Experimental Study of a PEMFC System with Different Dimensions of the Serpentine Type Channels from Bipolar Plate Flow Fields

Adriana ANDRONIE*, Ioan STAMATIN**, Valentin GIRLEANU***, Viorel IONESCU****, Nicolae BUZBUCHI***** *University of Bucharest, Faculty of Physics, 3Nano SAE Research Center, Bucharest, 077125, Romania, E-mail: adriana.andronie@gmail.com **University of Bucharest, Faculty of Physics, 3Nano SAE Research Center, Bucharest, 077125, Romania, E-mail: istarom@3nanosae.org ***University of Bucharest, Faculty of Physics, 3Nano SAE Research Center, Bucharest, 077125, Romania, E-mail: valentin.rata@ymail.com ****Department of Physics and Electronics, Ovidius University, Constanta, 900527, Romania, E-mail: ionescu.vio@gmail.com ****Doctoral School of Mechanical Engineering and Mechatronics, Maritime University, Constanta, 900663, Romania *****Department of Engineering Sciences, Maritime University, Constanta, 900663, Romania, E-mail: nicolae_buzbuchi@yahoo.com


Introduction
Proton exchange membrane fuel cell (PEMFC) represent at this moment one of the most attractive clean energy source for automotive applications, being, for example, a fundamental component of the Fuel Cell Hybrid Electrical Vehicle(FCHV) with an energy system containing mainly a PEM fuel cell stack and a battery/ultracapacitor storage [1,2].PEMFC present some very interesting characteristics, like low-temperature operation (50-100C), high power density (40% -60%) and nearly zero pollutants compared to conventional internal combustion gasoline vehicles [3,4], water being the only by-product obtained from the electrochemical reaction at cathode.
The bipolar plates with special flow field patterns represent one important component of a PEM fuel cell.The effect of the various flow channels design on the PEMFC performance was intensively investigated in different numerical and experimental studies.Experimental tests performed in order to compare the PEMFC performance of four types of flow field designs: serpentine, parallel, pin, and interdigitated showed that serpentine configuration for the flow channels conducted to the highest output power of all [5,6].The serpentine flow field can offer an efficient water removal(preventing water flooding) with a moderate pressure drop along the channels, presenting also a good humidity in the cathode channels in order to avert the electrolyte membrane dehydration [7][8][9].
Many other research studies were oriented on the optimization of cross-section area for flow channels, especially the channel width and the distance between two adjacent channels (channel ribs).The influence of channel to rib width ratio (varied between 0.5 and 2) on the PEMFC electrical performance was experimentally investigated for a serpentine-type flow field and the ratio of unity presented best performances concerning the VI/PI electrical curves [10].For this study, the active area of the flow field was 5 cm 2 , with MEA based of Nafion 117 electrolyte membrane and CL having Pt loading of 1 and 0.5 mg/cm 2 on anode and cathode, respectively.
A numerical CFD study of a PEMFC system, based on a serpentine flow field with 25 channels and active area of 26 cm 2 indicated an optimal channel to rib width ratio of 0.25 [11].Another numerical study for PEMFC with a serpentine flow field of 25 cm 2 active area, performed with CFD Fluent 14.0 software and with Taguchi method integrated in MINITAB 17 software for design optimization relieved and optimum channel to rib width ratio of 1 [12].
Despite the large number of numerical investigations reported already for the flow field design in PEMFC, more experimental studies are needed in order to establish an optimal relation between channel and rib widths, generally accepted for a future development of bipolar plates.
The effects of backpressure on the performance of the PEMFC have been studied recently through experiment and by theoretical modeling.Increasing levels of backpressure resulted in increases in the exchange current densities of the electrochemical reactions, with the increase in the reactant partial pressures and determine an enhancement of the fuel cell performance by increasing the oxygen and hydrogen diffusion to the active sites of the catalyst layer (CL) [13,14].Higher local current densities at CL surface will be generated when pressure inside the PEMFC cell is increased [15].
In this experimental study we investigated the effect produced by three flow fields with serpentine-type channels having different rib to channel width ratios (1, 1.05 and 1.2) on VI/PI electrical curves and on the thermodynamic efficiency of the PEMFC unit cell.In order to establish an optimal flow field for the fuel cell working in different pressure conditions, the backpressure of the test system was modified at two different values (70 kPa and 30 kPa).

Experimental set-up
Fuel cell performance at different backpressures was measured using a BEKKTECH BT-552 PEMFC test station (based on a single PEM fuel cell unit) in the laboratory of Fuel Cell and Hydrogen Storage, 3Nano-SAE Research Center, Bucharest.The membraneelectrode assembly (MEA) placed inside the cell was Nafion 212 type and catalyst layers (CL) were characterized by Pt loading of 0.3 mg/cm 2 at anode and 0.6 mg/cm 2 at cathode, with 20 wt% percent of Pt/C catalyst particles.
The experimental set-up presented in Fig. 1 consists of primary components, i.e., Agilent 6060 B 300W electronic load, two MKS RS-485 mass flow controllers, inlet pressure gauges for H2, N2 and air, outlet pressure gauges for H2 and air, two heated/insulated gas lines, H2/N2/air tubes with various on/off valves, temperature controllers and two manually controlled back pressure regulators.The components of the PEM fuel cell unit from Fig. 1 are schematically represented in detail in Fig. 2, were: 1gas inlet tube, 2gas outlet tube, 3end plate, 4sealing gasket, 5 current collector, 6graphite bipolar plate, 7 -Teflon gasket, 8 -Membrane Electrode Assembly(MEA), 9fastening screws.

Fig. 1 BEKKTECH BT-552 PEM fuel cell station used in experiments
Reactant gases are humidified by a humidification system which includes dual temperature controller with output gas line heaters and on/off bypass valves.The relative humidity for H2 and air flows circulating through the cell was controlled by adjusting the dew point temperature of the inlet flows.
Operational parameters like temperature, flow rate, relative humidity of the supplied gases, stoichiometry along with current and voltage were recorded and controlled by using a LabVIEW based control and data acquisition system.
The operational parameters for the PEMFC system, chosen according to the automotive applications are shown in Table 1 and are maintained constant during exper-imental tests.Here, Pref,a and Pref,c are the operating pressures of hydrogen and air at the PEMFC system entrance, before humidification stage.The reference values for the cell temperature, inlet pressure for H2 and air before humidification and the "dew point" temperature (for 80% RH) used in experiments Parameter Value The pressures at the exit of the fuel cell system (backpressures) for the anode and cathode were: The pressure of humidified gases crossing the fuel cell (from entry to exit) will be considered constant during the test period and estimated in the form of an average value, according to the relations [13]: 0.61094 We obtained the following values for anode and cathode pressures: We obtained the following partial pressures for H2 and O2: The dimensions of the three experimentally tested flow field models are presented in Table 2.The purpose of increasing the distance s between the first two flow channels in the gas inlet area was to reduce the pressure drop across the next channels.
Some research studies reported a series of advantages regarding the effect of ratio w/a >1 on the PEMFC performance, like an enhanced mechanical support for membrane and Gas Diffusion Layer (GDL), higher water content in the membrane, improved ion conductivity of the membrane, improved heat transfer from membrane to GDL, uniform distribution of the current density and temperature across the membrane and improved performance at high current densities [17,18].A few drawbacks were also distinguished here: a less efficient removal of water as a reaction product through GDL, higher pressure drop across the flow field, a reduced contact area between reactants and GDL and higher overall voltage losses.So, starting from the optimal ratio w/a = 1 reported from some numerical investigations for serpentine-type flow field in PEMFC [10,12], we considered here another two ratios w/a higher, but close to 1 (Table 2) in order to establish the flow field with optimal water removal capacity through GDL, reduced pressure along serpentine regions and lowest voltage losses.

Results and discussions
In accordance with the test protocol adopted here [19], for each set point of the load current, all recorded values of the voltage Ui along the 9-minute range considered for each constant current density step (385 values) are converted to a mean Uavg.Then the Voltage Standard Deviation Ustdev was calculated for each set of Ui values according to the relation [19]: 5) where: m = 385 for the 9-minute time interval considered during experimental testing at all current density steps.
The real value of the experimental voltage was then calculated for each selected current density step, according to the relationship: where: n represent the number of constant current steps.
In Fig. 4 are presented the experimental polarization curves for models M1-M3 at two different backpressures.As we can see in Fig. 4, a, at a backpressure of 30 kPa, the fuel cell having the bipolar flow field M2 entered rapidly in the concentration loss region, starting with a current density of only 0.7 A/cm 2 .We could observe also here that at current densities over 0.75 A/cm 2 , the fuel cell polarization curves for flow fields M1 and M3 are close to each other, due to similar ohmic losses.From Fig. 4, b we noticed almost identical performances of the cell at low current densities (under 0.4 A/cm 2 ) for the flow fields M1 and M2, indicating similar activation losses.
From Fig. 4, b we can notice a reduction in the slope of the polarization curve for the M3 field model compared to the other two models at the backpressure of 70 kPa, thus indicating better performance of the bipolar plate with a w/a ratio (land to channel ratio) equal to 1.05.This behavior can be associated with achieving the best compromise between the advantages and disadvantages of using a flow field with w/a> 1, as mentioned in the literature [17,18].Also, by comparing the polarization curves of each bipolar plate model tested at two different backpressures (Fig. 4) we could see a decrease in the slope of the curves along the ohmic loss region at backpressures of 70 kPa, behavior which indicates an improvement in ion transport across the PEM at the increasing backpressure [18].
The thermodynamic efficiency of the combustion cell ηFC can be determined as the ratio between the net output power Wgross and the rate of hydrogen usage mH2 in accordance with the following relationship [20]: The variation of the power density for each test performed on PEMFC system is shown in Fig. 5. Fuel cell with flow fields M 1 and M 3 presented really similar power densities across the entire load current range at the backpressure of 30 kPa, as we could observe in Fig. 5, a.At the backpressure of 70 kPa, it was noticed an increasing with 10 -11% of the power density at current densities between 0.7 and 0.85 A/cm 2 for the model M3, by comparing with model M1 (Fig. 5, b).
In Fig. 6 was plotted the thermodynamic efficiency determined on the basis of the relationship (9) for the two experimental tests of the PEMFC system, with different types of bipolar plate flow fields.
In the case of model M 2 with the highest w/a ratio of 1.2 was observed a rapid exponential decrease of the thermodynamic efficiency starting from a power density of 0.25 W/cm 2 , from 0.48 to 0.33 at a backpressure of 30 kPa (Fig. 6, a).Model M3 presented a linear variation of thermodynamic efficiency along the entire power density domain at backpressure of 70 kPa, with values increased by 7 -14% at power densities between 0.115 -0.44 W/cm 2 by comparing with model M2, as we could notice in Fig. 6, b.

Conclusions
At a system backpressure of 30 kPa, with partial pressures of 145.8 kPa for H2 and 20.1 kPa for O2 inside the PEM fuel cell, the flow field M3 with ratio w/a = 1.05 presented the polarization curve with lowest activation and ohmic losses till at a current density of 0.75 A/cm 2 .After this value, at high current densities, the ohmic losses for the models M3 and M1 (with w/a ratio of 1) seemed to be similar.
At a backpressure of 70 kPa, with partial pressures of 165.8 kPa for H2 and 24.3 kPa for O2 through the cell, a big slope reduction for polarization curve of the model M3 was observed, suggesting a much better electrical performance of the fuel cell with this type of the gas flow field along the entire current density domain.
The power density curves for the cell with flow fields M1 and M3 proved to be near to each other at a backpressure of 30 kPa, but at a higher backpressure a clear enhancement of 10 -11% was observed for model M1 at current densities over 0.7 A/cm 2 .
Optimized model M3 presented the highest thermodynamic efficiency along the entire power density domain for both pressure conditions considered in the experimental testing.

Fig. 2
Fig. 2 Schematic representation of the PEM unit cell used in the BEKKTECH BT-552 testing system, with all components included Table 1 for Test no. 2.

Fig. 3
Fig. 3 Images of the bipolar graphite plates used at anode (ac) and geometric dimensions of the serpentine shaped channel fields for the M1, M2 and M3 models (df)

Fig. 4
Fig. 4 Experimental polarization curves for the PEMF fuel cell with three different fields for the flow channels, obtained at two backpressures Wgross = Vcell •I, W is calculated for a current level I, A considered.LHVH2 represents the "Lower Heating Value" coefficient, having the value of 120.1 MJ/kg.The output current is correlated with the H2 usage rate in accordance with the relation[21]: MH2 represent hydrogen molar mass, having the value of 2.016 x 10 -3 kg/mol and F is the Faradays constant with the value of 96485.33A•s/mol.After a correlation of relations (3) and (4) we obtain a simplified expression for thermodynamic efficiency:

Fig. 5 Fig. 6
Fig. 5 Variation of the power density for the PEM fuel cell with bipolar flow fields M1 -M3, working at two different backpressures

Table 2
Flow field models implemented on bipolar fields