A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments

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Review Article

A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments T. Sadhasivam a, K. Dhanabalan a, Sung-Hee Roh b, Tae-Ho Kim c, Kyung-Won Park d, Seunghun Jung e, Mahaveer D. Kurkuri f, Ho-Young Jung a,* a

Department of Environment & Energy Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of Korea b College of General Education, Chosun University, 309 Pilmoon-daero, Dong-gu, Gwangju, 61452, Republic of Korea c Advanced Materials Division, Center for Membranes, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon, 34114, Republic of Korea d Department of Chemical Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul, 06978, Republic of Korea e School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 500-757, Republic of Korea f Centre for Nano and Material Sciences, Jain University, Bangalore, 562112, India

article info

abstract

Article history:

From extensive reported analyses, we reviewed the limitations, challenges, and advanced

Received 25 August 2016

developments of the materials and components mainly used in the unitized regenerative fuel

Received in revised form

cell (URFC) system. URFC is a viable energy storage system owing to its high specific packaged

24 October 2016

and theoretical energy densities of 400e1000 Wh/kg and 3660 Wh/kg, respectively. Never-

Accepted 25 October 2016

theless, during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), the

Available online xxx

stability and durability of the URFC unit cell was severely affected by various degradation factors in the stacked cell. The certain issues are related to the (i) electrocatalysts (high cost,

Keywords:

aggregation, migration, and supportive material corrosion), (ii) dissolution and cracks in the

Unitized regenerative fuel cell

Nafion binder, (iii) physical degradation and higher cost of polymer membrane, and severe

Electrocatalysts

carbon corrosion in (iv) gas diffusion packing and (v) bipolar plates. Among these factors, the

Ion exchange membrane

critical challenges are the severe carbon corrosion and durability of the membrane in the unit

Bipolar plate

cell regions. The degradation occurs in the supporting material of the electrocatalyst, gas

Carbon corrosion

diffusion packing, and bipolar plate owing to carbon corrosion because of the high applied

Gas diffusion layer

potential in the water electrolyzer mode. Recent developments are significantly enhancing the durability and overcoming the limitations in the URFC system. In this comprehensive review, we have pointed out the limitations, challenges, and critical developments in URFC systems. Furthermore, built on our experimental and intellectual awareness in the context of URFC system developments, new strategies have been suggested to prepare novel structured materials and composites for advanced URFC applications. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Fax: þ82 62 530 1859. E-mail addresses: [email protected], [email protected] (H.-Y. Jung). http://dx.doi.org/10.1016/j.ijhydene.2016.10.140 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sadhasivam T, et al., A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.140

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of fuel cell technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamental concepts of unitized regenerative fuel cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages and limitations in URFC unit cell systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scope of this review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations of electrocatalyst materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High cost of electrocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sintering, aggregation, or migration of electrocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corrosion of supportive materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cracking and dissolution of Nafion binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of modified Nafion binder solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dimensional change and degradation in polymer membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stability of gas diffusion backing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas diffusion layer (GDL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microporous layer (MPL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Severe corrosion in bipolar plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future prospective for advanced developments in URFC system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

List of acronyms BP DOE FC GDB GDL MEA MPL ORR OER PEFC PEMFC PEM SEM TEM URFC USDOE WE XRD

Bipolar plates Department of Energy Fuel cell Gas diffusion packing Macro porous or gas diffusion layer Membrane and electrode assembly Mesoporous or microporous layer Oxygen reduction reaction Oxygen evolution reaction Polymer electrolyte fuel cell Polymer electrolyte membrane fuel cell Polymer electrolyte membrane Scanning electron microscopy Transmission electron microscopy Unitized regenerative fuel cell United States Department of Energy Water electrolyzer X-ray diffraction pattern

Units A cm2 Ampere/square centimeter Electron e ft-lb in.1 foot-pound J m1 Joule/meter kPa kilo Pascal MPa Mega Pascal N cm2 Newton/square centimeter V Volt S/m Siemens per meter Wh/kg watt hour per kilogram W (mK)1 watt/meter/Kelvin  C Celsius

Introduction Overview of fuel cell technology Alternative energy is currently an emerging area of research for industrial and mobile transportation applications. Owing to fossil fuel depletion, alternative energy fuels have gained the highest importance to overcome the shortage of conventional energy sources and carriers [1e3]. However, the alternative fuel needs to be an environmentally friendly clean and green energy fuel, so as to not contribute to global warming issues [3e5]. So far, researchers worldwide have developed different techniques to provide efficient alternative energies, such as fuel cells [6e11], batteries [12e16], and electrochemical capacitors [17e21]. Among the various promising techniques, significantly higher specific energy density has been achieved for fuel cells [22,23]. Fuel cell technology is a clean and renewable energy source. To determine the optimal energy conversion, research on the fuel cell technology began in the 18th century and developments are emerging to this date [24e28]. Fuel cell technology is a remarkable and efficient electrochemical technique to produce electrical energy from chemical energy [25]. At present, different types of fuel cell technologies have been developed based on the typical electrolytes used. The developed fuel cell technologies [29] can be categorized as (i) polymer electrolyte membrane fuel cells [30,31], (ii) solid oxide fuel cells [32,33], (iii) molten carbonate fuel cells [34,35], (iv) phosphoric acid fuel cells [36,37], (v) alkaline fuel cells [38,39], (vi) direct methanol fuel cells [40,41], (vii) direct ethanol fuel cells [42,43], (viii) direct ethylene glycol fuel cells [44,45], (ix)

Please cite this article in press as: Sadhasivam T, et al., A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.140

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microbial fuel cells [46,47], (x) enzymatic fuel cells [48,49], (xi) direct carbon fuel cells [50,51], (xii) direct borohydride fuel cells [52,53], and (xii) direct formic acid fuel cells [54,55]. Among these, the polymer electrolyte membrane fuel cell (PEMFC) has gained attention because of the environmental friendliness of hydrogen gas as a fuel source for the production of electricity. Moreover, efficient developments in PEMFC could be made through feasible techniques [29e31,56,57]. The PEMFC can be operated through FC mode only. Greater hydrogen production is needed to make PEMFC a viable fuel cell system. In hydrogen fuel technology, hydrogen production and storage have many associated problems such as production costs and storage capacity [58e61]. From this viewpoint, advanced developments of PEM-based unitized reversible or regenerative fuel cells (URFC) can be considered an optimum fuel cell system as they offer round-trip energy conversion [62]. Although the URFC technology is different from PEMFC due to reaction mechanism of water electrolyzer mode.

Fundamental concepts of unitized regenerative fuel cell Unitized regenerative fuel cell is a highly developed and advanced fuel cell technology [63e65]. In an URFC unit cell, the electricity is produced through the round-trip energy conversion of (i) the conventional fuel cell (FC) mode and (ii) the water electrolyzer (WE) mode [66e68]. The overall reaction mechanism of URFC is shown below.

In FC mode: 4Hþ þ O2 þ 4e / 2H2O

In WE mode: 2H2O / 4Hþ þ O2 þ 4e In the FC mode, the hydrogen and oxygen are used to produce the electricity. The hydrogen and oxygen are passed to the anode and cathode, respectively. The oxygen reduction reaction (ORR) and electricity production process takes place through the FC mode. Initially, the hydrogen molecules dissociated into protons (Hþ) and electrons in the hydrogen electrode. The proton moves to another side of the electrode through the polymer-based proton exchange membrane and the electron moves through the external circuit. The end of the FC mode reaction is the production of water and electricity. The WE mode is known as water-splitting process in the URFC unit cell. It involves the opposite reaction that undergoes in the FC mode. The water molecules are supplied to the oxygen electrode and the reaction starts by splitting water into 4Hþ, and O2 through the power supply. The produced oxygen and hydrogen gases can be stored and used in the FC mode as an energy carrier [66]. This is a most important achievement in fuel cell applications. The detailed reaction mechanism is shown in Sections 2 and 4 of this review. The schematic representation of round trip energy conversion of URFC is shown in supplementary information.

Advantages and limitations in URFC unit cell systems Along with the round-trip energy conversion, the URFC system has several potential advantages such as the follows: (i) it

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offers high specific energy density (packaged: 400e1000 Wh/ kg and theoretical: 3660 Wh/kg), (ii) it uses the abundant chemical compound H2O as a fuel carrier, (iii) it is a renewable and sustainable energy system, (iv) no harmful emission occurs during the process (v) it is light weight, and (vi) it is a high durable system [69,70]. Because of the above-mentioned potential properties, the URFC device is used in (i) spacecrafts, (ii) zero-emission vehicles, (iii) solar rechargeable aircrafts (iv) military applications (v) on-site energy storage system, and (vi) residential power sources, and is an especially prominent energy storage system for space applications [69,71e74]. Apart from the numerous advantages, there are a few drawbacks associated with the materials and components of the URFC unit cell. The limitations of the each component in the URFC system have described in the consecutive sections. Fig. 1 represents the schematic representation of the continual arrangement of a single URFC unit cell system. The polymer electrolyte membrane is the central part of the unit cell. The sequence of the components aligned from the center of the unit cell is the membrane, Nafion binder, electrode containing the electrocatalyst and supportive materials, gas diffusion backing (GDB) consisting of a micro- or meso-porous layer (MPL) and gas diffusion layer (GDL), and bipolar plate (BP). A similar alignment is present on either side of the unit cell. The stability and durability of the unit cell are major challenges in the URFC system. For operating the WE mode in URFC system, it needs higher resistant material, when compared to PEMFC system. The challenges and limitations associated with each component of the URFC device are (i) electrocatalysts: the high cost of the Pt electrocatalyst, carbon corrosion of supportive materials, and aggregation and migration of electrocatalysts [67,74,75], (ii) Nafion binder: dissolution or solubility of the Nafion binder solvents and cracking of the binder [76], (iii) polymer electrolyte membrane (PEM): dimensional change and low proton conductivity [77], and (iv) GDB layers and BP plates: severe carbon corrosion [78e81]. In addition, the ultimate limitation is the cost effectiveness of each component in the URFC unit cell. Fig. 2 shows the breakdown of the 2015 fuel cell stack costs at 1000 and 500,000 systems per year, and the 2020 cost targets of URFC unit cell system is shown in Table 1, reprinted from the US-DOE Hydrogen and Fuel cells program record [82]. The US-DOE targets can be achieved through lower cost of the novel structured materials and composites using new technologies for advanced developments in the URFC unit cell components. To accomplish the higher performance, the limitations of each component should be overcome in URFC system.

Scope of this review This review article aims to present a comprehensive survey of limitations, challenges, and developments in the URFC unit cell systems. This article has extensively explored the stability and durability of the materials and components commonly used in the URFC unit cell system. Compared to other reviews, this article has paid significant attention to the fundamental and critical barriers to the up-to-date developments in the conventional URFC system. Furthermore, possible novel approaches for developing an advanced URFC device have been discussed in the prospective section.

Please cite this article in press as: Sadhasivam T, et al., A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.140

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Fig. 1 e Schematic representation of the alignment of the URFC unit cell components.

Fig. 2 e Schematic illustration of the breakdown of the projected 2015 fuel cell cost [82].

Therefore, this comprehensive review will be useful to overcome the limitations in URFC system. Moreover, it will be helpful to researchers investigating fuel cells in furthering advanced developments and innovations in URFC and other fuel cell-based technologies.

Limitations of electrocatalyst materials Conventionally, catalyst materials are used to enhance the reaction kinetics and modify the chemical reactions without being consumed during the reaction process. Fig. 3 represents the reaction mechanism of a combined URFC system of FC and WE modes [83]. In the URFC (both WE and FC modes) unit cell operation, the reaction of hydrogen gas was performed in the one side of electrode and the oxygen gas on the other side of electrode. The same electrode always performed with the same gas in FC and WE modes [64,83]. The hydrogen and oxygen electrodes are corresponding to the anode and cathode in the FC mode reaction. The opposite reaction of FC mode is known as WE mode in the URFC system. In WE mode, the hydrogen and oxygen electrode works as cathode and anode, respectively. The electricity is produced in the FC

mode. In WE mode, the water molecules were spitted through the power supply. Catalysts are involved in the ORR to produce electricity and OER to produce energy carriers during the URFC unit cell operation. In fuel-cell-related research fields, advanced functional properties of catalyst materials have developed for enhancing the superior electrochemical reactions under optimum conditions. Numerous classes of electrocatalyst materials have been examined for URFC applications, such as Pt, PtIr, Pt3Ir, PtIrO2, and RuO2eIrO2/Pt

Table 1 e Projected cost status of each component at 500,000 systems per year compared with 2020 cost targets [82]. Component

Cost Status

2020 Cost Target

System Stack MEA Fuel cell membrane Bipolar plates Air compressor (CEM) Humidifier system Humidifier membrane

$53/kWnet $26/kWnet $17/kWnet $17/m2 $7/kWnet $750/system $81/system $20/m2

$40/kWnet $20/kWnet $14/kWnet $20/m2 $3/kWnet $500/system $100/system $10/m2

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Fig. 3 e Graphical illustration of the working principles of the fuel cell and water electrolyzer modes. Reprinted with permission from Ref. [83]. Copyright Elsevier (License number: 3930080490100). [66,74,84,85]. The Pt electrocatalyst shows remarkable electrochemical performance for the ORR; but, it shows lower electrocatalytic performance for the OER [86]. Different kinds of metal, metal oxides and composites/alloys were used as electrocatalyst materials to improve the OER performances in water splitting process [66,84e93]. Especially, RuO2 and IrO2 can be effectively considered as a promising OER candidate to enhance the WE mode performance [74,85,86,94e97]. For example, the electrical conductivity and URFC performances of the composite Pt/porous IrO2 catalyst are higher than that of the conventional Pt and Pt/commercial IrO2 electrocatalysts, leading to appreciably increased performances for the ORR and OER [86]. Compared to the IrO2 catalyst, the RuO2 can be considered as an efficient candidate to the OER process due to assist the low over potential and low cost [94]. Although, the stability of RuO2 was minimized during long term operation at high anodic potentials [94] and increase the oxidation state of ruthenium [98]. To overcome the limitations, the mixed state of IrO2 and RuO2 electrocatalysts were considered as a capable electrocatalyst material to enhance the OER performance [98] in URFC system [74]. Fundamentally, the metal and metal oxides are the efficient catalyst to enhance the ORR and OER process, respectively. The mixed states of metalemetal oxide are effectively considered as a prominent electrocatalyst for the URFC applications [66,74,84e86]. Nevertheless, various factors involving the electrocatalysts limit the URFC performance for the practical applications, such as (i) high cost, (ii) sintering, (iii) aggregation, (iv) corrosion of supportive materials, and (v) migration of catalysts.

High cost of electrocatalysts In fuel cell technology, Pt is the foremost electrocatalyst material for the ORR and hydrogen oxidation reaction. A Pt

electrocatalyst shows efficient bifunctional properties and significantly enhances the ORR performance compared to other known catalyst materials [66]. However, Pt cannot be recognized as a viable electrocatalyst material for practical applications owing to its high cost and rarity [75,99e101]. The estimated cost of various elements and annual production [75] are shown in Fig. 4. It can be observed that the cost of Pt is significantly higher than the other materials and the availability is considerably lower. To overcome the cost restrictions along with enhancement of the electrocatalytic performance, novel composite and structured materials were recognized to the URFC technologies. Many kinds of metal and metal oxides were composite with Pt to enhance the electrochemical performance. In URFC applications, different catalyst materials have been introduced with Pt to make alloys or composites such as PtIr [102], Pt/IrO2 [103], Pt/Porous-IrO2 [86], RuO2eIrO2/ Pt [74] to identify the most promising low-cost bifunctional electrocatalyst materials for enhanced performances of the ORR and OER. Though, lack of electrocatalyst studies were reported in the URFC system. To optimize the low cost of electrocatalyst material, numerous investigations should be identified with various kinds of catalyst materials in the URFC systems.

Sintering, aggregation, or migration of electrocatalysts Fundamentally, the performance of electrocatalyst was concerned through (i) sintering, (ii) aggregation, and (iii) migration during the cyclic performance. Nano-sized electrocatalyst materials are widely used to enhance the electrocatalytic activity in the URFC electrochemical process [84,85]. In spite of that the nano-sized particles were sintered, when the temperature and/or pressure are applied during the preparation of electrocatalysts and electrodes [104]. This is one of the major obstacles in the URFC system; because the sintered

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Fig. 4 e Annual production vs. price of the periodical elements. Reprinted with permission from Ref. [75]. Copyright The Royal Society of Chemistry (License number: 3934621465052).

nanoparticles cause to poorer electrocatalytic effect. Another major issue related to the aggregation of electrocatalyst materials in the URFC system. The agglomeration of catalyst materials has created a non-uniform distribution of electrocatalyst on electrocatalyst support material in electrodes. The non-uniform dispersion of electrocatalyst materials restricts the bifunctional catalytic performance due to deprived electrical conductivity. The aggregation of Pt nanoparticles results in larger-sized particles [105], which affect the electronic conductivity and degrade the URFC performance owing to low electron transportation during the unit cell operations. The Pt/ IrO2 electrocatalyst has been considered as a promising bifunctional electrocatalyst material for the URFC system [86]. Fig. 5(a and b) represents the ORR and OER polarization curves of Pt/IrO2 electrocatalysts. Higher electrochemical performance and onset potential were achieved with novel structured Pt/porous IrO2 composite electrocatalysts. Pt is a well-known electrocatalyst material in ORR reaction; however, the significant high OER performance was achieved by the addition of IrO2 [86]. Nevertheless, the agglomeration rate of the Pt/IrO2 catalyst was also significantly higher because of poor distribution of Pt and IrO2 materials. The agglomeration of the electrocatalyst resulted in a higher ohmic resistance and hindered the electron transportation in Pt particles [85]. To achieve the better electrocatalytic performances during the long cycle operation, the aggregation of electrocatalysts must be restricted. Despite the sintering and aggregation, the

catalyst migration perhaps a key issue in URFC system. Fig. 6(a and b) shows the TEM micrographs of the cross-sectional view of the electrocatalyst migrations in the MEA assembly of PEFC [106], which can also possibly happen in the URFC unit cell operation. Here, the catalyst particles have migrated towards the membrane and the interfacial region between the membrane and electrodes (in both the positive and negative electrodes). The migration of electrocatalysts is mainly due to the electro-osmotic water flux in the anode and Pt oxidation in the cathode under the high current density during long-term operations [106]. The migration of electrocatalysts (migration effect) significantly affects the stability of the MEA and limits the electrocatalytic performances. The aggregation and migration of electrocatalysts should be prevented in the MEA to enhance the unit cell performance during the long cycle process.

Corrosion of supportive materials In fuel cell technology, supporting materials of electrocatalysts have played a vital role in determining electrochemical performances [67,103,105]. The supporting material should possess (i) a large surface area to volume ratio for the uniform distribution of the smaller-sized noble metal and/or metal oxide nanoparticles, (ii) high electrical conductivity and (iii) corrosion resistance. The presence of supportive materials is helpful for enhancing the activity of the electrocatalyst

Fig. 5 e (a) ORR and (b) OER polarization curves of different electrocatalysts. Reprinted with permission from Ref. [86]. Copyright Elsevier (License number: 3934630270969). Please cite this article in press as: Sadhasivam T, et al., A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.140

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Fig. 6 e (a) Catalyst particles migrating toward the catalyst/membrane interfacial region after 500 h operation and (b) Pt electrocatalyst particles migrating into membrane after 1000 h operation [106]. Reproduced by permission of The Electrochemical Society.

materials and decreases the amount of catalyst loading [66]. Typically, carbon black is considered a traditional supportive material for fuel cell electrodes because of its high electronic conductivity and large surface area [107e109]. However, carbon corrosion in the supportive material is a prominent challenge in the URFC owing to the high potential applied in the WE mode [66]. To understand the effect of carbon corrosion, the comparative durability analyses were performed to the Pt electrocatalyst loaded with the supporting materials of graphitized carbon and carbon black [110]. The catalyst materials were homogeneously distributed on both the supportive materials. After a few cycles, the Pt particles were detached or migrated from the carbon black due to severe corrosion of the carbon surface. The crystalline nature of the graphitized carbon support confirmed its corrosioneresistant properties and higher durability throughout the fuel cell operations. In URFC systems, carbon corrosion/oxidation was observed as a result of the OER due to the high applied potential (~1.5 V) in the WE mode [66]. The surface of the carbon black supportive material oxidized to CO2 according to the equation C þ H2O / CO2 þ 4e þ 4Hþ during the process [67]. The supportive material corrosion/oxidation hindered the further developments in the URFC unit cell system. High stability of supportive materials is required to replace carbon black for advanced URFC system. In the past decade, a few novel structured materials have been developed as corrosionresistant supportive materials for the URFC system, such as graphitized carbon [110], carbon nanotube (CNT) [111], carbon free Ti, [84], TiC, TiCN [102], sulfonated silica [112], and Sbdoped SnO2 [103]. Fig. 7 represents the TEM micrographs of before and after analyses (stability) of different Ti compounds supported Pt3Ir catalyst [84]. The different compound of Ti support materials shows the higher stability against the corrosion during the URFC cell operation. The nitrogen attached compound of TiCN supported Pt3Ir electrocatalysts have higher electrochemical performance. In spite of that, the catalyst aggregation and surface corrosion have been observed in the electrocatalyst support materials. These

supportive materials [112] have high corrosion resistance and are preferred as advanced supportive materials for electrocatalyst materials. However, any one of the following demerit may limiting the feasibility for their uses in URFC processes: (i) process of material preparation (ii) high cost, (iii) passivation of the surface (iv) the electron conductivity of electrocatalysts being affected, and (v) decreasing durability during long cycle operations.

Cracking and dissolution of Nafion binder At present, researchers worldwide are developing new kinds of binder materials to overcome the technical barriers in fuel cell technology. Binder material is a key factor in electrochemical reaction performances. The basic role of the binder material is to (i) prevent the degradation of MEA assembly (ii) increase the proton transportation and (iii) avoid the aggregation of electrocatalysts during the unit cell operations. Currently, different types of binder materials such as Nafion, poly(vinylidene fluoride) (PVDF), and poly(tetrafluoroethylene) (PTFE) are used in batteries and fuel cells to prevent the catalyst aggregation and enhance the conductivity [113e117]. The usual binder materials have some fundamental degradation issues during the unit cell operations such as (i) dissolution (solubility), (ii) low proton conductivity, and (iii) cracking. Nafion is known as an effective binding material between the catalyst layer and membrane of the fuel cells [76,117,118]. It can create a strong interfacial interaction and adhesion between the electrodes and the membrane. Additionally, it has shown some useful features such as (i) higher interfacial adhesion in MEA assembly (ii) presence of the sulfonic acid (eSO3H) functional group, which leads to higher ionic conductivity, and (iii) higher Pt dispersion owing to increase in the active surface area. The above fundamental functional properties of Nafion make it a good choice for the binder material in MEA by increasing the proton conductivity and electrical conductivity. Certain limitations are associated with the conventional Nafion binder. During the unit cell

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Fig. 7 e TEM micrographs before and after durability analyses of the Ti compounds supported Pt3Ir electrocatalysts. Reprinted with permission from Ref. [84]. Copyright Elsevier (License number: 3975151485487).

operation, interfacial delamination occurs in MEA (between the electrode and membrane) owing to the lower and higher dimensional changes in the electrode and membrane, respectively [118]. These dimensional changes affect the proton conductivity and electrical conductivity in the MEA assembly because of the morphological changes in the Nafion binder. The minor degradation in MEA significantly affects the fuel cell performances during long cycle operations.

Advantages of modified Nafion binder solvents From our reported experimental analyses [76,118], the modified Nafion binder significantly enhances the MEA interface stability and URFC performances due to higher the interfacial adhesion between membrane and electrodes and higher proton conductivity. Fig. 8(a and b) represents the digital images of Nafion binders prepared using conventional and modified solvents [76,118]. Water/alcohol and dimethylacetamide (DMAc) were used as conventional and modified Nafion dispersion solvents, respectively. From optical observation of conventional and modified Nafion films, the conventional Nafion film was seen but no cracks could be observed on the surface of the modified Nafion. The cracked morphology was attributed to the low brittleness and reduced mechanical stability in the Nafion binder due to the weak interaction between the Nafion and dispersion solution. The non-cracked morphology of the homemade Nafion binder was ascribed to the strong interaction between the modified Nafion dispersion solution and micelles of the Nafion. Higher brittleness and mechanical stability of the Nafion binder leads to further developments in the MEA assembly. Fig. 8(c) shows the XRD crystalline characteristics of Nafion-117 and its recast films using the two different solvents [118]. After preparing the recast film, the crystallinity of Nafion-117 is decreased by the addition of the solvents. However, compared to the conventional recast Nafion film, the

crystallinity is significantly higher for the modified Nafion recast film. The highly crystalline nature has explored to the modified Nafion binder by the insoluble properties of DMAc in the methanol solution. The SEM micrograph (Fig. 9a and b) shows the dispersion and adhesion of the Pt electrocatalyst in the electrodes through the conventional Nafion and modified Nafion binder solutions [76,118]. The agglomerations of the electrocatalyst particles are significantly higher in the convention Nafion binder electrode. It is a clear evidence for electrocatalyst aggregation in the catalyst layer due to Nafion binder solutions. The similar approaches were used to prepare the modified Nafion binder for the PEMFC and URFC applications [76,118]. The cross-sectional surface morphology of the conventional and homemade Nafion binder MEA of PEMFC and URFC is shown in Fig. 9cef [76,118]. The degradation of the conventional Nafion binder-based MEA occurs due to lower integrity. After a long term operation, higher stability was observed for the modified-Nafion-binder MEA assembly with strong interfacial interaction between the electrode and membrane.

Dimensional change and degradation in polymer membrane In URFC applications, construction and stability of the MEA is an important factor for advanced potential applications. For attaining higher durability of MEA, various kinds of polymerbased membranes have been developed for fuel cells and redox flow batteries [119e123]. In general, polymer-based membranes have been used as ion-exchange candidates and separators between the anode and cathode in the URFC unit cell. The advanced functional membrane should possess a high ion exchange and electrical insulator properties for enhancing the unit cell performance and prevent short circuits, respectively. It means that the membrane is required to

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Fig. 8 e Recast Nafion films prepared with (a) conventional solvents and (b) modified solvents. (c) Wide angle XRD of Nafion and its recast films. Reprinted with permission from Ref. [118]. Copyright Elsevier (License number: 3934640191708).

transport the protons (Hþ), not the electrons (e). Furthermore, the fuel permeation should not take place in the polymeric membrane during the unit cell operation [65]. As shown in Fig. 10 (a and b), the URFC reaction mechanism of positive and negative electrodes and proton transportation in membrane is as follows,

In FC mode: H2 / 2Hþ þ 2e (Hydrogen electrode)

2Hþ þ 2e þ 1/2O2 / H2O (Oxygen electrode)

In WE mode: H2O / 2Hþ þ 2e þ 1/2O2 (Oxygen electrode)

2Hþ þ 2e / H2 (Hydrogen electrode) To complete the reaction mechanisms shown above, the protons pass through the membrane from anode to cathode and cathode to anode. Therefore, the membrane is a major component in the URFC unit cell. Among the various kinds of polymer-based materials, fluoropolymer-based

Nafion membranes have shown higher proton (ionic) conductivity properties due to the presence of sulfonic acid functional groups (eSO3H) in the polymer chain. Fig. 11 demonstrates the polymer chain of the Nafion membrane [124]. Apart from the possible advantages, there are a few major disadvantages associated with the Nafion-based membranes, such as (i) compared to other fuel cell components, the production cost is significantly higher; (ii) the durability is not efficient for long cycle unit cell operation due to the swelling nature of the membrane; and (iii) fuel permeability [82,123,125]. To overcome these barriers, many investigations have been undertaken in PEMFC applications [123,126e128]. Nonetheless, up to now, not many membranebased studies have been reported for URFC applications [63,77,104]. A low-cost, highly stable advanced compositestructured membrane should be proposed to replace the more expensive Nafion membranes for URFC applications.

Stability of gas diffusion backing Fundamentally, gas diffusion backing (GDB) consists of a prosperous structured mesoporous or microporous layer

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Fig. 9 e (aeb) SEM micrograph of the electrocatalyst dispersion in electrodes. SEM cross-sectional MEA images of PEMFC (ced) and URFC (eef), after long term operation. Conventional (a, c, e) and modified (b, d, f) solvents used for Nafion binder preparation. Reprinted with permission from Ref. [118 and 76]. Copyright Elsevier (License number: 3934640191708) and Springer (License number: 3975160230397).

(MPL) and macro porous or gas diffusion layer (GDL) [79,129e131]. In a URFC system, GDB provides the major functional properties such as homogeneous distribution of fuels, water management, and electrical conductivity between the electrodes and bipolar plates [78,132e134].

Gas diffusion layer (GDL) Fundamentally, carbon-based porous materials have been widely studied and used as a GDB (MPL and GDL) for fuel cell applications due to their high electrical conductivity and gas permeability [78,135,136]. However, in URFC applications, the unit cell performances were lacking because of carbon corrosion during the WE mode operation. Consequently, extensive developments have been reported for novel GDB

associated with metal or metal-precursor-deposited MPL and PTFE-coated GDL layers [78,132,134,135,137]. Moreover the pore size distribution of GDB plays a key role in enhancing URFC performances [133,134,138]. The optimized GDB materials should possess a hydrophobic nature in the FC mode to prevent the water flooding, and hydrophilic nature in the WE mode to supply the water to the oxygen electrode [132]. Usually, carbon paper or carbon cloth containing fine carbon particles in conjunction with a hydrophobic compound has been considered a promising GDB for fuel cell and URFC applications [132e134]. To enhance the hydrophobic nature of the carbon-based GDB, current synthetic efforts have utilized the advanced hydrophobic nature of PTFE. The synthetic fluoropolymer of PTFE-coated carbon cloth significantly enhances the corrosion resistance on GDB and improve the

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Fig. 10 e Graphical representation of proton transportation mechanism in MEA in (a) fuel cell mode (b) water electrolyzer mode.

Fig. 11 e Structure of the fluoropolymer-based Nafion membrane [124].

performances in the URFC [132,137]. Additionally, the Ti metal and its compounds have also received significant attention for the novel GDB due to their higher stabilities and porous structures [135,138]. Though, the surface oxidation of Ti lowers the electronic conductivity of the unit cell; this significantly affects the URFC performances.

Microporous layer (MPL) As mentioned above, the novel structured or composite of MPL would be a good option to build an advanced GDB. Apart from PTFE as a MPL or GDL, graphitized carbon [139], iridiumetitanium nitride (IreTiN) [70] and iridium oxide/titanium IrO2/Ti MPLs [78] have been particularly considered as corrosion resistant GDLs for URFC applications. Fig. 12 shows the URFC e ORR polarization curves of the (a) carbon based MPL and (b) IreTiN based MPL of GDB [70]. The stability of the GDB was identified through the cyclic process of the ORR polarization curves. IreTiN-based MPL shows an excellent stable performance compared to that of the conventional carbonbased MPL under identical operating conditions. It is a direct indication that the IreTiN-based MPL has a higher resistance against the corrosion behaviors. Until the 9th cycle, the ORR performance was significantly degraded for the carbon-based MPL due to severe corrosion in the MPL layer. Furthermore,

the carbon corrosion creates a contact resistance between the electrode and BP plates, and higher water flooding in the unit cell. In contrast, corrosion resistivity of the IreTiN-based MPL rendered it an optimum MPL material for the GDB. However, a minor degradation in performance was observed from the polarization curves of the IreTiN-based MPL. Hence, the novel structured and composite materials with uniform pore size distribution of MPL and GDL could be invented for a 100% corrosion-free GDB for advanced URFC applications.

Severe corrosion in bipolar plate Because of multifunctional behaviors, significant attention has been focused on the bipolar plates (BPs) in FC and URFC technologies [80,81,140]. An ideal BP has significant advantages such as the separator, current collector, and backbone of multi-cell stacking. During the unit cell operations, BPs facilitates the electrical current transportation and fuel spreading from cell to cell [80]. It has additional functional properties such as heat and water management during the unit cell operation. Despite these several advantages, a carbon corrosion environment has occurred in the WE mode due to higher applied potentials in the range of >1.5 VSCE [141]. This higher corrosion phenomenon in BPs has severely affected the unit

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Fig. 12 e MPL stability analyses through the fuel cell mode (ORR) polarization curves for the (a) carbon based MPL and (b) Ire TiN based MPL. Reprinted with permission from Ref. [70]. Copyright Elsevier (License number: 3934640400753).

cell performances. Recently, a few corrosion-resistant BP materials with higher electrical conductivity have been considered for enhancing the electrochemical performances in URFC system, such as gold (45.2  106 S/m), aluminum (37.7  106 S/ m), ruthenium (3.19  106 S/m), titanium (2.34  106 S/m), SS 316 (1.33  106 S/m), and graphite (1.27  105 S/m) [81]. Nonetheless, the following additional properties are sought for advanced BP materials [140,142e144]: ➢ DOE and Freedom CAR targets ❖ Higher electrical conductivity (>100 S cm1) ❖ Low (hydrogen) permeability (<2  106 cm3 cm2 s1) ❖ Higher chemical stability e Lower corrosion rate (<16  106 A cm2) ❖ Crush strength (4200 kPa) ➢ Plug Power targets e Superior mechanical and thermal stability ❖ Tensile strength: >41 MPa ❖ Flexural strength: >59 MPa ❖ Impact strength: >40.5 J m1 (0.75 ft-lb in.1) ❖ Thermal conductivity: >10 W(mK)1 ➢ Higher thermal stability (40 to 120  C), low thermal expansion, and suitable hydrophobicity (or hydrophilicity). ➢ Low interfacial contact resistance: <20 mU cm2 at 150 N cm2 ➢ Light weight, rapid manufacture, and low cost Fundamentally, the carbonaceous structures of graphite and porous graphite have been considered as promising BP materials [145]. Nevertheless, graphitic materials have certain hindrances in the WE mode due to severe corrosion at >1.5 V in the URFC [80,141,146]. Apart from graphitic materials, Ti metal can be considered as an ideal BP material in order to achieve higher performances in URFC systems [141]. Ti-based BP has the potential properties of corrosion resistance at high positive over potentials and considerable mechanical stability. Despite these advanced properties in Ti-based BP plates, contact resistance occurs between the BP plate and electrode due to the passive oxide layer formed on the BP surface, and it moderately affects the URFC performances. In this context, to overcome the barriers in the Ti-based BP plates, various kinds of metal-coated titanium BP (such as Ti substrate coated with

(Ti, Zr) N thin films [141], TieAg-deposited Ti [147], nanocomposite TieAgeN films [148], Pt-deposited Ti [149], and Aucoated Ti [150]) were reported for corrosion resistance and enhancement of the electrochemical performances in the URFC system. Fig. 13 shows the corrosion resistance (stability) and electrochemical performance of (a) carbon based BP plate and (b) platinum coated titanium BP plate [149]. In the comparative performance of the BP plate, the FC mode performances were carried out under identical conditions for both the BP plates. Over the first cycle, the ORR polarization curves showed similar performances for both the BP plates. In contrast, remarkably different performances were observed after the first cycle. Significantly degraded performance was observed for the carbon-based BP plate, but the Ti-based BP showed stable performance after the first cycle. In spite of that, a minor degradation in performance was noted for the Pt-coated Ti-based BP plate, which is common in the URFC performance during the cyclic performances. On the basis of the ORR performances in URFC, the degraded performance was significant for the carbon-based BP. The optical images (Fig. 13a and b) in the inset clearly show the severe corrosion of the carbon-based BP plate after the FC mode. The corrosion in the BP plate leads to the contact resistance between the electrode and BP plate. It is a major reason for the degraded URFC performance during the cycle analyses. However, using a Pt-coated Ti-based BP plate shows superior stability against the severe corrosion during the unit cell operation. From the reported studies, the corrosion resistance and enhanced URFC performances have shown the superior performance of the Ti-based novel metal deposited/composite BP plate [141,147e150]. Nevertheless, in the near future, cost-effective BP plate materials should be considered instead of high-cost noble metals for optimal URFC unit cell systems.

Future prospective for advanced developments in URFC system In the previous sections, we have evidently explained the limitations, challenges, and developments in the URFC unit

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Fig. 13 e The cyclic performance and corrosion stability of the (a) carbon-based BP plate and (b) Pt-deposited Ti BP plate. Reprinted with permission from Ref. [149]. Copyright Elsevier (License number: 3934640535520).

cell system. Here, based on the current developments in the URFC systems, we have proposed our ideas for constructing advanced URFC systems. So far, high-cost Pt-based electrocatalysts have been considered as promising materials for enhanced electrochemical reactions. Various metals and their derivatives have been added with Pt electrocatalysts for reducing the Pt loading in the electrodes and to lower the manufacturing cost of the electrodes [97e99,151e154]. For making viable low-cost bifunctional electrocatalysts, novel structured or composite of non-Pt based unsupported electrocatalysts such as metal and metal oxide (porous IrO2, RuO2) could provide efficient electrocatalyst materials for URFC applications. The advanced structured materials should have the multiple functional properties of electrocatalysts and supportive materials. The novel electrocatalyst must contain the synergistic effect properties for enhancing the electrochemical

reactions, protons and electrical conductivity. As well, the corrosion resistances of supportive materials should also contribute to enhancing the electrochemical performances [151,154,155]. The schematic representation in Fig. 14 shows the advanced technical view of novel electrocatalyst functions. Introducing sulfonation (eSO3H functional group) on the supportive materials will be a priority choice for the advanced URFC systems, because sulfonation in the supportive materials can considerably enhance the electrochemical reactions due to the rapid proton transportation [112]. The severe corrosion in supportive materials can be controlled by the highly crystalline nature of novel structured materials. If the electrocatalyst support material contains the ORR and/or OER properties, it will be a more beneficial to the advanced URFC system. In URFC technology, the next major challenge is to produce highly stable, low-cost membranes for replacing the

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Fig. 14 e Schematic representations of optimum electrocatalyst materials for advanced URFC developments.

conventional Nafion membrane. In the previous decade, various kinds of polymer-based membranes have been studied for different fuel cell applications. Though, no extensive analyses have been undertaken for URFC applications. Fig. 15 represents an advanced hybrid membrane for the URFC system. Compared to the conventional Nafion membrane, the proposed organic-inorganic hybrid composite structure will be a promising membrane for advanced URFC applications. Sulfonated polyether ether ketone (sPEEK) and sulfonated poly phenylene oxide (sPPO) can be considered as efficient organic compounds instead of conventional Nafion due to their low cost, availability, and easy sulfonation processes [156e158]. The sulfonated polymer membrane can increase the proton conductivity, flexibility, and stability of the membrane. In order to augment the advanced properties in the membrane, functionalized inorganic materials must be dispersed in the polymer membrane matrix. The inorganic materials can

Fig. 15 e Graphical illustration of advanced organiceinorganic composite hybrid membrane for URFC systems. Please cite this article in press as: Sadhasivam T, et al., A comprehensive review on unitized regenerative fuel cells: Crucial challenges and developments, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.140

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enhance the mechanical and thermal stability of the polymer membrane due to strong interfacial interaction between the polymer main chain and inorganic functional properties. Furthermore, the incorporation of inorganic material can act as a barrier for the fuel crossover in the membrane. From the above mentioned advanced properties, the hybrid organicinorganic membrane will be a potential source for the further developments in the URFC systems. In URFC, carbon corrosion is also a major limitation in the BP plate and GDB. In order to prevent the carbon corrosion in GDB (GDL and MPL) and BP plates, the highly crystallite nature of graphitized carbon can be considered as a higher corrosion-resistant material [139]. On the other hand, the durability of graphitized carbon-based BP might be decreased during the long time operation. At present, graphite-based metal composite plates [144,145] and metal-based PB plates, especially stainless steel and Ti [141,147e150], have the promising properties for enhancing the stability of the BP plate. For the advanced technologies, low cost of novel mesoporous or nano-rod structured metal particles such as Ti, IrO2, and novel alloys incorporation on graphitized carbon will be promising BP plates for the URFC applications. The proposed materials will be more effective for the advanced developments in the near future for viable URFC applications.

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The current trend shows several advantages for the novel structured materials, composites, and advanced techniques in the URFC unit cell. However, low-cost and highly durable materials and components need to be invented for creating advanced and optimum URFC systems. This comprehensive analysis and prospective suggestions is supportive of initiating advanced developments in URFC applications.

Acknowledgements This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resources by the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20153030031670). This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015061146).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.10.140.

Concluding remarks The URFC system has been considered a viable advanced and alternative energy system for the upcoming future. The URFC is an optimum fuel cell system owing to its specific high energy density. This comprehensive review highlighted the major challenges, limitations, recent progress, and further developments in the context of URFC systems. From the materials perspective, the degradation and poor stability of the materials in the unit cell is a significant challenge in the URFC device. Researchers have made noteworthy analyses and developments for commercializing the URFC system. In addition, novel scientific ideas have been suggested in the prospective section to develop and commercialize the URFC system with optimum operating conditions. For further sophisticated developments of URFCs, the following difficulties should be overcome in the near future. The major limitations associated with the MEA of URFCs are the high cost and poor stability of the materials. In MEA, the prominent Pt electrocatalyst and Nafion membranes have the highest production costs. Moreover, another critical challenge is the stability and durability of the MEA. In long-term operations, the electrocatalyst reaction has suffered owing to the agglomeration, migration, and sintering due to corrosion of supportive materials. The stability of the membrane has degraded through the dimensional change and inside penetration of electrocatalyst because of migration effect. The dissolution of Nafion binder has also hindered and caused the degraded performances in the unit cell because of electrocatalyst aggregation and diminished the interfacial interaction between electrodes and membrane. Apart from the major issues in MEA, carbon corrosion should be overcome in the GDB (MPL and GDL) and bipolar plates. Nevertheless, it is possible to develop feasible URFC unit cell systems using the novel structured materials.

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