Interfacial contact resistance in polymer electrolyte membrane fuel cells: Recent developments and challenges

Renewable and Sustainable Energy Reviews 115 (2019) 109351

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Interfacial contact resistance in polymer electrolyte membrane fuel cells: Recent developments and challenges

T

Amit C. Bhosale, Raghunathan Rengaswamy∗ Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, 36, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Gas diffusion layer Bipolar plate Contact resistance Contact pressure Surface morphology Surface modification Clamping pressure

Interfacial contact resistance in fuel cells is one of the primary challenges that needs to be overcome in the commercialization of fuel cells. This paper summarizes the governing parameters of the resistance such as surface morphology, contact pressure at the interface, electrical conductivities and corrosion resistance of gas diffusion layer and bipolar plate. Also, researchers’ contributions in modifying the cell components and clamping techniques are discussed in detail. Gas diffusion layer is found to be a crucial factor to control the contact resistance and hence its modification techniques are discussed. Moreover, surface of bipolar plate is prone to oxidation in acidic environment of the fuel cell. Different materials, anticorrosive coatings as well as surface modification techniques for the bipolar plate are therefore discussed in detail. Effect of clamping pressure on distribution of contact pressure is highlighted and various clamping techniques suggested by researchers are summarized. Effect of gasket selection on contact pressure distribution is considerable and therefore included in the paper. The review would be helpful for researchers in understanding the importance, reasons of existence and factors influencing the contact resistance at the interface of gas diffusion layer and bipolar plate.

1. Introduction

important barriers in their commercialization. A fuel cell typically houses a membrane electrode assembly (MEA) sandwiched between a pair of gaskets, bipolar plates (BPP) and end plates on both sides. Although hydrogen is used as a primary fuel for most of the fuel cell types, other fuels such as methanol, dilute light hydrocarbons such as methane (CH4), carbon monoxide (CO, for high temperature fuel cells like SOFC) can also be used in other class of fuel cells. CO, though a poison for low temperature fuel cells, can be used to generate hydrogen when reacted with water. The H2 formed can then be utilized electrochemically. Also, CH4 in dilute proportion can steam reformed to produce H2 in SOFCs [15]. Hydrogen gas, when supplied to anode, gets oxidized to protons and electrons. Protons pass through the electrolyte i.e. membrane, whereas electrons flow through the external circuit constituting to the current. Oxygen gets combined with them at cathode to produce water (Fig. 1). Nafion® has been widely used as an electrolyte for PEMFCs [16]. Carbon based gas diffusion layers (GDLs) along with graphite based BPPs have been conventionally used in the cell due to their better connectivity at the GDL-BPP interface and high corrosion resistance [17]. However, use of graphite based BPPs would decrease the volumetric power density of the device. Therefore, alternative materials have been employed to reduce the machining cost. Metal based BPPs are found to be much better compared to conventional graphite plates.

Fuel cell [1] is an important component in the mix of the energy technologies that has the potential to meet the burgeoning global energy demand. There are different types of fuel cells classified based on the temperature of operation, electrolyte and fuel used viz. alkaline fuel cells (AFCs), microbial fuel cells (MFCs), solid oxide fuel cells (SOFCs), unitized regenerative fuel cells (URFCs) etc. [2–5]. AFCs, due to alkaline nature of the electrolyte, can use nonprecious metal catalysts and characterized by low overvoltages [6]. MFCs, on the other hand, use bacteria to oxidize the waste in the water and simultaneously produce electricity. Such cells have primarily been focused for waste water management [2]. When generating power at higher temperature range (650–1000 °C), SOFCs make it convenient while using a hydrocarbon based fuel, thereby operating in combined heat and power mode (CHP) [7]. Its often convenient to use the surplus energy available from primary energy sources such as PV modules in generating hydrogen and oxygen by electrolyzing the water; and use same gases to power the load using a fuel cell. Such combination of modes can be made possible in URFCs that can possibly find applications in remote areas [8,9]. Out of these types, polymer electrolyte fuel cells (PEMFCs) are seen to be the potential candidates in automobile [10,11] and standalone applications [12]. However, cost and durability [13,14] still remain ∗

Corresponding author. E-mail address: [email protected] (R. Rengaswamy).

https://doi.org/10.1016/j.rser.2019.109351 Received 23 February 2019; Received in revised form 21 August 2019; Accepted 21 August 2019 1364-0321/ © 2019 Published by Elsevier Ltd.

Renewable and Sustainable Energy Reviews 115 (2019) 109351

A.C. Bhosale and R. Rengaswamy

Fig. 1. Schematic presentation of a PEMFC with reactions at respective electrodes.

However, formation of oxide layer on their surfaces has led to application of different anticorrosive coatings [18–20] for the improvement of the contact. The contact resistance at the electrode/GDL and GDL/BPP interface predominantly adds an ohmic drop along with that of the membrane and electrical connections. Such ICR is almost equivalent to that of membrane (Fig. 3) for an appropriately assembled cell and can cause severe losses during the cell performance if not paid enough attention. There have been quite a good literature available in highlighting ICR and correspondingly reducing methods, however, not summarized to best of authors’ knowledge. The aim of this paper is, therefore, to review the work done in terms of lowering the ICR by carrying out the improvements in GDLs, BPPs or equivalent methods.

Fig. 3. Resistances of different materials and their contacts contributing ohmic drop. Reproduced with permission from Ref. [26].

2.1. Irreversible losses The total energy that can be extracted from hydrogen combustion is equal to the enthalpy of the reaction (−286 kJ/mol, Eq. (1)). However, due to entropy loss, the energy available at STP conditions is equal to o Gibb's free energy and corresponds to 1.23 V i.e. Erev (Eq. (2)) [23].

E TN = o Erev =

2. Performance limiting factors in a PEMFC

−ΔH nF

(1)

−ΔGo (2)

nF

However, fuel cells are normally operated at temperature and pressure more than that of STP conditions for better catalytic activities. M. Perez-Page et al. [24] observed an increase in current density of 100 mA/cm2 at the 20 cell stack voltage of 10 V when the cell temperature was increased from 30 to 70 °C. This deviation in operating conditions brings in more loss that is highlighted by Nernst equation (Eq. (3)).

Fuel cells despite being a promising technology, are often limited by several losses which can be broadly classified into 2 categories viz. reversible and irreversible ones (Fig. 2 [21]). Irreversible losses are incurred in the system primarily due to thermodynamics and therefore cannot be eliminated. Such irreversible losses account for entropy loss and loss due to deviation of operating conditions from STP conditions. Reversible losses, on the other hand, are a result of materials, designs and manufacturing methods employed in the cell preparation [22].

o EN = E T , P = Erev

1 ⎡ T ⎤ {T − 298.15} × {Δcp − Δs o} − T Δcp ln 298.15 ⎦ nF ⎣ RT ln Q − nF

−

{

}

(3)

2.2. Reversible losses Hydrogen being a small molecule (Kinetic diameter ≈ 2.89 Å [25]), sometimes reaches the cathode through the electrolyte without being oxidized at anode. Such leaked hydrogen gets oxidized to protons and electrons at the cathode which bring down the potential of the cell. Loss due to such fuel crossover can be minimized by optimizing the operating conditions and membrane thickness. Once the current is drawn from the cell, the initial drop in potential is sudden and is mainly due to the energy lost in activating the kinetics of the reaction. This activation overpotential (ηact ) is sum of individual reactions happening at respective electrodes; however, mainly dominated by the cathodic loss. An appropriate catalyst chosen to support the reactions can be used to reduce ηact . The ohmic overpotential is the sum of potential lost due to resistance offered by the membrane and that of the other components and their connections. The membrane resistance (Rmembrane ) is governed by its resistivity, active area and thickness and is given by Eq (4).

Fig. 2. Different losses incurred in a typical fuel cell operation. 2

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Rmembrane = (ρ × t ) A

between GDL and BPP must be enhanced. This way, more pathways are generated for electrons to flow from one surface to the other thereby lowering the ICR and the voltage drop incurred. The metallic BPPs are therefore often coated with highly conductive and anticorrosive coatings [33–35]. GDL is also coated with microporous layer (MPL) on both sides to have better contact with BPP [13]. These surface modifications lower the roughness of the surfaces, increase the area in contact at the interface and thus reduce the ICR (Fig. 4b). ICR can be dependent primarily on three phenomena, viz. surface morphology of components, electrical conductance and corrosion resistance and contact pressure at the interface of the components (Fig. 5). Each of these factors are explained in detail in the following subsections.

(4)

where.

ρ is resistivity of the membrane, Ω-cm t is the thickness of the membrane, cm and. A is active area in cm2. Hence, thickness of membrane plays a very important role in controlling the membrane resistance, assuming other factors remain constant for a given cell. The minimum thickness offers least resistance however, it also increases the probability of fuel crossover. Hence, selection of membrane satisfying both conditions is very essential. Another important contributor to lowering the performance is ICR. It is a measure of electronic resistance offered by cell components as well as their interconnections. As ICR is mainly observed at the interface of GDL and BPP, surface roughness, materials and their contacts are major factors that need to studied. When the cell is operated beyond maximum power density point, the increase in the demand of reaction species at the reaction site is fulfilled partially due to water blockage. This situation further reduces the potential of the system, which is normally denoted as mass transfer overpotential (ηmass transfer ). The actual operating potential (E Actual ) is calculated by substracting sum of all the losses from EN or E T , P (Eq. (5)).

E Actual = E T , P − ηact − ηohmic − ηmass transfer

3.1. Surface morphology The surfaces of cell components such as BPP and GDL play a very important role in controlling ICR. Smoother surfaces of such components when they come in contact with each other introduce multiple contact points/surfaces. Such regions facilitate electron transfer and thus reduce the resistance incurred. The conventional GDL and BPPs used for fuel cells normally have rough surfaces at the interface. The GDL, in general, is coated with a MPL on one side (facing catalyst) to improve diffusion of gases and water management [36]. However, such GDL causes fibrous structure to come in contact with machined surface of BPP, thus inducing relatively low contact. Hence to enhance the contact area at the interface, coating MPL on both sides of GDL could help improve the contact [37] (Fig. 6a and b). Similarly, the metal BPPs as they are machined, have rough surfaces. Such a high roughness could be lowered by making the surface smoother using milling or similar operation.

(5)

where,

ηohmic = ηmembrane + ηICR

(6)

Importance of decreasing ICR was studied by Netwall et al. [26]. The authors examined the contribution of such resistance to total ohmic drop with the help of state-of-the-art cell components such as MEAs, GDLs and BPPs. With the testing protocols, almost 55% of ohmic resistance was found to come from contacts between membrane-catalyst layer; GDL-catalyst layer and GDL-BPP. The materials offered close to 45% to ohmic resistance (Fig. 3).

3.2. Electrical conductivity and corrosion resistance of surface coatings As pointed out earlier, contacting surfaces of the bodies need to be smooth for allowing more area in contact for electron transfer. However, these surfaces at the same time, should be highly conductive for easy transfer of electrons across the interface. In case of carbon based GDL and BPP, the change in material properties is minimal and therefore leads to minimum ICR. The electrical conductivities of conventional GDL and BPP are not very high [38] and therefore should be modified with materials [39–41] having better electrical conductivities (Table 1) [42].

3. Emergence of ICR BPPs are important components in the cell as they conduct the electrons to/from the diffusion layer; guide the flow of gases and remove excessive heat from the cell [17]. Considering such multitasking, BPPs occupy most of the volume; more than 40 and 80% of stack cost and weight respectively [27]. Besides being cost effective, the expected material for BPPs should possess

3.3. Contact pressure at the interface Since a PEMFC comprises of different components such as MEA, gaskets, BPPs and end plates, it is required to have an appropriate clamping mechanism to hold the components firmly. Such clamping pressure ensures a better sealing contact pressure at the interface of electrode and BPP [43]. However, excessive contact pressure is not encouraged as it may end up in generation of hot spots in MEA due to uneven production of current thereby leading to the MEA puncture [44]. Alizadeh et al. [45] modeled the cell for the investigation of contact pressure distribution as a measure of clamping pressure, thickness of end plates and hardness of the gasket. They found that these parameters control the distribution and should be considered in the cell design (Fig. 7). Zhou et al. [46] modeled the fuel cell for analyzing the effect of contact pressure on ICR and proposed an evaluation model with coefficients to represent the assembly process. Ratio of Channel/rib width of BPPs was also found to be a key factor to change ICR significantly [47]. Qiu et al. [48] developed a model for electrical contact resistance against contact pressure taking into consideration the porous nature of GDL and a solid structure of a BPP. Bhosale et al. [9] experimentally found the relation between ICR

• excellent electrical conductivity, • low permeability to gases, • good machinability and • good stability against corrosion. Graphite and its composites have been conventionally used as the preferred materials for BPPs since they satisfy most of the desired properties mentioned above [27]. However, their brittleness, gas permeability and poor machinability have led to use of other materials such steel, aluminum, nickel etc. [28–30]. The metal BPPs, especially steel, reduced the stack volume and offered more options for channel designs due to better machinability [31]. The bare metals, due to high surface roughness offered more resistance for electrons to flow across the interface of GDL and BPP (Fig. 4a). Secondly, the passive layer formed on the surface of BPP due to acidic environment in PEMFCs was a major contributor to the resistance offered to the electron transfer. It is found that 59% of total power loss from the cell is due to contact resistance at the interface of electrode and BPP [32]. Hence, in order to reduce the ICR, the contact points/surfaces 3

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Fig. 4. (a) High roughness of surfaces in contact induce higher ICR and (b) surface modifications realize better contact and thereby lower ICR.

Fig. 5. Factors influencing ICR.

the pathway for electrons to flow across the interface. Thus, lowering the temperature does not guarantee the same path followed by ICR against temperature (Fig. 9a). Increase in the relative humidity of gases (0–100%), on the other hand, has very little effect on ICR probably due to higher water uptake by membrane causing its swelling (Fig. 9b). This may have led to increase in thickness of membrane causing GDL to deform against BPP to help decrease ICR. Moreover, time of operation of the cell may highlight oxidation of anticorrosive coating/surface of BPP over time that might have guided the ICR to increase [50].

and average contact pressure between carbon paper and Titanium nitride (TiN) coated BPP sample (Fig. 8). The increase in pressure at the interface forced the crests and troughs of surfaces to come in contact with each other due to which the ICR was found to decrease significantly. However, further increase in contact pressure does not usually decrease ICR as there are no further additions of pathways for electrons to flow across the interface [49]. 3.4. Temperature, humidity and time of operation When the cell is operated at higher temperatures, the thermal expansions of GDL and BPP play a very vital role in reducing the ICR by bringing in more area in contact. Such thermal expansion in case of a GDL is irreversible due to the presence of large degree of hysteresis even after the completion of the temperature cycle. Such phenomenon affects the contact area between the GDL and BPP and therefore hinders

4. Developments in cell components and assembling techniques 4.1. Gas diffusion layer The gas diffusion layer (GDL) as mentioned earlier, promotes the 4

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Fig. 6. Surface views of (a) GDL substrate and (b) one coated with MPL. Reproduced with permission from Ref. [37]. Table 1 Electrical conductivities of different materials chosen as base materials for GDL and BPP [42]. Material

Electrical Conductivity (S/m)

Gold Aluminum Titanium SS 316 Graphite GDL (Toray-TGP-H-120, through plane conductivity)

45.2 × 106 37.7 × 106 2.34 × 106 1.33 × 106 1.27 × 105 2.3 × 105

diffusion of gases, electrons and water transport from triple phase boundary (TPB) to ribs and channels respectively [51,52]. Conventionally, a MPL supported gas diffusion backing (GDB) forms the GDL. Wettability in GDLs has been found to be a key factor in controlling the water management as well as flow of reactants. The selection of GDB (carbon paper/cloth) and Polytetrafluoroethylene (PTFE) content decide the quality of catalyst deposition due to variation in electrochemically active surface area (EASA) [53]. Therefore, GDL acts as a mediator between catalyst layer and a BPP. Thus, the contact resistances between catalyst layer/GDL and GDL/BPP interfaces need to be addressed appropriately for better performance and life [54]. Conventionally carbon paper or cloth along with wet proofed Vulcan carbon has been used as a substrate for GDL and MPL respectively in

Fig. 8. ICR for carbon paper and SS316 coated with TiN. Reproduced with permission from Ref. [9].

fuel cells [55]. The phenomenon of carbon corrosion (Eq. (7)) is an issue mainly at MPL region leading to cracks at the MPL surface [14]. This subsequently reduces the hydrophobicity in the region and increases the contact resistance at the interface of catalyst layer and MPL.

Fig. 7. Importance of contact pressure and its distribution over active area of the cell. Reproduced with permission from Ref. [45]. 5

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Fig. 9. Dependence of ICR on (a) temperature and (b) relative humidity of supplied gases. Reproduced with permission from Ref. [50].

Table 2 Various combinations used to prepare GDL.

GDB

paper • Carbon • TiS.S.foam • PAN based GDB • Carbon cloth etc. [58–71] •

MPL

Preparation method

black • Carbon • TiATOpowder • MPL made from acetylene black and PTFE • Carbon nanofiber sheet (CNFS) etc. •

followed by carbonization • Polymerization coating • spray method • chemical casting • freeze • Electrodeposition • electrospinning followed by hot pressing on GDB etc.

Fig. 10. Classification of materials for BPP. Reproduced with permission from Ref. [75].

4.2. Bipolar plate

Researchers have, therefore, demonstrated better materials than Vulcan carbon [56,57].

C + 2H2 O → CO2 + 4H+ + 4e− (E0 = 0.207 V / NHE )

Bipolar plate (BPP) is the key component in PEMFC responsible for

(7)

Table 2 summarizes the different actions taken to modify GDL in terms of better transfer of reactants, improve stability against corrosion and minimize ICR.

1) separation and thereby electrical connection between anode and cathode,

6

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machinability. The limitation further created a major concern for graphite BPPs in automobile applications. Also, minimum thickness (4–6 mm) for graphite based BPPs due to molecular structure, flexural strength and porosity, must be maintained, which further led to lower volumetric power densities for the cells/stacks [74]. Fig. 10 classifies the materials used for BPP fabrication as metal based and carbon based [75]. Coatings such as TiAlN or Pt have the metallic bond and therefore offer better electrical conductivity than TiN/TiC with the chemical bond. However, noble metals being costly, are outplayed by other metal coatings [76]. Since, none of the materials satisfies the properties discussed earlier, trade-offs must be made to make the best combination of materials to serve the purpose. The different drawbacks of graphite forced researchers across the globe to look for alternative materials, specially metals [77–79] due to their excellent machinability and lower cost compared to graphite [13]. However, usage of bare metals results in the formation of passive oxide layer. Rate of such oxide layer formation gets rapidly decreased initially and then remains constant underlining the complete formation of the layer [80]. Hence, modification of surface without coating the anticorrosive layers was also attempted by researchers. Fig. 11 summarizes the research done across the globe in terms of number of articles published since 2011. It should be noted that articles dedicated to improvement of electrical contact of BPP have been considered, which include different preparation methods, materials used, surface modifications etc. (Reference: Scopus). It's quite interesting to note that almost 40% of substrate materials chosen for further modification is steel among other metals, nonmetals and composites. This underlines that steel is considered as reference material in metals for BPPs. Lin et al. [81] modified the surface of steel using active screen plasma nitriding technique wherein gas mixture of 25% N2 and 75% H2 was used. The same group further used active screen plasma co-alloying treatment to modify the surface of 316 SS with N2 and Pt [82].

Fig. 11. Bibliographic analysis of articles published in the view of BPPs.

2) supply fuel and oxidant to electrodes, 3) remove reaction byproduct and 4) remove excess heat generated during the reaction [17]. The listed purposes are met when the material selected for BPP has an excellent electrical conductivity, high impermeability to the gases, possibilities for better flow designs and high resistance to corrosion in acidic environment of fuel cells [72]. Department of Energy (DOE) has already set targets for BPPs such as weight (< 0.4 kgkW−1), cost (3 $kW−1), corrosion (< 1 μAcm−2), electrical conductivity (> 100 Scm−1), flexural strength (> 25 MPa), area specific resistance (0.01 Ωcm2) etc. [73] for the improvement of PEMFCs. Graphite set itself as a benchmark material for the BPPs as it satisfied almost all the requirements except for its brittleness and

Fig. 12. Surface topology of (a) bare steel substrate and (b) one coated with carbon. Cross-sectional images of AIN-TiN samples coated with (c) 5 shots, (d) 10 shots and (e) 20 shots respectively. Reproduced with permission from Refs. [85,86]. 7

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Table 3 Combinations with steel plate as substrate. Ref.

Coating used

Coating method

Time (min)

Temperature (oC)

Average Thickness (μm)

[88] [89]

Polylaminate TaN/Ta Au

– 5

– 38

3 1

[90] [91] [92] [33] [93] [94] [95] [96] [97] [98] [99] [100]

Cr2N – C/CrN Cr–C TiCx/a-C Zr–C/a-C TiN Cr2O3/C Mixture of Cr, NH4Cl and Al2O3 ZrN PbO2 Au–Ti and Au–Ni

110 15 40 10 120 90 – 255 120 appx 9.4 4 –

1000–1100 Ambient – 10 – – – 350 700 75 Ambient 200

10–15 400 1.22 1.3 0.156 100 0.2 0.01–0.015 – 0.036 0.1 2

[101] [102] [103] [104,105] [106] [107] [108] [109] [110]

– 70 0.16–0.33 60 – 420 84 – 960

– 800–900 – 30 450 −20 – 110

110 0.5 130 0.4 22 1 1 1 76

[111] [112] [113] [114] [115] [116] [117] [118] [119]

C–Ni Cr/a-C Graphite CrN/Cr Ni–Mo and Ni–Mo–P N and Ag Ta/TaN CrNx Graphite:CB:Epoxy (45:5:50 vol %) C/Al–Cr–N Chromium oxide Cr/CrN/Cr fluorine-doped tin oxide (FTO) Nb–N TiZr Vanadium oxide (V2O3) Cr2N and (Cr,W)2N ZrN, TiN

Magnetron sputtering Electroplating of Au followed by enabling thin layer using additive manufacturing Single step back cementation process BPPs prepared by electro etching process Cathode arc-ion plating Electrodeposition Bias voltage strategy Magnetron sputtering Magnetron sputtering Chemical, heating and electrochemical method Chromization with rolling treatment Atomic layer deposition Electrodeposition Electrodeposition of Au followed by electron beam evaporation of Ti or Ni plasma blowpipe with internal arc Magnetron sputtering Heating and compression Pulsed bias arc ion plating; magnetron sputtering method Electrodeposition active screen plasma co-alloying technology Magnetron sputtering Arc ion plating Spray coating followed by hot pressing to cure the binder.

85

–

2.3

[120] [121]

W-doped carbon Pd–Co

Magnetron sputtering Reaction of N2 with H2O Arc ion plating electron cyclotron resonance-metal organic chemical vapor deposition Surface diffusion alloying technique Cathodic arc evaporation technique Dip-coating process Cathodic arc evaporation technique Physical vapor deposition (PVD) on in-house developed BPPs using stamping and hydroforming CFUBMSIP coating system Electrodeposition

10 – – < 80 – – 28 – 120 525 100 290 Outsourced for the coatings 175 8

– 35

0.75 0.3 6 3.66 0.394 0.086 1 Few nm 5.5

resistance and durability of the BPP. The combinations of steel and anticorrosive coatings have been compared with graphite and found to be better in most of the cases. Hence, ICR values achieved in these references have been avoided in the table. Researchers explored different materials other than steel to investigate the effect of fabrication and/coating methods on ICR. Compression molding was used to fabricate composite BPP made from conducting fillers and thermosetting resin [122]. Authors optimized the molding parameters i.e. 187 °C, 119 bar and 5 min for an electrical conductivity of 107.4 Scm−1. Another composite BPP of thickness 600 μm using epoxy-carbon fiber prepregs was fabricated using compression molding technique. The said composite had in-plane and through conductivity of 172 and 38 Scm−1 respectively [123]. Hung et al. [124] developed a 3 PEMFC stack with Al BPPs coated with chromium carbide using the thermal spray. The coating maintained the stability for almost 750 h. Table 4 lists the possible combinations of substrates (other than steel) with coating materials and methods in order to reduce the ICR.

Homogeneous and dense layer of Pt3Fe formed on the surface resulted in better corrosion resistance and electrical conductivity satisfying DOE's targets. Nitrogen supersaturated S-phase layers were observed on the surface of the substrate lowering the ICR with increase in treatment temperature. Steel (201 SS) with low Ni content was explored as a possible BPP material compared with that of 316L SS in terms of corrosion resistance and performance in simulated fuel cell environment [83]. Authors found both types to perform similar in cathodic environment and had similar ICR. N2 and Nb were used to modify the surface of steel using active screen plasma nitriding process [84]. The ICR for the modified surface was found close to 9 mΩ cm2 for the compaction pressure of 140 Ncm−2. Application of anticorrosive coatings was found as a better option than surface modification of the substrate as it resulted in enhancement of uniformity of surface as well as removal of pits (Fig. 12a and b [85]). Omrani et al. [86] deposited a thin layer of AIN-TiN on SS316 using plasma focus device (Fig. 12c, d and e). The thickness was varied in the range of 5–15 μm. The ICR of the coated sample was found 5 times lower than that of TiN sample with 20 shots from the device. Cr–Al–N films were coated on steel sample to observe the improvement in the conductivity with lower Al content [87]. Authors used closed unbalanced magnetron sputter ion plating (CFUBMSIP) to deposit the film. ICR as low as 5 mΩ cm2 for the compaction pressure of 1.4 MPa was recorded by the group. Addition of Al in CrN film lowered the ion diffusion in the electrolyte and improved the durability of the samples. Table 3 summarizes the different coating used on steel substrate along with the coating method adopted to improve corrosion

4.3. Clamping pressure End plates have been conventionally used to clamp the cell components as well as act as an inlet port for the BPPs. Clamping pressure is normally applied to the cell using nut-bolt pairs in the form of torque with position of the bolts underlining the effectiveness of the pressure [145,146]. Application of such appropriate torque leads to decrease in the contact resistance, lowering the porosity of GDL and therefore increasing the performance [147]. Reduction in ohmic as well as charge 8

Renewable and Sustainable Energy Reviews 115 (2019) 109351

1000 80–125 170 – 160 50 90 600 – 20 copper – Titanium Titanium Plain weave carbon/epoxy composite Graphene Carbon composite plates carbon/PTFE/TiN TiN Graphite [137] [138–140] [141] [142,143] [144]

Chemical vapor deposition (CVD) compression molding hydrothermal and impregnation process DC magnetron sputtering method Lamination

Tin Oxide

Ethylene-propylene reinforced with graphite particles Graphite foil or PET films Fe–Ni–Cr alloy [132] [133,134] [135]

[136]

Au-PTFE/Ni–P [131]

[130]

transfer resistance are also observed [148]. However, further increase in the clamping pressure would certainly affect the efficiency of the cell [149]. Also, the force exerted by a nut-bolt pair acts on the perimeters of the end plates resulting in deflection of the plate. Such deflected plates offer non-uniform contact pressure to the cell components. A pneumatic clamping system, therefore, developed had a piston cylinder arrangement (Fig. 13a) for the end plate [32]. This method of clamping was found to have better distribution of clamping pressure than the conventional one giving rise to a uniform contact pressure distribution at the interface of GDL and BPP (Fig. 13b and c). The method was proved efficient using structural modeling as well. A composite end plate made of carbon and glass fiber reinforced composite with precurvature was found suitable to provide uniform clamping pressure [150]. The plate was fabricated using residual thermal deformation and pre-curvature was decided based on clamping tests. Ghosh [150] invented a new clamping system for fuel cell assembly arranged in a matrix pattern. The system apart from providing the clamping force also detected the failure or change required in the force. Grot [151] used a pair of plates which had a plurality of threaded screws through the upper plate spaced uniformly. The screws provided a uniform pressure when tightened selectively. R. Montanini et al. [152] investigated the relation between actual pressure distribution, clamping torque and the deformation of end plate with the simultaneous measurement using a dedicated sensing device. The authors concluded that better pressure distribution could be obtained if stiffer end plates were used taking into account the matching of gasket and MEA. Toghyani et al. [153] modeled the effect on channel width against assembly pressures and the performance of the cell. They observed that the decrement in the GDL penetration with decrease in channel width led to increase in flow velocity and improvement in the performance. Also, such intrusion of GDL into the rib had more effect on performance than the decrease in GDL thickness and porosity. Moreover, based on GDL thickness, optimum clamping pressure (0.39, 0.69 and 1.1 MPa for 0.11, 0.254 and 0.37 mm thick GDL respectively) should be applied as the pressure higher than the optimum would decrease the water fraction and lower the performance [154]. Distribution of current and thereby temperature is of utmost importance as their improper distribution could lead to MEA puncture. Hence, magnitude of clamping pressure and gasket should be correctly selected for nearly isothermal conditions in the cell [155]. Alex Bates et al. [156] simulated and experimentally analyzed the contact pressure distribution between GDL and BPP for a single cell as well as stack of 16 cells. An optimum average contact pressure of 1 MPa was suggested by the authors. Net drag coefficient (measure of electro-osomatic drag) was investigated for different clamping pressures. It was dependent more on current density than clamping pressures [157]. However, increasing clamping pressure increased the mass transport resistance, which allowed more back diffusion of water through the membrane thus decreasing the net drag coefficient. Mason et al. [158] compared the decrease in contact resistance for both carbon cloth and the paper under varying compaction pressures. They underlined the importance of clamping to reduce the ICR at the interface of GDL and BPP. The group also highlighted the trend of decrease in total resistance with increase in compaction pressure; however elastic region was not found to exist for carbon paper. Shiro Tanaka et al. [159] developed a model mimicking the use of a metal GDL with an MPL in fuel cell to observe the effect of contact resistance against compression pressure. The model was found to have a good fit with that of experiment in terms of characterization of the cell. Also, MPL helped decrease the electrical resistance of the cell.

No quantification in the paper Few nm 200 2.3 0.2 1000 Room temperature Arbitrary time carbonaceous bipolar plate

2000 25 2000 60 90 15 – Carbon composite –

360 80–125 65

5 85–90 45 Aluminium alloy 5052

50 25–400 Aluminium

Immersion of Al plate in aqueous solution of graphene oxide followed by a thermal treatment Electroless nickel coating followed by Au-PTFE coating using immersion technique Compression molding Compression molding Vacuum induction method (VIM); surface treatments done using HNO3, HF and HCl. Liquid phase deposition (LPD)

1080

– 130–200 – 120 Aluminium – [127] [128,129]

Electroless technique Compression method

40 750

10 – ⁓50 86–220 2 60 multi-arc ion plating technique Compression molding [125] [126]

CrN A composite plate made of graphite, CB, CF and graphene to show better corrosion resistance in fuel cell environment Ni–P Epoxy along with carbon fibers to fabricate a carbon composite BPP using compression molding; Area specific resistance (ASR) of the BPP to satisfy DOE standards. Graphene

Fe–Cr alloy –

Average Thickness (μm) Coating method Coating used Ref.

Table 4 Overview of different combinations of coating materials and substrates used as BPPs in fuel cell applications.

Substrate material

Time (min)

Temperature (oC)

A.C. Bhosale and R. Rengaswamy

4.4. Gasket PEMFCs are made leak-proof using gaskets to avoid loss/mixing of 9

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Fig. 13. (a) Schematic diagram of pneumatic clamping system with distribution of contact pressure for (b) a conventional and (c) pneumatic clamping mechanism. Reproduced with permission from Ref. [32].

Fig. 14. (a) Ideal case of cell assembly (b) loss of fuel due to thinner gasket and (c) high ICR due to thicker gasket. Reproduced with permission from Ref. [8].

Gatto et al. [166] evaluated the performance of three gaskets subjected to the different clamping torques based on the contact pressure. Authors observed different optimum torque values for the gaskets due to their varying physical and chemical properties. Depth of sealant groove was also found to be an important parameter to control the contact pressure between GDL and BPP with minimum compression pressure of 2 MPa for a leak-proof assembly [167]. Thicker end plate was shown to have better distribution of pressure due to its minimum bending/deformation. The authors optimized thickness of end plate, clamping pressure and depth of channel to be 10 mm, 10 MPa and 0.12 mm respectively.

fuel for better performance of the cells [160]. They are typically made from Teflon (PTFE) or silicon [161–163]. Appropriate selection of material and thickness of gasket is very important as it also governs the contact pressure between the electrode and BPP (Fig. 14a). A thinner gasket leads to fuel leakage whereas thicker one provides insufficient contact at the interface of the electrode and BPP (Fig. 14b and c). This fact is often neglected when the cells are scaled up [8]. Selection of an appropriate gasket along with the GDL and clamping pressure leads to a better contact pressure and lower ICR. Most of the research in gaskets is dedicated to mechanical and chemical stability of gaskets in the view of acidic environment of fuel cells [161,164,165]. 10

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5. Concluding remarks

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The review highlights the importance of ICR along with its various causes and possible solutions to minimize the same. It is undoubtedly true that ICR, despite noted efforts in making good MEAs, can bring down the performance of PEMFCs single handedly. Hence, it is very important to understand and analyze ICR to minimize its effect. Following conclusions can be drawn from the review:

• Surface morphology severely affects the electron transfer, hence

•

•

both GDL and BPP should have surfaces as smooth as possible. MPL on both sides of GDL is preferred. Metal based BPPs have gained considerable attention due to their availability and maturity in fabrication processes; however they are susceptible to corrosion. This has necessitated the employment of corrosion resistive coatings such as Pt, Au, TiN etc. . It can be observed that despite having higher through plane conductivity of metallic BPPs, their performance is inferior with respect to graphite based BPPs. This fact can be underlined due to higher values of ICR of metallic BPPs due to the natural presence of the oxidative layer. This brings to the appropriate selection of anticorrosive coatings as discussed in detail in the previous sections. Composite based BPPs such as carbonaceous composites and graphite conductive composite BPPs have been observed to offer better strength than graphite; however, they are inferior in terms of electrical conductivity. The contact pressure plays a vital role in controlling the ICR. The optimized value at the interface of GDL and BPP is suggested as 1 MPa. Such pressure is extremely sensitive towards clamping techniques, size and design of fuel cells and can vary tremendously from minimum at center to maximum at outer edge. Therefore, the combination of clamping pressure, gasket and GDL should be made in such a way that contact pressure is close to the optimized value and is distributed well around the electrode surface. The focus of decreasing ICR in future should highlight the amount of energy saved. It should be noted that even 100 mΩcm2 of increase in ICR drops the potential of the cell of geometrical area 100 cm2 by 100 mV if operated at 1 A/cm2. Hence, appropriate materials for the cell components with interconnects should be chosen.

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Abbreviations ATO: Antimony doped tin oxide BPP: Bipolar plate CFUBMSIP: Closed unbalanced magnetron sputter ion plating CNFS: Carbon nanofiber sheet CVD: Chemical vapor deposition DOE: Department of Energy EASA: Electrochemically active surface area GDB: Gas diffusion backing GDL: Gas diffusion layer ICR: Interfacial contact resistance MEA: Membrane electrode assembly MPL: Microporous layer PAN: Poly-acrylonitrile PEMFC: Polymer electrolyte fuel cell PTFE: Polytetrafluoroethylene TiN: Titanium nitride TPB: Triple phase boundary

Nomenclature A : Active area of MEA, cm2 Δcp : Change in specific heat, Jmol−1 E Actual : Actual operating potential of the cell, V o Erev : Reversible potential, V EN = ET , P : Nernst potential at a given temperature and pressure, V ETN : Thermoneutral potential, V ηact : Activation overpotential, V ηmass transfer : Mass transfer overpotential, V

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R : Universal gas constant, 8.314 Jmol−1K−1 Rmembrane : Membrane resistance, Ω Δs o : Change in entropy at STP conditions, Jmol−1 T : Operating temperature, K t : Membrane thickness, cm Q : Reaction quotient

ηmembrane : Overpotential due to membrane resistance, V ηICR : Overpotential due to interfacial contact resistance, V ηohmic : Ohmic overpotential, V F : Faraday's constant, 96485 Cmol-1 ηG o : Gibb's free energy at STP conditions, kJmol−1 ΔH : Enthalpy of the reaction, kJmol−1 n : Number of electrons per mole of H2

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