Performance Studies of Proton Exchange Membrane Fuel Cells with Different Flow Field Designs – Review

Proton Exchange Membrane Fuel Cell (PEMFC) is majorly used for power generation without producing any emission. In PEMFC, the water generated in the cathode heavily affects the performance of fuel cell which needs better water management. The flow channel designs, dimensions, shape and size of the rib/channel, effective area of the flow channel and material properties are considered for better water management and performance enhancement of the PEMFC in addition to the inlet reactant's mass flow rate, flow directions, relative humidity, pressure and temperature. With the purpose of increasing the output energy of the fuel cell, many flow field designs are being developed continuously. In this paper, the performance of various conventional, modified, hybrid and new flow field designs of the PEMFC is studied in detail. Further the effects of channel tapering, channel bending, landing to channels width ratios, channel cross‐sections and insertion of baffles/blockages/pin‐fins/inserts are reviewed. The power density of the flow field designs, the physical parameters like active area, dimensions of channel/rib, number of channels; and the operating parameters like temperature and pressure are also tabulated.


Fuel Cell
Air pollution is a major problem throughout the world, which is caused by solid/liquid particles and certain gases like Nitrogen oxide (NOx), Carbon dioxide (CO 2 ), Sulphur oxide (SOx) etc. The transport sector is one of the largest contributors for carbon emissions. The pollution control has been initiated from the different sectors and adopts them through suitable sustainable alternatives. The fuel cell is one of the green power systems to overcome the air pollution by replacing IC engines. [1,2] Fuel cell technology is majorly upgraded than coal/oil-burning power plants, diesel power plants, internal combustion engines, and nuclear power plants. Fuel cell is working like battery, but it does not need recharging. [3] There are different types of fuel cells like Alkaline Fuel cell (AFC), Phosphoric Acid Fuel Cell, Proton Exchange Membrane Fuel Cell (PEMFC), Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC). Among several types of fuel cells, the PEMFC is preferred due to its high electrical efficiency and simple design compared to others. [4] The PEMFC is an electrochemical device which employs hydrogen gas and oxygen gas as fuels along with a special proton exchange membrane (Nafion). The schematic diagram of PEMFC is shown in Figure 1. The outputs of the electrochemical reaction in PEMFC are water, electricity, and heat. The electrochemical reactions in PEMFC are given in Eq. (1)(2)(3).
At anode catalyst layer: At cathode catalyst layer: Overall reaction: As the PEMFCs are having a high energy density [5] and operating at atmospheric pressure and temperature with zero emissions, they are widely used in Fuel Cell Electric Vehicles (FCEV). [6] As FCEV gives a lot of environmental benefits in urban regions, the major automakers like Toyota, General motors, Hyundai, Honda, Ford, Audi, Mercedes are majorly involving in the development of FCEV. [7] The need for hydrogen which used as a main fuel in PEMFC is also rising rapidly. The hydrogen demand for FCEV in 2030 at UK will be 254000 tons per annum as per the roadmap report. [8] The European Strategic Energy Technology commission targeted the fuel cell technology, and its allied new FCEV production will be increased by 40 % by 2030 for achieving zero-emission vehicles. [9] Figure 2 shows the fuel cell car market upto 2030. [10] Hence FCEV will contribute to the reduction of long period emissions and the pollution control worldwide.
The hydrogen storage and handling, high cost of the component (Platinum catalyst), instability during dynamic conditions, thermal and water handling issues in single-cell and stacks are some of the drawbacks of PEMFC. Many efforts have been carried out to solve the above problems like onboard generation of hydrogen, development of new catalysts at reduced cost [11][12][13] and the design modifications of components for thermal and water handling issues. The design parameters of components (flow field, MEA, current collectors, supporting plates) and the operating parameters of reactants (pressure, temperature, humidity and mass flow rate) are affecting the water management and performance of PEMFC [14][15][16][17] in addition to the material properties of the components. [18] The generated water at cathode side is to be maintained at optimum level to avoid flooding, dehydration, hotspot and drop in performance which are mainly affected by the flow field design. This can be overcome through effective flow field

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Chem. Rec. 2021, 21, 663 -714 design and this review is to focus on the flow field design aspects of the PEMFC.

Importance of Flow Field in PEMFC
The flow field is an important component which is designed and machined to supply the required amount of the oxygen and hydrogen to the Gas Diffusion Layer (GDL) and the Catalyst Layer (CL) with minimum pressure drop on cathode and anode sides respectively. In case of stack, the flow channels are machined on both sides of the plate which is called as a bipolar plate where one side is acting as anode and another side is cathode. The essential properties of flow field materials are high electric and thermal conductivities, high mechanical strength to withstand clamping pressure, non-toxic, water resistant and ease machinability. Usually plain graphite or stainless steel materials are used for this purpose and in which the graphite material is majorly used. The flow field plates perform several functions like carrying the electrons and water to and from the CL in addition to the supply of reactants on cathode and anode sides. The better design of the flow field will help to avoid the water flooding in the cathode and for better water management and higher performance.

Most Popular Flow Field Designs
In PEMFC, the single straight, parallel, serpentine, interdigitated, pin, spiral, cylindrical, radial, natural inspired, square tubular and fractal flow field designs are generally used. The most popular flow field designs of PEMFC are shown in Figure 3. The design of flow field is highly affecting the performance of PEMFC which leads to many modifications to improve its performance further. This article is to review the effects of various conventional flow field designs, modified flow field designs, hybrid flow field designs, new flow field designs, channel tapering, channel bending, landing to channels width ratios, channel cross-sections and insertion of baffles/blockages/pin-fins/inserts on channel and ribs on the performance of PEMFC.

Effects of Different Flow Field Designs
The various design modifications have been made on the conventional flow fields (straight, parallel, serpentine, interdigitated, spiral, cylindrical, radial, bionic and square tubular flow fields) using grooves, blockages, sub channels, microdistributors, orientations, etc.

Straight Flow Field Design
The straight flow field is having only one channel with a straight path. It has uniform cross-section (normally square or rectangle) throughout the path. The provision of traps, open channels, and different shapes of blockages on the flow path are influencing the performance of straight flow fields.

Design Modifications in Straight Flow Field
By modifying the straight channel, the newest trap shaped flow channel (Figure 4a) was designed. [19] The channel with 8 mm long trap (among 2, 5, 8, 11, 14 mm traps) gave higher current densities and improved O 2 cum water distributions over CL. So, the trap shaped channel produced more power   [19], (b) PEMFC with Ducted and Ribbed channels. Drawn from [20], (c) Cross-sectional views of straight, wave, bio-inspired wave, and M-flow channels. Drawn from [22,23]. than the conventional straight channel. The cathode side channel is opened to atmosphere for breathing of air directly in Air Breathing Fuel Cell (ABFC) or Air Breathing-PEMFC (AB-PEMFC). This open channeled (or Ribbed) flow field and the conventional close channeled (or ducted) flow field as shown in Figure 4b were compared for vertical and horizontal orientations. [20] The ribbed design and vertical orientation of channels gave a better performance due to easy breathing of air and easy removal of water by the gravitational effects on water respectively.

Effects of Blockages in Straight Flow Field
In the wave channels, the wave shapes were formed along the straight channels to provide a uniform temperature distribution, high reactant velocity, an improved convective heat transfer, catalytic electrochemical reactions and power density compared to the straight channels. [21] The design of wave channels were further modified by using bioinspired wave form which was copied from the fins of cuttlefish. [22] The low resistance to reactant flow in the wavy channel improved the performance compared to straight channel. Compared to the regular wave channel, the bioinspired wave channel reserved the flowing area of the channel with low flow resistance and high efficiency. The optimized wave channel with 3.05 × 10 À 5 m central amplitude and 3.52 wave cycles gave 2.2 % improved power density. The wave channels were further improved by the M-flow channels which generated lower entropy with same pumping energy and 21.3 % high power density. [23] In M-flow channels, the thickness of blockage and height of obstacle increased the reactant flow in the direction of the wall along with higher mass and heat transfer rates. The cross-sectional views of straight, wave, bio-inspired wave and M-flow channels are shown in Figure 4c.
The various rectangular cylinders (N = 1, 3, 5, 7 as shown in Figure 5a) were transversely installed at the axis of flow channel to estimate the optimal parameters of PEMFC. [24] Among the four designs, the channel with five rectangular cylinders (N = 5) gave high performance and nominal pressure drop. Two transverse pin-fins with little hydraulic diameter along the transverse section of cathode channel (Figure 5b) were inserted. [25] The friction factor is changing in direct and indirect proportional to the clearance ratio (ratio of flow height below the fin and channel height) and GDL porosity, respectively. Further, increasing of Reynolds number increased the diffusion of gases through GDL. Use of Pin-fins with optimum design parameters increased the cell performance.
Six rectangular shaped obstacles (size: 0.4 mm × 0.8 mm) were placed close to the exit of both anode and cathode channels of 50 mm long. [26] Each obstacle was placed at 5 mm gap from the exit. These obstacles in channel increase the reactant distribution along the channel and GDL, thereby increasing the electrochemical reactions and performance of PEMFC at higher (more than 1.5 A/cm 2 ) current densities. The triangular and rectangular Pin-fins of different sizes and tilt angles (4°, 6°and 8°) were inserted in the single channel as shown in Figure 5c to improve the reactant distribution over GDL. [27] The increased radial velocity of reactant and the highest mixed flow created near the interfacial area of channel and GDL are developed by Pin-fins. Hence the reactants are forced towards the GDL. Further the reactant distribution over GDL and performance by rectangular fins were higher than triangular. The rectangular fins with 3 mm of both width and height and the H 2 velocity of 4.22 m/s were the optimum values.

Parallel Flow Field (PFF) Design
The PFF is one in which various straight channels are arranged parallel to each other with single/multiple inlet and outlet. This design is simple and easy to manufacture.

PFF with Varying Inlets
Based on the inlet and outlet positions, the PFF are categorized as U-type, Z-type, Multi Z-type (Multipass parallel), Mixed Z and U-type, 2 U-type and 4 U types as shown in Figure 6a. Though the pressure drop of reactants is reduced in PFF than Serpentine Flow Field (SFF), few of the channels are starved of the reactants due to the maldistribution of flow. The maldistribution is caused strongly due to the design parameters such as the header, channel, and rib dimensions. In the Utype, the flow distribution and flow rate are independent. So, the U-type configuration gave better performance than the Ztype at high flow rates. Both multiple U-type and multiple Ztype have a lower non-uniformity flow index, but multiple Z has much more pressure drop. [28,29] Mixed Z and U-type has better flow distribution than conventional PFF. Dividing the flow field into two halves and providing two inlets have reduced non-uniformity flow factor upto 50 %. [30] The PFF was modified with single and double in/out and varying channel widths of 3 to 1 mm as shown in Figure 6b, where the width of the channel was decreased from entrance to middle and then increased up to exit for even distribution of reactant. [31] The single in/out PFF has equal channel length than double in/out PFF, which resulted in even distribution of reactants. The modified PFF with a single in/out showed high performance than the conventional PFF due to even distribution of reactant. Further the uniform reactant flow and uniform current generation in the modified PFF have reduced the hotspots. [32] The PEMFCs having planar and ducted flow field designs of ABFC with different cathode open areas and different cell orientations (45°inclined cell with cathode upward/downward, horizontal and vertical) on cathode were analysed and found that the vertical orientation of cathode flow field was the best for both designs, due to better water removal characteristics and higher mass and heat transfer. [33]

PFF with Baffles/Blockages
The baffles are the flow obstructing vanes used for increasing the efficiency of fluid flow systems. The different shapes of baffles/blockages and the use of high porous materials in flow channels instead of plain channels are influencing the performance. The various indentations with Trapezoidal, Semicircular and Square shapes were made along the cathode channel (6 dents at 10 mm distances along 80 mm cathode channel). These indentations increased the amount of reactant at CL and current due to synergy effect provoked by advection and diffusion mechanisms. [34] The variation of the dent height, dent locations (parallel and staggered) and rib sizes have increased upto 25 % of the net power. Further, the Trapezoidal indentation showed better performance than others.
The small rectangular baffles and a plain Nickel foam were introduced in the channels of PFF [35] as shown in Figure 7a. In this design, the Nickel metal foam (80 % porosity) was used as a flow distributing layer with total replacement of the channels and rib on the cathode. The current density and oxygen concentrations were increased by the addition of baffle plates and metal foam. The high porosity of metal foam enhanced the current density. Further, the current density was increased by decreasing the flow channel depth. The power density of porous Nickel foam (90 % porosity and 10 À 8 m 2 permeability) as flow field was higher upto 50 % than PFF and 10 % than SFF and Interdigitated Flow Field (IFF) at medium and higher humidity values. [36] Instead of the rectangular blockages, the trapezoidal plates (4 Nos., 0.8 mm height and 45°slope angles) were located in parallel and staggered arrangements along the channels of PFF which in turn they named as parallel trapezoid baffle plate flow field (PTBPFF or FF : 1) and staggered trapezoid baffle plate flow field (STBPFF or FF : 2) respectively as shown in Figure 7b. [37] The cross flow of reactants (the over rib convection) between adjacent channels by staggered arrangement of trapezoidal plates, better water removal rate and lower pressure drop are the advantages of FF : 2 than FF : 1. This resulted in 2.54 % and 6.39 % higher net power density for FF : 2 than FF : 1 and PFF, respectively. The baffle plate flow  [35], (b) PFF with inline and staggered trapezoidal blockages. Drawn from [37], (c) PFF with inline and staggered rectangular blockages, [39] (d) PFF with wire coil inserts. Drawn from [42]. fields commonly result in larger flow resistance and pressure drop compared with the conventional PFF. The Eq. (4) shows the net power calculation of the PEMFC system.
where W GP is the gross power, W PP is the pumping power. For better water management, the first attempt to apply the carbon foams in channels of PEMFC was carried out. [38] So, the carbon foams (80 & 100 PPI pore sizes and 3 % porosity) were inserted along two parallel micro channels. Here the carbon foam was selected than carbon cloth/paper, due to its high mechanical strength. Self-regulation of water content and very low flow maldistribution are the advantages, whereas the very high-pressure drop of reactants is the disadvantage of porous inserted channels compared to hollow channels.
The two PFF with inline and staggered blockages (3 blockages of rectangular shape in each channel) ( Figure 7c) were analyzed. [39,40] These blockages induced the reactant velocity in the direction of GDL, thereby increasing the amount of reactant at CL, reaction rate and performance. Even though the pressure drop due to staggered blockages was high (160 %) compared to conventional PFF, the additional pumping energy needed was very less. The parallel channel with staggered blockage gave the higher performance of 28 % and 18 % than the parallel channels without blockages and with inline blockages respectively due to both over rib and over block convections. The effect of wave gas channel (GC) on PFF, SFF and IFF were analyzed. [41] It was found that the wave GC on PFF improved the gas transport into GDL and performance, whereas the effect of wave GC on SFF and IFF was weakened by sub rib convection.
The wire coils (0.9 mm diameter, 1.3 mm pitch, Silver material, 28 coils) were inserted inside the cathode flow channels of PFF as shown in Figure 7d. [42] It has produced 41 % more power density than the channel without coil due to the generation of circumferential flow of reactant near GDL, thereby forcing the reactants towards GDL and increasing the electrochemical reactions. The effect of inserting small cylindrical shaped obstacles (Each of 0.8 mm diameter in 9 rows with zigzag manner) inside the input header of PFF (having 32 channels) was analyzed. [43] These obstacles have reduced the maldistribution factor (MF) upto 3.8, thereby maintaining almost equal mass flow rates in all the channels.

PFF with Sub Channels and Microgrooves
The addition of sub channels, transition areas, micro distributors, and grooves in PFF is also affecting the performance of PFF. The SFR (Sub-channel Flow Rate) is the quantity of air from the sub channel entry as a percentage of the entire flow rate in cathode. The performance of PFF with SFR and SIP (Sub-channel Inlet Position) on cathode (Figure 8a) was investigated. [44] The SFR with 70 % and SIP at 30 % (both among 30 %, 50 %, and 70 % values) along the length of the main channel inlet had minimized 70 % of water vapour which was used for humidification. The PFF with optimum SFR and SIP values gave a better performance and water removal than the conventional design at high current densities.
The effect of micro distributor of 0.2 × 0.2 mm 2 sectional area at the inlet of each channel on PFF was tested. [45] This modified PFF (Figure 8b) gave 22.8 % higher performance than the conventional PFF and it showed a comparable output performance with the SFF. Further, the performance was increased by decreasing the size of the micro distributor. The reduction of uneven supply of O 2 in the interfacial area of GDL and CL can be achieved by the increased pressure drop between the main in and out channels.
The channel with microgrooves on rib walls and top of channels ( Figure 8c) were designed. [46] The microgrooves (width and depth in mm of 0.2 × 0.2 and 0.3 × 0.2 with 20°, 30°and 45°inclinations along the flow direction) were made in two lateral sides and top of channel. Due to shear forces and capillarity of air flow, the water was taken away from GDL-Landing interfacial area along microgrooves to channel and 3.2 % of power output was increased at 8 m/s air velocity. The Laser Induced Fluorescence method (LIF) was used to measure the water characteristics in the microgrooves. Further, 20°i nclination of microgrooves showed better water removal characteristics. [47] A novel 3D cathode flow field with Porous bottom landings, main channels, sub-channels, and transition areas was investigated. [48] The performance of the 3D flow fields was much better than the conventional PFF. Further, the species transport was better in this new flow field. Porous bottom landings increased the gas transfer to GDL/CL from the channel with minimum pressure drop, thereby improving the performance. The PFF with four main channels were cut with additional sub channels, inclinations, and transition areas for mixing and redistribution of reactant. This 4 × 3 structure of 3D flow field with 4 channels, 3 segments in each channel and height reduction in the linear axis is shown in Figure 8d. [49] This modified 3D flow field increased the Effective Mass Transfer Coefficient (EMTC) and cell performance compared to PFF.

PFF with Novel Designs
In PFF with conventional arrangement of inlet, the distribution of reactants is not uniform in all channels. To solve this issue and for the uniform supply of gases to all channels, the funnel shaped in/out was introduced on 225 cm 2 PEMFC. [50] Further the straight channels were bent to form a Zigzag PFF with single funnelled in/out. The current and power densities of zigzag PFF with single funnelled in/out are much more than the straight PFF with single funnelled in/out due to improved consumption of reactants, optimum distribution of water in the electrolyte and better removal of excess water from the cathode channels.
When the channel count is exceeding 13 in PFF with single inlet (with 500 ml/min flow rate), the uniform flow of reactants was disturbed. To improve the uniform supply of gases to all channels of zigzag PFF with larger active areas, the double funnelled inlet were introduced on 100 cm 2 PEMFC. [51] This zigzag PFF with funnelled in/out is also termed as sinuous flow field. The funnelled inlet and outlet made the uniform distribution of reactants to all channels at entry and effective removal of generated water at exit of flow field. The sinuous FF has increased the power density by 7.7 % than SFF. The various modifications in PFF with funnelled in/out are shown in Figure 9a.
In case of large sized PEMFC, the separate cooling flow channels are provided in bipolar plates. A new zigzag wavy cooling flow channel having the cooling plate with a pitch length of 4 mm and a channel width of 2 mm (Figure 9b) were analysed. [52] The performance of the zigzag channels was higher and about 23 %, 5 %, and 8 % reductions were detected, in surface temperature difference, maximum surface temperatures, and uniformity indices respectively, compared to  [45], (c) Channels with and without microgrooves, [46] (d) PFF and 3D flow field with 4 × 3 structure. Drawn from [49]. the straight channel. The thermal performance of PEMFC was improved with zigzag cooling flow channel.
The PFF and sinusoidal flow field (channels are same as zigzag flow channels) were compared. [53] Even though the pressure drop of sinusoidal flow field was 6 times more than PFF with more power consumption, this was helpful in the removal of water at higher current densities. Further, the performance of the sinusoidal flow field was better at concentration loss region, due to uniform reactant distribution and better water management.
A novel flow field-Through-Plane Array (TPA) (Figure 9c) was designed which had the advantages of high performance at high current density and low back pressure than PFF and SFF. [54] A construction of Printed Circuit Board (PCB) fuel cell was used without the need of end plates. In TPA design, the small holes of diameter less than 0.01 mm were incorporated on the landing areas of the PFF using laser beam. By forcing the oxygen to go into GDL in order to exit, the gas was exposed to under rib areas. This enhanced the easy removal of water and improved the performance.
A novel PFF (Figure 9d) was designed to have two sets of parallel channels which were run at different operating pressure. [55] The controlling pressure gradient in between the nearby channels, was obtained in this novel FF which increased the reactant concentrations and water removal under the rib at cathode.

Serpentine Flow Field (SFF) Design
The serpentine flow field (SFF) is one in which the flow path is continuous from inlet to outlet and the channel arrangement resembles the form or movement of a serpent/snake. The pressure drop of reactants is increased in SFF than PFF. As SFF is efficient in distributing the reactant throughout the active area of PEMFC, this is majorly used in PEMFC. The SFF gave a better performance than the straight FF. [56]

Effects of Multi-passes in SFF
In general, SFF has a single channel path from inlet to outlet. The arrangement of two or more paths parallel to the first serpentine path is known as a multi path SFF (MPSFF or FF : 3). The single pass SFF (1-SFF), two pass SFF (2-SFF), cyclic 1-SFF and symmetric 1-SFF ( Figure 10a) were analysed. [57] In Cyclic 1-SFF and Symmetric 1-SFF, the total length of single serpentine path of 1-SFF was split with five segments with separate in/out and varying flow directions. The performance of 2-SFF was higher at higher humidity than  [50,51], (b) Zigzag flow channel. Drawn from [52], (c) Novel flow field-TPA, [54] (d) Novel PFF having two sets of parallel channels. [55] other three designs, whereas the cyclic 1-SFF and symmetric 1-SFF produced lower pressure drop and higher performance at lower humidity.
From the comparison of 2, 3 and 4 pass SFF of same rib and channel size, it was found that the performance of 3 pass SFF was better compared to others. [58] Similarly from the comparison of 3, 6 and 9 pass SFF, it was found that the performance of 6 pass SFF was better compared to others. [59] The performance of 4-SFF and 1-SFF with parallel, counter and cross flows were investigated. [60] Compared to cross and counter flows, the parallel flow increased the performance of 4-SFF and 1-SFF at low and high voltages, respectively. However, the performance of 4-SFF was better than 1-SFF, when working under the same operating conditions. The 3-SFF was analysed for evaluating the reactant humidification, water control and performance. [61] It was found that the humidity in the reactant gases was the major factor for increasing the cell performance. When the anode side humidity was increased, then the mass transfer of hydrogen, the chemical reaction and cell performance were increased in the triple-SFF. The FF : 3 with common in/out header for each set flow channels is known as parallel-in-series (PIS) design. In this way, the 3-SFF with common mixed inlet path, called Triple mixed SFF (Figure 10b) was analysed for High Temperature PEMFC (HTPEMFC) at various operating conditions. [62] Due to high proton conductivity and quick reaction kinetics, the performance of the fuel cell was improved with increase in temperature.
Due to higher flow velocity of reactants, the water management and performance of 3PIS and 5PIS designs were higher than FF : 3. [63] The FF : 3 with the effect of parameters like the channel width (w) and height (h), channel length (L), the numbers of parallel channels (n), the rib between two adjacent channels (d) and serpentine channel turns (s) were studied. [64] It was found that the most excellent possible SFF configuration for the running condition with geometrical specifications as 1, 1, 0.4, 249, 5, 4 for w, d, h, L, n, and s, respectively.
While comparing the PFF (17 channels), 1-SFF and 7-SFF, there was a considerable balance among the flow distribution and pressure drop in 7-SFF with robust operation of PEMFC. [65] The PEMFC with 3, 6, 13, 26 and symmetric 26 channelled MPSFF or FF : 3 was analysed to find the effects of channel path length and more number of passes. [66] The shorter path length gave uniform current distributions and less condensed water and flooding, compared to the longer path length. The 13-channel was better than 26-channel (Figure 10c), because of the differences in membrane hydration. Generally, the 90°bends are made on the channels of SFF. The single, double and triple SFF configurations namely 1-S, 2-S and 3-S respectively, having curved bends were studied. [67] When operated below 0.7 V only, the performance of the cell increased by decreasing the number of flow channel passes. This estimated the 1-S gave superior performance than the 2-S and 3-S.

SBFF
To enhance the under rib convection, the three pass serpentine flow field was redesigned with dead end which was named as a serpentine-baffle flow field (SBFF or FF : 4). [68] The FF : 4 design ( Figure 11a) showed a higher performance than SFF at low cell voltages, whereas the same performance was obtained in both designs at high cell voltages. The baffles in SFF induced the higher-pressure differences between adjacent channels over the entire active area, thereby increasing the under-rib convection, mass transport, limiting current density and cell performance.
A Parallel Serpentine-Baffle Flow Field Plate (PSBFFP or FF : 5) which was the modification of the FF : 4 for improving the water management was proposed. [69] Refer Figure 11b-11c. The FF : 5 design had the advantages of both Parallel Serpentine Flow Field Plate (PSFFP or FF : 6) and IFF Plate. The FF : 5 channels are the best suited for the operating temperature more than 100°C because it avoids water loss from the flow channels. The FF : 5 which is the FF : 6 along with interdigitated features on anode side of DE-PEMFC was analysed. [70] The discontinuous path of channels in FF : 5 improved the heat transfer between reagent and plate, thus increased the reactions and the performance.

CESFF
The continuous path of SFF is redesigned to enhance its performance. The under-rib convection is an important factor which is affecting the flow and distribution of reactants, water removal and performance of PEMFC. A Convection-Enhanced Serpentine Flow Field (CESFF or FF : 7) as shown in Figure 11d for DMFC was analyzed. [71] In FF : 7, the first and third paths carry the reactant from top to bottom of flow field, whereas the second and fourth paths carry reactants from bottom to top. Due to the large pressure difference between the nearby flow channels induced by FF : 7, the mass flow rate of reactants to and from the CL has increased along with better water management. The under-rib convection in FF : 7 removes the water from the active area by raising the local pressure variation between adjacent pairs of channels. The FF : 7 gave better performance than the conventional SFF.
Five different FF : 7 designs are analysed and compared with SFF to enhance the under rib mass transport of SFF. [72] The CESFF or FF : 7 are designated by its name followed by the number of paths and design number as CESFF 3-1, 3-2, 4-1, 4-2, and 4-3. The CESFF 4-3 configuration (Figure 11e) gave 22.6 % increase in power when compared to the SFF. A cascade type of SFF ( Figure 11f) with different channel widths of 0.8 mm, 1 mm, 1.2 mm, and 2.2 mm was designed. [73] The membrane flooding and drying problems are affecting the life of Membrane Electrode Assembly (MEA). The channel width, channel depth and rib width of 1.2 mm, 0.8 mm and 0.8 mm respectively, confirmed the best performance with uniform current density and better water management in cascade FF.
A Compensated SFF ( Figure 11g) with 5 channels, 6 turns to have GDL area and electrode area of 49 cm 2 and 46 cm 2 respectively was proposed by the modification of SFF for HT-PEMFC. [74] The compensated SFF gave 27 % higher current density at 0.57 V than SFF, due to more even distribution of current over the electrode area. A CDMD (Current density measurement device) was fabricated and used for current distribution. The simple design and low cost are the main features of CDMD compared to other devices.

MSFF
Nine different Modified Serpentine Flow Fields (MSFF or FF : 8) were analysed based on number segments and channels. [75] The MSFF-1-7 and MSFF-3-7 (Figure 12a-12b) gave a better performance than PFF. The increased channel path number and the secondary flow in the channel (due to the serpentine path turn and the reactant transportation under the landing) have improved the performance of the MSFF-3-7 than parallel type.

ECSSFF
With the aim of reducing the pressure drop of reactants and parasitic losses of three pass SFF, the Enhanced Cross flow Split Serpentine Flow Field (ECSSFF or FF : 9) was designed. [76] This FF : 9 has three different serpentine flow channels arranged individually from a single inlet to reach a common outlet so that the same pressure drop is occurring between the entry and exit of each serpentine channel. This FF : 9 was useful in the evacuation of water in U bends. Further, the pressure drop of FF : 9 was reduced upto 30 times than Single Serpentine Flow Field (SSFF or FF : 10. [77] The FF : 9 ( Figure 12c) gave 7.51 % of higher power density than Triple Serpentine Flow Field (TSFF or FF : 11) due to better water management and less parasitic losses. [78] Further the scaling up studies of FF : 9 with active areas of 50,

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Chem. Rec. 2021, 21, 663 -714 100, 150, 200 cm 2 were analysed. [79] While increasing the size of the active area from low to high, the power was increased 4.5 to 13.5 % in FF : 9 compared to FF : 11. The performance of Single Variable section Serpentine Flow Field (SVSFF or FF : 12), Single Serpentine Flow Field (FF : 10) and 2-1 SFF on cathode at cold start conditions were investigated. [80] In 2-1 SFF, two serpentine channels were combined into one channel at the mid of active area. The 3-SFF engraved on the PCB was used on anode side. The FF : 12 showed better cold start performance below À 5°C with higher pressure drop and lower outstanding water in the flow field. The 2-1 SFF (Figure 12d) showed the better performance at cold start above À 5°C due to the most uniformity of current density.
The compound FF was designed which was the mixture of 5 pass SFF and PFF (Figure 12e). [81] Even though the performance of compound FF and SFF were approximately similar, the prevention of water flooding by compound FF was better than SFF.

SFF with Zigzag and Wavy Designs
A novel modified SFF called W flow field (Figure 13a) was developed and compared with seven Z and W types of flow fields. [82] As this novel design covered the maximum active surface and produced the uniform current, the performance was superior to other designs. The straight channel of SFF was bent to form a zigzag channel which was named as a Zigzag SFF as shown in Figure 13b. The straight zigzag channel showed better water removal and high performance with a minimum drop in the pressure when compared to the other three types namely PFF, Zigzag PFF and straight SFF.
Three sinusoidal wavy serpentine flow channels (Figure 13c) having the same wave length of 6.28 mm and three different amplitudes of 0.25 mm (C1), 0.5 mm (C2) and 0.75 mm (C3) were compared with the conventional serpentine channel (C4) for different cell temperature and H 2 flow rates. [83] The C1 showed higher results up to 20.15 % than the base model (C4). At all flow rates, the C1 exhibited good performance than C2, C3 and C4. Further, the increase in reactant flow rates, have increased the diffusion and power.
The novel three pass Waved Serpentine Flow Fields (WSFF or FF : 13) with bend radius of 0.5 mm and three different slope angles ( Figure 13d) on channels were developed and compared with SFF of zero slope angle. [84] The wave channels affect the flow velocity, flow direction and pressure which cause the secondary and cross overflows thereby increasing the mass transfer in GDL and CL. Hence, the O 2 transport through GDL and the water management were better in FF : 13 with 17.5 % rise in power density than the  [75], (c) ECSSFF Drawn from [78], (d) 2 to 1 SFF. Drawn from [80], (e) Compound FF. Drawn from [81].

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Chem. Rec. 2021, 21, 663 -714 SFF (slope angle is zero). Further, the lower pressure drops over the entire current density range and better water removal are the important characterises of FF : 13 over SFF.

SFF with Varying Channel Width/Height and Subchannels
The effects of changing the length and height of the serpentine channel ( Figure 14a) was analyzed. [85] With the length reduction ratio of 0.3 and height reduction ratio of 0.4, the gas velocities, water removal and cell performance were increased. The effect of varying the channel width was analysed by designing four convergent serpentine flow slabs (TS28, TS36, TS44, TS52) and compared with One common-type (OS) design ( Figure 14b). [86] Because of the proper mixing of fuels, the TS36 gave 1.35 times higher current density than OS. The SFF of 5 passes and 4 turns without and with subchannels at landings (Figure 14c) were investigated. [87] Due to the easy removal of water, less power drop and uniform current density, the power output of the SFF having subchannels was higher than the conventional SFF.

Effects of Orientations of Serpentine Channel
The different horizontal configurations (VÀ HÀ D, VÀ HÀ U, VÀ VÀ D, VÀ VÀ U) and vertical configurations (HÀ HÀ U, HÀ HÀ D) of single SFF on two-phase flow were investigated to find the effects of orientations with different inlet/outlet. [88] The horizontally located channels along with upper side inlet manifolds showed the lowest pressure drop cum parasitic losses, easy water removal and higher performance. The serpentine multi-pass channels with horizontal and vertical orientations were investigated and found that the cell with horizontal orientation gave better performance (voltage gain of 20-35 mV at 10 A), superior membrane hydration and lower liquid blockages than the vertical orientation. [89]

Effects of Insert Technology in SFF
To enhance the performance of SFF with better water management, a specially made porous carbon inserts (PCI) were inserted along the cathode ribs [90,91] in uniform and staggered arrangements (Figure 15a,b). The PCI are made of Vulcan carbon. The water between the interfacial area of GDL and the rib was absorbed by the PCI and delivered through the lateral sides into the channel. Due to the globalized water absorption throughout the active area by PCI and better water management, the staggered arrangements of PCI along the rib of the cathode flow field increased the performance up to 11.56 % compared to SFF. Further, the scaling up studies on PCI accommodated PEMFC [92] with three active areas 25, 36 and 70 cm 2 showed that the 36 cm 2 was the optimum active  [82], (c) Wavy shape serpentine channel (a -amplitude). Drawn from [83], (d) Straight and waved flow channels of WSFF. Drawn from [84].

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Chem. Rec. 2021, 21, 663 -714 area which produced higher power density than 25 and 70 cm 2 .
To enhance the performance of SFF with better water management further, a specially made porous sponge inserts (PSI) were inserted along the cathode ribs in uniform and staggered arrangements. The PCI made of phenolic foam were inserted in uniform and staggered arrangements. The two flow fields namely Modified Serpentine flow field with Inline arrangement of porous inserts (MSI) and Modified Serpentine flow field with Staggered arrangement of porous inserts (MSS) with Porous Sponge Inserts were analysed. [93] Because of good water absorption feature of PSI than PCI, the MSS with PSI yielded good performance than the MSI (Figure 15c).
When the nickel foams of 0.95 porosity, 110 PPI and different thicknesses (1 to 3 mm) were used to replace the cathode SFF, and keeping SFF on anode side, the performance was increased upto 6 % and the internal impedance was reduced than graphite SFF. [94] Here the performance was analysed by Electrochemical Impedance and Electrochemical active surface area (EASA).  [86], (c) SFF without and with sub channels. Drawn from [87].

SFF with DEA
Sometimes the anodic flooding occurs in PEMFC, due to the back diffusion of water from cathode which causes voltage drop. [95] So periodic purging is necessary to avoid this. The H 2 pulsation is majorly affecting the performance of DEA-PEMFCs for portable applications. [96] The influence of Dead End Anode (DEA) on PFF, IFF and SFF in anode side was analysed. For SFF with anode exit reservoir and IFF without anode exit reservoir, the steady-state operation of PEMFC with DEA can be obtained. But the PFF with DEA was not fitted for PEMFC. [97]

Interdigitated Flow Field (IFF) Design
The flow field with dead ended channels, called interdigitated flow field (IFF) was proposed by Nguyen. [98] This design changed the diffusion transfer mechanism of the gases to/from CL into forced convection mechanism. [99] As the convection process is faster than diffusion process, the electrochemical reactions at CL of IFF are majorly increased. Further the water trapped inside the electrodes are taken away by the shear force of reactant flow.

Effects of Baffles in IFF
The insertions of baffles are changing the design parameters and performance of IFF. The PFF was modified into the IFF by using baffles at end of the path (Type I) and at the mid path (Type II) as shown in Figure 16a. [100] The blockage of channels increased the reactant flow transportation and cell performance. Further, the use of oxygen instead of air in the cathode side provided better results. A mid-baffle IFF gave 1.3 times more power output than the conventional IFF when air was used as reactant on the cathode. [101] When back pressure was raised from 1 to 2 bar, the power density of the mid-baffle IFF was improved by 24 %. The number of rectangular parallelepipeds of N = 1,3,5,7,8,9 and their different positions in inlet and outlet channels (Figure 16b) on the IFF were investigated. [102] The arrangement of rectangular parallelepipeds on outlet channels with the optimum combination of process parameters on the novel flow field gave 26 % more power than the smooth-walled channel. The scaling up and stacking up studies on IFF and SFF with two C : L ratios (1:1, 2:2) showed that the IFF gave higher performance than SFF. [103] When back pressure was varied from 0 to 1.5 bar, the power density was raised by 65 % for IFF with C : L ratios (1:1). The scaling up and stacking up of PEMFC has decreased the cell performance due to poor water management.

Design Modifications in IFF
The new slotted-IFF for stamped metal bipolar plates with uniform flow distribution and improved cell performance was proposed and optimized. [104] The two slots of each two adjacent channels in new slotted-IFF gave better performance due to uniform flow distributions. [105] The impacts of channel length on IFF was investigated and found that the fuel cell with 5 cm length gave better performance than 25 cm length. Further, there were a maldistribution in the longer cell and homogenous cross flow in the shorter cell. The mass transport and electrochemical reactions of PEMFC were evaluated. [106] For the conventional straight flow channel, the least amount of current density was found beneath the land area. The current density of the IFF was comparatively higher than the straight flow field. The two pass IFF was analysed at various temperature and C : L values and found that the C : L of 2 : 1 yielded the maximum power density. [107]

Hybrid Designs in IFF
A novel flow field (Figure 16c) was developed which can be exchanged between PFF and IFF. [108] This FF has two flow paths with separate in/out and set of valves. By opening and closing of valves, the FF can be exchanged as PFF and IFF. The IFF gave higher current density and power at peak current density with low overvoltage.
The novel Hybrid Serpentine Interdigitated (HSI or FF : 14) flow fields namely one inlet-one outlet (1-IO) FF : 14, two inlets-two outlets (2-IO) FF : 14 and one inlet-two outlets (1I-2O) FF : 14 were developed and compared with one channel serpentine (1S). [109] The FF : 14 flow field has the advantages of both SFF and IFF. At 0.6 V, the 2-IO FF : 14 flow field (Figure 16d) showed better cell performance than other 1S and other two FF : 14 flow fields due to uniform gas distributions, temperature and current.

Spiral, Cylindrical, and Radial flow fields
The multiple concentric spirals for the flow field with 1, 2, 3, 4, 6 and 8 concentric spirals channels were investigated. [110] The flow field with 3 and 4 channels (Figure 17a) gave better performance, whereas the 6 and 8 channels gave the worst performance. Due to better distribution of gases, the 4-channel design was the best one. Five inlet and outlet spiral-shaped flow channels with radius of 28.2 mm (Figure 17b) were investigated. [111] The spiral flow field produced secondary vortices which led to the increased performance than the SFF. With the smaller outlet channel than the inlet channel, there was an improvement of water flooding on CL.
The designs of parallel, serpentine, and interdigitated in a cylindrical configuration (Figure 17c) were analysed. [112] The results showed that the cylindrical or tubular SFF had better performance than others. Because of twist and gradual declination of flow path angle in tubular SFF, there were a low-pressure drop along the channel designs, easy removal of water from GDL and high O 2 concentration at CL. The cylindrical PEMFCs are having high gravimetric and volumet-ric power density compared to the planar PEMFC. Even though the design of cylindrical PEMFC is simple, its fabrication is complex. The 40 % total power density was increased with the optimization of anode flow rate and cathode landing width. [113] An innovative radial flow field with control rings showed a superior flow distribution and water removal, better utilization of active area and lower pressure drop than parallel and serpentine types. [114]

Natural Inspired Flow Field Design
The flow field designs that are inspired from the nature like veins of plant leaves and human lungs are known as Natural inspired or Bio inspired flow fields. The various types of Natural inspired flow fields like bio-inspired with and without subchannels/obstacles, biomimetic, Ginkgo and Net leaf  [112], (c) Cylindrical designs of PFF, IFF and SFF. Drawn from [113].

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Chem. Rec. 2021, 21, 663 -714 designs were developed. The Bio-inspired (BI) flow field ( Figure 18a) which was developed from the leaf flow patterns, showed the higher power densities of 26 % and 56 % at various current densities. when compared to SFF and PFF respectively, [115] due to more uniform flow and velocity distribution.
The impact of the bionic flow field (Figure 18b) using different branching structures through biomimetic analogy theory were investigated. [116] The location and number of branches played a vital role in exit velocity, which had impacts on the discharge of water. Due to secondary channels, more gas consumption ratio, even reactant distribution and low ohmic losses, the bionic flow field had high performance than the conventional flow field. A novel Biomimetic flow field with main, sub and third-level channels of varying width and height was designed based on Murray's Law as shown in Figure 18c. [117] When compared to serpentine and parallel types, the biomimetic flow field yielded higher power, uniform distribution of gases and easy removal of water.
Based on the human lungs and leaf veins of natural network system, the lung and two bio-inspired leaf flow fields were designed (Figure 18d) using Murray's law on DMFC. [118] While testing the various combinations of flow fields on anode and cathode, the combinations of SFF on anode and second leaf design on cathode showed the highest performance. Further the leaf FF showed higher performance on cathode and the lung FF showed lower performance on both anode and cathode. The lung FF yielded 34 % higher power than triple SFF. [119] Three Bio-inspired IFF (Figure 19a) were designed based on Murray's law and these bio-inspired designs substantially improved the fuel cell performance by 25 % compared to the conventional IFF and 5 channeled parallel-in-series designs. [120] The various biologically inspired IFF (Figure 19b) were designed using Murray's law and Pitchfork pattern. [121] The biologically inspired IFF with 9 entry channels had higher current density distributions and lower pressure drops.
The Ginkgo and Net leaf flow fields were analyzed ( Figure 19c) and found that the water removal of the Ginkgo FF was better than PFF. [122] Further, the peak power density of Ginkgo FF was 40 % higher than PFF and 7 % lower than SFF. A semi cylindrical shaped obstacles along the daughter channels of natural inspired flow field which is called as a Nature inspired design with obstacle (NIDO) of PEMFC has Figure 18. (a) Bio inspired (BI) flow field. Drawn from [115], (b) Biomimetic flow field. Drawn from [116], (c) New Bionic Flow fields. Drawn from [117], (d) Bio-inspired Leaf FF-1, Leaf FF-2 and Lung FF. Drawn from [118].

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Chem. Rec. 2021, 21, 663 -714 improved the diffusion of reactant into the GDL and 41 % higher performance than SFF (Figure 19d). [123]

Square Tubular Flow Field
The square tubular FF is having a squared cross-sectional area. Compared to the planar design, the tubular design is having the advantages of even pressure distribution on MEA. The effects of different architectures of GDL, membrane and CL Figure 19. (a) Bio-inspired IFF with constant channel width, Bio-inspired Non-IFF with constant channel width, Bio-inspired IFF with varying channel width, [120] (b) Murray interdigitated-2 design (left) and Interdigitated pitchfork design (right). Drawn from [121], (c) Ginkgo and Net leaf FF, [122] (d) Obstacles along the daughter channels. [123]

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Chem. Rec. 2021, 21, 663 -714 with square tubular PEMFC under the cases of Square Bisectors (SB), Square Chordal (SC) and Square Peripheral (SP) as shown in Figure 20a were investigated. [124] Among the three models, SC architecture had higher power density, and higher consumption of hydrogen and oxygen mass fractions.
The results showed that the three-square architectures gave better performance than the conventional model. Four square tubular flow fields namely simple square tubular (without intermediate electrode), Double Bisectors Intermediate Electrode (DBIE), Triple Parallel Intermediate Electrode (TPIE) and Double Parallel Intermediate Electrode (DPIE) were investigated. [125] DPIE and DBIE are shown in Figure 20b. Due to less mass transport losses in tubular flow fields and the additional intermediate electrode, the current density and cell performance were increased. Further, the performance of DPIE was greater than DBIE.

Mesh Type and Fractal FF Designs
The mesh-type designs (also known as pin or land-pillar or grid type) are made by two orthogonal sets of parallel flow channels. The intersectant FF (Figure 21a) with optimum porosity and channel depth of 0.5 and 0.3 mm respectively gave superior performance than the SFF. [126] The perforated FF consisting of 169 holes of each diameter 2 mm (Figure 21b) was analysed [127] and found that the perforated FF generated 5.2 %, 6.5 % and 5.8 % of more power densities at 1, 0.5 and 0 bar pressure respectively on comparison with SFF. The honeycomb FF (Figure 21c) having the hexagonal pins was analysed [128] and found that the hexagonal pins increased the diffusion rate of oxygen through GDL by 10 times. The land-pillar flow fields with different channel width (CW) of 2 and 3 mm and single/multiple inlets were optimized and it is found that the flow field with CW of 3 mm, single inlet and 76 % flow area showed higher performance due to uniform reactant distributions. [129] When compared to the uniform pin type, the zigzag pin type flow field (Figure 21d) showed higher performance of up to 3 % and 12 % on 25 cm 2 and 70 cm 2 PEMFCs respectively, due to even distribution of reactant on active area.
The cascade FF with matching and crossed design configurations were analysed [130] and found that the large number of electrochemical reactions were carried out near the H 2 entry and thereby more water was formed. To improve the performance, it was suggested to give a greater number of H 2 inlets. Further, the area of catalyst under the rib was being unused, thereby it reduced the performance of PEMFC.
For the commercialization of PEMFC, the cost of the stack is to be reduced which is possible with large sized FF plates. So, the large-shaped branch channel with broad active area of 300 cm 2 and flow characteristics was proposed. [131] The branching factor (f) was ranged from 1 to 0.5 at the branch outlet. Even though the performance of branch channel was 1.7 % lower than SFF, its pressure drop was also 12.47 % lower than SFF. So, the PEMFC with branch channel FF reduced the blower's power consumption and increased the system efficiency. The branched channel with f = 0.5 gave 3.55 % higher performance than the serpentine channel due to less electric power consumption of blower.
In the air-breathing cathode with three-channel widths and fixed rib width of 1 mm, the air was brought into the channels by a natural convection. [132] It was found that the 3 mm channel width and 75.9 % open ratio gave the best performance from the open ratios of 67.5, 75.9 and 80 % and channel widths of 2, 3 and 4 mm. The three dimensional flow field (3DFF) with trapezoidal baffles of different inclination angles (90°, 15°, 10°and 5°) increased the reactant transport and the power (upto 15 %) than PFF. [133] Further, the baffles with small inclination angle were the best for the high water and high relative humidity conditions.
The performance of a fractal design (Figure 22a) was higher than the PFF, but lower than the SFF. [134] The constructal flow distributor with 16 and 64 outlets using fractal structure (Figure 22b) had a good hydrodynamic performance. [135]

Effects of Different Landing and Channels Width Ratios of Channels
The channel in FF is the flow path for the reactants and the landing is the wall separating the channels. The landing and channels width ratios are influencing the reactant flow, current conduction and performance of PEMFC. The performance of a fixed channel height (0.55 mm) and varying channel/rib ratios (0.9/0.9, 1/0.7, 0.7/1) were investigated for different flow channel designs (Figure 23a). [136] The channel with high rib ratios showed high performance and better heat transfer. The six Channel to Rib width ratios (η) (Figure 23b) for SFF, IFF and PFF having 25 channels were investigated. [137] While increasing the rib width and decreasing the channel width, the performance of the cell was improved. At η = 0.25 than 2.66, the performance was increased as 120 %, 45 % and 23 % for SFF, IFF and PFF respectively. SFF with different (horizontal and vertical) cross-section aspect ratio of the channel as shown in Figure 23c was investigated. [138] The ratio between the height and width is known as the aspect ratio. The cell was operated under aspect ratios ranging from 0.07 to 15. The channel with a high crosssection aspect ratio gave better performance than the channel with low cross section aspect ratio. The aspect ratios of 10 : 06 and 12 : 05 gave the best results.
The different aspect ratios (0.8 and 1.25) and crosssectional areas (2, 5 and 8 mm 2 ) of flow channels were investigated. [143] The cell performance is directly proportional to the aspect ratio and inversely proportional to the crosssectional area. The reduction of the rib area has increased the oxygen molar concentrations between GDL and CL. The 5 pass SFF with different channel widths (1, 1.25, 1.5 and 1.75 mm) and different channel heights (0.34, 0.5, 0.67 and 0.83 mm) were investigated. [144] Increasing the channel height has increased the area and water accumulation; and has decreased the cell performance. Further, increasing of channel width caused the membrane dehydration.

Effects Of Different Shapes of the Channel
The cross-sections of the flow channels are also influencing the performance of PEMFC. The various cross-sectional shapes like rectangular, trapezoidal and parallelogram, triangular, stepped, semi-circular, hemisphere, slanted, parallelogram and combinations of above were developed and analysed.
From the analysis of three channel shapes (rectangular, trapezoidal and parallelogram) as shown in Figure 24a, the rectangular channel gave higher cell voltage than other channels. [145] The rectangular channel offered elevated cell potentials, but the trapezoidal channel offered uniform current density distributions. The triangular and rectangular crosssections ( Figure 24b) were used with and without PTFE coating to increase the surface static contact angle. [146] The water accumulation in the channel was largely affected the performance of the cell. With the use of PTFE coated membrane, the water clogging was reduced to an optimal level. Further, the triangle channel kept low water than rectangular.

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Chem. Rec. 2021, 21, 663 -714 The square, trapezoidal and stepped channels (Figure 24c) were investigated. [147] The square channel exhibited better electric performance than the trapezoidal and stepped channels, whereas the trapezoidal and stepped channels showed better water management than the square channel. The semicircular, trapezoidal and triangular channels and rectangular channel were analyzed. [148] Based on cell performance, the channel cross-sections can be ordered from high to low range as triangular, semi-circular, trapezoidal and rectangular (Figure 24d). At low cell voltages, the rate of electrochemical reactions and water formation were more. So, there was the effect of channel cross-sections only in low voltages, not in high voltages. The effect of shape and channel dimensions were analyzed. [149] The triangle and hemisphere shapes gave 9 % more H 2 consumption than rectangular shape. The optimum values of the channel depth, channel width and land width were found as 1.5, 1.5 and 0.5 mm respectively at 80 % H 2 consumption. It also showed that the H 2 consumption at the anode was increased with decreasing of land width. The increase in the H 2 consumption has increased the performance.

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Chem. Rec. 2021, 21, 663 -714 triangular and rectangular of same area (3 cm 2 ) were analyzed. [151] It is found that the rectangular channel offered higher cell performance compared to others. The 25 new channel cross-sections with the combinations of Rectangular (Rct), Trapezoidal (Trp), Half elliptical (Hlp) and Triangular (Trg) shapes in straight and inverted positions were designed and analysed. [152] The Half elliptical, Trapezoidal, Rectangular-Half elliptical, Rectangular-Trapezoidal, Rectangular-Triangular and Trapezoidal-Inverted Trapezoidal shapes (Figure 24e) produced more power density than a rectangular shape. Among 25 new channels and 5 conventional channels, the Trapezoidal-Inverted Trapezoidal shape gave higher power density up to 4.65 %.
A channel with 35°inclined slanted grooves (Figure 24f) were designed and analysed and it is found that the downward slanting of the channel on anode stimulated the back diffusion of water from cathode to anode. [153] This increased the membrane conductivity, hydration, and performance.

Effects of Taper in Flow Fields
The tapering in channel width/height, inlet header of various flow fields is affecting its performance.

Taper in Straight Flow Field and PFF
Two novel PFF with channels tapered in height and width ( Figure 25a) were proposed. [154] The reduction on the height of the cathode channel outlet increased the fuel consumption efficiency and performance. Further, increasing the width of the tapered channel has increased the contact area between GDL and channel, thereby giving better performance. The highest performance was obtained for 0.5 and 1.8 values of the height and width taper ratios, respectively. The channel taper angle of 0.75°(among 0°, 0.5°, 0.75°on 50 mm long channel) increased the power density by 8 % due to better O 2 distributions and heat transfer along the taper channels. [155] The increase in taper is directly increasing the pressure drop factor in straight flow field.
The converging-diverging flow channels having convergence angle of 0.2°and divergence angle of 0.3°as shown in Figure 25b were designed. [156] At the channel angle of 0.3°, the electrical power output was increased by 16 % in comparison with the conventional parallel flow channels. Due to the pressure variations between adjacent channels, a transverse over rib velocity was produced which in turn increased the O 2 distribution over the CL and increased the cell performance. The inlet header with reducing width (PFF with tapered inlet manifold as shown in Figure 25c) has greatly increased the  [148], (e) New channel cross-sections. Drawn from [152], (f) Slanted channels. Drawn from [153]. reactant flow. In this design, 70 % of gas is flowing through 80 % of the active area, whereas in the conventional PFF, 80 % of gas is flowing through only 20 % of the active area. [30] An innovative cathode flow field was designed which showed a greater water maintaining capability on the membrane compared to conventional PFF. [157] This design supplied an additional air to the cooling channels. Further, it decelerated the air in the gas channels and accelerated the coolant air in the cooling channels at the same time.

Taper in SFF and IFF
In the converging-diverging flow channels, the depth of the channel was varied from inlet to outlet by varying the angle of inclination. [158] The different depths of channels used in Inlet-Outlet were 3.5-0.5 mm, 3-1 mm, 2.5-1.5 mm and 1.5-0.5 mm. The high-power density was obtained for the design having less channel width between the inlet and outlet. Further, the current density and water management were better in a convergent flow channel. The transport phenomena of Convergent Serpentine (SC) and Divergent Serpentine (SD) flow fields with variable channel depth were compared with a conventional serpentine flow field (S). [159] Due to low concentration of water and high O 2 transport over the active surface on the cathode; and high-pressure distribution along the channel, the SC (Figure 25d) gave higher performance than the S and SD.
A taper SFF was designed with starting and ending height of 1 mm and 0.5 mm respectively in each channel. [160] With continuous loading of 5 hours, the tapered SFF resulted in 15 % of higher current at 0.5 V than conventional SFF. The tapered arrangement induced the reactant velocity in the direction of GDL, thereby increasing the amount of reactant at CL, reaction rate and performance. Bode and Nyquist plots were used to confirm the performance. The width of the outlet channel was made narrower than the conventional channel in taper IFF. [161] The results showed that there was better oxygen transport and higher performance in the narrower channels than the conventional ones.

Effects of Channel Bending
The performance of serpentine channels without slope and with different slope turns of normal, small and large sectional areas (Figure 26a) were investigated and found that the slope and contact angle of 120°and 160°respectively gave better water removal for the given condition. [162] Due to the hydrophobic of slope turns, the water particles were removed effectively from the MEA surface. From the analysis of the SFF with 1, 2, 3, 4, 5, 6 channels and different channel curvatures (Figure 26b), it was found that the 6-channel flow field with a sharp channel curve had 25 % of more perform-

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Chem. Rec. 2021, 21, 663 -714 ance than the 4-channel flow field, as it contained larger areas of gas flow, high secondary flow and velocity. [163] The effects of the single sharp bend, 180°bend, curved bends and serpentine channels were investigated. [164] The various curvature ratios of channel bend (C = 0, 1.41, 2.82 and 5.66) were studied. In the fuel cell stacks, the pressure losses of reactants are key factors for overall fuel cell efficiency. The optimization of the channel configurations (i. e. height, width, and gap) can improve the water management and the cell performance through a desired pressure drop. From the analysis of three bend sizes of 0.8, 1 and 1.2 mm on serpentine channels (Figure 26c), it was found that the 1.2 mm bend size had less pressure drop and uniform temperature distribution. [165] So the current density was more uniform at this channel which increased the power density.

Effects of Compression on the Performance
The clamping pressure of the fuel cell is also affecting the performance of a single cell assembly. Chang et al. [167] studied the contact resistance among the GDL, and the flow field was measured under different clamping pressures for a 25 cm 2 active area. While increasing the clamping pressure, the interfacial resistance was reduced between them, and it improved the electrochemical performances of the single cell. The optimal clamping pressure of carbon paper was found between 10 and 20 bar, and it was reduced up to 20-30 % in thickness in that pressure range. Meanwhile, a higher clamping pressure reduced the ohmic resistance and caused harmful effects such as GDL damage, loss of porosity, and tenting. [168][169][170][171] Wang et al. [172] designed a pair of fixture plates to study the pressure distribution in a single cell and fuel cell stack and concluded that the optimum pressure was delivered  [165], (c) Different bend sizes. Drawn from [166].

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Chem. Rec. 2021, 21, 663 -714 better. PEMFC with an active area of 25 cm 2 and the clamping pressure of about 1.38 MPa showed higher current density for the new end plates. When the flow path was partially blocked during non-uniform clamping, the membrane got deflected and extended. The high interfacial contact area between the channel surface and the carbon paper (GDL) ending with an exponential increase in the contact at higher compression. These will significantly affect the relationship between the contact resistance and cell performance. [173] Xing et al. numerically evaluated the clamping pressure on GDL's porosity, conductivity, and the contact resistance between flow field and GDL. When the clamping pressure was raised from 0 to 3 MPa, the plane conductivity increased by 52.9 % and 71.3 %, whereas the permeability non-linearly decreased by 69.5 % and 87.5 % for SGL-10-BB and SGL-10-BA, correspondingly. [174] Chen et al. [175] conducted experimental studies to find the cell's optimum clamping pressure and concluded that 1 MPa was optimum. The clamping pressure of 0.5 MPa showed the least performance, whereas, in 3.5 MPa, GDL thickness was reduced by 31.6 % from the original state. There is a linear variation (resistance and conductivity) in cell performance due to the increased clamping pressure (above 3 MPa) from the overall observations. Low clamping pressure (below 1 MPa) leads to rectants leakage, and High clamping pressure reduces the thickness and porosity of the GDL.

Performance Comparison of Various Flow Fields by using Numerical Analysis and Experimental Evaluation
The performance evaluation by using Numerical analysis for Trap channel, straight channel, bio inspired, wave and M channels is shown in Figure 27. Similarly, the performance comparison by using Numerical analysis and Experimental evaluation for Metal foam and PFF; FF : 1 (PTBPFF) and FF : 2 (STBPFF); and PFF with and without blockages is shown in Figure 28. PFF with rectangular baffles and plain Nickel foam had maximum power density of Figure 27. Power density curves of (a) Trap channel (blue line) and straight channel (square shaped curve), [19] (b) Wave and straight channels, [21] (c) Bio inspired wavy (optimal channel) and straight channel, [22] (d) Wave and M channels. [23]  Similarly, the performance comparison by using Numerical analysis and Experimental evaluation for PFF with subchannels and micro distributor, Serpentine/Straight zigzag FF and Sinuous FF is shown in Figure 29. PFF with subchannels and micro distributer (Figure 29a-b) yielded a higher performance due to the influence of sub channels and the reduced size of the micro distributor respectively. The performance of straight zigzag FF was higher than serpentine zigzag FF as shown in Figure 29c. Similarly, the Serpentine-Sinuous FF combinations on anode and cathode respectively showed a higher performance (Figure 29d) than Sinuous-Sinuous and Serpentine-Serpentine combinations due to the enhanced under rib convection and better water removal.

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
The performance comparison by using Numerical analysis and Experimental evaluation for Sinusoidal and PFF; TPA, PFF and SFF; PIS; and FF : 4 (SBFF) flow fields ise shown in Figure 30. Sinusoidal FF showed the enhanced performance due to the uniform reactant distribution (Figure 30a). The performance of TPA was higher than PFF and SFF (Figure 30b). Similarly, PIS increased the performance of cell with the increase of temperature (Figure 30c). Increasing the number of Baffles in FF : 4 induced the higher pressure differences between the adjacent channels than SFF (Figure 30d).
The performance comparison by using Numerical analysis and Experimental evaluation for FF : 5 (PSBFFP), FF : 6 (PSFFP), FF : 7 (CESFF), Cascade FF and Compensated SFF si shown in Figure 31. Discontinuous path of channels in FF : 5 gave better performance with good membrane hydration, and enhanced heat transfer (Figure 31a). FF : 7 had the enhanced performance compared with SFF due to the underrib convection which removed the water from the active area ( Figure 31b). Also, Cascade FF showed better performance due to the optimum values of channel width, channel depth and rib width (Figure 31c). Compensated SFF showed 27 % higher performance than SFF (Figure 30d).

Conclusion
Use of the fuel cell technology is playing a vital role in pollution control for the benefit of people throughout the world. Even though the PEMFCs are majorly used in Fuel Figure 28. Power density curves of (a) Metal foam and PFF, [36] (b) STBPFF and PTBPFF, [37] (c) PFF with and without blockages, [39] (d) PFF with 2 blockages and without blockages. [40]

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
Chem. Rec. 2021, 21, 663 -714 Figure 29. Power density curves of (a) PFF with and without subchannels, [44] (b) PFF with and without micro distributor, [45] (c) Serpentine zigzag and Straight zigzag FF, [50] (d) Sinuous FF and SFF. [51] Figure 30. Power density curves of (a) Sinusoidal and PFF, [53] (b) TPA, SFF and PFF, [54] (c) PIS at different temperature, [62] (d) FF : 4 (SBFF) and SFF. [68]

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
Chem. Rec. 2021, 21, 663 -714 Cell Vehicles like cars and buses, still many research works are being carried out to rectify the major issues in it. In this regard, the design of the flow field of fuel cell is also highly affecting the water control on the interfacial between the GDL and catalyst layer of the cathode side and the performance of PEMFC. Each flow field design has pros and cons which in turn make it for different applications. The design and operating parameters along with the power density values of various flow field designs which are analysed experimentally and numerically are given in Table 1 and Table 2 respectively. From the wide literature, the following important points are derived. 1. As the trap shaped flow channel is simple, inexpensive, and having high performance, it will be one of the potential designs for the coming generations. Further the vertical orientations of AB-PEMFC are the best due to easy removal of water at cathode side. The wave, bioinspired wave and M-shaped blockages and the arrangement of pin-fins and cylinders in flow channel have improved the performance. 2. The PFF design is simple and easy to manufacture.
Though the pressure drop of reactants is reduced in PFF than SFF, few of the channels are starved of the gases due to the maldistribution of flow. The performance of PFF can be improved by the following ways; * The staggered arrangement of rectangular and trapezoidal blockages along the channel (due to the cross flow of reactants and over rib convection) * The use of metal foams with high porosity * Provision of sub channels, transition areas, inclinations, microgrooves in rib walls and micro distributor * Provision of bends and curves in channels, number of funnelled in/out, small holes in landing/rib areas of the flow field and two sets of parallel channels operating at different pressures * Generation of circumferential flow of reactant near GDL by the insertion of wire coils in channels 3. Due to the high cell performance, reliability and durability, the SFF is widely used for many applications and it is considered as the "Industry standard". [176] Hence many design modifications are being made on serpentine design for further enhancement of fuel cell performance.  Figure 31. Power density curves of (a) PSBFFP and PSFFP, [69] (b) CESFF and SFF, [72] (c) Cascade SFF at different channel width, [73] (d) Compensated SFF and SFF. [74]        Experimental/ Numerical Shorter length of channel was better with 12.7 % more power density than long length channels.

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
[105] PFF/IFF with valves The increase in current density was 11.9 %.
[111]               NA -Not Available wavy SFF, Sinusoidal SFF, MSI, MSS of plate type FF and the tubular cylindrical FF are some of the designs developed from SFF. The important points obtained from SFF are as follows; * There is a considerable balance among the flow distribution and pressure drop with robust operation of PEMFC in FF : 3 than single pass SFF * The under rib convection in FF : 4 and FF : 7 and the enhanced cross flow in FF : 9 have increased the cell performance. Further, the pressure drop of the reactants can be greatly reduced by using FF:9 * FF : 5 design has the advantages of both Parallel Serpentine Flow Field Plate (FF : 6) and IFF Plate * The bends and wavy shapes on SFF have improved performance over the conventional SFF. The subchannels and DEA in SFF have increased the cell performance * To enhance the performance of SFF with better water management, a specially made PCI and PSI can be inserted along the cathode ribs at different positions and sizes. Due to globalized water absorption, the staggered arrangement of PCI and PSI showed better performance in MSS than MSI 4. The IFF design changes the diffusion transfer mechanism of the gases to/from CL into the forced convection mechanism. Even though the performance of IFF is higher than SFF, the pressure drop of reactants is much more, due to the dead end of channels. Further, the use of oxygen instead of air in the cathode side provides better results. The important points obtained from IFF are as follows; * The FF:14 (HSI) design has the advantages of both SFF and IFF The FF : 14 can be used for commercial applications due to the simple design and manufacturability * The proper position of baffles in inlet and outlet channels on the IFF has improved the cell performance * The novel flow field which can be exchanged between PFF and IFF was designed 5. The spiral, concentric spiral, radial, cylindrical, and tubular flow fields showed higher performance than planar flow fields, but their design and fabrication are complex. * The cylindrical SFF has better performance than cylindrical PFF and cylindrical IFF * The cylindrical PEMFC are having high gravimetric and volumetric power density compared to the planar PEMFC * The innovative radial flow field with control rings showed a superior flow distribution and water removal, utilization of active area and lower pressure drop than parallel and serpentine types 6. The Natural inspired or Bio inspired flow fields namely BI, Bionic, Biomimetic, Lung, Bio-inspired IFF, Ginkgo and Net leaf flow fields, etc. were developed from the nature designs. * The Bio-inspired (BI) flow field developed from the leaf flow patterns, showed the higher power densities than SFF and PFF * The Bio-inspired IFF designed based on Murray's law has improved the fuel cell performance than IFF and 5 channeled parallel-in-series designs * The interdigitated designs with a Pitchfork pattern has uniform current density distributions * The peak power density of Ginkgo flow field is higher than PFF * A semi cylindrical shaped obstacle along the daughter channels of natural inspired flow field is higher 7. The intersectant FF with optimum porosity gave superior performance than the SFF. The perforated FF which consists of 169 holes generated more power densities on comparison with SFF. The honeycomb FF having the hexagonal pins increased the diffusion rate of oxygen through GDL by 10 times. Compared to the uniform type, the zigzag pin type FF showed a higher performance due to even distribution of reactant. The fractal designs of flow fields have lower performance than serpentine. For portable applications, the tubular cylindrical configurations are deemed as a possible choice than conventional designs. 8. The length and number of channels, width and height of landing and channel and number of inlets have an influence on the cell performance. The bending curvature, slope and width have an influence on fuel cell performance. In automotive applications, a wide channel with narrow width and the co-current flow gives higher performance, whereas, for stationary applications, a narrow channel with wide width and the co-current flow gives higher performance. 9. The rectangular cross-section of the channel with sharp curve resulted in better performance when compared to other cross-sections like triangular, trapezoid, stepped channel etc. The Trapezoidal-Inverted Trapezoidal shape gained greater power density upto 4.65 %. 10. For commercial applications of fuel cell, the flow field with active area of 100, 200 and 300 cm 2 and above are more suitable. For increasing the output voltage and current, the cells can be connected in series and parallel respectively to form a stack. 11. The Converging-Diverging flow channels, the convergent serpentine channels and the taper in width and height of channels have improved the reactant flow and cell performance. The performance of the taper channels is better than the conventional ones. 12. With proper channel design, operating parameters and orientations of channels, the performance of fuel cell can be improved. Finally, almost care must be given while choosing the correct flow field design for higher performance of PEMFC.