1. Introduction
The wide application of hydrogen energy was considered to be one of the effective ways to solve energy shortage and environmental pollution. Proton exchange membrane fuel cell (PEMFC) uses hydrogen as fuel, and the product is water, which is one of the effective ways to solve carbon emissions. At the same time, it has attracted much attention in the field of automobile and power generation due to its high energy conversion rate and high output power [1–5]. PEMFC consisted of bipolar plates (BPPs) and membrane electrode assembly (MEA) [6,7]. And MEA consisted of gas diffusion layer (GDLs), catalytic layer (CLs) and a proton exchange membrane (PEM) [6,7]. The mainly key functions of BPPs (as the core component of PEMFC) were isolated the oxidant and gas fuel and discharge the reaction products and heat in time, conducted current between each battery unit, and play a supporting role in the entire battery module [8,9]. The electrode reaction in PEMFC was affected by the distribution of reactants on the CLs, while the distribution of reactants on the CLs was obviously affected by the structure of flow field in BPPs [10]. Therefore, the optimization of flow field structure as an efficient way to manage the mass distribution and transfer in BPPs has received widespread attention. Chen et al. studied the reactants transfer behavior in convection and diffusion processes, and showed that the main transfer process of reactants was convection [11]. Due to its structural characteristics, the pressure drops of traditional straight channel was low, and convection effect under the ribs was weak, which made the reaction gas cannot fully react [12,13]. Enhancing the convection effect in traditional straight channel and improving the utilization rate of the reactants had become wide-spread concern of many scholars. Numerous studies had shown that adding obstacles in channel can effectively enhance the transport of reactants to porous media (GDLs and CLs), enhance the convection and diffusion phenomenon of oxygen, and improve the output characteristic and water management ability of PEMFC [14–16].
The research on obstacles generally focused on the number, shape and size of the baffles (blocks). Huang et al. designed different shape of baffles and showed that the mutation caused by the disturbance effect of the baffle made the reaction fluid obtain more effective convection effect, and the larger baffle volume can better promote the mass transfer and obtain better performance [17]. Zhang et al. studied the obstacles distributed horizontally in the channel, and found that as the obstacles were aligned distribution and obstacle number was 36, the PEMFC obtained the best performance. The net power density and drainage performance increased about 47.8% and 32.6%, respectively [18]. Afshari et al. numerically analyzed baffle number, shape and size effect on fuel cell mass transfer, and found that baffle height had the most significant effect on the mass transfer [19]. Perng et al. compared trapezoidal baffles and blocks effect on PEMFC, and shown that whether it was baffle or block, PEMFC performance would be improved with trapezoidal angle and height increased. Compared with the baffle, the block obstacle had better performance in PEMFC output characteristics and the uniformity of the distribution of reactants and products [20,21]. Blocky obstacles had larger volume than baffles, which was more sufficient to promote local mass transfer, and blocky obstacles had higher developability, which had gradually become a hot spot in obstacle research. Cai et al. found that the cross-sectional shape, size and interval distance of obstacles can significantly affect the mass transfer effect [22]. Bahar et al. compared semi-circular and rectangular blocks, and found that rectangular blocks obtained better performance, and the performance improvement increased with the height of the blocks increased [23]. Ebrahimzadeh et al. compared the effect of trapezoidal, cylindrical, triangular, and rectangular obstacles on current density, and shown that triangular and rectangular obstacles obtained higher mass transfer and current density, compared with the rectangular scheme, the triangular scheme obtained lower pressure drops [24]. Guo et al discussed the effect of obstacle with different shapes on the power loss in channel region, and found that rectangular obstacles provided higher performance than other shapes, but also caused higher power loss [25]. Yin et al. discussed the forward angle and number of trapezoidal plugs effect on single channel PEMFC performance. It was found that larger diversion angle led to higher vertical gas velocity, thus enhancing the convective transport effect of oxygen. The performance of PEMFC first increased with the increasing of the number of blocks, but if the number of blocking blocks was too many, it would hinder the material transfer along the channel and reduce PEMFC performance [26]. Wang et al. studied the design of obstacles with regular arrangement, and found that the uneven spacing of obstacles can better promote the uniform distribution of reaction gas and applicate the unequal height obstacles would further affect the mass in PEMFC, by increasing the height of obstacles along the flow direction can improve the diffusion of oxygen, water removal and PEMFC performance. At the same time, it has relatively lower pressure drops than uniform height obstacles [27]. Tiss et al. through experiments verified that partial blockage of the flow channel has significant improvement on PEMFC performance [28]. Heidary et al. discussed the effect of partial blockage and completed blockage of obstacles on PEMFC, and showed that partial blockage imposed lower pressure lose, in general, the net power obtained by completely blocking the channel was greater than that of partially blocking [29]. However, for the whole flow field, when discussing the degree of blockage of obstacles, most studies did not consider channel complete blockage, because when obstacle number was large enough, channel complete blockage will seriously hinder the PEMFC internal material transfer, and even damage the MEA. The above research discussed the number and size of regular-shaped obstacles, but there are also some studies of irregular-shaped obstacles. Chen et al. designed the single-oriented flow channel structure with streamlined blocks and porous blocks, and shown that the larger streamlined blocks could better discharge water, and the porous blocks enhance the power loss in the flow channel. The streamlined plugs with streamlined upstream and inclined downstream sections can further reduce the excessive increasing in power loss [30]. Karthikeyan et al. compared the effects of porous carbon inclusions and porous sponge inclusions on PEMFC through experiments. The experimental results shown that the porous sponge inclusions had higher performance, and increased inclusions size can improve the performance of PEMFC [32].
Most of the current studies on obstacle shape, number and size were discussed in single-channel of PEMFC, but rarely in the whole flow field structure. Gain generated by obstacles closely related to the size of the channel and obstacle itself. Obstacles with the same shape and number would have different effects due to the different sizes of the channel and the obstacle itself. More importantly, only discussing the gain of obstacles in single-channel PEMFC was not meaningful in practice. In other words, the performance gain generated by obstacles in single-channel PEMFC was not necessarily applicable to the entire flow field structure. Based on the above research, this paper discusses the obstacle number, shape and blockage degree in flow field of PEMFC effect on its performance in the whole basin. The flow field was divided into regions based on the discussing of the optimal number and shape of obstacles and the degree of channel blockage, and the effect of the blockage degree of different regions on the performance of PEMFC was studied. Finally, a set of obstacle design schemes with the largest performance gain was obtained, which provided reference for the future discussion of the enhancement benefits of obstacles in the whole flow field structure.
2. Experimental
2.1 Physical model
Fig. 1 was the 3d steady-state model (including anode and cathode channels and MEA) established in COMSOL software fuel cell module, and the shapes of obstacles in Fig. 1(a–e) were Trapezoidal-I (right-angled trapezoid), Trapezoidal-II (isosceles trapezoid), Triangular-I (right-angled triangle), Triangular-II ( isosceles triangle) and Rectangular, respectively. The effective reaction length of the model was 32 mm, and the height and width of the flow channel were 1 mm. Fig. 2 was the shape and size of obstacles. The parameters of the model were shown in Table 1.
2.2 Mathematical model
2.2.1 Mass conservation
Where, ρ is the density (kg·m−3). ɛ is the porosity; ū is the velocity vector (m·s−1); Sm is the quality source term. In different regions of PEMFC, the value of Sm is different. At the gas diffusion layer and outlet flow channel of the two poles, Sm is set to zero. For the catalytic layer of the two poles, the value of Sm is calculated by the following formula:
Where, MH2, MO2, MH2O is the molar mass of H2, O2, H2O, respectively (kg·mol−1). F is Faraday constant, 96485 C·mol−1. i is the current density, (A·m−2). Subscripts a and c represent the anode and cathode, respectively.
2.2.2 Momentum conservation
Where, P represent the pressure (Pa). μ is the dynamic viscosity (N·s·m−2). su is the source term of momentum conservation. ɛ is 1. Equation (4) is simplified as flowing formula based on Darcy’s theorem:
Where, kP represent the gas permeability of GDLs and CLs (m2).
2.2.3 Energy conservation
Where, CP is the specific heat capacity (J·kg−1·K−1). T is the operating temperature of PEMFC (K). keff is the effective thermal conductivity of the material (W·m−1·K−1). SQ is the energy source term. is is the surface current density (A·m−2). R0hm is resistivity. β is the ratio of chemical energy to heat energy. sH2 is the gaseous water formation rate. hreaction is the reaction enthalpy, J·kg−1·mol−1. rw is the water phase transition rate. hlg is water phase change enthalpy, J·kg−1 ·mol−1. sa,c is the exchange current density (A·m−2), subscripts a and c represent anode and cathode, respectively. η is the overpotential (V).
2.2.4 Electric charge conservation
According to the existing potential form, potential can be defined as solid phase potential and membrane phase potential.
Where, σsol is the ionic conductivity; φsol is the electrode potential; φmem is the membrane potential; Rsol is the volume transfer current of the bipolar catalytic layer.
2.2.5 Component conservation
Where, Ck is the component concentration, mol·m−3;
D k e f f
At the gas diffusion layer and outlet flow channel of the two poles sk is 0; For the catalyst layer of the two poles, sk are as follows:
2.2.6 Butler-Volmer equation
Where,
j a r e f j c r e f
2.3 Assumption
The operating parameters of the PEMFC was listed in Table 2.
2.4 Model verification
2.4.1 Grid independence verification
The traditional straight channel as the object of grid independence test and all models were meshed by COMSOL grid division module. Table 3 shown the limiting current density of different grid cell number schemes. From Table 3, when the grid number increased from 1538242 to 2681453, the value increased was very small, proving that the grid cells effect on PEMFC output characteristics was negligible after reaching a certain number, and the error between Scheme 3 and Scheme 5 was only 0.1%, which was sufficient to prove the grid independence of the model. Therefore, the subsequent model would use the grid division mode of scheme 3. The meshing results of the traditional scheme in the scheme 3 meshing mode were shown in Fig. 3.
2.4.2 Experimental verification
Mojica et al. investigated the effects of four structures (straight, curving, 2D-Nozzle, 3D-Nozzle) on the performance of fuel cell. In this research, the straight channel structure is verified by referring to the boundary conditions and operating parameters set by Mojica [42]. Fig. 4 shown the comparison of experimental and simulation data. As shown in Fig. 4, the experimental data were in good agreement with the simulation data, so the simulation results also have high reliability.
3. Results and Discussion
A series of obstacles with longitudinal distribution were filled in the channel, obstacles number (obstacle number is 6, 8, 10, 12 in single channel), channel blockage degree (20%, 40%, 60%, 80%) and shape (Trapezoidal-I, Trapezoidal-II, Triangular-I, Triangular-II and Rectangular) effect on PEMFC performance were discussed. Obstacle blockage degree in channel was set to 50% as discussing obstacle number. The whole basin was divided into region 1, region 2, and region 3, and studied the effect of blockage degree of different regions on PEMFC performance.
3.1 The effect of obstacle number on PEMFC performance
Many studies have shown that the optimized PEMFC performance is mainly verified by polarization curves. [43]. Fig. 5(a–e) showed the polarization (V-I) curve and power density (P-I) curve of different number schemes of Trapezoidal-I, Trapezoidal-II, Triangular-I, Triangular-II and Rectangular obstacles. From Fig. 5, regardless of obstacle shape, the output characteristics of PEMFC would be improved with obstacle number increased. The more the number of obstacles, the greater the diffusion resistance generated during the reaction gas transmission, but it also enhanced the mass transfer. As previously mentioned, the mass transfer mode of the reaction gas in the channel under the obstacle schemes was mainly convection [11]. The presence of obstacles will produce certain diffusion resistance but also will significantly enhance the sub-rib convection effect of the reactants, forcing the reactants to CL. And the mass transfer PEMFC also enhanced. For the shape of the obstacle, the enhancement benefit of rectangular obstacle was the largest, followed by trapezoidal obstacle and triangular obstacle. Compared with the traditional scheme, the scheme with more obstacles in each shape had significant effect on PEMFC output characteristic. From Fig. 5(a–e), no matter what shape was the obstacle, the output characteristics were significantly improved as obstacle number increased. However, when the number of obstacles increased to a certain extent (10 to 12), the improvement of fuel cell output characteristics was negligible. When the shape of obstacle was rectangular and trapezoidal, the lifting was the smallest, and the two curves with 10 and 12 obstacles in single channel almost overlap. Considering the power lose generated by the pressure drop, the pumping power wP can be calculated from the following formula:
Where, wP is the pumping power, wout is the maximum output power of the fuel cell, wNet is the net output power, P is the pressure drop, Qin is the intake flow rate, η is the pumping efficiency, and in this research η is set to 0.8 [44]. Fig. 6(a,b) was the pressure drops and net power density of different number schemes, respectively. According to Fig. 6(a), the pressure drops would increase as obstacle number increased regardless of obstacle shape. When obstacle number increased from 10 to 12, the pressure drop increased the most. Under the same number, the pressure drops from high to low was: rectangle, trapezoid, and triangle. This was because the pressure drops were related to obstacle volume. In this research, the bottom areas of different shapes of obstacles were the same with the same blocking degree. The volume of rectangular obstacles was greater than that of trapezoids and triangles. For the same volume of trapezoid (Trapezoidal-I and Trapezoidal-II) and triangle (Triangular-I and Triangular-II), the pressure drop was slightly different with the same obstacles number (n = 6, 8, 10), but the deviation value was the same and remained small, which was in line with the law. As shown in Fig. 6(b), regardless of obstacle shape, the net power density increased as obstacle number increased with obstacle number increase from 6 to 10. Rectangular obstacle scheme obtained the highest net power density because the rectangular obstacle has the largest flow suppression area in the channel, which can make more reaction gas enter the CL and promote local current density. Trapezoidal-I had more higher net power density than that of Trapezoidal-II with the same volume because Trapezoidal-I has better flow suppression, which pushed more reaction gas from the GDL to the CL to participate the electrochemical reaction. However, when obstacle number increased from 10 to 12, pressure drops increased obviously lead to the increasing of pumping power, the net power density of rectangular obstacle schemes decreased. Therefore, adding too many obstacles in the channel of PEMFC, the output performance will not achieve an ideal improvement.
Fig. 7 shown the oxygen and water distribution on CL surface at 0.6 V. Due to the structural characteristics of the traditional straight channel, the distribution of reactants and products in the flow field shown good symmetry. From Fig. 7(a), the oxygen distribution on CL surface was gradually improved with obstacle number increased. Rectangular obstacles and trapezoidal obstacles have better improvement, especially rectangular obstacles. The phenomenon of ‘oxygen starvation’ gradually disappears as obstacle number increased. For the triangle scheme, the increasing of obstacle number was significantly less effective than rectangle and trapezoid in improving the oxygen distribution. The distribution of products was highly related to reactants distribution, and as the electrochemical reaction continues, oxygen was gradually consumed and large amount of water was produced. Study shown higher gas flow velocity was effective to separate water from the flow field cathode GDL. As the number of obstacles continues to increasing, the pressure drops increased, enhanced the gas flow velocity that diffused to CL, promoted water transfer from CL to channel through the GDL. Fig. 8 showed oxygen minimum molar fraction and water maximum molar fraction on the CL surface. Rectangular obstacle scheme had the best mass transfer effect, which was significantly higher than that of trapezoid and triangle. The gain effect was obvious when obstacle number in single channel increased from 6 to 10, and the gain amplitude decreased significantly when obstacle number increased from 10 to 12. For trapezoidal obstacles, there was a similar situation. For triangular obstacles, with obstacle number increased, the PEMFC internal mass transfer on the cathode side gradually improved. Compared with rectangular and trapezoidal obstacle scheme, obstacle number increasing of triangular scheme was stable for the improvement of the internal mass transfer effect, but the overall improvement was much smaller than that of rectangular and trapezoidal obstacles.
Through the above analysis, it was known that whether the fuel cell output performance or the cathode side mass transfer effect, obstacle number increasing will produce corresponding gain, but the pressure drop will also increase. However, the number of obstacles increased too much will not get the ideal gain effect. For rectangular obstacle, the overall performance of PEMFC was the most ideal when obstacle number was 10. For trapezoidal obstacles, when obstacle number increased to 12, the promotion in output characteristic and mass transfer were small. For triangular obstacles, the promotion in output characteristic and mass transfer were considerable, but the promotion was far less than rectangle and trapezoidal obstacle. When discussing the effect of obstacles, obstacle number should be reasonably designed. In the subsequent research, obstacle number was set to 10.
3.2 Channel blockage degree effect on PEMFC performance
Channel blockage degree (obstacle height) had a markedly effect on PEMFC performance [45]. Fig. 9 shows the V-I curves and P-I curves of different blockage schemes. The output characteristic of regardless obstacle was continuously improved when channel blockage degree increased from 20% to 60%. However, when it continued to increasing to 80%, the output performance would be reduced accordingly. This was related to the volume of the obstacle, at a lower blockage degree, the larger obstacle can produce larger normal gas velocity, so that more reaction gas entered the CL to react and promoted the generation of current in the local region. However, at a high blockage degree, gas flow resistance caused by larger obstacles was also large, which seriously hindered the transmission of reaction gas and affects PEMFC performance. For rectangular and trapezoidal (Trapezoidal-I and Trapezoidal-II) obstacles, PEMFC output characteristic was significantly reduced at 80% blockage degree, indicated that at 80% blockage, the transfer of oxygen along the flow direction was seriously hindered, and oxygen concentration in the back of the basin was low, which affect the electrochemical process. However, for triangular (Triangular-I and Triangular-II) obstacle scheme, when the channel was 80% blocked, the impact on the fuel cell was much smaller than other shapes. Fig. 10(a) shown the pressure drops of different blockage schemes. The pressure drops increased as channel blockage degree increased. Regardless of obstacle shape, the pressure drops increasing was small for low blockage degree. However, when the blockage degree was 80%, the pressure drops increased sharply, resulting in a greater pumping power required for the reaction gas. Fig. 10(b) shown the net power density of different blocking schemes. It was known that the maximum was obtained at 60% blockage degree regardless of obstacle shape. The rectangular obstacle had the largest net power density, which was about 29.2% higher than that of traditional scheme. The net power density of Trapezoidal-I, Trapezoidal-II, Triangular-I, and Triangular-II scheme was increased by 28.1%, 25.9%, 25% and 22.4%, respectively. When the blockage degree was 80%, the net power density decreased, and the rectangular and trapezoidal (Trapezoidal-I and Trapezoidal-II) schemes decreased seriously, which were 12.7%, 21.9% and 16.1% lower than that of the traditional scheme, respectively.
Fig. 11 shows the oxygen and water distribution on CL surface of different blockage degree. For rectangular and trapezoidal (Trapezoidal-I and Trapezoidal-II) schemes, when the blockage degree in the channel increased from 20% to 60%, oxygen and water uniformity were gradually improved, and the mass transfer effect was the best when the blockage degree was 60%. When the blockage was 80%, the second half of the basin was almost ‘paralyzed’, the gas flow rate in this region was almost zero. Therefore, oxygen hardly flow to the rear section under the obstruction of obstacles, water would be blocked in GDL/CL without the carrying of gas flow rate, thus seriously endangering MEA. For the triangular (Triangular-I and Triangular-II) schemes, as the blockage degree increased, the mass transfer effect on the cathode side was gradually improved, and the mass transfer effect was best under 80% blockage degree. Fig. 12 shown oxygen minimum mole fraction and water maximum mole fraction on CL surface of different blocking schemes. If the minimum mole fraction of oxygen was used as a standard to represent the uniformity of oxygen distribution, from Fig. 12(a), the rectangular and trapezoidal (Trapezoidal-I and Trapezoidal-II) schemes had the best distribution effect when the channel was 60% blocked, while the triangular (Triangular-I and Triangular-II) schemes had the best distribution effect when the channel was 80% blocked. Throughout all schemes, rectangular scheme had the best oxygen distribution uniformity at 60% blockage degree. If the maximum mole fraction of water as a standard to represent PEMFC drainage performance, from Fig. 12(b), rectangular and trapezoidal (Trapezoidal-I and Trapezoidal-II) schemes had the best drainage effect when the channel was 60% blocked, which was 33.9%, 30.6% and 26.9% higher than that of traditional scheme, respectively. Triangular (Triangular-I and Triangular-II) schemes had the best drainage effect when the channel was 80% blocked, which was 17.9% and 24.4% higher than that of the traditional scheme. Throughout all schemes, rectangular schemes had the best drainage effect at 60% blockage degree, which was 33.9% higher than that of traditional scheme.
3.3 Obstacle shape effect on PEMFC performance
The optimal obstacle number and channel blockage degree were discussed in the previous section. Fig. 13 shown the V-I curves and P-I curves with different obstacle shape. As mentioned earlier, the mass transfer rate of the fuel cell in the concentration polarization region determined the generation of the current, and the shape of the obstacle was closely related to the mass transfer rate of the fuel cell. From Fig. 13, the output characteristic from high to low was Rectangular, Trapezoidal-I, Trapezoidal-II, Triangular-I, Triangular-II. The obstacle was mainly composed of the diversion part and the flow suppression part in the channel, the diversion part was responsible for guiding the reactants to continue to transfer downward in the channel, and the flow suppression part pushed the reactants entered the CL. The more the reaction gas entering the CL, the higher the current density generated locally, which can greatly improve the utilization efficiency of the reactants. Therefore, the flow suppression part of the obstacle was the main factor affect fuel cell mass transfer. Rectangular obstacle produces higher vertical gas velocity component in flow stage, thereby enhanced the reactants transfer to GDL. For the Trapezoidal-I and Trapezoidal-II with the same volume, Trapezoidal-I has larger diversion anteversion angle than Trapezoidal-II. Trapezoidal-I produces higher vertical gas velocity component, so the gain effect of Trapezoidal-I was higher than that of Trapezoidal-II. Similarly, for triangle obstacles. The different shapes of obstacles lead to different channel pressure drop, Fig. 14 shown the pressure drop and net power density of different obstacle shape. Adding obstacles increased channel pressure drop greatly, the pressure loss caused by different obstacle shapes was also different, the volume of rectangular obstacle was larger than that of trapezoid and triangle, so the pressure loss was the largest. Rectangular obstacle had the highest net power density, which was about 29.2% higher than that of conventional scheme. Trapezoidal-I, Trapezoidal-II, Triangular-I, Triangular-II was increased by 28.1%, 25.9%, 25% and 22.4%, respectively.
Fig. 15 shows the velocity distribution of different obstacle schemes. For conventional straight channel, the largest gas flow rate was obtained in main channel, and a low velocity in intermediate channel. The region where obstacle placed would cause pressure difference between the region at both ends of the obstacle, and enhanced the gas acceleration phenomenon in the local region. Rectangular and trapezoidal schemes were more obvious. Huang et al. found that the change of gas velocity was related to the volume of obstacles, the larger the volume of obstacles, the greater the change of velocity direction, and the greater the eddy current [46]. Compared with traditional straight channel, the gas velocity in channel of obstacle schemes was obviously enhanced, and the gain effect from high to low was Rectangular, Trapezoidal-I, Trapezoidal-II, Triangular-I, Triangular-II. Fig. 16(a,b) shows the oxygen and water distribution on CL surface of different obstacle schemes, respectively. For obstacle scheme, rectangular obstacle had the best distribution of reactants and products, followed by trapezoidal obstacle scheme, and triangular obstacle scheme was the worst. Obstacle flow suppression part determined the amount of oxygen entering the CL, rectangular obstacle had the largest flow suppression area, which can make oxygen produce large normal velocity component, more oxygen enter the CL reaction, and the resulting water was carried into the channel at high gas velocity. For trapezoidal obstacle, Trapezoidal-I flow suppression part was larger than Trapezoidal-II, so the distribution of reactants and products was more uniform. Similarly, the same for triangular obstacle.
Based on the above analysis, whether it was the fuel cell output performance or mass transfer effect, rectangular obstacle had the most improvement, followed by the trapezoidal obstacle, and the triangular obstacle was the worst. The main factor affected the mass transfer effect was the flow suppression part of the obstacle. The flow suppression part determined how much the reactant entered the CL to participate in the reaction, thus affected fuel cell local performance.
3.4 Blockage degree effect on PEMFC performance
Through the above discussion, the optimal number (n=10), shape (rectangle) and blockage degree (60%) of obstacles were determined. On this basis, the parallel flow field was divided into regions according to the direction of gas flow. Region 1 was near the inlet end and each sub-channel contained three obstacles. Region 2 was the intermediate region and each sub-channel contained four obstacles. Region 3 was near the outlet end and each sub-channel contained three obstacles. The specific division was shown in Fig. 17. For rectangular obstacle scheme, PEMFC performance was best under 60% blockage degree, therefore, based on 60% blockage degree, six groups of schemes with different blockage degree were designed as shown in Table 4. By analyzing PEMFC output characteristic and mass transfer of each scheme to find the best design scheme for blockage degree.
Fig. 18 shown the V-I curves and P-I curves of different regional height distribution schemes. Except for scheme 4, the output characteristic of other schemes was not as high as those of the whole region with 60% blockage degree. Through scheme 4 and the whole region 60% with blockage degree, it can be known that with 60% blockage degree of the channel in region 2 and region 3 can obtain higher output power. For the traditional straight channel, the region that determined PEMFC electrochemical performance was mainly region 2 and region 3, because of region 1 was close to the inlet end, and the gas flow rate in this region was relatively high, which can fully react. Therefore, the degree of blockage in region 1 had little effect on the overall electrochemical reaction process. By comparing the scheme 2, 4 and the scheme with 60% blockage degree in the whole region, it was known that the effect of placing obstacles with a blockage degree of 40% in region 1 was the best. Fig. 19 shown the cathode side pressure drops and net power density of each scheme. Through previous analysis, the pressure drop was highly correlated with obstacle blockage degree. The scheme with 60% blockage degree in the whole region had the largest pressure drop, and the pressure drop could be effectively reduced by reducing the degree of blockage. Through scheme 4 and the whole region with 60% blockage degree, it can be known that 60% blockage degree in region 2 and region 3 can obtain higher net power density. Among all the schemes, scheme 4 had the largest net power density, which was about 29.8% higher than that of traditional scheme and 0.6% higher than that of the whole basin with 60% blockage degree. For the PEMFC output performance, the scheme with 40%, 60% and 60% blockage degree in region 1, region 2 and region 3 was the best.
Fig. 20 shown the oxygen and water distribution on CL surface of each scheme. Compared with other schemes, scheme 4, scheme 6 and 60% scheme in the whole region had better water and oxygen distribution, which indicated that PEMFC mass transfer effect was related to the overall blockage degree, the larger blockage degree in the whole region was more conducive to mass transfer. By comparing scheme 4 with scheme 6, it was found that when the overall blockage degree was the same, the higher blockage degree in region 2 and region 3 can better promote mass transfer. Throughout all schemes, scheme 4 (40%-60%-60%) has the most uniform of water and oxygen distribution.
Fig. 21 shows the maximum molar fraction of oxygen and the minimum molar fraction of water in the cathode side of different blockage degree. From Fig. 21, it can be seen that compared with other schemes, with the blockage degree of 60% have the highest molar fraction of oxygen and the smallest molar fraction of water, so they have the best performance in the distribution of water and oxygen concentration in PEMFC, followed by scheme 4.
3.5 Relative humidity effect on PEMFC performance
Relative humidity had prominent effect on PEMFC electrochemical performance. In this research, the effect of cathode relative humidity at 25%, 50%, 75%, and 100% on PEMFC performance was discussed under the operating temperature of 353.15 K and anode relative humidity of 100%. Fig. 22 shown the V-I curves and P-I curves of the four relative humidity schemes. From Fig. 22, when the anode relative humidity was 100%, with the increasing of anode humidity, the output performance of the fuel cell reached the best when the relative humidity was 75%. When the humidity continues increasing to 100%, the performance decreased. This was because the increasing of humidity will lead to the increasing of the conductivity of the membrane, which will improve the output of the fuel cell. When the humidity reaches 100%, although the conductivity increased, the water content in the membrane will increase significantly, which will lead to flooding and affect the performance of the fuel cell.
Fig. 23 shown the water distribution at the cathode side membrane-CL interface of the four schemes. When the relative humidity of the anode was 100%, the increasing of the cathode relative humidity caused the water concentration of the cathode to be higher than that of the anode side, and the reverse diffusion of water occurs [47]. It can be clearly seen that when the relative humidity of the cathode was 100%, the water content at the membrane-CL interface increased significantly and highly concentrated in large range.
4. Conclusions
A series of obstacles with longitudinal distribution were filled in the channel. The effect of obstacles number, channel blockage degree and shape on PEMFC performance were discussed. The whole basin was divided into region 1, region 2, and region 3, and studied the effect of height distribution of different regions on PEMFC performance. The main conclusions were as follows:
Obstacle longitudinally distributed can significantly promote reactants transfer to GDL, compared with obstacle number and shape, the effect of channel blockage degree on PEMFC performance much more significantly.
Obstacle number should be designed reasonably, PEMFC performance first increased as obstacles number increased, when obstacle number reached certain number, the performance improvement reached the bottleneck period. As obstacle number was too large will hinder the transmission process of the reaction gas and reduce PEMFC performance. Considering the processing technology and cost, the optimal obstacle number in this research was 10.
The blocking part of the obstacle was the main factor affecting PEMFC mass transfer. The size of the blocking part was different with the shape of the obstacle. The larger blocking part can make the reaction gas produce larger normal velocity, so that more reaction gas enters CL through GDL to react. Among the shape schemes in this research, rectangular scheme had the highest output performance and the most uniform distribution of reactants and products, followed by the trapezoidal scheme and the triangular scheme.
The degree of channel blockage significantly affects PEMFC performance, appropriate blockage of channels by obstacles will significantly improve PEMFC performance. However, excessive blockage will not only reduce the performance, but also damage MEA. In this paper, all obstacle schemes had the best performance when the degree of blockage was 60%, rectangular and trapezoidal obstacle scheme was obviously worse when the degree of blockage reached 80%. Although fuel cell output characteristic decreased when the triangle obstacle was blocked by 80%, the mass transfer effect improved. Rectangular obstacle scheme had the best output performance when the channel was blocked by 60%, which was 29.2% higher than traditional scheme.
The regions of the obstacle height had certain influence on PEMFC performance. The regions that determined PEMFC electrochemical performance were mainly region 2 and region 3. The 60% blockage degree in region 2 and region 3 can obtain higher net power density and better mass transfer. The region 1 was close to inlet end, so the gas flow rate in region 1 relatively large, and the reaction gas can fully react, so the degree of blockage in region 1 had little effect on the electrochemical reaction process. On the whole, the mass transfer effect of region 1 with 40% blockage degree was better than that of 60% blockage degree. The best PEMFC performance was obtained with the blockage degree of 40%, 60% and 60% in region 1, region 2 and region 3, respectively. The net power density was 29.8% higher than traditional scheme and 0.6% higher than that of the whole basin with 60% blockage degree.
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