Effect of flow channel cross-section shape on the performance of proton exchange membrane electrolysis cell

To study the thermal and mass distribution in proton exchange membrane electrolytic cells (PEMEC), a two-phase numerical model of a single-channel electrolytic cell was established. The polarization performance and gas-heat distribution in a single-channel electrolytic cell under five different flow channel cross-sections (rectangle, trapezoid, inverted trapezoid, triangle, and circle) were studied. The findings indicate that the inverted trapezoidal flow channel’s gas buildup effect is rather severe in both the catalytic and diffusion layers. In the trapezoidal flow channel, the temperature of the proton exchange membrane region is lower, and the electrochemical performance of the rectangular flow channel electrolytic cell is the best, with an improvement rate of up to 11.47%. The electrochemical performance of the triangular and circular flow channels is the worst.


Introduction
Hydrogen energy has many benefits as a clean, safe, and zero-carbon energy source, including plentiful supplies, a high energy density, and extended storage.It is thought to be the 21st century's brightest potential secondary energy [1] .The beneficial characteristics of proton exchange membrane electrolysis (PEM) technology include its compact size, quick reaction, lack of alkali pollution, high efficiency (75%-85%), and good adaptability to power fluctuation [2] .It is internationally recognized as the development trend and research focus for the application of electric hydrogen production technology in the power grid [3] .
The fluid channel's configuration and shape in PEMEC directly affect the distribution of hydrogen and oxygen components and provide an essential effect on the electrolysis cell's functionality.Toghyani et al. [4] established five different PEM electrolysis cell flow field models and studied the component and pressure distributions in the electrolysis cells of different flow field models.Li et al. [5] found that the fluid spread of the liquid phase in a columnar flow field electrolysis cell was investigated, and it was discovered that the liquid dispersion in each flow channel is uneven, which may cause the local temperature of the electrolysis cell to be too high.Olesen et al. [6] compared the thermal mass distribution of different interdigitated PEM electrolysis cells and found that the hot spots are all related to the uneven distribution of two-phase flow and current density.
In this paper, a three-dimensional numerical simulation model of a single-channel PEMEC is established, the effects of five-channel structures with rectangular, trapezoidal, inverted trapezoidal, triangular, and circular cross sections on the electrochemical properties of the PEMEC under the same flow channel area are studied, and the thermal and mass distribution has been studied.

Geometrical model
Based on the repeatability of the flow channel in the flow field of the PEMEC and the symmetry of a single flow channel, a single flow channel was selected for modeling and solution.Figure 1 shows the model calculation area, which includes the anode plate (ACP), anode flow channel (ACH), anode diffusion layer (AGDL), anode catalytic layer (ACL), proton exchange membrane (PEM), cathode plate (CCP), cathode flow channel (CCH), cathode diffusion layer (CGDL), and cathode catalytic layer (CCL).The geometric parameters of the single-channel PEM electrolysis cell are shown in Table 1.When keeping the cross-sectional area unchanged, five single-channel electrolytic cell models with flow channel cross-sectional shapes of rectangular, trapezoidal, inverted trapezoidal, triangular, and circular were constructed.The cross-section shape structure and grid diagram of single-channel electrolytic cells are shown in Figure 2.

Governing equations
When the electrolysis cell is operating, liquid water enters flow channels from the inlet and diffuses through the GDL to the catalyst layer.Under the action of the catalyst, water is electrolyzed to produce hydrogen and oxygen.The generated gas flows out of the outlet with the water.The basic governing equations include electrochemical, mass, energy, momentum, and component conservation equations.
( ) where ζ is active specific surface area; γ is concentration dependence; jref is reference exchange current density; F is the Faraday constant; η is the local activation overpotential; α is the transfer coefficient; R is the gas constant; T is the temperature; ε is the porosity of porous media; S is the source term; ρ is the fluid density; cp is the specific heat at constant pressure; k is the effective thermal conductivity; Ck is the molar concentration of Substance k; Dk is the effective diffusion coefficient.

Multiphysics and initial condition parameter settings
Figure 3 shows the boundary conditions involved in the model.Anode potential: The electrolytic potential E_cell is applied at the boundary of the ACP; Cathode potential: The potential of CCP is 0; Wall: The inner wall of the flow field is set as a flow with no-slip boundary, the side surface of the electrolysis cell has symmetric boundary conditions, and the other walls are adiabatic boundaries.
Table 2 lists the calculation parameters of single-channel PEM electrolysis cells.The calculation parameters are not given and they are all introduced into the model calculation through the coupling of several physical fields or the material library module function via variables.Figure 4 shows the oxygen distribution cloud map at the center section position of the flow channel of the single-channel electrolysis cell at a voltage of 2.0 V. Figure 5 shows the hydrogen volume fraction distribution cloud map.The gas volume fraction in the diffusion layer is higher than that in the flow channel.The further the proton exchange membrane is, the lower the gas volume fraction is.There is a phenomenon of gas accumulation below the ridge, especially in the inverted trapezoidal flow channel, and the accumulation of bubbles is not conducive to the electrolytic reaction in the membrane electrode.Compared with the triangular channel and circular channel, the fraction of oxygen and hydrogen in the rectangular channel increased significantly, the average volume fraction of hydrogen increased from 27.77% and 26.35% to 32.06%, and the rate of hydrogen production was faster.

Influence of cross-section shape on temperature distribution
Figure 6 shows the temperature of the central part of the PEMEC when the voltage is 2.0 V.The high-temperature area of the single-channel electrolytic cell is mainly concentrated in the proton exchange membrane and its two sides.The PEM has the highest heat production and temperature, and water is both a reacting and cooling medium in A/CGDL and A/CCH.The cooling capacity of water in the A/CGDL on both sides is poor.The closer to the center of the runner it gets, the lower the temperature is.The temperature in the exchange film region from high to low is inverted trapezoidal channel, rectangular channel, trapezoidal channel, triangular channel, and circular channel.As electrolysis is a heat-absorbing process, the higher the temperature is, the more concentrated the distribution will be, which is more conducive to the electrolytic reaction.Figure 7 shows the polarization curves of PEMEC.When the voltage is the same, the current density of the rectangular channel is the highest, which is slightly higher than that of the trapezoidal channel and the inverted trapezoidal channel, due to the relatively low temperature of the PEM in the trapezoidal channel.The gas produced by electrolysis in ACL, CCL, AGDL, and CGDL of the inverted trapezoidal channel cannot be quickly discharged and there is an agglomeration effect, both of which are not conducive to the occurrence of electrolytic reaction.The current density of the rectangular channel increases by about 0.8%.Compared with triangular and circular channels, the current density of rectangular channels increases by 0.91%~9.32%and 0.95%~11.47%respectively.The higher the voltage is, the more obvious the lifting effect is.Therefore, the electrochemical properties of PEMEC are best when the flow channel cross-section is rectangular.

Conclusions
This paper numerically simulates the electrochemical reaction and mass and heat transfer process in a PEMEC, establishes the computational model for a single-channel electrolytic cell considering five cross-sectional shapes, and studies the composition distribution, temperature distribution, and polarization properties of the cell.The research found that the electrochemical properties of a single-channel electrolytic cell are rectangular, inverted trapezoidal, trapezoidal, triangular, and circular in order from high to low.The increase in current density at the same voltage is about 0.85% to 11.47%.Considering the temperature distribution and gas distribution of the generated gas, the flow channel structure of the electrolytic cell with a rectangular cross-section should be optimized.

Figure 1 .
Figure 1.Schematic diagram of single-channel electrolytic cell model.

Figure 3 .
Figure 3. Boundary conditions of single-channel electrolytic cell model.

Figure 4 .
Figure 4. Oxygen volume fraction distribution of the electrolytic cell at different cross-section shapes.

Figure 5 .
Figure 5. Hydrogen volume fraction distribution of the electrolytic cell at different cross-section shapes.

Figure 6 .Figure 7 .
Figure 6.Temperature distribution of PEMEC at different cross-section shapes.3.3Influence of cross-section shape on polarization curve

Table 1 .
Geometric parameters of single-channel PEM electrolysis cell.

Table 2 .
Calculation parameters of single-channel PEM electrolysis cell.