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A review on unitized regenerative fuel cell technologies, part-A:Unitized regenerative proton exchange membrane fuel cells

Citation for published version:Wang, Y, Leung, DYC, Xuan, J & Wang, H 2016, 'A review on unitized regenerative fuel cell technologies,part-A: Unitized regenerative proton exchange membrane fuel cells', Renewable and Sustainable EnergyReviews, vol. 65, pp. 961–977. https://doi.org/10.1016/j.rser.2016.07.046

Digital Object Identifier (DOI):10.1016/j.rser.2016.07.046

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Published In:Renewable and Sustainable Energy Reviews

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Download date: 18. Aug. 2021

A review on unitized regenerative fuel cell technologies, part-A: unitized

regenerative proton exchange membrane fuel cells

Yifei Wanga, Dennis Y.C. Leunga, Jin Xuana,b, Huizhi Wanga,b

aDepartment of Mechanical Engineering, The University of Hong Kong, Pokfulam Road,

Hong Kong

bSchool of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14

4AS, United Kingdom

Corresponding author: Dennis Y.C. Leung

Tel: (852) 28597911

Fax: (852) 28585415

Email: ycleung@hku.hk

Abstract

Energy storage and conversion is a very important link between the steps of energy

production and energy consumption. Traditional fossil fuels are natural and unsustainable

energy storage medium with limited reserves and notorious pollution problems, therefore

demanding for a better choice to store and utilize the green and renewable energies in the

future. Unitized regenerative fuel cell (URFC), a compact version of regenerative fuel

cell with only one electrochemical cell, is one of the competent technologies for this

purpose. A URFC can produce hydrogen fuel through an electrolysis mode to store the

excess energy, and output power in a fuel cell mode to meet different consumption

requirements. Such a reversible system possesses several distinctive advantages such as

high specific energy, pollution-free, and most importantly, the decoupled energy storage

capacity with rated power. Based on the different electrolytes utilized, current available

URFC technologies include the most common proton exchange membrane (PEM)-based

URFC, and other types of URFC such as the alkaline, solid oxide and microfluidic

URFCs. This part of the URFC review emphasizes on the PEM-based URFC.

Specifically, the research progress on both cell components and systematic issues is

introduced. Benefiting from its fairly mature technology stage, the PEM-based URFC has

already been applied in aerospace and terrestrial areas. However, for large-scale

application, their cost and efficiency are still the obstacles when competing with other

energy storage technologies. As for the alkaline, solid oxide and microfluidic types of

URFC, their research progress is reported independently in part B of this review.

Keywords

Unitized regenerative fuel cell, proton exchange membrane, bifunctional catalyst, gas

diffusion layer, bipolar plates, practical applications

Abbreviations

ATO: Sb-doped SnO2

BHC: Bifunctional hydrogen catalyst

BOC: Bifunctional oxygen catalyst

BOP: Balance of plant

BPPs: Bipolar plates

EC: Electrolysis cell

FC: Fuel cell

GDB: Gas diffusion backing

GDE: Gas diffusion electrode

GDL: Gas diffusion layer

HER: Hydrogen evolution reaction

HOR: Hydrogen oxidation reaction

LLNL: Lawrence Livermore National Laboratory

MEA: Membrane electrode assembly

MH: Metal hydride

MPL: Microporous layer

OER: Oxygen evolution reaction

ORR: Oxygen reduction reaction

PEM: Proton exchange membrane

PEMEC: Proton exchange membrane electrolyzer cell

PEMFC: Proton exchange membrane fuel cell

PPD: Peak power density

PV: Photovoltaic

RFC: Regenerative fuel cell

RFB: Redox flow battery

R-PEMFC: Regenerative proton exchange membrane fuel cell

UPS: Uninterrupted power supply

URFC: Unitized regenerative fuel cell

UR-PEMFC: Unitized regenerative proton exchange membrane fuel cell

UR-AFC: Unitized regenerative alkaline fuel cell

UR-SOFC: Utilized regenerative solid oxide fuel cell

UR-MFC: Utilized regenerative microfluidic fuel cell

ZEV: Zero-emission vehicles

1. Introduction

Nowadays environmental issues, such as global climate change and air pollution, have

strongly attracted world’s attention. Traditional fossil fuels are still the predominant

energy source for human activity, which are responsible for many well-known

environmental problems, typically the global warming due to massive CO2 emissions and

the air pollution due to the emissions of SOx and NOx. In addition, fossil fuel reserves are

limited. According to a prediction in 2009, coal can last for about 107 years, crude oil for

about 35 years, and nature gas for about 37 years [1], while world population and energy

demand still keep increasing. When facing these problems, either the optimization of

existing fossil fuel utilization facilities or the development of renewable energy sources is

promising, of which the latter one is more sustainable considering the limited reserve of

fossil fuels. Renewable energies such as solar, wind, geothermal etc. are getting more and

more attention and investment in recent years, with a growing capacity and proportion in

world energy supply. However, the intermittent nature of renewable energies has

restrained their application prospect. In order to balance with their energy production

intermittency, either secondary batteries or regenerative fuel cells (RFC) are proposed as

auxiliary energy storage & conversion component for them.

As shown in Figure-1(a), a RFC is composed of three main parts: a fuel cell for power

generation, an electrolysis cell for fuel production, and a fuel storage component. In

addition, an oxygen storage part can also be added for a completely-closed RFC system.

When coupled with renewable power sources, the electrolyzer can utilize the redundant

and inferior power input to split water for hydrogen generation, while during the power

output period hydrogen fuel will flow back to fuel cell and output stable electric power.

To make a more compact system, the fuel cell part and the electrolyzer part can be

unitized into a single electrochemical cell, which functions alternatively as fuel cell and

electrolyzer, as shown in Figure-1(b). This unitized regenerative fuel cell (URFC) holds

several advantages against the conventional discrete RFC, such as lower capital cost,

simpler structure, higher specific energy, no need for auxiliary heating [2], etc.

Till now, secondary batteries are widely used for energy storage purpose due to their

high round-trip efficiency (around 80%) [3], but their drawbacks are also evident. The

durability of secondary batteries is not very satisfactory when facing deep cycling, and

their specific energy is constrained by the heavy weight. Moreover, the coupling of

energy storage capacity and rated power has made secondary batteries less efficient to

scale up. To solve this problem, redox flow batteries (RFBs) are proposed as an

alternative choice. Unlike the conventional secondary batteries which store reactants

within the cell, the RFBs utilize electro-chemical reactants dissolved in electrolytic

solutions which are stored in external tanks and circulated on the electrode surface during

operation. In this way, the energy storage capacity and rated power of RFBs are

decoupled [4]. By enlarging the electrolyte storage tank the capacity can be easily

increased, while the rated power can be enhanced by enlarging the electrodes’ area or

through stacking. However, due to the bulk electrolyte solution contained in the system,

the specific energy of RFBs is generally much lower. Similar to RFBs, URFCs also store

the fuel and oxidant, generally H2 and O2, externally in separated gas tanks, therefore

achieving decoupled storage capacity and output power, but their specific energy is much

higher due to the absence of liquid electrolyte, which is about 0.4 to 1.0 kWhkg-1

including the mass of the hydrogen and oxygen gas tanks [5]. In addition, URFCs can be

totally charged and discharged without damaging the durability compared with secondary

batteries. These advantages have made URFCs very competitive against secondary

batteries and RFBs. However, URFCs generally achieve lower round-trip efficiency than

batteries due to the sluggish oxygen reactions. Other issues such as high cost, hydrogen

storage, and relatively low technology readiness, have also hindered their applications.

Same as the classification of fuel cells, URFCs can also be classified by the

electrolytes they employ, such as the unitized regenerative proton exchange membrane

fuel cell (UR-PEMFC), unitized regenerative alkaline fuel cell (UR-AFC), utilized

regenerative solid oxide fuel cell (UR-SOFC) and utilized regenerative microfluidic fuel

cell (UR-MFC). The principal difference among these different types of URFC exists in

the electrolyte where different ions are transferred to connect the oxidation and reduction

reactions. Figure-2 gives the schematic diagrams of different types of URFC operated in

both modes. The conducting ions in the electrolyte are H+ and OH- for UR-PEMFC and

UR-AFC, respectively, while for UR-SOFC two different ions, i.e. O2- and H+, can both

be utilized. As for UR-MFC, it is not included in this figure due to its flexible electrolyte

choice. In addition, Table-1 gives a brief comparison of different URFC technologies,

including their main feature, working temperature, efficiency, and R&D status.

In this part of review, recent research progress on UR-PEMFC is reported first, which

is currently the most-developed URFC technology. Research works on different cell

components will be introduced separately, from the inner component, i.e. the bifunctional

catalysts, to the outermost component, i.e. the bipolar plates. In addition, systematic

issues such as electrode configuration, fuel cell stacking, heat and water management, etc.

are also presented. With all these efforts, UR-PEMFCs have already achieved an early

application stage in many areas, including aerospace and aviation, renewable energy,

power supply, and transportation. which are also introduced at the end of this review. As

for the other URFC technologies, detailed report will be given in part-B of this review.

2. Unitized regenerative proton exchange membrane fuel cell (UR-PEMFC)

Proton exchange membrane fuel cell (PEMFC) is one of the most developed fuel cell

technologies with abundant R&D efforts, since its first appearance in the early 1960s by

Willard Thomas Grubb and Leonard Niedrach of General Electric [6]. It can convert

chemical energy contained in fuels such as hydrogen, methanol, ethanol etc. directly into

electrical energy under mild reaction conditions, i.e. 20-100℃ and moderate pressure.

The reverse of this reaction to produce hydrogen fuel by water electrolysis is also

possible with the device called proton exchange membrane electrolysis cell (PEMEC),

which was proposed at nearly the same time with PEMFC [7]. One possible way to

utilize these two devices is to combine them into a single system, namely the regenerative

proton exchange membrane fuel cell (R-PEMFC), which can store excess energy as

hydrogen fuel by the electrolysis cell component and output electrical energy by the fuel

cell component. To make the system simpler and more compact, the PEMFC component

and the PEMEC component were further integrated into one bifunctional electrochemical

cell, i.e. the unitized regenerative proton exchange membrane fuel cell (UR-PEMFC),

which was first put forward in the early 1960s [8] and successfully tested for space

missions in 1973 [9].

Since its first appearance, UR-PEMFC has earned numerous interests because of its

distinct advantages over other energy storage technologies. With much higher specific

energy (0.4-1.0 kWhkg-1 [5]), long-term storage ability due to the elimination of self-

discharge, and most importantly, the decoupling between power output ability and energy

storage ability, UR-PEMFC is deemed to be promising for practical applications. In the

late 1990s, Mitlitsky et al. [5, 10-12] from the Lawrence Livermore National Laboratory

(LLNL) have done great works to improve cell performance and cycle stability of UR-

PEMFCs with a relatively low catalyst loading (1mgcm-2). Negligible degradation was

observed for 2010 cycle tests. Light-weight pressure vessel was also designed to improve

the systematic specific energy. As for commercial applications, Proton Energy Systems,

Inc. has developed their product UNIGENTM which performed identically to dedicated

electrolyzers in electrolysis cell (EC) mode and to dedicated fuel cells in fuel cell (FC)

mode [13]. Despite of these great efforts, UR-PEMFC technology has still not ushered

large-scale commercial application stage. Continuous efforts have been made to further

improve their performance, cycle stability, round-trip energy efficiency and cost-

effectiveness.

This section will mainly introduce recent research progress on different UR-PEMFC

components, including bifunctional hydrogen catalyst (BHC), bifunctional oxygen

catalyst (BOC), gas diffusion layer (GDL), and bipolar plates (BPPs). Moreover, research

works on electrode configuration, single cell stacking, and other systematic issues such as

heat & water management will also be reported. Considering the commonly employed

commercial Nafion membrane in UR-PEMFCs, membrane development is not included

in this review. Table-2 has summarized the existing UR-PEMFC prototypes

demonstrated to date in the literature, with both their cell parameters and performance

specified, such as the catalyst/electrolyte/GDL material, operation temperature, peak

power density (PPD), round-trip efficiency, and cycle stability. Figure-3 gives a

schematic diagram of the main components in a UR-PEMFC single cell, including a

Nafion membrane successively sandwiched by the catalyst layer, GDL and BPPs. In

addition, micrographs of the catalyst dispersion on catalyst support, catalyst layer

structure, GDL micro structure, and commercial BPPs are also included in this figure.

2.1 Bifunctional hydrogen catalyst (BHC)

Pt has long been utilized and is still the most efficient and stable BHC for UR-

PEMFCs. Great efforts have been made to decrease its loading to less than 0.5mgcm-2,

considering the scarcity and high cost of Pt [14-20]. However, for large-scale applications,

noble metal-free catalysts are still preferred to replace Pt. Goff et al. [21] grafted nickel

catalyst on carbon nanotubes to mimic the active sites of hydrogenase enzymes. This

synthetic catalyst not only resulted in low overvoltage (20mV) in hydrogen evolution

reaction (HER) and excellent stability (over 100,000 turnovers), but also exhibited high

efficiency for hydrogen oxidation reaction (HOR) with comparative current densities to

the hydrogenase-based materials. Andrews et al. [22] built up a novel UR-PEMFC

prototype called the “proton flow battery” with an integrated metal hydride (MH) as

hydrogen electrode. This MH electrode can not only catalyze the HOR and HER, but also

store the generated hydrogen fuel in the form of “proton”, thus eliminating both the Pt

catalyst and the outside hydrogen storage component. However, its cell performance and

hydrogen storage capacity are yet to be improved.

2.2 Bifunctional oxygen catalyst (BOC)

As shown in Figure-3, the BOC layer lies between the PEM and the GDL, and is

directly responsible for both oxygen reduction reaction (ORR) and oxygen evolution

reaction (OER). Competent BOCs should have such properties as high catalytic activity,

long-term durability, high electronic conductivity and low cost. Currently, the choice of

BOCs in UR-PEMFCs is still limited to noble metal catalysts. Various research works

have been done to improve their properties, including both the optimization of catalyst

composition and catalyst support. In 2006, Pettersson et al. [23] have published a review

paper focusing primarily on BOC development. Based on their work, this section will add

more recent related research works ever since.

2.2.1 Catalyst composition:

To date, most of the BOCs utilized in UR-PEMFCs are still noble metal-based by the

combination of platinum (Pt), ruthenium (Ru), iridium (Ir) and their oxides. Pt, the

preferred catalyst for ORR, is not suitable for OER, while the preferred catalysts for OER

such as Ru, Ir and their oxides are not suitable for ORR. Therefore, a compromise is

made by combining Pt with Ru, Ir or their oxides as composite BOCs.

Research works have been carried out to figure out the best catalyst combination for

BOC. Chen et al. [24] screened 715 unique combinations of five elements (Pt, Ru, Os, Ir,

and Rh) to evaluate their activity and stability as BOC. They found that the Pt-Ru rich

region of the Pt-Ru-Ir ternary exhibited high activity towards ORR/OER with good

anodic corrosion resistance, among which the Pt4.5Ru4Ir0.5 is more active than Pt1Ir1. This

conclusion was also confirmed by Rivas et al. [25]. Yim et al. [26, 27] screened several

composite catalysts including Pt, Pt-Ir, Pt-RuOx, Pt-IrOx, Pt-Ru and Pt-Ru-Ir. They found

that the FC performance was in the order of Pt black > Pt-Ir > Pt-RuOx > Pt-Ru ∼ Pt-Ru-

Ir > Pt-IrOx, while the EC performance was in the order of Pt-Ir ∼ Pt-IrOx > Pt-Ru > Pt-

Ru-Ir > Pt-RuOx ∼ Pt black, among which the Pt-Ir possessed the best cycle performance

and highest round-trip efficiency. As for Pt4.5Ru4Ir0.5, they found that its activity was just

similar to Pt-Ru, which was contradictory to Chen and Rivas’s findings [24, 25].

Regardless of this divergence, Ru and its oxides are found to be unstable under the

operating conditions [28]. Therefore, the combination of Pt and Ir or its oxides is

currently the mainstream choice for BOC. Various investigations towards their elemental

ratio, catalyst preparation method, microstructure etc. have been conducted. In addition, a

recent related review by Antolini et al. [29] has introduced Ir as catalyst and co-catalyst

for ORR/OER.

As shown in Table-2, there seems no specific optimal ratio between Pt and Ir, of which

the proportion of Ir (or IrO2) can range from 1wt.% to 50wt.% in different studies. This

divergence might be due to the different catalyst/electrode preparation methods in

different studies. In general, the proportion of Pt should be more than half, due to the

incapability of Ir towards ORR. As for the preparation of composite BOCs, mechanical

mixing is an easy and direct method that is commonly employed, which however cannot

finely disperse the different catalyst components, so agglomeration tends to happen after

the mixing. To solve this problem, different synthesize methods were developed instead

of direct mixing, including Pt deposited on IrOx [20, 30], IrOx deposited on Pt [15, 31,

32], or a further mixture after catalyst deposition [19, 33, 34], all of which were reported

to obtain a better bifunctional performance than the mixed Pt-IrOx catalyst. Figure-4 also

gives a graphic description of the above-mentioned deposited BOCs. In general, by

depositing one catalyst component on the surface of the other, better particle dispersion

and less agglomeration can be achieved, leading to enhanced electrochemical active

surface area and electronic conduction for the BOC.

2.2.2 Catalyst support:

Catalyst support plays very important roles in electro-catalysis. First it provides a

physical surface for the catalyst particles to disperse on, leading to a larger catalyst

surface area and therefore higher electro-catalysis rate. The catalyst loading can also be

lowered down with catalyst support. Moreover, catalyst support helps to control the

wettability and provide better electronic conductivity for the electrode. A good catalyst

support should have such features as high surface area, good electronic conductivity, high

corrosion resistance, and low cost. For PEMFC, carbon black is commonly used as

catalyst support in both hydrogen and oxygen electrode, which is no longer suitable in

UR-PEMFCs especially on the oxygen side. This is due to the fact that the high anodic

potential at the BOC in EC mode will cause severe carbon corrosion via the following

reaction [35], which is, unfortunately, accelerated by the dispersed catalyst on electrode

surface.

C + 2𝐻2𝑂𝑃𝑡→ 𝐶𝑂2 + 4𝐻+ + 4𝑒− (𝐸𝑜 = 0.118𝑉 𝑣𝑠. 𝑅𝐻𝐸 𝑎𝑡 25℃)

Efforts have been made to develop suitable catalyst support for the BOC in UR-

PEMFCs. One possible choice is to use a portion of the catalyst itself as support, as

mentioned in Section 2.2.1. However, the surface area provided by either Pt or IrO2 is not

high enough, and it is also not cost-effective to use noble metals as catalyst support.

Instead, non-noble metal-based supports such as titanium oxides, titanium carbides,

graphitized carbon etc. have been proposed, which is summarized in Table-3. Titanium

oxides are considered to be promising as UR-PEMFC catalyst support [18, 36], whose

performance can be further improved by niobium-doping [37-39]. In addition, titanium

carbide (TiC) and nitride (TiN) have also been investigated [40-43], among which the

TiCN-supported catalyst has demonstrated remarkable catalytic activity and corrosion

resistance [42, 43]. However, particle agglomeration was found to be aggravated with

higher nitrogen content in these catalyst supports [43]. Other catalyst support materials

were also found to be suitable in UR-PEMFCs, such as the graphitized carbon [44] and

Sb-doped SnO2 [45].

2.3 Electrode preparation

After synthesizing bifunctional catalysts, different methods have been proposed to

fabricate the electrode. Traditional roll press method uses PTFE to bond the catalyst

powder into a self-standing porous electrode, which however would lead to a high

catalyst loading up to 10mg cm-2 [31, 46] and a poor mass transport due to the thick

electrode. To reduce the catalyst loading, paste method, also called the decal method or

transfer printing method, is proposed which uses a PTFE sheet to support the thin film

electro-catalyst before pasting it onto the membrane. Catalyst loading can be decreased to

2-4mgcm-2 [47], and could even reach 0.4mgcm-2 [14]. To use this method, a catalyst ink

or slurry is prepared first by mixing the catalyst with Nafion solution and isopropanol,

and then dispersed by ultrasonic treatment. The Nafion plays the role of catalyst binder

while the isopropanol is used for better catalyst dispersion. In addition to the paste

method, the catalyst ink can also be applied directly onto the membrane or GDL by either

brushing [15-17] or spraying [18-20]. Other methods such as screen printing [48] and

chemical reduction [49] have also been proposed in the literature.

2.3.1 Catalyst binder

Nafion is commonly used as catalyst binder when preparing electrodes. It plays

important roles in providing proton conductivity between catalyst and membrane,

adhesion of catalyst on membrane/GDL, and catalyst uniform dispersion. However,

commercial Nafion binder has several problems such as brittleness, low proton

conductivity, and vulnerable to alcohols [50]. Jung et al. [50] modified the Nafion binder

by using dimethylacetamide as dispersion solvent, and compared its property with

commercial Nafion binder. The modified Nafion binder showed much lower dissolution

in methanol solution, higher proton conductivity and better water uptake than the

commercial one. When applied in a UR-PEMFC, more stable performance was achieved

in both FC and EC mode. UR-PEMFCs with Nafion binder can also get flooded easily at

high current densities in FC mode due to the high hydrophilicity of Nafion. In order to

improve the water management and mass transport in FC mode, Chen et al. [51]

developed a novel membrane electrode assembly (MEA) by the Nafion-pyrolyzed

method. The Nafion binder used in the oxygen electrode was pyrolyzed in nitrogen at 320℃

for an hour before the MEA was formed. UR-PEMFC with this novel MEA exhibited a

high water electrolysis performance and a much higher fuel cell performance than that of

the conventional MEA. Fuel cell efficiency and round-trip efficiency is improved by 13.5%

and 10.8% at 700mAcm-2 compared with the cell with normal Nafion binder.

2.3.2 Catalyst layer structure

The effect of different catalyst layer structures on cell performance and stability has

also been investigated in the literature. Liu et al. [52] proposed a composite electrode

with an extra layer of catalyst applied on the GDL, which improved the UR-PEMFC

stability and cycle life for over 25 cycles. Wittstadt et al. [53] investigated the effect of

different modifications to MEA especially on the oxygen side. By sputtering extra Pt onto

the BOC, the MEA achieved better performance in both modes. By sputtering extra Pt

directly onto the membrane (oxygen side), impermeability of the MEA was improved

which leads to a higher potential at low current density region, but the power density at

high current density was reduced due to the weakened contact between electrode and

electrolyte. Chen et al. [54] investigated the effect of different catalyst layer structures on

UR-PEMFC performance and round-trip efficiency. The highest round-trip efficiency

was achieved by the structure of “Pt sprayed on GDL and then IrO2 sprayed on Pt”, while

the lowest efficiency was achieved by the structure of “Pt sprayed on membrane and IrO2

sprayed on GDL”. Altmann et al. [55] also investigated various BOC layer structures,

including single catalyst layer of Pt-IrO2 mixture, multilayered Pt and IrO2, and

segmented catalyst layer of Pt and IrO2, as shown in Figure-5. It was found that the single

layer structure performed the best among the three different structures.

2.4 Gas diffusion layer (GDL)

As shown in Figure-6(a), a gas diffusion layer (GDL) generally consists of two parts: a

thin and porous substrate called gas diffusion backing (GDB), and another porous layer

coated on GDB called microporous layer (MPL). The GDB provides supporting substrate

and gas channels for the electrode, while the MPL helps in water management and mass

transport by modifying the pore structure of GDB. In some cases, the catalyst is directly

deposited on GDL to form a gas diffusion electrode (GDE), as shown in Figure-3.

Table-4 has summarized the existing GDL materials suitable for working in UR-

PEMFCs. For PEMFCs, hydrophobic carbon paper or carbon cloth is commonly adopted

as GDB because of its high electronic conductivity, low cost and easy processing.

However, such kind of GDB is no longer suitable for UR-PEMFCs because of the severe

carbon corrosion in EC mode and the required balance between hydrophilic and

hydrophobic properties of the GDB [56]. Instead, titanium material is proposed as a

promising candidate for GDB because it is highly conductive and corrosion-resistant

even under high-potential and acidic environment. Both bare Ti-felt GDB [53, 56-59] and

Ti-felt GDB coated with Ti-powder MPL [60, 61] have been applied in UR-PEMFCs, as

shown in Figures 6(b) and 6(c), respectively. It is found that by adding an appropriate

amount of PTFE into the Ti-felt GDB, both cell performance and stability can be

improved [53, 56], while a larger fiber diameter of Ti-felt can harm the EC performance

but improve the FC performance [57, 58]. By coating Ti-powder MPL to Ti-felt GDB,

EC performance is not affected, but FC performance is significantly improved under fully

humidification condition attributed to the hydrophobic meso-pores in GDL brought by

the Ti-powder, which can improve mass transport in FC mode [60, 61].

Even with so many advantages, titanium GDB can also be oxidized in EC mode and

form a low-conductive oxide surface layer, leading to an elevated cell resistance.

Moreover, titanium material is quite expensive, and it is difficult and costly to make it

thinner, lighter and highly porous at present [62]. Conventional carbon paper can still be

used as GDB in UR-PEMFCs if coated with a protective MPL, which can combine the

active oxygen species (oxygen atom and hydroxyl free radicals [63, 64]) to form oxygen

molecules rapidly before they diffuse to the carbon GDB and corrode it. Such MPL is

generally composed of an OER-active component (Ir or IrOx) and a corrosion-resistant

component (Ti or TiOx), such as the Ti-powder supported IrO2 [65], IrTiOx-doped TiC

[62], Ir-TiN composite [66], etc. In addition, graphitized carbon MPL is also proved to be

able to improve the stability of the carbon paper GDB [16].

2.5 Bipolar plates (BPPs)

As the outermost cell component, BPPs support the MEA for single cells and serve as

separator for the stack. In addition, BPPs also help in fuel & oxidant distribution, current

collection, and heat and water management [67-72]. Competent BPPs should have such

properties as non-permeability, high thermo-conductivity, low interfacial contact

resistance, good mechanical strength, high corrosion-resistance, lightweight, cost-

effectiveness, etc. [67-72]. A more specific review paper on suitable BPPs for UR-

PEMFC has been published by Dihrab et al. [73] in 2009.

Three candidate materials have been proposed for BPPs in the literature: graphite,

polymer blended with carbon, and metal [72]. Graphite is commonly used as BPPs in

PEMFCs because of its excellent electrical conductivity, high chemical stability, and

good hydrophobicity, but its shortcomings are also evident, including poor mechanical

strength and manufacturability. As for UR-PEMFCs, the high positive overpotential in

EC mode and the acidic environment can cause severe corrosion to graphite BPPs,

leading to not only a weakened electrical contact between electrodes and BPPs but also a

decreased BPPs hydrophobicity, which can increase the ohmic resistance and mass

transport losses. Using polymer blended with carbon can improve the poor

manufacturability of graphite, but suffers from weakened electrical conductivity. Instead

of carbon-based materials, metal-based BPPs are proposed as a good alternative because

of their high mechanical strength, non-permeability, excellent manufacturability and cost-

effectiveness. The metal includes titanium, stainless steel, aluminum, copper alloy, nickel,

etc.

Table-5 summarizes the suitable BPPs for UR-PEMFCs. Titanium BPPs have been

extensively investigated for UR-PEMFCs because of their excellent corrosion resistance

and light weight, which, however, suffer from passivation problem due to the titanium

oxide layer formed on their surface under oxidation environment. The same problem also

occurs for stainless steel BPPs. Therefore, surface modification is needed in order to

further improve their corrosion resistance. Jung et al. tried both Au-coated [67] and Pt-

coated [68] Ti BPPs for UR-PEMFC, which achieved much better stability than the

carbon-based BPPs. As shown in Figure-7, the Au-coated Ti-BPPs exhibited much better

stability than the carbon-based BPPs with a 5 times lower voltage degradation rate.

Nevertheless, Au or Pt coating can be quite expensive considering the high surface area

of BPPs. To lower down the cost, Ti-Ag film [69], Ti-Ag-N film [71], or (Ti,Zr)N

coating [72] was utilized instead, which can also improve the cycle stability of UR-

PEMFCs.

Apart from corrosion resistance improvement, flow channel optimization is another

important aspect for BPPs, in order to achieve a better management of fuel, oxidant and

water. Hwang et al. [70] compared three different flow channels of BPPs on the oxygen

side, including the serpentine-single shape, serpentine-dual shape, and parallel shape as

shown in Figure-8. It was found that the serpentine-single flow channel can achieve the

highest FC performance and comparable EC performance among the three channel

shapes.

2.6 Electrode configuration

All the above-mentioned research works deal with the UR-PEMFC with the so-called

“H2 and O2 electrode configuration”, which is currently the mainstream of UR-PEMFC

research. With this configuration, each electrode will deal with the same reactant, namely

the BHE and BOE as shown in Figure-9(a). In addition to this configuration, Figure-9(b)

shows another type of UR-PEMFC which utilizes the same electrode to deal with the

same redox reaction, which means that the “oxidation electrode” deals with OER and

HOR, while the “reduction electrode” deals with ORR and HER [74-77]. This is called

the “reduction and oxidation electrode configuration”. The “H2 and O2 electrode

configuration” has the advantage of simple gas management. However, carbon-based

material is no longer suitable on the oxygen side due to carbon corrosion under the EC

mode, and less efficient ORR catalyst (i.e. Pt-IrOx composite catalyst) has to be utilized

rather than Pt, leading to a compromised FC performance. The advantage of the

“reduction and oxidation electrode configuration” is that conventional PEMFC

components such as Pt catalyst and carbon-based materials can be directly utilized on the

cathode side, but the gas compartments need to be purged every time before mode

switching.

2.7 Stacking

The energy storage rate and power output ability of a single cell is normally

insufficient to satisfy practical demands due to its limited cell voltage and electrode area.

Hence, it is necessary to stack multiple UR-PEMFC single cells together in series or in

parallel to improve the fuel generation rate and power level. Figure-10 shows some stack

prototypes demonstrated in the literature, including both commercial products from

Proton Inc. [11], LLNL [11], Lynntech, Inc. [78], and in-lab products by Sone [79],

Grigoriev et al. [80], and Millet et al. [81]. Table-6 also summarizes the main features

and stack performance of the demonstrated UR-PEMFC stacks in the literature, including

both single cell parameters and stack designs. In addition, more details on UR-PEMFC

stack development can be found in a recent review by Gabbasa et al. [82]

Su et al. [83] designed a planar 6-cell UR-PEMFC stack (in series) as a mini power

source. Under the EC mode, the hydrogen generated can be stored in a hydrogen storage

alloy and provide fuel for the subsequent FC mode operation. Test results showed an

OCV of 4.9V and a maximum power density of 74.8mWcm-2 for this stack. As for

durability, the stack can only work under 20mAcm-2 for 40min due to the limited

hydrogen storage capacity. Cycle test showed that the stack voltage was constant during

10 cycles of operation. Sone [79] demonstrated a 100W-class URFC stack prepared by

Takasago Thermal Engineering Co., Ltd. This URFC stack had 17 cells in series. In FC

mode the nominal current level was 15A at 12V, and in EC mode the nominal current

was less than 30A at 28V. Moreover, the performance of each single cell was very

uniform in both working modes: the difference in cell voltage was within 20mV when

operated at 40~60℃. Grigoriev et al. [80] developed a 7-cell UR-PEMFC stack based on

the “reduction and oxidation electrode configuration”. Tested under 80℃, nominal

electric power consumption of 1.5kW in EC mode and nominal electric power production

of 0.5kW in FC mode is achieved. At 0.5Acm-2, the mean cell voltage is 1.74V in EC

mode and 0.55V in FC mode, leading to a 30% round-trip efficiency. In the GenHyPEM

project (2005-2008), a 2-cell UR-PEMFC stack, also based on the "reduction and

oxidation electrode configuration", was developed by Millet et al. [81]. Performance of

this UR-PEMFC stack in both modes are very close to the data measured on individual

fuel cell and electrolysis devices, but the long-term stability under alternative operation in

FC and EC modes is still not sufficiently good.

2.8 System-level development considerations

As shown in Figure-11, an integrated UR-PEMFC system is much more complicated

than the stack alone, including other auxiliary components such as gas storage, pressure

control, water storage, heat pipe, system control interface, etc. [84] Therefore, in addition

to single cell and stack optimization, other system-related issues also need to be carefully

considered, among which the heat and water managements are especially important to

ensure a stable operation of the UR-PEMFC system.

In order to maintain a constant working temperature in UR-PEMFCs, waste heat

generated during FC and EC mode operation should be removed efficiently from the cell,

which is generally accomplished by a heat exchanger or a radiator. However, such

devices can lower down the system’s specific energy and cause parasitic power

consumption. Instead of using external heat exchangers or radiators, loop heat pipes were

employed which are coiled around gas tanks and covered with a thin layer of thermally

conductive carbon composite, in order to transfer the waste heat from the cells to the gas

tank surface as system radiator [84]. This novel heat exchanger is not only lightweight,

but also consumes less parasitic power. Furthermore, it can also be applied as

regenerative gas dryers/humidifiers (RGD/H) to dry the generated gases during EC mode

operation, and humidify the reactant gases during FC mode operation [85].

Water management is another important issue in a UR-PEMFC system, which is,

however, quite complicated. In general, a hydrophobic environment is needed in FC

mode to prevent water flooding, while a hydrophilic environment is requisite in EC mode

to provide sufficient water for the electrolysis reaction [47]. Therefore, a delicate balance

between the proportion of hydrophobic PTFE and hydrophilic Nafion should be made in

both the catalyst layer and the GDL, especially on the oxygen side where water is

generated in FC mode and consumed in EC mode. Ioroi et al. [47] found that PTFE

content can only affect the FC performance while Nafion content can affect both the FC

and EC performance. In conclusion, 5-7 wt.% PTFE and 7-9 wt.% Nafion were believed

to be the most appropriate ratio. Jung et al. [16] concluded that 5wt.% PTFE in the

electrode is optimum in terms of both cell performance and long-term stability. Zhuo et al.

[86] found that the optimum amount of PTFE in the GDL (oxygen side) is 26.95%

considering both FC and EC performance. These divergent conclusions might be due to

the different catalyst compositions and different electrode structures employed in

different research works, indicating that the appropriate ratio of PTFE and Nafion should

be specifically tailored for a specific UR-PEMFC system. In addition to this, water

content in the membrane electrolyte is also of great importance to its ionic conductivity,

which is tightly coupled with cell thermal management. In general, the reaction heat

should be removed efficiently from the MEA to prevent its temperature from elevating,

e.g. by utilizing BPPs with high thermo-conductivity. Otherwise, the membrane

electrolyte will get easily dehydrated, resulting in an elevated ohmic resistance and

deteriorated cell performance.

3. Applications

As shown in Figure-12, URFCs can be potentially applied as energy storage &

conversion devices in various areas, including aerospace and aviation, renewable energy,

power supply, transportation, and portable applications. Among all these areas, URFCs

are especially promising for aerospace applications. The mass of an aircraft is crucial and

can be minimized by employing URFCs, benefited from their high specific energy.

URFCs can also be integrated with solar/wind energies in remote off-grid areas.

Considering their intermittency of electricity generation, URFCs can help solar/wind

energies to smooth their output, by storing solar/wind energy in EC mode and outputting

stable electrical energy in FC mode. As for power supply, URFCs can either be employed

as grid supplement to balance the different power demands during on-peak and off-peak

periods, or as backup power/uninterrupted power supply (UPS) when off-grid. In addition,

they are also popular in electric vehicle applications with zero emissions.

Till now, most of the reported URFC applications are based on UR-PEMFCs

considering their higher technical maturity against other URFC technologies. However,

due to their relatively high cost, most of current applications are still limited to aerospace

and military fields. Similar to PEMFCs, the high cost of UR-PEMFC stacks is mainly

resulted from the expensive cell components utilized, including the PEM, noble metal

catalysts, GDL, and metal-based BPPs. Moreover, cost from the balance of plant (BOP)

can constitute more than half in the total system cost, including the management cost of

the fuel, oxidant, water, heat, etc. [87]. Furthermore, the continuous cycle operation of

UR-PEMFCs between FC and EC mode can lead to a faster component degradation rate

hence a shorter life time, which further raises their maintenance cost for component

replacement. Therefore, UR-PEMFCs are mostly applied in high cost-tolerable areas at

present. In order to promote their civil applications, novel cost-efficient substitutes are

requisite in replacement of the expensive cell components, and the cost from BOPs

should also be greatly lowered. In addition, their cycle stability should also be improved

in order to achieve a much longer life time.

Nevertheless, tentative applications and studies of UR-PEMFCs for civil purposes,

such as renewable energy, grid supplement and automobiles, are still available in the

literature, which will be introduced in the subsequent sections. Due to the ambiguity

between RFC and URFC in some of the reports, both of them will be included

considering the potentially available replacement of RFCs with URFCs.

3.1 Aerospace and aviation

Fuel cells such as PEMFCs and AFCs have long been applied in the field of aerospace

as auxiliary power sources [88]. As for URFCs, additional electrolysis function is added

to better satisfy the demands from different mission profiles, especially for geostationary

satellites, solar aircrafts, and high altitude long endurance aircrafts [11]. Early in the

1960s, patent concerning UR-AFC for the Apollo program was filed by Ludwig [89]. In

the 1990s, Mitlitsky et al. from the LLNL have published several reports on UR-PEMFCs

for aerospace and aviation purpose [5, 10-12]. Afterwards, more and more applications of

UR-PEMFCs in this area were reported.

In 2004, Connecticut-based Distributed Energy Systems have developed a multi-kWe

capable, closed-loop, lightweight RFC for high-altitude airships, which can generate

pressurized hydrogen and oxygen electrochemically without mechanical compression

[90]. NASA Glenn Research Center has also demonstrated a closed-loop RFC for a solar

electric aircraft in 2006 [91]. This RFC could store the input electrical energy and output

a steady power of 5kWe for at least 8 hours. Its transition of different working modes was

also demonstrated successfully. During 2010-2011, Japanese industrial conglomerate IHI,

collaborated with Boeing company, produced a prototype RFC for aircrafts as auxiliary

energy supplier of the engine generators [92]. This RFC can help optimize the aircraft

electricity production, lowering the demand for energy supply and making the energy

system much lighter.

For mission profiles with short discharge duration or low power requirement,

secondary batteries still work better than URFCs so far, due to their higher efficiency and

considering the disproportional mass of ancillary equipment in UR-PEMFCs to the mass

of the spacecraft [93].

3.2 Renewable energy

In 2003, a collaborative Canadian project has developed a stationary 2kWe RFC

system integrated with renewable energy such as photovoltaic (PV) or wind energy to

generate sustainable hydrogen energy [94]. Maclay et al. [95, 96] developed a model

which integrated RFC with a PV and battery system for solar-hydrogen powered

residence application. It was found that by combining RFC with battery in the hybrid

energy system, PV utilization, battery power density and efficiency, and the energy

storage capacity were all increased. However, since RFCs generally operate at lower

efficiencies than batteries, the overall system efficiency was decreased due to this

combination. In the University of Zacatecas, Mexico, a 2.5kW hybrid energy system was

developed for standalone application by Chavez-Ramırez et al. [97]. This hybrid system

included a PV panel, a micro wind turbine, a UR-PEMFC and batteries. Control strategy

for this system was also developed to achieve the highest performance and efficiency, by

selecting which renewable source to feed the battery and which to meet the load demand

autonomously. Raj et al. [98] compared the Standalone PV-diesel system with PV-H2

system from economic point of view and found that a system with UR-PEMFC and metal

hydride storage offered the most cost-effective solution.

3.3 Power supply

For power supply purpose, UR-PEMFCs can either be connected to the grid as peak

shaver, or work independently as backup power/UPS. When connected to the main grid,

UR-PEMFCs can utilize the redundant electricity from the grid during night time for

hydrogen production and storage, while during the day time they can generate electricity

back to the grid to deal with possible power shortage. Benefited from their decoupled

power & capacity and long-term energy storage ability, UR-PEMFCs can effectively

coordinate with local grid. For instance, Proton Energy Systems Inc. has built a 5kWe

UR-PEMFC system for a telecoms site based on their product Unigen® [90]. During the

on-peak period, this system can work as a peak shaver for the local grid and utilize the

extra electric power to produce hydrogen.

As for off-grid power supplier, UR-PEMFCs are also very promising as backup

powers or UPS for various applications, such as computers, data centers,

telecommunication equipment [90], etc. The produced hydrogen can be stored for a long

time without self-discharge, and the electricity supply capacity can be easily raised by

increasing the hydrogen tank volume, resulting in a flexible backup power device for

different missions. Proton Energy Systems Inc. has provided a 15kWe Unigen® UR-

PEMFC system as backup power unit for the electric division of the Town of Wallingford

in its control room battery system [99]. In the case of power failure, this system can use

the high-pressure hydrogen to provide at least 8hrs of backup power at the rated load.

Moreover, a 4kWe Unigen® URFC was also built as a UPS at Mohegan Sun Casino’s

Energy, Environment, Economics & Education Center in 2005. [99].

3.4 Transportation

UR-PEMFCs have also been proposed for transportation applications such as the zero-

emission vehicles (ZEVs) [11]. During the off-peak period, hydrogen fuel can be

produced by connecting to the grid, while electricity can be provided to drive the electric

motor during vehicle operation. Such a system eliminates the pollutions related to

internal combustion engine-based vehicles, while retaining the quick refueling ability

compared with battery-based vehicles, providing that hydrogen refueling infrastructure is

mature. Unfortunately, the high price of present UR-PEMFCs has hindered their

application in ZEVs. Instead, the URFC-battery-engine hybrid vehicles are proposed

considering both the cost-efficiency and the possible reduction of gasoline consumption

[100, 101]. Under this scheme, the battery pack is charged and hydrogen fuel is produced

by a UR-PEMFC during the nighttime. When the vehicle works in the daytime, battery

electricity is used first, then it is recharged by the UR-PEMFC with the stored hydrogen,

and finally gasoline is consumed by the combustion engine. With this hybrid car, both the

battery pack size and the power demand of the UR-PEMFC can be decreased, leading to

a lower capital cost.

In addition to the ZEVs, UR-PEMFCs are also potentially applicable to other

transportation applications such as buses, ships, airplanes, etc. as long as sufficient power

is provided. However, this would be less cost-effective considering the high cost of UR-

PEMFCs at present. Therefore, practical applications of UR-PEMFCs in these fields can

hardly be found in literature.

4. Conclusion

This part of review has introduced the research progress on UR-PEMFC technology in

detail. Great efforts on the development of different cell components are described singly,

from the inner bifunctional catalysts, to the outmost BPPs. In addition, developments on

several systematic issues such as electrode configuration, single cell stacking, and heat

and water management are also presented.

UR-PEMFCs have shown their tremendous application prospect in areas of aerospace

and aviation, renewable energy, power supply and transportation. Benefited from their

high specific energy, deep cycling ability, and decoupled energy storage capacity with

rated power, UR-PEMFCs have exhibited superior advantages for long-term energy

storage and power output purposes. However, till now their application is still limited to

aerospace and military fields due to their high-cost components such as noble metal

catalysts, PEM, BPPs, and the cost from BOPs. Therefore, continuous research efforts are

still needed to find cost-efficient substitutes, in order to realize their large-scale civil

applications. Moreover, the round-trip efficiency and cycle stability of the present UR-

PEMFCs also require further improvement to achieve higher competitiveness against

other energy storage technologies.

Acknowledgements

This project is supported by the Hong Kong Research Grant Council CERG #714313.

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Figure captions:

Fig.1 Comparison between RFC and URFC: (a) RFC; (b) URFC.

Fig.2 Schematic diagram of various URFC technologies operated in FC and EC modes:

(a) UR-PEMFC; (b) UR-AFC; (c) UR-SOFC with oxygen ion conducting electrolyte; (d)

UR-SOFC with proton conducting electrolyte.

Fig.3 Composition of a typical UR-FEMFC single cell: (a) main cell components; (b)

TEM micrograph of Pt supported on carbon [77]; (c) catalyst layer deposited on PEM

[47]; (d) Carbon paper GDL [58]; (e) Commercial BPPs (from http://www.mgm-

carbon.com/products/carbongraphitefelt/9.html)

Fig.4 Various structures of the deposited Pt-IrOx composite BOCs: (a) IrO2 deposited on

Pt [31]; (b) Pt deposited on IrO2 first and then mixed with Pt [19]; (c) Pt supported on Ir-

deposited IrO2 [33]; (d) Pt supported on IrO2 [30]; (e) RuO2 and IrO2 supported on Pt

[15]; (f) Ir@Pt nanodendrites [32]

Fig.5 Various catalyst layer structures on the oxygen side [55]: (a) one-layer composite

electrode; (b) multilayered electrode; (c) segmented electrode. (FC stands for fuel cell

catalyst, i.e. Pt; EC stands for electrolysis cell catalyst, i.e. IrO2)

Fig.6 GDL in UR-PEMFCs: (a) schematic diagram of the GDL structure; (b) Ti-felt GDB

[61]; (c) Ti-felt GDB coated with Ti-powder MPL [61]

Fig.7 Stability of UR-PEMFC performance after running cell at 2.0V for 1 h every day

[68], with: (a) carbon-based BPPs; (b) Pt-deposited Ti-BPPs.

Fig.8 Different flow channels of the BPPs on the oxygen side [70]: (a) serpentine-single

channel; (b) serpentine-double channel; (c) parallel channel.

Fig.9 Two different electrode configurations for UR-PEMFCs: (a) hydrogen and oxygen

electrode configuration; (b) reduction and oxidation electrode configuration.

Fig.10 UR-PEMFC stacks reported in the literature: (a) in-lab stacks by Sone [79],

Grigoriev et al. [80], and Millet et al. [81] (From left to right); (b) Commercial stacks by

Proton Inc. [11], LLNL [11], and Lynntech, Inc. [78] (From left to right).

Fig.11 UR-PEMFC systems: (a) a schematic model [84]; (b) a practical model [79]

Fig.12 Schematic representation of UR-PMFC applications in various fields (Figures of

renewable energy, electric grid, laptop and cellphone from: http://all-free-download.com;

Figures of space shutter, satellite, airplane, cars, and UPS from http://openclipart.org)

Tables:

Table-1 Comparison between various types of URFC technology

Types of

URFC

Electrolyte Main feature Working temp.

(℃)

Round-trip

efficiency

Technical

Status

UR-PEMFC PEM Noble-metal catalyst 20-100 40-50% Mature

UR-AFC Alkaline solution;

AEM

Non-noble metal catalyst 20-120 30-40% Developing

UR-SOFC Proton-conducting ceramics;

Oxygen ion-conducting ceramics

High-temperature operation

High energy efficiency

Non-noble metal catalyst

500-1000 60-80% Developing

UR-MFC Acid/Alkaline solution Membraneless structure

Low capital cost

20-80 60% for

vanadium-species

Early stage

Table-2 Overview on various UR-PEMFC prototypes and their performances demonstrated to date

References BHC Electrolyte BOC Temp.

(℃)

PPD Round-trip

efficiency

Stability

Shao et al.

[14]

Pt

0.4mgcm-2

Nafion 115 Pt-IrO2 (1:1 wt. ratio)

0.4mgcm-2

80 40.9% at 400mAcm-2 Stable for 4 cycles

Ioroi et al.

[46]

Pt

8-10mgcm-2

Nafion 115 Pt-IrO2 (10-30mol% IrO2)

8-10mgcm-2

80 49% at 300mAcm-2

42% at 500mAcm-2

Ioroi et al.

[31]

Pt Nafion 115 IrO2/Pt (20at.% IrO2) 80 51% at 300mAcm-2

44% at 500mAcm-2

Ioroi et al.

[47]

Pt

2-4mgcm-2

Nafion 115 Pt-Ir (10at.% Ir)

2-4mgcm-2

80 ~50% a at 200mAcm-2

Yim et al. [26]

Pt 4mgcm-2

Nafion 1135 Pt-Ir (1:1 wt. ratio) 4mgcm-2

60 53% at 200mAcm-2 46% at 500mAcm-2

Stable for 3 cycles

Yim et al.

[27]

Pt

4mgcm-2

Nafion 1135 Pt-Ir (99:1 wt. ratio)

4mgcm-2

60 53% at 200mAcm-2

47% at 500mAcm-2

Zhang et al. [15]

Pt/C 0.4mgPtcm−2

Nafion 115 (RuO2–IrO2)/Pt (25:25:50, wt. ratio) 2mgcm-2

80 50% at 400mAcm-2 43% at 500mAcm-2

Stable for 10 cycles at 0.4 or 0.5Acm-2

Jung et al.

[16, 17]

Pt

0.5mgcm-2

Nafion 112 Pt-Ir (85wt.% Pt)

4mgcm-2

70-75 49% at 500mAcm-2

41% at 1Acm-2

Stable for 4 cycles at 500mAcm-2 (120hrs)

Zhang et al. [19]

Pt/C 0.2mgPtcm-2

Nafion 212 (Pt/IrO2)-Pt (5:95:100, wt. ratio) 1mgcm-2

80 1.16Wcm-2

Huang et al.

[18]

Pt

0.5mgcm-2

Nafion 112 (Pt/TiO2)-(Ir/TiO2) (Pt/Ir=85/15 wt.

ratio)

1mgPtIrcm-2

75 0.93Wcm-2 50.3% at 500mAcm-2

42.2% at 1Acm-2

Rivas et al.

[25]

Pt/C

0.5-1mgPtcm-2

Nafion 115 Pt4.5Ru4Ir0.5

3-5mgcm-2

80 FC performance decrease after WE operation

Cruz et al.

[20]

Pt/C

0.3mgPtcm-2

Nafion 115 Pt/IrO2 (1:1 wt. ratio)

0.5mgcm-2

Room

temp.

0.1Wcm-2 47% at 50mAcm-2

37% at 100mAcm-2 30% at 200mAcm-2

Stable for 100hrs at 1.8V, with FC

performance unchanged

Huang et al.

[36]

Pt

0.5mgcm-2

Nafion 112 Pt/TiO2 75 0.94Wcm-2 Very stable for 200hrs at 1.2V

Pai et al. [44]

Pt 0.25mgcm-2

Nafion 212 Pt/Graphitized carbon 0.25mgPtcm-2

190mWcm-2 37.5% at 100mAcm-2 Stable for 4 cycles at 100mAcm-2 (56hrs)

Cruz et al.

[45]

Pt/C

0.4mgPtcm-2

Nafion 115 Pt-IrO2 (1:1 weight ratio)/ATO

0.5mgcm-2

80 48% at 50Ag-1

43% at 100Ag-1 31% at 200Ag-1

Acceptable stability for 400min at 0.6V in

FC mode and 1.55V in EC mode, respectively.

Chen et al.

[51]

Pt/C Nafion 115 Pt-IrO2

2.1mgPtcm-2 0.9mgIrO2cm-2

80 474mWcm-2 60.3% at 100mAcm-2

52.1% at 300mAcm-2 44.2% at 500mAcm-2

37.8% at 700mAcm-2

Jung et al.

[50]

Pt

4mgcm-2

Nafion 112 Pt-Ir

4mgcm-2

70, 90 Degradation rate of 0.6mVh-1 at 2Acm-2 in

FC mode Degradation rate of 0.13mVh-1 at 0.3Acm-2 in

EC mode

Liu et al. [52]

Pt 0.8mgcm-2

Nafion 115 Pt-IrO2 2.2mgPtcm-2

1.7mgIrO2cm-2

20,40, 60,80

Stable performance of 25 cycles (75hrs)

Wittstadt et

al. [53]

Pt

2mgcm-2

Nafion 117 Pt-Ir (1:1)

2mgcm-2

5, 25,

50, 80

Better stability achieved with a PTFE-treated

Ti GDL

Chen et al.

[54]

Pt/C

0.5mgcm-2

Nafion 1035 Pt-IrO2

0.5mgPtcm-2

0.5mgIrO2cm-2

80 ~60% a at 100mAcm-2

42.1% at 800mAcm-2

Altmann et al. [55]

Pt 0.7mgcm-2

Nafion 1135 Pt-IrO2 0.73mgPtcm-2

0.73mgIrO2cm-2

80-85 95

Only short-term stability was achieved

Lee et al. [49]

Pt 0.38mgcm-2

Polypyrrole/Nafion composite membrane

Pt 0.38mgcm-2

80

Pettersson et

al. [48]

Pt

3-4mgcm-2

Nafion 115 Pt-Ir

3-4mgcm-2

Room

temp.

44% at 50mAcm-2

35% at 100mAcm-2

Stable for 3 cycles

Ioroi et al. [56]

Pt 3-4mgcm-2

Nafion 115 Pt-Ir (85:15 atomic ratio) 3-4mgcm-2

80 ~46.5% a at 0.5Acm-2 GDB with 16mgcm-3 of PTFE coating achieved good stability for 4 cycles

Song et al.

[65]

Pt/C

0.5mgcm-2

Nafion 112 Pt-IrO2 (70wt.% Pt)

3mgcm-2

80 Stable for 20 cycles at 300mAcm-2 (40hrs)

Chen et al. [62]

Pt/C 0.5mgcm-2

Nafion 112 Pt-IrO2 (70wt.% Pt) 3mgcm-2

80 Stable for 15 cycles at 200mAcm-2 (60hrs)

Ito et al. [57,

60]

Pt Nafion 115 Pt-IrO2 80

Hwang et al. [58, 61]

Pt Nafion 115 Pt-IrO2 80

Huang et al.

[66]

Pt

0.5mgcm-2

Nafion 212 (Pt/TiO2)-(Ir/TiO2)

1mgPt-Ircm-2

80 0.93Wcm-2 50.3% at 0.5Acm-2

42.2% at 1Acm-2

Stable for 9 cycles at 0.7V in FC mode and

1.5V in EC mode (36hrs)

Jung et al. [67]

Pt/C 0.5mgPtcm-2

Nafion 112 Pt-Ir (85wt.% Pt) 4mgcm-2

75 5 times lower degradation rate than URFC with carbon-based BPPs

Jung et al.

[68]

Pt

0.5mgcm-2

Nafion 112 Pt-Ir (85wt.% Pt)

4mgcm-2

75 Stable for 4 cycles at 300mAcm-2 (100hrs)

a: Estimated value from the figure in the literature; For the composite BOCs, “A-B” means mixed A and B, while “A/B” means A supported on B

Table-3 Suitable catalyst supports for BOCs reported in the literature

References Catalyst support Performance

Chen et al. [37, 38] Ti0.9Nb0.1O2 Quite good stability against electrochemical oxidation due to

its non-defective oxygen lattice

Ganesan et al. [39] Nb-doped TiO2 Better stability than carbon support at more anodic potentials

Huang et al. [18,

36] TiO2 High fuel cell performance and ultrahigh stability at high

potentials; Round-trip efficiency was improved by the catalyst

support.

Sui et al. [40] TiC Pt-Ir/TiC catalyst prepared by the plasma process was more active than that by the chemical process.

García et al. [42] TiC and TiCN TiCN support showed better corrosion resistance than TiC

support at anodic potentials. Roca-Ayats et al.

[43] TiC, TiCN, and

TiN TiCN supported catalyst yielded the highest activity towards

ORR & OER in addition to the best compromise between

catalytic activity and stability. Fuentes et al. [41] TiC No short-term degradation; Round-trip efficiency as high as

65% was achieved at the lower kinetic current density region. Pai et al. [44] Graphitized carbon Higher round-trip efficiency and better stability than carbon

black catalyst support Cruz et al. [45] Sb-doped SnO2

(ATO)

Stability was quite good for 400min FC operation at 0.6V and

400min EC operation at 1.55V.

Table-4 Suitable GDLs for UR-PEMFCs reported in the literature

Reference GDL Performance

GDB MPL

Ioroi et al. [56] Ti fiber A stable operation can be achieved with the appropriate PTFE

loading on the Ti GDB.

Wittstadt et al. [53] Ti foam PTFE coating on the Ti foam led to better stability than the

untreated Ti foam.

Ito et al. [57] Ti felt Larger fiber diameter of Ti felt harms the EC performance but

improves the FC performance.

Ito et al. [60] Ti felt Ti powder MPL has little effect on the EC performance, but can significantly

improve the FC performance under fully humidification

condition.

Hwang et al. [58] Ti felt The highest round-trip efficiency was achieved by the Ti felt

GDB with large fiber diameter (80μm) under a temperature of

80℃. Hwang et al. [61] Ti felt Ti powder MPL has little effect on the EC operation, but can significantly

improve FC performance under fully humidified condition.

Song et al. [65] Carbon paper IrO2/Ti FC performance was as good as the conventional carbon-based

GDL, but EC performance was much higher; The cycle stability

was improved for over 20 cycles.

Chen et al. [62] Carbon paper IrTiOx-doped

TiC

Similar initial performance with the conventional GDL with

carbon black MPL, but achieved much better stability during the

cycle test for over 60 hrs.

Huang et al. [66] Carbon paper Ir-TiN

composite

Much better stability in the cycle test for 36hrs.

Jung et al. [16] Carbon paper Graphitized

carbon

Similar FC and EC performance with conventional GDL, but with

much better stability.

Table-5 Suitable BPPs for UR-PEMFCs reported in the literature

References BPPs Performance

Jung et al. [67] Au-coated Ti BPPs Five times lower voltage degradation rate than the

carbon-based BPPs.

Jung et al. [68] Pt-coated Ti BPPs Improved FC and EC performance compared with

bare Ti-BPPs; Cycling stability was also very good

for more than 100 hrs.

Zhang et al. [69] Ti-Ag film coated on Ti

BPPs

Interfacial contact resistance was much smaller than

the bared Ti BPPs; Anti-corrosion ability was also

improved; Better water management was achieved

due to the low surface energy.

Zhang et al. [71] Honeycomb-like

nanocomposite Ti-Ag-N

film coated on Ti BPPs

Superior conductivity attributed to the evenly

embedded Ag nanoparticles in Ti2N matrix; Great

corrosion-resistance was also achieved.

Lin et al. [72] (Ti,Zr)N-coated Ti BPPs Corrosion resistance was improved by 200 times.

(Ti,Zr)N-coated stainless

steel BPPs

Corrosion resistance was improved by 215 times.

Table-6 UR-PEMFC stack development reported in the literature

References Stack features Single cell features Temp. OCV Performance Stability

BHC Electrolyte BOC Area

Su et al.

[83]

6 cells in series;

Shared membrane;

Hydrogen storage alloy

Pt/C (40 wt.%)

0.4mgPtcm-2

Nafion 112 Pt-IrO2 (60:40 wt.

ratio)

0.7mgPtcm-2

1cm2 25℃ 4.9V 74.8mWcm-2 at 2.2V. Good stability for 10

cycles.

Sone et al.

[79]

17 cells in series;

Closed system

Pt-IrOx 28.5cm2 Above

25℃

~18V 15A at 12V in FC mode;

Less than 30A at 28V in EC

mode.

Grigoriev

et al. [80]

7 cells in series;

Reduction and oxidation

electrodes;

Pt/C

(HER&ORR)

0.8mgPtcm-2

Nafion

1135

Pt-Ir (1:1 wt. ratio,

HOR&OER)

1mgPtcm-2

256cm2 80℃ ~6.5V Round-trip efficiency of

30% at 0.5Acm-2.

Millet et al.

[81]

2 cells;

Reduction and oxidation

electrodes;

Pt/C

(HER&ORR)

0.8mgPtcm-2

Pt-Ir (1:1 wt. ratio,

HOR&OER)

1mgPtcm-2

250cm2 80℃ ~1V In EC mode, 80%

efficiency at 0.5Acm-2.

Degradations are observed

after several hundred

cycles of operation.

Fig.1 Comparison between RFC and URFC: (a) RFC; (b) URFC.

Fig.2 Schematic diagram of various URFC technologies operated in FC and EC

modes: (a) UR-PEMFC; (b) UR-AFC; (c) UR-SOFC with oxygen ion conducting

electrolyte; (d) UR-SOFC with proton conducting electrolyte.

Fig.3 Composition of a typical UR-FEMFC single cell: (a) main cell components; (b)

TEM micrograph of Pt supported on carbon [77]; (c) catalyst layer deposited on PEM

[47]; (d) Carbon paper GDL [58]; (e) Commercial BPPs (from http://www.mgm-

carbon.com/products/carbongraphitefelt/9.html)

Fig.4 Various structures of the deposited Pt-IrOx composite BOCs: (a) IrO2 deposited

on Pt [31]; (b) Pt deposited on IrO2 first and then mixed with Pt [19]; (c) Pt supported

on Ir-deposited IrO2 [33]; (d) Pt supported on IrO2 [30]; (e) RuO2 and IrO2 supported

on Pt [15]; (f) Ir@Pt nanodendrites [32]

Fig.5 Various catalyst layer structures on the oxygen side [55]: (a) one-layer

composite electrode; (b) multilayered electrode; (c) segmented electrode. (“FC”

stands for fuel cell catalyst, i.e. Pt; “EC” stands for electrolysis cell catalyst, i.e. IrO2)

Fig.6 GDL in UR-PEMFCs: (a) schematic diagram of the GDL structure; (b) Ti-felt

GDB [61]; (c) Ti-felt GDB coated with Ti-powder MPL [61].

Fig.7 Stability of UR-PEMFC performance after running cell at 2.0V for 1 h every

day [68], with: (a) carbon-based BPPs; (b) Pt-deposited Ti-BPPs.

Fig.8 Different flow channels of the BPPs on the oxygen side [70]: (a) serpentine-

single channel; (b) serpentine-double channel; (c) parallel channel.

Fig.9 Two different electrode configurations for UR-PEMFCs: (a) hydrogen and

oxygen electrode configuration; (b) reduction and oxidation electrode configuration.

Fig.10 UR-PEMFC stacks reported in the literature: (a) in-lab stacks by Sone [79],

Grigoriev et al. [80], and Millet et al. [81] (From left to right); (b) Commercial stacks

by Proton Inc. [11], LLNL [11], and Lynntech, Inc. [78] (From left to right).

Fig.11 UR-PEMFC systems: (a) a schematic model [84]; (b) a practical model [79]

Fig.12 Schematic representation of UR-PMFC applications in various fields (Figures

of renewable energy, electric grid, laptop and cellphone from: http://all-free-

download.com; Figures of space shutter, satellite, airplane, cars, and UPS from

http://openclipart.org)