Investigation of PEMFC Stack Operating at Elevated Temperature

Proton exchange membrane fuel cell (PEMFC) operating at elevated temperature has many advantages compared to low temperature prototypes. With the adoption of short side chain, low equivalent weight (EW) perfluorosulphonic acid (PFSA) membrane, a 100W PEMFC stack was built and evaluated at an elevated temperature of 95°C. For the purpose of studying the effect of anode relative humidity (RH) on fuel cell performance, the stack was tested with (1) 60% RH air and 70% RH H2, (2) 60% RH air and dry H2, respectively under different temperatures. Furthermore, the CO tolerance of the stack was investigated at low and elevated temperatures in the presence of 5ppm, 10ppm and 20ppm CO/H2, respectively. The results demonstrated that the stack performance with dry H2 was more than 30% inferior to that with H2 of 70% RH, and the cell consistency deteriorated under higher temperature and lower anode RH. The polarization curves of the stack employed fuel rich in CO indicated that elevating the operating temperature to 95°C could improve the anode catalyst tolerance to CO, and the CO poisoning effect on cell polarization is slight at low current densities.


Introduction
Proton exchange membrane fuel cell (PEMFC) usually works at 60-80℃.Some issues existing at low operating temperature can be alleviated by elevating the temperature [1][2][3][4][5].Elevated temperature is able to enhance cathode kinetics, simplify water and heat management, as well as mitigate the poisoning effects of CO and other impurities on Pt catalyst.Therefore, PEMFC operating at elevated temperature becomes a hot research topic worldwide at present, which may promote the commercialization of PEMFC and its application in transportation.
For high-temperature (HT) PEMFC working above 100℃, the key components are membrane, catalyst layer, membrane electrode assembly (MEA) and flow fields.Conventional perfluorosulphonic acid (PFSA) membranes will dehydrate and their conductivity decreases dramatically at high operating temperature, so it is of great importance to develop polymer membranes capable of retaining high proton conductivity and possessing chemical and structural stability at elevated temperature.These developments could be classified into three groups [6][7][8][9]: (1) modified PFSA membranes, which incorporate hydroscopic oxides and solid inorganic proton conductors; (2) sulfonated polyaromatic polymers and composite membranes, such as PEEK, SPEEK, and PBI; (3) acid-base polymer membranes, such as phosphoric acid-doped PBI.There are two conventional approaches to prepare catalyst layers and MEAs, of which the catalyst-coated membrane (CCM) method is generally used because catalyst layer can be made thinner and catalyst loading can be reduced.The design of flow fields is significant, since its configuration and dimensional parameters markedly impact PEMFC performance.Up to now, many kinds of flow fields have been developed by researchers for HT-PEMFC.
Compared with high-temperature prototypes, the PEMFC operated at 90-95℃ provides a beneficial choice.On the one hand, under these temperatures, the rigid material requests for membrane, catalyst, gas diffusion layer (GDL) and gasket under temperature higher than 100℃ can be avoided.On the other hand, the shorter side chain membranes, such as Dupont's Nafion ® -DFC1 and Solvay's Aquivion™ E79-03S are available on the market, which can be used up to 100℃ [10].
In this contribution, with the adoption of short side chain, low equivalent weight (EW) PFSA membrane (Dongyue), the operating temperature of a 100W PEMFC stack was elevated to 95℃ to evaluate the stack performance and tolerance to CO.

Stack preparation
The active area of each MEA was 50 cm 2 .The catalyst ink was prepared by mixing 40%Pt/C (Johnson Matthy, JM) with a solution of 5 wt% Nafion ® (Dupont) and isopropanol, then sonicated for 4 hours.The ratio of Pt/C catalyst to Nafion ® was 3:1.The catalytic ink was sprayed directly to the membrane at 100℃ to form the catalyst layer giving a Pt loading of 0.3 and 0.4 mg cm -2 for anode and cathode, respectively.This was followed by just physically placing gas diffusion layers (SGL SIGRACET ® 25BC carbon paper) onto the anodes and cathodes, without a hot-pressing process.The 100W PEMFC stack consisting of 5 cells (Fig. 1) was fabricated employing bipolar plates with single serpentine channel as the anode flow field and four serpentine channels as the cathode flow field.

Test conditions
Through the control of back pressure valves, the pressures for anode and cathode were set at 0.4bar and 0.3bar, respectively.For stack humidification, bubbler humidifier and membrane humidifier were adopted for H 2 and air, separately, and the stoichiometric ratios of fuel and oxidant were maintained at 1.4 and 2.5, respectively.Relative humidity (RH) was tested with a humidity sensor (HMT330, VAISALA).
To investigate the CO tolerance of anode catalyst under elevated temperature, the stack was operated at 80℃ and 95℃, respectively.High purity H 2 (99.99%) with different CO concentration, i.e. 5ppm, 10ppm and 20ppm, was fed to the stack.N 2 was introduced into the anode at the end of each test to remove the CO remaining in the stack.

Impact of anode relative humidity on stack performance
Fig. 2 presents the polarization curves when 60% RH air and 70% RH H 2 are employed under different temperatures.It can be seen that the stack demonstrates good performance at elevated temperatures.The peak cell power density (PCPD), 476.4 mW cm -2 , can be reached under 90℃ at 1000 mA cm -2 .In addition, the PCPD can still achieve 453mW cm -2 under 95℃.This temperature-related phenomenon is distinct from that with the commonly used Nafion ® 212 as the electrolyte (figure not provide here), where no stable voltage could be reached at current density higher than 1000 mA cm -2 under 85℃.
The performance of single cells is illustrated in Fig. 3. Preferable cell consistency is observed under low temperatures, i.e. 80℃, and the uniformity of cell performance suffers recession with elevating temperature.This may be attributed to the aggravation of non-uniform temperature distribution in the stack, which leads to diverse performances of different cells.
The polarization curves of the stack supplied with 60% RH air and dry H 2 is showed in Fig. 4. From the figure, it is evidently that the stack performance declines with dry H 2 .The PCPD under 90℃ is only 319.2 mW cm -2 , which is 157.2 mW cm -2 lower than that with 70% RH H 2 , and the attenuation ratio is 33%.Similar result could be obtained under 95℃, when the attenuation ratio reaches 36.9%.These results Current density / mA cm -2

CO tolerance of the stack under different temperatures
The stack tolerance to CO in anode stream was investigated at both low and elevated temperatures.The stack was operated at 600 mA cm -2 for the duration of 180 mins at the given temperature, while polarization curve tests were taken before and after each test.
The polarization curves of the stack with different CO/H 2 concentrations are proposed in Fig. 6.In the presence of 5 ppm CO/H 2 , the PCPD at 80℃ decreases from 473 mW cm -2 to 330 mW cm -2 , and the attenuation rate is 30.23%.While at 95℃ the PCPD drops from 458.6 mW cm -2 to 406.1 mW cm -2 and the attenuation rate is 11.45%,only 38% of that at 80℃.Similar results are noticed with 10 ppm and 20 ppm CO/H 2 , when the PCPD attenuation rates at 95℃ are 56% and 72% of those at 80℃, respectively.
The results demonstrate that elevating the operating temperature can effectively enhance the CO tolerance of PEMFC, for the CO absorption on Pt is an exothermic process, which means that the absorption favors low temperature, while increasing the temperature will have negative effect.Compared with CO, H 2 adsorption is less exothermic, and operation at elevated temperatures leads to a beneficial shift towards higher H 2 coverage at the expense of CO coverage.
As shown in Fig. 6, when the stack works at low current density, especially below 200 mA cm -2 , the effect of CO on cell polarization is insignificant; yet the polarization aggravates with increasing current density.The reason may be that CO is absorbed on Pt surface and forms Pt=CO bond, which leads to inadequate active sites for H 2 oxidation reaction, especially at higher current density.However, no severe polarization will be caused at low current density (≤200 mA cm -2 ), since smaller active surface area is needed.

Fig 2 :
Fig 2: Performance of the stack with 60% RH air and 70% RH H 2

Fig. 5
Fig. 5 demonstrates the performance of single cells with 60% RH air and dry H 2 under different temperatures.Compared with Fig 3, the deterioration in single cell consistency can be observed.The phenomenon is probably due to the dehydration of membrane with dry H 2 and the uneven distribution of water inside the stack, which may seriously impact the proton conductivity of the membrane, thus results in discrepant performances of different cells.

Fig 4 :
Fig 4: Performance of the stack with 60% RH air and dry H 2

Fig 5 :
Fig 5: Performance of single cells under different temperatures with 60% RH air and dry H 2

Fig 6 :
Fig 6: Polarization curves of the stack at 80℃ and 95℃ with different concentrations of CO/H 2 , (a) 5ppm, (b) 10ppm, (c) 20ppm Moreover, elevating the stack operating temperature could improve the anode catalyst tolerance to CO.The attenuation rate of peak cell power density with 5ppm CO/H 2 at 95℃ is 38% of that at 80℃, and the same trend is observed in the presence of 10ppm and 20ppm CO/H 2 .This is because the CO absorption on Pt catalyst is associated with negative entropy, and temperature rising interferes with CO absorption, consequently alleviates deterioration of fuel cell performance.The polarization curves illustrate that the CO poisoning on cell polarization is slight at low current densities but aggravated with growing current densities due to inadequate HOR active sites.Daijun Yang received Ph.D. degree in environment engineering from East China University of Science and Technology, China, in 2006.His research interests are fuel cell technologies.Jianxin Ma was awarded a Ph.D. degree in chemical engineering from Clausthal University of Technology, Germany, in 1989.Now he works as a professor in Tongji University.His research interests are production, storage and charging of hydrogen and fuel cell technologies.