Design and characterization of bi-functional electrocatalytic layers for application in PEM unitized regenerative fuel cells

international journal of hydrogen energy 35 (2010) 5070–5076

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Design and characterization of bi-functional electrocatalytic layers for application in PEM unitized regenerative fuel cells S.A. Grigoriev a,c,*, P. Millet b, K.A. Dzhus c, H. Middleton d, T.O. Saetre d, V.N. Fateev a a

Hydrogen Energy and Plasma Technology Institute, Russian Research Center ‘‘Kurchatov Institute’’, Kurchatov sq. 1, 123182 Moscow, Russia b Institut de Chimie Mole´culaire et des Mate´riaux, UMR CNRS n 8182, Universite´ Paris Sud 11, Baˆtiment 410, 91405 Orsay Cedex, France c Moscow Power Engineering Institute (Technical University), Krasnokazarmennaya 14, 111250 Moscow, Russia d University of Agder, Faculty of Engineering and Science, Grooseveien 36, NO-4876 Grimstad, Norway

article info

abstract

Article history:

Results concerning the development and characterization of bi-functional electrocatalytic

Received 13 July 2009

layers for application in unitized regenerative fuel cells (URFCs) based on proton

Received in revised form

exchange membrane (PEM) technology are reported. Carbon-supported hydrophobic

20 August 2009

(10 wt.% of PTFE) Pt catalysts (40 wt.% of Pt), and Pt and Ir black powders of large specific

Accepted 30 August 2009

areas have been synthesized. Their structure, morphology and electrochemical properties

Available online 15 October 2009

have been investigated using SEM, TEM, XRD analysis, and by measurements of polarization curves and cyclic voltammograms. Current–voltage curves have been recorded

Keywords:

during water electrolysis and H2/O2 fuel cell experiments to evaluate their performances

Unitized regenerative fuel cells

at the lab-scale (7 cm2 active area). Results show that, among various Pt–Ir compositions,

Proton exchange membrane

the best URFC current–voltage performances are obtained with anodic electrocatalytic

Water electrolysis

structures made of two adjacent porous layers, a first one made of Ir (50 wt.%) in direct

H2/O2 fuel cells

contact with the polymer membrane and a second one made of Pt (50 wt.%) coated over the first one. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Unitized regenerative fuel cells (URFCs) can potentially be used in different energy management systems for transport, domestic and space applications [1–10]. The concept of reversible PEM cell was put forward for the first time in the early sixties of the 20th century [11]. In 1972, PEM-URFC technology was successfully tested by the US firm General Electric Co., leading to the development of a URFC prototype for application as power management unit in a space satellite [10,12]. In the nineties, R&D on reversible cells was pursued at the Lawrence Livermore National Laboratory (LLNL) and at

AeroVironment of Monrovia [5,10,13]. A reversible system having a specific power density of ca. 450 Wh kg1 has been successfully developed for application in pilot-less highaltitude airships [5,10]. Then, LLNL and Hamilton Standard (a branch of United Technologies Co.) carried out research on URFC-based cars [5]. Commercially available Unigen reversible modules [13] and related systems have been developed by Proton Energy Systems (ex Distributed Energy Systems) [14] since 1998 [15]. Unigen modules consuming 15 kW in electrolysis mode and producing up to 5 kW of electric power in fuel cell mode have been used for demonstration in decentralised power supply units [16], in high-altitude (20–30 km)

* Corresponding author at: Hydrogen Energy and Plasma Technology Institute of Russian Research Center ‘‘Kurchatov Institute’’, Kurchatov sq. 1, 123182 Moscow, Russia. Tel.: þ7 499 196 94 44; fax: þ7 499 196 62 78. E-mail address: [email protected] (S.A. Grigoriev). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.081

international journal of hydrogen energy 35 (2010) 5070–5076

planes [15], and also for specific needs in naval–air base in California [17]. R&D on URFC was also performed by the Canadian company Green Volt Power Corp [18,19], by the American companies Lynntech Inc., Glenn Research Center [20], Giner Inc. and some others. Experimental developments on URFC have also been reported in France at the Toulouse University (LAPLACE Laboratory) [21]. As in conventional H2/ O2 fuel cells and water electrolysers, the structure and composition of electrocatalytic layers in URFCs are of particular importance and strongly impact current–voltage performances and cell efficiencies. The structure of electrocatalytic layers used in URFC is even more complicated due to the specific requirement of bi-functionality. Potentially, there are two different ways to operate a URFC cell (Fig. 1). The first concept (A in Fig. 1) is based on truly reversible oxygen and hydrogen electrodes (i.e. the same electrode deals with the same gas in both water electrolysis and fuel cell modes). This is the approach followed by the majority of research groups [22–30]. The advantage of this concept is a simpler management of gases. A significant drawback however comes from the oxygen electrode. Carbon-based material commonly used as gas diffusion electrode and catalysts carrier in PEM fuel cell technology are corroded under oxygen evolution during operation in electrolysis mode [31,32]. Hence, they must be replaced by metal black and hydrophilic porous titanium current collectors. As a consequence, electrochemical performances of the URFC during operation in fuel cell mode are lower than conventional fuel cells and unstable, due to the higher sensitivity of the oxygen electrode to flooding and to the use of less-efficient catalyst. The second concept (B in Fig. 1) is based on electrodes which do not change their redox function when the operation mode of the cell is switched [33– 35]. The advantage of this approach is that conventional fuel cell gas diffusion and electrocatalytic materials can be used at the cathode [33]. The main drawback is that it is necessary to purge URFC cell compartments before switching from one mode of operation to the other. This paper concerns the

A

B WATER ELECTROLYSIS MODE

2H++2e→H2

H2O→1/2O2+2e+2H+

2H++2e→H2

H2O→1/2O2+2e+2H+

cathode

anode

cathode

anode

Pt

Pt+Ir

anode H2→2H++2e

cathode

Pt

cathode

1/2O2+2e+2H+→H2O 1/2O2+2e+2H+→H2O

Pt+Ir

anode H2→2H++2e

FUEL CELL MODE

hydrogen electrode

oxygen electrode

reduction electrode

oxidation electrode

Fig. 1 – Schematic representation of the two possible types of operation of a reversible PEM cell. (A) – Chemically reversible electrodes (so-called oxygen and hydrogen electrodes); (B) – electrodes which do not change their redox function when operation mode of the cell is switched (oxidation and reduction electrodes).

5071

optimization of bi-functional electrocatalytic layers for application in URFCs with electrodes of the second kind.

2.

Experimental section

2.1.

Catalysts synthesis

An impregnation/reduction method described in detail in Ref. [36] has been used to prepare VulcanXC-72 (Cabot Co.)supported platinum electrocatalysts for the cathode. A hydrophobic material (10 wt.% of PTFE) has been added to promote the removal of liquid water during operation in fuel cell mode. Pt and Ir black powders used at the anode have been synthesized by chemical reduction of H2PtCl6$6H2O (99.8%, Aldrich) and H2IrCl6$6H2O (99.8%, Aldrich) using NaBH4 (98%, Merck Chemicals) as chemical reducer. Briefly, Irpowder was prepared by adding H2IrCl6 to de-ionized water followed by dropwise addition of 0.5 M KOH solution in order to bring the pH of the solution in the 13–13.5 range. This mixture was then stirred at room temperature and a solution of NaBH4 in 1 M NaOH was added. The solution was constantly stirred until the end of gas evolution. Then the remaining deposits (Ir black powder) were thoroughly washed off (several times) using bi-distilled water in order to bring the pH of the downtake solution within the 6–6.5 range. Finally, catalysts were dried at 60–70  C.

2.2.

Characterization tools and techniques

Structure and morphology of catalyst particles were investigated using electron microscopy and X-ray analysis. Crystal structure and crystallite size of the catalysts were determined by XRD analysis, using a Siemens D5005 powder X-ray diffractometer (Cu Ka, l ¼ 0.15406 nm). Measurements were carried out in the range 30 < 2q < 90 degrees. The equipment was calibrated using a LaB6 standard. The diffraction patterns were fitted to a pseudo-Voigt profile. Crystal size distributions were calculated using a Warren–Averbach model (Crysize software). TEM analysis was performed using a JEOL 2010F electron microscope equipped with a field emission gun. The sample was prepared by dispersing electrocatalyst particles in ethanol using an ultrasonic bath. Then, the suspension was deposited onto a holey carbon copper grid. SEM-micrographs were obtained using a Zeiss SUPRA 55 VP microscope. Catalysts have been characterized by recording cyclic voltammograms (CVAs) in de-aerated electrolyte using a Gamry potentiostat/ galvanostat (Gamry Instruments, Inc) in the potential domain ranging from 0.2 to þ1.1 V vs. SCE, at a scan rate of 20 mV/s. Experiments have been made using a conventional threeelectrode cell in 1 M H2SO4. A KCl saturated Hg/Hg2Cl2 electrode (SCE) and a platinum wire were used as reference and counter electrodes respectively. Catalytic slurries were prepared by mixing the catalyst particles (80 mg) in ethanol with 5 wt.% Nafion alcoholic solution (Aldrich Co.), using an ultrasonic bath. Then, these well mixed slurries were sprayed over the surface (1 cm2) of a glassy carbon disk. Electrochemical activity of catalytic mixtures has also been determined by measuring polarization curves with regard to the oxygen evolution reaction (OER) and hydrogen oxidation reaction (HOR).

5072

international journal of hydrogen energy 35 (2010) 5070–5076

2.3. Preparation of reversible membrane-electrode assemblies Reversible membrane-electrode assemblies (MEAs) have been prepared using Nafion-115 (E.I. du Pont de Nemours Co.) membranes as solid polymer electrolyte (SPE). Membranes were thoroughly washed in 5 vol.% H2SO4 and de-ionized water, in order to remove various surface and bulk traces of impurities. Catalytic inks (a mixture of Pt40/VulcanXC-72 and 10 wt.% of PTFE for the cathodes and Pt–Ir black powders for the anodes) were ultrasonically mixed with 15 (Pt40/VulcanXC-72) or 5 (Pt–Ir blacks) wt.% of a solution of perfluorinated ionomer (wt.% values are expressed with respect to the weight of catalyst). These mixtures have then been used as ink and sprayed over the surfaces of the SPEs in order to prepare 7 cm2 area MEAs. MEAs with typical cathodic catalyst loadings of 0.35 mg Pt cm2, and typical anodic loadings of 1.5 mg cm2 of Ir and Pt blacks have been obtained. Finally, the MEAs have been clamped between two current collectors/gas diffusion layers and hot-pressed for 5 min at T ¼ 120  C and P ¼ 50 kg cm2. Porous titanium sheets have been used as anodic gas diffusion layer/current collector and carbon paper Sigracet 10bb with micro-porous sub-layers have been used as cathode gas diffusion electrode. Two reference MEAs having the same active area (7 cm2) have also been prepared, one to perform PEM water electrolysis measurement and one to perform H2/O2 fuel cell measurements. They have been used to produce reference polarization curves against which the performances of the reversible cells were evaluated. The reference MEA for water electrolysis contained metallic iridium at the anode (1.5 mg cm2), and Pt40/VulcanXC-72 at the cathode (0.35 mg Pt cm2). Porous titanium disks have been used as gas diffusion layer/current collector on each side of the MEA. The MEA for the reference fuel cell has been prepared as described above, with the difference that Pt40/ VulcanXC-72 (0.35 mg Pt cm2) and Sigracet 10bb have been used both at the anode and the cathode. The morphology of the MEAs has been investigated by SEM analysis, to check the homogeneity of the deposits and to determine the thickness of the catalytic layers.

3.

Results and discussion

3.1.

Characterization of electrocatalysts and MEAs

A typical XRD spectrum measured on the VulcanXC-72 cathodic catalyst is shown in Fig. 2. Broad peaks are observed, outlining the amorphous structure of the powder. The position of diffraction peaks, characteristic of the face-centered cubic lattice of Pt, confirm the successful reduction of Ptprecursors to the metallic state. The ratio of peak intensity (111)/(200) in Fig. 2 is larger than the one measured on Pt JCPDS (bulk platinum). This is an indication that the (111) plan is prevailing. This is confirmed by cyclic voltammetry analysis as reported elsewhere [36]. (111) H adsorption peaks are clearly seen on cyclic voltammograms. The diffraction pattern of Fig. 2 has been modeled to estimate the mean crystallite size. Electrocatalyst particles were found to have a very narrow crystal size distribution centered

Fig. 2 – XRD-pattern measured on the Pt40/VulcanXC-72 catalyst.

around d ¼ 2.3 nm. A homogeneous distribution of Pt nanoparticles over the surface of the carbon-carrier is obtained (Fig. 3). An average diameter of 2.5–3.5 nm is measured, in accordance with data of Fig. 2. These results are slightly better than those obtained with commercially available Pt–carbon products, which have clusters in the 3.5–3.9 nm range for Pt contents of ca. 40 wt.% [22]. Therefore, samples prepared as detailed in this paper show higher specific surface area per gram of platinum, i.e. platinum is more efficiently used. A typical SEMmicrograph of the Ir catalyst is shown in Fig. 4. A highly porous structure is obtained. The powder is not only made of isolated metal particles. Porous particles are interconnected, offering a better overall electronic conductivity. With such structure, highly efficient electrochemical performances are expected regarding the OER (oxygen evolution reaction). The electrochemical active surface area (EAS) of the different catalysts has been determined from CVAs, using a technique described in Ref. [36]. For Pt40/VulcanXC-72, a large value close to 70 m2 g1 has been obtained, indicating that most platinum, if not all, is in the form of small

Fig. 3 – TEM micrograph of a particle of Pt40/VulcanXC-72 catalyst used at the cathode of the URFC.

international journal of hydrogen energy 35 (2010) 5070–5076

5073

Fig. 4 – SEM-micrograph of Ir-powder used at the anode of the URFC.

Fig. 6 – SEM picture showing the cross-section of a reversible MEA.

(nanometer thick) particles, available for the electrochemical reaction. CVAs obtained for different Pt- and Ir-based catalysts are plotted in Fig. 5. The shape of the curves is characteristic of these metals. The Pt–Ir mixtures present a large specific area. For example, the value for the iridium powder is ca. 55 m2 g1. This is an unexpectedly high value for an unsupported catalyst. These results were found reproducible from one batch to the other. A typical SEM-micrograph showing the cross-section of a reversible MEA is pictured in Fig. 6. Uniform and homogeneous catalysts deposits are obtained. A typical thickness of ca. 5 mm is obtained at the anode. This is sufficient for the hydrogen oxidation reaction (HOR) in fuel cell mode and for the OER during water electrolysis. At the cathode, where the oxygen reduction reaction (ORR) is rate-determining, a thicker catalytic layer (ca. 15 mm) is required to increase the area of the gas–electrolyte interface (3D electrode).

electrocatalyst at the cathode of the reversible cell to promote the HER during water electrolysis and the ORR during fuel cell operation. On the contrary, the structure and composition of the anode require optimization because two significantly different anodic reactions can take place: the OER during water electrolysis and the HOR during fuel cell operation. Polarization curves obtained for both the OER and the HOR, using electrodes containing various Pt–Ir compositions, are plotted in Figs. 7 and 8 respectively. Concerning the OER, best results are obtained with pure iridium. As can be seen from Fig. 7, the higher the platinum content, the lower the performances. This is largely consistent with all data published in the literature on the subject [37,38]. Since the OER is the slowest and the less-efficient of the two reactions, a significantly high iridium content is required at the anode to promote the OER. Concerning the HOR (Fig. 8), the difference between the different catalysts is less significant than for the OER, especially for low overvoltages (<100 mV) although it can

3.2. Electrochemical performances of the anode of the reversible cell As discussed above, in the B configuration (Fig. 1), carbonsupported nanoparticles of platinum can be used as

Fig. 5 – Cyclic voltammograms recorded for electrodes with various IrxPty catalytic compositions in 1 M H2SO4 at 20 mV/s and 20 8C.

Fig. 7 – Polarization curves of the OER measured for electrodes with various IrxPty catalytic compositions in 0.5 M H2SO4 at 1 mV/s and 80 8C.

5074

international journal of hydrogen energy 35 (2010) 5070–5076

2.2 2.1 2.0 1.9 1.8 1.7 1.6

ref. electrolyser

1.5

ref. fuel cell

1.4

Ir

1.3

Ir0.75Pt0.25 a

U, V

1.2

Ir0.5Pt0.5 a

1.1

Ir0.5Pt0.5 b

1.0

Fig. 8 – Polarization curves of the HOR measured for electrodes with various IrxPty catalytic compositions in 0.5 M H2SO4 at 1 mV/s and 80 8C.

Ir0.5Pt0.5 c

0.9

Ir0.25Pt0.75 a

0.8

Pt

0.7 0.6

be said that platinum provides the best results. At high overvoltages where transport limitations appear, significantly different catalytic efficiencies are observed as a function of the iridium content. From these results, it can be concluded that iridium is required for the OER and platinum should be preferred for the HOR. Therefore, the composition of the Pt–Ir mixture must be optimized. From our experiments, it turned out that the optimal composition depends mainly on BET surface areas of individual Pt and Ir-powders. Logically, a higher surface area can compensate a lower electrochemical activity. For example, for experiments made with Pt and Ir blacks having BET surface areas of respectively 30 and 32 m2/g, the optimum weight ratio is close to 0.5/0.5. However, when a Pt powder with a reduced surface area (15.7 m2/g) is used, Ir amounts larger than 50 wt.% are recommended. Also, if the surface area of the Ir black is large (about 55 m2 g1) and the Pt surface area is significantly lower, higher Pt/Ir ratios are required. Typical BET surfaces measured on different Pt–Ir mixtures using two different gases are compiled in Table 1. These Pt–Ir catalysts with different Pt–Ir compositions were obtained by mixing Pt powder (z20 m2.g1) and Ir-powder (z13 m2 g1). As a result, surface area of the mixtures is proportional to the composition.

Table 1 – Surface areas of various PtxIry compositions measured using BET method by H2 and CO adsorption. Catalyst, wt. ratio

Pt Pt0.9Ir0.1 Pt0.7Ir0.3 Pt0.5Ir0.5 Pt0.3Ir0.7 Pt0.2Ir0.8 Pt0.1Ir0.9 Ir

SBET, m2/g of the catalyst by H2

by CO

20.3 20.5 15.6 16.8 16.1 16.5 15.1 13.3

17.3 16.8 19.4 16.0 12.1 15.7 13.6 11.4

0.5 0.4 0.3 0.2 0.1 0 0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

i, A/cm2 Fig. 9 – Current–voltage relationships measured using reference PEM fuel cell, a reference electrolysis cell, and reversible cells with anodes made of various IrxPty catalytic compositions. S [ 7 cm2. Fuel cell mode: Tcell [ 80 8C, PH2 [ 2.8 bar and PO2 [ 3.0 bar; humidification temperature for H2 [ 85 8C; H2 and O2 flow rates 160 ml minL1; Water electrolysis mode: Tcell [ 90 8C; PH2 [ PO2 [ 1 bar.

3.3. Electrochemical performances measured on reversible cells Pt–Ir mixtures of Table 1 have been tested as bi-functional electrocatalysts at the anode of reversible cells: true Pt–Ir mixtures with different weight ratios and separate Pt and Ir layers. Polarization curves measured in both water electrolysis and fuel cell modes of operation with different catalysts are plotted in Fig. 9. In each case, Pt40/VulcanXC-72 has been used at the cathode for the HER and the ORR. In Fig. 9, typical polarization curves measured on a conventional PEM water electrolysis cell (B) and on a conventional H2/O2 PEM fuel cell (C) are also plotted. They are used as reference curves to evaluate the efficiency of the reversible cells. As a general rule, true Pt–Ir mixtures did not provide satisfactory results, electrochemical performances being largely inferior to those measured on references cells. Best performances were obtained using a first thin and porous layer of metallic iridium deposited directly against the SPE and a second layer of platinum placed on top. Using this two-layer

5075

international journal of hydrogen energy 35 (2010) 5070–5076

Table 2 – Voltages measured on the reversible cell (i) UWE in water electrolysis mode and (ii) UFC fuel cell mode at i ¼ 0.5 A/cm2 for various compositions of anodic electrocatalytic layer. Overall efficiency is given for each catalyst composition. Operating conditions in water electrolysis mode: T [ 90 8C, PO2 [ PH2 [ 1 bar; operating conditions in H2/O2 fuel cell mode: T [ 85 8C, PO2 [ 3 bar; PH2 [ 2 bar; total Pt and Ir loading at the anode: 1.5 mg/cm2, 5 wt.% of ion-exchange polymer; Pt loading at the cathode: 0.35 mg/cm2; 15 wt.% of ion-exchange polymer; Nafion-115 membrane. Composition of the catalytic layer, wt. ratio UWE, V UFC, V Overall efficiency, %

Pt

Ir0.25Pt0.75a

Pt0.5Ir0.5a

Pt0.5Ir0.5b

Ir0.5Pt0.5c

Ir0.75Pt0.25a

Ir

1.993 0.684 34.3

1.673 0.622 37.2

1.610 0.545 33.9

1.747 0.593 33.9

1.591 0.673 42.3

1.654 0.588 35.6

1.572 0.619 39.4

a Mixture of Pt and Ir. b One first layer of Pt contacting the SPE and a second layer of Ir on top. c One first layer of Ir contacting the SPE and a second layer of Pt on top.

structure, best current–voltage performances, both in water electrolysis and fuel cell modes, were obtained using 50 wt.% Pt and 50 wt.% Ir. Detailed results are compiled in Table 2. The benefit of this two-layer configuration can be analyzed as follows. The iridium layer placed against the membrane reduces the anodic overvoltage during the OER (electrolysis mode) as in conventional water electrolysis cells. Platinum particles located in the second layer are in direct contact with the gas phase at the backside of the electrode. They promote the oxidation of hydrogen with a reduced overvoltage during the HOR (fuel cell mode) possibly because of the larger surface mobility of hydrogen ad-atoms. Typical cell voltages of ca. 1.70 V (water electrolysis) and 0.55 V (fuel cell operation) have been obtained with the reversible cell at a current density of 1 A cm2 (Fig. 10). Such electrochemical performances are very close to those measured on conventional water electrolysis cells and H2/O2 fuel cells with the same noble metal loadings and similar operating conditions. For comparison, cell voltages of ca. 1.68 V and 1.70 V were obtained during water electrolysis at 1 A cm2 with the standard water electrolysis and reversible cells respectively. The voltage measured on the reversible cell during fuel cell operation is slightly lower (ca. 50 mV) than the voltage measured on the standard fuel cell at 1 A cm2 using O2 (and not air) and Nafion-115 as SPE. Finally, it is worth noting that, as can be seen from Fig. 9, the catalysts used in this work are sufficiently efficient to reach high current densities (up to 2 A cm2) in both water electrolysis and fuel cell modes of operation.

Stability tests performed at constant current density (1 A cm2) over more than 100 h of intermittent cell operation have been performed. Some data are reported in Fig. 10. During the first 70 h of operation, the reversible cell has been operated in electrolysis mode. The cell was switched off at night time. Voltage fluctuations are mostly due to temperature changes. After that, the reversible cell has been purged under a flow of nitrogen and then operated intermittently in H2/O2 fuel cell mode for 50 more hours. A few (3) cycles have been made, to check that stable performances can be obtained. Before switching from one mode to the other, care was taken to purge the cell with water-saturated nitrogen for several minutes. However, the effect of cycling on the long term is still under investigation and the ability of the cell to switch rapidly from one mode to the other has not been investigated.

4.

Conclusions

Bi-functional electrocatalysts have been synthesized and characterized for application in PEM-URFCs. It is shown that best current–voltage performances are obtained using Ir0.5Pt0.5 at the anode (for water oxidation in electrolysis mode and hydrogen oxidation in fuel cell mode) and Pt/VulcanXC-72 at the cathode (for hydrogen evolution in electrolysis mode and oxygen reduction in fuel cell mode). Electrochemical performances are close to those measured on conventional water electrolysis and H2/O2 fuel cells with the same noble metal loadings and similar operating conditions. Stable results were obtained over 100 h of continuous operation in both water electrolysis and fuel cell mode of operation. Investigation of long-term performances and detailed evaluation of degradation performances are still underway. These results open the way to the development of reversible cells of larger active area and URFC stacks of larger power capacity.

Acknowledgements

Fig. 10 – Results of stability tests of URFC in water electrolysis and fuel cell modes at a current density of 1 A/cm2 (operating conditions: similar to those of Fig. 9).

This work has been financially supported by the Federal Agency for Science and Innovations of the Russian Federation (Federal Principal Scientific-Technical Programme ‘‘Researches and development on priority directions in development of scientific technological complex of Russia for

5076

international journal of hydrogen energy 35 (2010) 5070–5076

2007–2012), and by the Global Energy International Prize NonProfit Foundation (Grant No. MG-2008/04/3). Support from the European Commission (GenHyPEM STREP program, FP6) is also acknowledged. Authors also wish to acknowledge Dr. Mikhail Tsypkin (NTNU, Norway) for his help in the characterization of the catalysts and for fruitful discussions, and Dr. Osina M.A. (MPEI, Russia) for her help with the electrochemical measurements.

references

[1] Mitlitsky F, Myers B, Weisberg AH, Molter TM, Smith WF. Reversible (unitised) PEM fuel cell devices. Fuel Cells Bull 1999;2:6–11. [2] Barbir F, Molter T, Dalton L. Efficiency and weight trade-off analysis of regenerative fuel cells as energy storage for aerospace applications. Int J Hydrogen Energy 2005;30:351–7. [3] Barbir F. PEM fuel cells: theory and practice. Birlington: Elsevier Academic Press; 2005. p. 382–91. [4] Maclay JD, Brouwer J, Samuelsen GS. Dynamic analyses of regenerative fuel cell power for potential use in renewable residential applications. Int J Hydrogen Energy 2006;31:994– 1009. [5] Hauff S, Bolwin K. System mass optimization of hydrogen/ oxygen based regenerative fuel cells for geosynchronous space missions. J Power Sources 1992;38:303–15. [6] Pat. EP 0755088. Regenerative power system. [7] Pat. Appl. 20050008904, USA, Regenerative fuel cell technology. [8] Suppes GJ. Plug-in hybrid with fuel cell battery charger. Int J Hydrogen Energy 2005;30:113–21. [9] Bents DJ, Scullin VJ, Chang BJ, Johnson DW, Garcia CP, Jakupca IJ. Hydrogen–oxygen PEM regenerative fuel cell development at Nasa Glenn Research Center. Fuel Cells Bulletin 2006;1:12–4. [10] Mitlitsky F, Myers B, Weisberg AH. Regenerative fuel cell systems. Energy Fuels 1998;12:56–71. [11] Bone JS, Gilman S, Niedrach LW, Read MD. Ion-exchange regenerative fuel cells. In: Proceedings of the 15th annual power source conference, Ft. Monmouth, N.J.; May 9–11, 1961. p. 47–9. [12] Chludzinski PJ, Danzig IF, Fickett AP, Craft DW. Regenerative fuel cell development for satellite secondary power. Technical Report AFAPL-TR-73–34. General Electric Company; June 1973. [13] Smith W. The role of fuel cells in energy storage. J Power Sources 2000;86:74–83. [14] F9 investments acquired proton energy systems, hydrogen technology innovator from distributed energy systems Corp., Proton Energy Systems Press Release. Available from: http:// www.protonenergy.com. [15] Funding, demo for regenerative fuel cell. Fuel Cells Bulletin 2004;11:7–8. [16] Contracts for proton regenerative fuel cells. Fuel Cells Bulletin 2004;12:10. [17] Proton energy ships system to China lake. Available from: http://www.fuelcells.org/archives/03/update05.html#. [18] GreenVolt Power Corp. http://www.greenvolt.com. [19] Farret FA, Simoes MG. Integration of alternative sources of energy. A John Wiley & Sons, Inc.; 2006. p. 167. [20] Burke KA. Unitized regenerative fuel cell system development. Glenn Technical Report NASA/TM-2003–212739, http://gltrs. grc.nasa.gov/reports/2003/TM-2003-212739.pdf; 2003.

[21] Rabih S, Rallieres O, Turpin C, Astier S. Experimental study of a PEM reversible fuel cell. In: Proceedings of the intern. conf. on renewable energy and power quality (ICREPQ’08), Valencia; March 11–13, 2008. p. 268. [22] Yim S-D, Lee W-Y, Yoon Y-G, Sohn Y-J, Park G-G, Yang T-H, et al. Optimization of bifunctional electrocatalyst for PEM unitized regenerative fuel cell. Electrochim Acta 2004;50: 713–8. [23] Yim S-D, Park G-G, Sohn Y-J, Lee W-Y, Yoon Y-G, Yang T-H, et al. Optimization of PtIr electrocatalyst for PEM URFC. Int J Hydrogen Energy 2005;30:1345–50. [24] Pettersson J, Ramsey B, Harrison D. A review of the latest developments in electrodes for unitised regenerative polymer electrolyte fuel cells. J Power Sources 2006;157: 28–34. [25] Ioroi T, Kitazawa N, Yasuda K, Yamamoto Y, Takenaka H. IrO2-deposited Pt electrocatalysts for unitized regenerative polymer electrolyte fuel cells. J Appl Electrochem 2001;31: 1179–83. [26] Ioroi T, Yasuda K, Siroma Z, Fujiwara N, Miyazaki Y. Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2002;12: 583–7. [27] Ioroi T, Oku T, Yasuda K, Kumagai N, Miyazaki Y. Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells. J Power Sources 2003;124:385–9. [28] Jorissen L. Bifunctional oxygen/air electrodes. J Power Sources 2006;155:23–32. [29] Chen G, Bare SR, Mallouka TE. Development of supported bifunctional electrocatalysts for unitized regenerative fuel cells. J Electrochem Soc 2002;149:A1092–9. [30] Chen G, Zhang H, Cheng J, Ma Y, Zhong H. A novel membrane electrode assembly for improving the efficiency of the unitized regenerative fuel cell. Electrochem Commun 2008;10:1373–6. [31] Song S, Zhang H, Ma X, Shao Z-G, Zhang Y, Yi B. Bifunctional oxygen electrode with corrosion-resistive gas diffusion layer for unitized regenerative fuel cell. Electrochem Commun 2006;8:399–405. [32] Dihrab S, Sopian K, Alghoul MA, Sulaiman MY. Review of the membrane and bipolar plates materials for conventional and unitized regenerative fuel cells. Renew Sustain Energy Rev 2009;13:1663–8. [33] Ahn J, Holse R. Bifunctional electrodes for an integrated water-electrolysis and hydrogen–oxygen fuel cell with a solid polymer electrolyte. J Appl Electrochem 1992;22: 1167–74. [34] Tsypkin MA, Lyutikova EK, Fateev VN, Rusanov VD. Catalytic layers in a reversible system comprising an electrolyzing cell and a fuel cell based on solid polymer electrolyte. Russ J Electrochem 2000;36:545–8. [35] Ledjeff K, Mahlendorf F, Peinecke V, Heinzel A. Development of electrode/membrane units for the reversible solid polymer fuel cell (RSPFC). Electrochim Acta 1995;40:315–9. [36] Grigoriev SA, Millet P, Fateev VN. Evaluation of carbonsupported Pt and Pd nanoparticles for the hydrogen evolution reaction in PEM water electrolysers. J Power Sources 2008;177:281–5. [37] Solid polymer electrolyte water electrolysis technology. DOE/ ET/26 202–1, Development for large-scale hydrogen production. US Department of Energy; 1981. [38] Millet P, Durand R, Pineri M. Preparation of new solid polymer electrolyte composites for water electrolysis. Int J Hydrogen Energy 1990;15(4):245–53.