- Email: [email protected]
Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 6 4 e4 1 6 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology S.A. Grigoriev a,*, P. Millet b, V.I. Porembsky a, 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 no 8182, Universite´ Paris Sud, baˆt 410, 91405 Orsay Cedex, France
article info
abstract
Article history:
Results related to the development and testing of a unitized regenerative fuel cell (URFC)
Received 14 April 2010
based on proton-exchange membrane (PEM) technology are reported. A URFC is an elec-
Received in revised form
trochemical device which can operate either as an electrolyser for the production of
1 July 2010
hydrogen and oxygen (water electrolysis mode) or as a H2/O2 fuel cell for the production of
Accepted 1 July 2010
electricity and heat (fuel cell mode). The URFC stack described in this paper is made
Available online 9 August 2010
of seven electrochemical cells (256 cn2 active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production
Keywords:
in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water
Unitized regenerative fuel cell
electrolysis at 0.5 A cm2 (85% efficiency based on the thermoneutral voltage of the water
Proton-exchange membrane
splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell
Electrochemical stack
operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A unitized regenerative fuel cell (URFC) is a reversible electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode WE) or as a H2/O2 fuel cell for the production of electricity and heat (fuel cell mode FC). URFCs are potentially interesting for a large number of energy management applications [1,2]. In particular, they could play a significant role for the promotion of renewable energy sources. First attempts to built URFCs using proton-exchange membrane (PEM) technology were made in the 1960s [3]. At that time, systems suffered from inacceptable low electrochemical performances. This was due to problems with membrane and electrocatalysts. More promising results have been obtained
in 1972 at General Electric Co. [4]. Later, in the 1990s, a prototype URFC stack having a specific power density of ca. 450 Wh kg1 has been developed and tested at the Lawrence Livermore National Laboratory (LLNL) by AeroVironment of Monrovia and Hamilton Standard [4]. In 1998 Proton Energy Systems has developed a commercial product (Unigen reversible module) consuming 15 kW in electrolysis mode (WE) and producing up to 5 kW of electric power in fuel cell (FC) mode [4]. R&D on URFC has also received attention from different US companies (Lynntech Inc., Glenn Research Center, Giner Inc.), from the Canadian company Green Volt Power Corp and some others [4e7]. Experimental optimization of membrane-electrode assemblies (MEAs) for application in URFCs is reported in Refs. [8,9]. More recently, results related to the modeling and characterization of URFS have been
* Corresponding author. Tel.: þ7 499 196 94 44; fax: þ7 499 196 62 78. E-mail address: [email protected] (S.A. Grigoriev). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.011
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 6 4 e4 1 6 8
published [10e12], showing that this technology is the subject of an increasing attention. It should be emphasized that the above mentioned technological developments are all using a similar concept of socalled ‘hydrogen and oxygen electrodes’, i.e. H2 is produced (WE mode) or consumed (FC mode) in the same cell compartment and O2 is produced (WE mode) or consumed (FC mode) in the other cell compartment. This requires the development of ‘reversible electrodes’, a non-trivial task, especially on the oxygen side since the H2O/O2 redox system is significantly irreversible in the temperature range (from 20 C to e100 C) of operation used in PEM technology. Despite some obvious advantages (in particular a simpler management of gases), this concept presents some additional drawbacks, mainly a problem of material stability, the corrosion of carbon-based materials (carbon was used as catalyst carrier, gas diffusion layer and bipolar plate [13]) at the anode during water electrolysis being responsible for rapid losses of performances. In this paper, an alternative design of URFC cells based on the so-called concept of ‘reduction and oxidation electrodes’ [7,14e16] has been used to develop a URFC stack. One cell compartment is designed for the promotion of oxidation processes (water oxidation in WE mode and H2 oxidation in FC mode), and the other is designed for the promotion of reduction processes (H2 evolution in WE mode and O2 reduction in FC mode). Despite the fact that this concept requires the purge of electrode chambers with an inert gas before switching from one mode of operation (WE) to the other (FC) and vice-versa, it greatly simplifies the choice of material and the structure of electrodes. In particular, conventional carbon-based gas diffusion and electrocatalytic materials (with an adequate degree of hydrophobicity) used in PEM fuel cell technology can also be used at the cathode of the URFC. Results reported here concern the development and testing of a URFC stack. A nominal electric power of 1.5 kW is required to perform water electrolysis in electrolysis mode and to produce hydrogen and oxygen of electrolytic grade in nominal conditions of operation. The same system can be used as a H2/O2 fuel cell and deliver an output electric power of 0.5 kW.
2.
Experimental section
2.1.
Preparation of membrane-electrode assemblies
MEAs have been prepared using a technique described elsewhere [7]. Briefly, Nafion-1135 (89 microns thickness, exchange capacity of 0.89 meq/g) has been used as solid polymer electrolyte (SPE). At the cathode, platinum nanoparticles coated onto VulcanXC-72 (Pt40/VulcanXC-72) and mixed with 10 wt.% of PTFE have been used as electrocatalysts for the promotion of the hydrogen evolution reaction (HER) in electrolysis mode and for the promotion of the oxygen reduction reaction (ORR) in fuel cell mode. Typical loadings of 0.8 mg Pt cm2 have been used. At the anode, mixtures (50e50 wt.%) of iridium and platinum powders have been used as electrocatalysts for the promotion of the oxygen evolution reaction (OER) in electrolysis mode and for the promotion of the hydrogen oxidation reaction (HOR) in fuel cell mode.
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Iridium and platinum catalyst layers have been deposited separately: a first layer of iridium black has been deposited directly against the SPE and a second layer of Pt powder has been deposited over the first iridium layer. Typical metal loadings of 1 mg cm2 were used for each metal. Catalytic structures (Pt40/Vulcan XC-72 on cathodic sides and Pt-Irblack powders on anodic sides) were mixed with 15 (cathodes) or 7 (anodes) wt.% of the ion-exchange polymer (wt.% values are expressed with respect to the weight of catalyst). Porous titanium sheets (256 cm2 surface area and 0.9 0.02 mm thick) have been used at the anode as gas diffusion layer/current collector. Carbon paper Sigracet 10bb (SGL Carbon Group) 0.42 mm thick with micro-porous sub-layers have been used as gas diffusion electrode at the cathode.
2.2.
Description of the URFC stack
A filter-press URFC stack containing seven MEAs (256 cm2) has been designed and constructed (Figs. 1 and 2). Individual PEM cells are separated by titanium (BT-1-0) bipolar plates. Supply/ removal of water and gases to/from MEAs is achieved using parallel U-shaped flow channels. Spacers made of titanium expanded grid have been placed between bipolar plates and current collectors to allow water circulation and collect biphasic mixtures. MEAs are fixed between end-plates using stud-bolts to facilitate the assembly and obtain a homogeneous distribution of pressure in the stack. Elastic rubber sealants are used to avoid gas leakage. Individual cell voltages can be measured during operation directly on the bipolar plates, using a data acquisition unit. All input/output fluid pipes are located on the end-plates and have the same tubular geometry (Fig. 2).
2.3.
Test bench
The experimental setup pictured in Fig. 3 has been used to evaluate the electrochemical performances of the stack. The setup gathers the URFC stack, a feed water unit, an hydrogen and oxygen supply/removal unit, a temperature management unit, a gas humidification system, a DC power supply, an automated electric load system and a process supervision unit.
Fig. 1 e Assembling of URFC stack.
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Ø an electrical load for adjusting the impedance of the external circuit and perform measurements in fuel cell mode. The resistance box (rheostat) is used manually. Alternatively, an electronic and adjustable load is scheduled for automatic operation. Ø the control system and measurement unit used to control key operating parameters. Automatic control concerns: gases pressure, flow rate and gases humidity, temperature and electrical parameters (current and voltage). Ø pneumo-hydraulic scheme includes also valves (VE1eVE13), relief valves (RV1eRV2) and control valve (CV).
Fig. 2 e URFC stack.
A detailed flow diagram is provided in Fig. 4. The different sub-units are: Ø the reversible PEM cells stack (URFC stack). Ø the hydrogen supply systems, consisting of hydrogen vessel V3, reservoir-separator RS2, pressure regulator PR3, pressure sensor PS2. Ø the oxidant supply system (oxygen or air), including a vessel with oxygen V2, pressure regulator PR2, pressure sensor PS1, the compressor C with system of air cleaning. Ø the water supply system, including separators of gases C1 and C2, makeup tank MT, pumps P1, P2 and P3, ionexchange filters F1 and F2. Ø the thermoregulation system, including the liquid thermostat of the TS of circulating type. Ø the humidifying system, including humidifiers H1 and H2 in hydrogen and oxygen contours of the test bench with regulators and sensors of gases humidification.
Fig. 3 e URFC stack with thermal insulation installed in the test bench.
The different units (reversible cell, humidifiers, pressure regulators, manometers, sensors) have been assembled on a vertical support or placed in its vicinity (the thermostat, a rheostat, monitoring and control devices, gas vessels, etc.). The temperature of the URFC was regulated in the 20e92 C temperature range using a liquid water thermostat. In electrolysis mode, deionized water (1.0 MU cm) is circulated on the anodic circuit. The electric power supply can deliver a working current density up to 1.0 A cm2. The humidification unit is used to monitor the level of gas humidity in FC mode of operation. A maximum relative humidity of 92% was used for hydrogen and a maximum value of 65% was used for oxygen. Gas humidity was adjusted by controlling the temperature of gas humidifiers. The test bench was placed under the supervision of a personal computer for remote control, automated data acquisition and data processing. Pressure sensors have been used to measure pressure. Pressure regulation was achieved in the 0.1e3.0 bar pressure range. Hydrogen and oxygen flow rates were regulated using regulating electromagnetic valves. The electronic load used to perform FC measurements can vary in the 0.1e250 A current range and voltage in the 0.1e12 V voltage range.
3.
Results and discussion
Electrochemical performances of the URFC stack have been measured at 80 C, in both WE and FC modes of operation. Typical currentevoltage polarization curves are plotted in Fig. 5. Maximum current densities of 0.5 A cm2 have been obtained in both modes. Main operating parameters are summarized in Table 1. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm2, corresponding to 85% efficiency (based on the thermoneutral voltage of the water splitting reaction, i.e. the higher heating value of hydrogen). A mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density, corresponding to a 37% electric efficiency. Such electrochemical performances are rather close to those measured on conventional water electrolysis cells and H2/O2 fuel cells with the same noble metal loadings and under similar operating conditions. Some preliminary stability tests have been performed at constant current density (0.5 A cm2). A total of less than 100 h of intermittent WE and FC operation has been cumulated so far. Before switching from one mode to the other, care was taken to fully purge the stack. Before water electrolysis operation, thermostated liquid water was
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Fig. 4 e Scheme of URFC test bench.
circulated into each (anodic and cathodic) compartment until thermal equilibrium was reached. Then, the stack voltage was gradually increased until nominal value and maintained for 1 h. Then the polarization curve was recorded by stepping the cell voltage down to zero and measuring the equilibrium current every 50 mV. Before fuel cell operation, each compartment was first dried with dry argon to remove liquid water. Then humidified argon was circulated (50% relative humidity). Then hydrated hydrogen has been circulated for at
Table 1 e Technical performances of the URFC stack.
Fig. 5 e Typical currentevoltage polarization curves measured on the URFC stack in (A) WE and (B) FC modes of operation. Operating conditions are compiled in Table 1.
Nominal stack voltage, V Nominal current, A Nominal electric power, W Operating temperature, C Operating pressure of hydrogen, bar Operating pressure of oxygen, bar Humidification of hydrogen, % Humidification of oxygen, %
WE mode
FC mode
12.2 128 1562 80 0 0 e e
3.85 492 2.8 3 80 e
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Acknowledgements 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 2007e2012), and by the Global Energy International Prize Non-Profit Foundation (Grant No. MG-2008/04/3).
references
Fig. 6 e Currentevoltage polarization curves measured during WE and FC modes of operation. (C) cell 2 of the URFC stack in the present work; (A) data taken from Ref. [16].
least 1 h, until the open voltage of each cell was stabilized. Then, the polarization curve of the stack was recorded by stepping down the value of the resistance. It is expected that quite different and less efficient results will be obtained if the stack is switch rapidly from one mode of operation to the other without these precautions. Experiments are still continued to assess the potentialities of the stack on the long term and to determine the highest operational frequency of switch. Since the concept of URFC discussed in this paper has not been the subject of detailed reports in the literature, it is difficult to make quantitative comparisons with state-of-the-art systems. However, the currentevoltage curves measured on the stack were found slightly more efficient than those reported for a lab-scale monocell URFC of similar concept described in Ref. [16]. Results are compared in Fig. 6.
4.
Conclusion
URFCs are promising reversible electrochemical devices which can operate either as a water electrolyser for the production of electrolytic hydrogen and oxygen or as a H2/O2 fuel cell for the production of electricity and heat. A seven-cell URFC stack has been developed. A special cell design has been used to solve the problem associated with the non-reversibility of the watereoxygen electrode. Electrochemical performances measured during water electrolysis and fuel cell operations are close to those measured on individual systems. This is a significant result which highlights the promising potential of such devices. An overall energy conversion efficiency (including water electrolysis and fuel cell contributions) of w30% has been obtained at 0.5 A cm2. These preliminary results are encouraging. Additional tests are in progress to assess the potentialities of the stack on the long term.
[1] Suppes GJ. Plug-in hybrid with fuel cell battery charger. Int J Hydrogen Energy 2005;30:113e21. [2] 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: 994e1009. [3] Bone JS, Gilman S, Niedrach LW, Read MD. Ion-exchange regenerative fuel cells. In: Proceedings of the 15th annual power source conf. N.J.: Ft. Monmouth; May 9e11, 1961. p. 47e9. [4] Mitlitsky F, Myers B, Weisberg AH, Molter TM, Smith WF. Reversible (unitised) PEM fuel cell devices. Fuel Cells Bull 1999;2:6e11. [5] 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;30:1345e50. [6] Zhang Y, Wang C, Wan N, Mao Z. Deposited RuO2eIrO2/Pt electrocatalyst for the regenerative fuel cell. Int J Hydrogen Energy 2007;32:400e4. [7] Grigoriev SA, Millet P, Dzhus KA, Middleton H, Saetre TO, Fateev VN. Design and characterization of bi-functional electrocatalytic layers for application in PEM unitized regenerative fuel cells. Int J Hydrogen Energy 2010;35:5070e6. [8] Wittstadt U, Wagner E, Jungmann T. Membrane electrode assemblies for unitised regenerative polymer electrolyte fuel cells. J Power Sources 2005;145:555e62. [9] 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;10:1373e6. [10] Doddathimmaiah A, Andrews J. Theory, modelling and performance measurement of unitised regenerative fuel cells. Int J Hydrogen Energy 2009;34:8157e70. [11] Xiaojin L, Yu X, Zhigang S, Baolian Y. Mass minimization of a discrete regenerative fuel cell (RFC) system for on-board energy storage. J Power Sources 2010;195:4811e5. [12] Yao W, Yang J, Wang J, Nuli Y. Chemical deposition of platinum nanoparticles on iridium oxide for oxygen electrode of unitized regenerative fuel cell. Electrochem Commun 2007;9:1029e34. [13] Salwan 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;13:1663e8. [14] 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;22:1167e74. [15] Ledjeff K, Mahlendorf F, Peinecke V, Heinzel A. Development of electrode/membrane units for the reversible solid polymer fuel cell (RSPFC). Electrochim Acta 1995;40:315e9. [16] 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;36:545e8.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology S.A. Grigoriev a,*, P. Millet b, V.I. Porembsky a, 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 no 8182, Universite´ Paris Sud, baˆt 410, 91405 Orsay Cedex, France
article info
abstract
Article history:
Results related to the development and testing of a unitized regenerative fuel cell (URFC)
Received 14 April 2010
based on proton-exchange membrane (PEM) technology are reported. A URFC is an elec-
Received in revised form
trochemical device which can operate either as an electrolyser for the production of
1 July 2010
hydrogen and oxygen (water electrolysis mode) or as a H2/O2 fuel cell for the production of
Accepted 1 July 2010
electricity and heat (fuel cell mode). The URFC stack described in this paper is made
Available online 9 August 2010
of seven electrochemical cells (256 cn2 active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production
Keywords:
in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water
Unitized regenerative fuel cell
electrolysis at 0.5 A cm2 (85% efficiency based on the thermoneutral voltage of the water
Proton-exchange membrane
splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell
Electrochemical stack
operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A unitized regenerative fuel cell (URFC) is a reversible electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode WE) or as a H2/O2 fuel cell for the production of electricity and heat (fuel cell mode FC). URFCs are potentially interesting for a large number of energy management applications [1,2]. In particular, they could play a significant role for the promotion of renewable energy sources. First attempts to built URFCs using proton-exchange membrane (PEM) technology were made in the 1960s [3]. At that time, systems suffered from inacceptable low electrochemical performances. This was due to problems with membrane and electrocatalysts. More promising results have been obtained
in 1972 at General Electric Co. [4]. Later, in the 1990s, a prototype URFC stack having a specific power density of ca. 450 Wh kg1 has been developed and tested at the Lawrence Livermore National Laboratory (LLNL) by AeroVironment of Monrovia and Hamilton Standard [4]. In 1998 Proton Energy Systems has developed a commercial product (Unigen reversible module) consuming 15 kW in electrolysis mode (WE) and producing up to 5 kW of electric power in fuel cell (FC) mode [4]. R&D on URFC has also received attention from different US companies (Lynntech Inc., Glenn Research Center, Giner Inc.), from the Canadian company Green Volt Power Corp and some others [4e7]. Experimental optimization of membrane-electrode assemblies (MEAs) for application in URFCs is reported in Refs. [8,9]. More recently, results related to the modeling and characterization of URFS have been
* Corresponding author. Tel.: þ7 499 196 94 44; fax: þ7 499 196 62 78. E-mail address: [email protected] (S.A. Grigoriev). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.011
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 6 4 e4 1 6 8
published [10e12], showing that this technology is the subject of an increasing attention. It should be emphasized that the above mentioned technological developments are all using a similar concept of socalled ‘hydrogen and oxygen electrodes’, i.e. H2 is produced (WE mode) or consumed (FC mode) in the same cell compartment and O2 is produced (WE mode) or consumed (FC mode) in the other cell compartment. This requires the development of ‘reversible electrodes’, a non-trivial task, especially on the oxygen side since the H2O/O2 redox system is significantly irreversible in the temperature range (from 20 C to e100 C) of operation used in PEM technology. Despite some obvious advantages (in particular a simpler management of gases), this concept presents some additional drawbacks, mainly a problem of material stability, the corrosion of carbon-based materials (carbon was used as catalyst carrier, gas diffusion layer and bipolar plate [13]) at the anode during water electrolysis being responsible for rapid losses of performances. In this paper, an alternative design of URFC cells based on the so-called concept of ‘reduction and oxidation electrodes’ [7,14e16] has been used to develop a URFC stack. One cell compartment is designed for the promotion of oxidation processes (water oxidation in WE mode and H2 oxidation in FC mode), and the other is designed for the promotion of reduction processes (H2 evolution in WE mode and O2 reduction in FC mode). Despite the fact that this concept requires the purge of electrode chambers with an inert gas before switching from one mode of operation (WE) to the other (FC) and vice-versa, it greatly simplifies the choice of material and the structure of electrodes. In particular, conventional carbon-based gas diffusion and electrocatalytic materials (with an adequate degree of hydrophobicity) used in PEM fuel cell technology can also be used at the cathode of the URFC. Results reported here concern the development and testing of a URFC stack. A nominal electric power of 1.5 kW is required to perform water electrolysis in electrolysis mode and to produce hydrogen and oxygen of electrolytic grade in nominal conditions of operation. The same system can be used as a H2/O2 fuel cell and deliver an output electric power of 0.5 kW.
2.
Experimental section
2.1.
Preparation of membrane-electrode assemblies
MEAs have been prepared using a technique described elsewhere [7]. Briefly, Nafion-1135 (89 microns thickness, exchange capacity of 0.89 meq/g) has been used as solid polymer electrolyte (SPE). At the cathode, platinum nanoparticles coated onto VulcanXC-72 (Pt40/VulcanXC-72) and mixed with 10 wt.% of PTFE have been used as electrocatalysts for the promotion of the hydrogen evolution reaction (HER) in electrolysis mode and for the promotion of the oxygen reduction reaction (ORR) in fuel cell mode. Typical loadings of 0.8 mg Pt cm2 have been used. At the anode, mixtures (50e50 wt.%) of iridium and platinum powders have been used as electrocatalysts for the promotion of the oxygen evolution reaction (OER) in electrolysis mode and for the promotion of the hydrogen oxidation reaction (HOR) in fuel cell mode.
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Iridium and platinum catalyst layers have been deposited separately: a first layer of iridium black has been deposited directly against the SPE and a second layer of Pt powder has been deposited over the first iridium layer. Typical metal loadings of 1 mg cm2 were used for each metal. Catalytic structures (Pt40/Vulcan XC-72 on cathodic sides and Pt-Irblack powders on anodic sides) were mixed with 15 (cathodes) or 7 (anodes) wt.% of the ion-exchange polymer (wt.% values are expressed with respect to the weight of catalyst). Porous titanium sheets (256 cm2 surface area and 0.9 0.02 mm thick) have been used at the anode as gas diffusion layer/current collector. Carbon paper Sigracet 10bb (SGL Carbon Group) 0.42 mm thick with micro-porous sub-layers have been used as gas diffusion electrode at the cathode.
2.2.
Description of the URFC stack
A filter-press URFC stack containing seven MEAs (256 cm2) has been designed and constructed (Figs. 1 and 2). Individual PEM cells are separated by titanium (BT-1-0) bipolar plates. Supply/ removal of water and gases to/from MEAs is achieved using parallel U-shaped flow channels. Spacers made of titanium expanded grid have been placed between bipolar plates and current collectors to allow water circulation and collect biphasic mixtures. MEAs are fixed between end-plates using stud-bolts to facilitate the assembly and obtain a homogeneous distribution of pressure in the stack. Elastic rubber sealants are used to avoid gas leakage. Individual cell voltages can be measured during operation directly on the bipolar plates, using a data acquisition unit. All input/output fluid pipes are located on the end-plates and have the same tubular geometry (Fig. 2).
2.3.
Test bench
The experimental setup pictured in Fig. 3 has been used to evaluate the electrochemical performances of the stack. The setup gathers the URFC stack, a feed water unit, an hydrogen and oxygen supply/removal unit, a temperature management unit, a gas humidification system, a DC power supply, an automated electric load system and a process supervision unit.
Fig. 1 e Assembling of URFC stack.
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Ø an electrical load for adjusting the impedance of the external circuit and perform measurements in fuel cell mode. The resistance box (rheostat) is used manually. Alternatively, an electronic and adjustable load is scheduled for automatic operation. Ø the control system and measurement unit used to control key operating parameters. Automatic control concerns: gases pressure, flow rate and gases humidity, temperature and electrical parameters (current and voltage). Ø pneumo-hydraulic scheme includes also valves (VE1eVE13), relief valves (RV1eRV2) and control valve (CV).
Fig. 2 e URFC stack.
A detailed flow diagram is provided in Fig. 4. The different sub-units are: Ø the reversible PEM cells stack (URFC stack). Ø the hydrogen supply systems, consisting of hydrogen vessel V3, reservoir-separator RS2, pressure regulator PR3, pressure sensor PS2. Ø the oxidant supply system (oxygen or air), including a vessel with oxygen V2, pressure regulator PR2, pressure sensor PS1, the compressor C with system of air cleaning. Ø the water supply system, including separators of gases C1 and C2, makeup tank MT, pumps P1, P2 and P3, ionexchange filters F1 and F2. Ø the thermoregulation system, including the liquid thermostat of the TS of circulating type. Ø the humidifying system, including humidifiers H1 and H2 in hydrogen and oxygen contours of the test bench with regulators and sensors of gases humidification.
Fig. 3 e URFC stack with thermal insulation installed in the test bench.
The different units (reversible cell, humidifiers, pressure regulators, manometers, sensors) have been assembled on a vertical support or placed in its vicinity (the thermostat, a rheostat, monitoring and control devices, gas vessels, etc.). The temperature of the URFC was regulated in the 20e92 C temperature range using a liquid water thermostat. In electrolysis mode, deionized water (1.0 MU cm) is circulated on the anodic circuit. The electric power supply can deliver a working current density up to 1.0 A cm2. The humidification unit is used to monitor the level of gas humidity in FC mode of operation. A maximum relative humidity of 92% was used for hydrogen and a maximum value of 65% was used for oxygen. Gas humidity was adjusted by controlling the temperature of gas humidifiers. The test bench was placed under the supervision of a personal computer for remote control, automated data acquisition and data processing. Pressure sensors have been used to measure pressure. Pressure regulation was achieved in the 0.1e3.0 bar pressure range. Hydrogen and oxygen flow rates were regulated using regulating electromagnetic valves. The electronic load used to perform FC measurements can vary in the 0.1e250 A current range and voltage in the 0.1e12 V voltage range.
3.
Results and discussion
Electrochemical performances of the URFC stack have been measured at 80 C, in both WE and FC modes of operation. Typical currentevoltage polarization curves are plotted in Fig. 5. Maximum current densities of 0.5 A cm2 have been obtained in both modes. Main operating parameters are summarized in Table 1. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm2, corresponding to 85% efficiency (based on the thermoneutral voltage of the water splitting reaction, i.e. the higher heating value of hydrogen). A mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density, corresponding to a 37% electric efficiency. Such electrochemical performances are rather close to those measured on conventional water electrolysis cells and H2/O2 fuel cells with the same noble metal loadings and under similar operating conditions. Some preliminary stability tests have been performed at constant current density (0.5 A cm2). A total of less than 100 h of intermittent WE and FC operation has been cumulated so far. Before switching from one mode to the other, care was taken to fully purge the stack. Before water electrolysis operation, thermostated liquid water was
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 6 4 e4 1 6 8
Fig. 4 e Scheme of URFC test bench.
circulated into each (anodic and cathodic) compartment until thermal equilibrium was reached. Then, the stack voltage was gradually increased until nominal value and maintained for 1 h. Then the polarization curve was recorded by stepping the cell voltage down to zero and measuring the equilibrium current every 50 mV. Before fuel cell operation, each compartment was first dried with dry argon to remove liquid water. Then humidified argon was circulated (50% relative humidity). Then hydrated hydrogen has been circulated for at
Table 1 e Technical performances of the URFC stack.
Fig. 5 e Typical currentevoltage polarization curves measured on the URFC stack in (A) WE and (B) FC modes of operation. Operating conditions are compiled in Table 1.
Nominal stack voltage, V Nominal current, A Nominal electric power, W Operating temperature, C Operating pressure of hydrogen, bar Operating pressure of oxygen, bar Humidification of hydrogen, % Humidification of oxygen, %
WE mode
FC mode
12.2 128 1562 80 0 0 e e
3.85 492 2.8 3 80 e
4168
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 6 4 e4 1 6 8
Acknowledgements 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 2007e2012), and by the Global Energy International Prize Non-Profit Foundation (Grant No. MG-2008/04/3).
references
Fig. 6 e Currentevoltage polarization curves measured during WE and FC modes of operation. (C) cell 2 of the URFC stack in the present work; (A) data taken from Ref. [16].
least 1 h, until the open voltage of each cell was stabilized. Then, the polarization curve of the stack was recorded by stepping down the value of the resistance. It is expected that quite different and less efficient results will be obtained if the stack is switch rapidly from one mode of operation to the other without these precautions. Experiments are still continued to assess the potentialities of the stack on the long term and to determine the highest operational frequency of switch. Since the concept of URFC discussed in this paper has not been the subject of detailed reports in the literature, it is difficult to make quantitative comparisons with state-of-the-art systems. However, the currentevoltage curves measured on the stack were found slightly more efficient than those reported for a lab-scale monocell URFC of similar concept described in Ref. [16]. Results are compared in Fig. 6.
4.
Conclusion
URFCs are promising reversible electrochemical devices which can operate either as a water electrolyser for the production of electrolytic hydrogen and oxygen or as a H2/O2 fuel cell for the production of electricity and heat. A seven-cell URFC stack has been developed. A special cell design has been used to solve the problem associated with the non-reversibility of the watereoxygen electrode. Electrochemical performances measured during water electrolysis and fuel cell operations are close to those measured on individual systems. This is a significant result which highlights the promising potential of such devices. An overall energy conversion efficiency (including water electrolysis and fuel cell contributions) of w30% has been obtained at 0.5 A cm2. These preliminary results are encouraging. Additional tests are in progress to assess the potentialities of the stack on the long term.
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