### 1. Introduction

*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].

*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].

### 2. Experimental

### 2.1 Physical model

### 2.2 Mathematical model

#### 2.2.1 Mass conservation

*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:

*M*

_{H}_{2},

*M*

_{O}_{2},

*M*

_{H}_{2}

*is the molar mass of H*

_{O}_{2}, O

_{2}, H

_{2}O, 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

*P*represent the pressure (Pa).

*μ*is the dynamic viscosity (N·s·m

^{−2}).

*s*

*is the source term of momentum conservation.*

_{u}*ɛ*is 1. Equation (4) is simplified as flowing formula based on Darcy’s theorem:

*k*

*represent the gas permeability of GDLs and CLs (m*

_{P}^{2}).

#### 2.2.3 Energy conservation

*C*

*is the specific heat capacity (J·kg*

_{P}^{−1}·K

^{−1}).

*T*is the operating temperature of PEMFC (K).

*k*

*is the effective thermal conductivity of the material (W·m*

^{eff}^{−1}·K

^{−1}).

*S*

*is the energy source term.*

_{Q}*i*

*is the surface current density (A·m*

^{s}^{−2}).

*R*

_{0}

*is resistivity.*

_{hm}*β*is the ratio of chemical energy to heat energy.

*s*

_{H}_{2}is the gaseous water formation rate.

*h*

*is the reaction enthalpy, J·kg*

_{reaction}^{−1}·mol

^{−1}.

*r*

*is the water phase transition rate.*

_{w}*h*

*is water phase change enthalpy, J·kg*

_{lg}^{−1}·mol

^{−1}.

*s*

_{a}_{,}

*is the exchange current density (A·m*

_{c}^{−2}), subscripts

*a*and

*c*represent anode and cathode, respectively.

*η*is the overpotential (V).

#### 2.2.4 Electric charge conservation

*σ*

*is the ionic conductivity;*

_{sol}*φ*

*is the electrode potential;*

_{sol}*φ*

*is the membrane potential;*

_{mem}*R*

*is the volume transfer current of the bipolar catalytic layer.*

_{sol}#### 2.2.5 Component conservation

*C*

*is the component concentration, mol·m*

_{k}^{−3};

*s*

*is component source term; The subscript*

_{k}*k*is the component code.

*s*

*is 0; For the catalyst layer of the two poles,*

_{k}*s*

*are as follows:*

_{k}#### 2.2.6 Butler-Volmer equation

^{−2}.

*C*

_{H}_{2}is the molar concentration of H

_{2}, mol·m

^{−3}.

*C*

_{O}_{2}is the molar concentration of O

_{2}, mol·m

^{−3}.

*C*

_{H}_{2,ref}and represent the reference molar concentrations of H

_{2}and O

_{2}, respectively, mol·m

^{−3}.

*γ*

*= 0.5,*

_{a}*γ*

*= 1.*

_{c}*α*is the coefficient of transmission.

*R*is the gas constant, J·kg

^{−1}·mol

^{−1}.

*T*is the temperature, K.

### 2.3 Assumption

### 2.4 Model verification

#### 2.4.1 Grid independence verification

#### 2.4.2 Experimental verification

*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

### 3.1 The effect of obstacle number on PEMFC performance

*w*

*can be calculated from the following formula:*

_{P}*w*

*is the pumping power,*

_{P}*w*

*is the maximum output power of the fuel cell,*

_{out}*w*

*is the net output power,*

_{Net}*P*is the pressure drop,

*Q*

*is the intake flow rate,*

_{in}*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.

### 3.2 Channel blockage degree effect on PEMFC performance

### 3.3 Obstacle shape effect on PEMFC performance

*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.

### 3.4 Blockage degree effect on PEMFC performance

### 3.5 Relative humidity effect on PEMFC performance

### 4. Conclusions

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.