathode sides of the fuel cell. Based on the operating conditions (e.g., pressures, temperatures, relative humidities, etc.) and the total amount of water accumulated at the outlets of the test station, water balance calculations were performed fo defermine the net water drag coefficient. Janssen and Overvelde [171] used this method to observe how different operating conditions and fuel cell maferials affected... 

Figure 10. Current distribution (A/m ) in a low humidity fuel cell at 0.65 V or average current density of 1.1 kicw with the same operating conditions as in Figure 9.

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