J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 1 8 (1 9 9 9 ) 1807 – 1809

CaH2 containing halide electrolytes and fuel cells B. ZHU Department of Chemical Engineering & Technology, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden E-mail: [email protected]

The utilization of hydrogen energy is an important subject for a sustainable development of the society in the 21st century, where proton conducting and hydrogen semipermeable materials play a key role. Protonconducting fuel cells for converting the energy of hydrogen containing fuel have some advantages compared with oxygen-ion conducting solid oxide fuel cells (SOFCs). Solid state proton conductors as electrolytes are very promising for fuel cells operating at intermediate temperatures (say 400 to 800 ◦ C) [1–4]. However, there is a need for even higher proton conductivity in order to improve the power output. The task to develop new proton conducting electrolytes is always important. Fuel cells using halides-based electrolytes, e.g., chlorides and fluorides, have been demonstrated very recently, which may meet the requirements for developing advanced ceramic fuel cell technology [3, 4]. New discoveries of proton and oxygen ion conduction in non-oxide ceramics, chlorides- and fluoride-based materials open a new academic area [5]. For developing practical fuel cell devices an improvement of the electrical properties for these halide-based electrolytes is needed. A low concentration of protons and oxygen ions in original chloride- and fluoride-based electrolytes may not be satisfactory for a sufficient ionic (proton or oxygen ion) transport number and conductivity involved in the electrode reaction and fuel cell process. As a consequence, a relatively large voltage loss was observed in the fuel cell. How to increase the proton or oxygen ion concentration in the chloride- and fluoride-based electrolytes, and how to enhance their ionic transport thus become a critical issue. New development of the CaH2 containing materials, i.e., hydrochlorides and hydrofluorides, may solve this problem. This paper reports these CaH2 containing halide electrolytes and their promise in development of new advanced ceramic fuel cell technology. In order to make a good mechanical property of the electrolyte, a halide composite with alumina was always used. Materials used for fuel cell construction are: electrolytes of the type MXx (M = Li, Na, Ba, X = Cl, F, x = 1, 2), CaH2 (A. R., Aldrich Chemical Company, Inc., USA) and alumina (A. R., Merck); electrodes of the type platinum (Leitplatin 308A, Hanau, Germany) paste; stainless steel was used for the bipolar plates and the fuel cell device holder. The hydrotype halides were prepared in the following procedure: chlorides or fluorides (MXx ) were mixed with Al2 O3 and CaH2 in various molar ratios and well ground. The mixture was then heat-treated at 700 ◦ C in a pure hyC 1999 Kluwer Academic Publishers 0261–8028 °

drogen gas atmosphere for two hours to produce hydrochloride or hydrofluoride, MXx -CaH2 -Al2 O3 , composites, which were used for fuel cell construction and measurement. Fuel cells were constructed using an electrolytesupported technique with Pt electrodes pasted on both sides of the MXx -CaH2 -Al2 O3 electrolyte disc (thickness ∼1–2 mm). The assembly of the fuel cell with the electrolyte and electrodes was heat-treated in H2 at 600 ◦ C for 30 min and then mounted into the device/holder with the following configuration: anode (H2 , fuel chamber)/electrolyte/cathode (2% O2 in Ar, oxidant chamber), the cell size being normally 13 mm in diameter and 1.0–2.0 mm thick. The fuel cells were tested in the intermediate temperature region, 400 to 800 ◦ C. In fuel cells, only H+ (H− ) or O2− are non-blocking mobile ionic species, provided by the reversible gas electrodes from the external resources, which contribute the steady-state fuel cell current ouput; while F− ions are blocking, they would not make contribution to the steady-state current under fuel cell operation. The fuel cells using hydro-type halides as the electrolytes two kinds of reactions at the hydrogen electrode of the devices may exist: H2 + 2e− = 2H−

(1)

H2 − 2e− = 2H+

(2)

and

These two reactions generate opposite electromotive forces. Reaction 1 provides the hydrogen electrode as positive, while reaction 2 makes it negative. There is thus a simple judgment for which reaction is dominating according to signs of the cell potentials, correspondingly we know which charge carrier is dominating during fuel cell operation. If proton conduction is predominating in the electrolytes, reaction 1 for producing H− ions will reduce the fuel cell potential (voltage) due to causing a positive polarity of the fuel cell anode. There is always a thermodynamic equilibrium between reactions 1 and 2. The measured fuel cell voltage is thus a sum of the net potential that contributes an open circuit voltage (OCV) of the fuel cells. The voltages for the fuel cells using hydrochloride and hydrofluoride electrolytes were measured at the open circuit to be normally between 0.8 to 1.05 V, depending mainly on the electrolyte composition and temperature. These cell voltages agree with the sign of 1807

Figure 1 I -V characteristics for fuel cells using NaCl-Al2 O3 and NaClCaH2 -Al2 O3 electrolytes at 680 ◦ C.

Figure 3 I -V characteristic for the LiF-CaH2 -Al2 O3 electrolyte fuel cell at 800 ◦ C.

the proton-conducting-type fuel cell, not the H− -ionconducting type, although a number of reports claimed that hydrofluorides were the H− ion conductors [6–8]. However, there were also influences from the hydride ions. For example, the voltages were measured usually somehow to be lower than those obtained for pure chloride- and fluoride-based electrolyte fuel cells. The cell voltages also decreased as the temperature increased, since the H− ions were highly mobile at high temperatures [6]. Figs 1 and 2 show typical I -V characteristics for fuel cells using MXx CaH2 -Al2 O3 as the electrolytes, where NaCl- or NaFAl2 O3 as the electrolytes are inserted for comparison, since they are the best among the MXx -Al2 O3 fuel cells. It can be seen clearly from Figs 1 and 2 that the CaH2 content has improved significantly fuel cell performance. NaCl- and LiF-based electrolyte fuel cells are typical examples. The short circuit current densities were increased from 12 mA cm−2 for NaCl-Al2 O3 to 140 mA cm−2 for NaCl-CaH2 -Al2 O3 fuel cells at 680 ◦ C, and from 20 mA cm−2 for LiFAl2 O3 to 400 mA cm−2 for LiF-Al2 O3 -CaH2 fuel cells at 750 ◦ C. At 800 ◦ C the LiF-CaH2 -Al2 O3 fuel cell has so far reached the best performance with the short circuit current density of 850 mA cm−2 and the peak power of 160 mW cm−2 under 500 mA cm−2 (0.32 V), see Fig. 3. The conductivity of the electrolytes was obtained from measurements of the fuel cell I -V characteristics

in subtraction of the influence from the electrodes. In the I -V characteristics, the linear section in the central region reflects the IR loss mainly caused by the electrolyte, from which the resistance, and thus the conductivity of the electrolyte can be calculated. Fig. 4 shows the temperature dependences of conductivities for NaF- and LiF-based electrolytes. The CaH2 content shows also strong influence on the electrical properties, e.g., to enhance the conductivity one to two orders of magnitude higher from LiF-Al2 O3 to LiF-CaH2 -Al2 O3 and reduce the activation energy from 0.95 eV for LiFAl2 O3 to 0.65 eV for LiF-CaH2 -Al2 O3 . The conductivity enhancement may be caused by an increase of H+ concentration from the CaH2 content. However, it can not yet be explained how the H− ions can be converted to H+ in the fuel cell process, which might arouse a great interest in fundamental research. It was reported before that the H− can be converted to H atoms by xray or UV irradiation, and these hydrogen atoms were very mobile [9]. It is reasonable to assume that under an electrical field of the fuel cell device, the H− can be possibly converted to the H or H+ state, causing an increase of the hydrogen concentration and an enhancement of proton conduction. Further investigation is under progress. In addition, chloride and fluoride solid solutions, e.g., MX-NX2 -Al2 O3 (M = Li, Na, N = Ca, Ba, X = Cl, F) can usually improve significantly fuel cell performance compared with single chloride and fluoride electrolytes

Figure 2 I -V characteristics for fuel cells using various MFx -CaH2 Al2 O3 electrolytes at 750 ◦ C, where NaF-Al2 O3 electrolyte fuel cell is inserted for comparison.

Figure 4 Temperature dependence of the conductivity for MF-Al2 O3 and MF-CaH2 -Al2 O3 (M = Li, Na) electrolytes.

1808

as previously reported [3, 4]. The chloride and fluoride solid solutions containing CaH2 as the electrolytes can also further improve the fuel cell performance. For example, LiF-BaF2 -CaH2 -Al2 O3 fuel cells showed a short circuit current density close to 1000 mA/cm2 , and a peak power density of about 180 mW/cm2 at 300 mA/cm2 (0.6 V) and 750 ◦ C; while the fuel cells using the MX-NX2 -Al2 O3 as the electrolytes reached only less half performance. Fuel cell applications have demonstrated proton conduction is predominating in these new hydro-type halide electrolytes, especially, hydrofluorides. The high proton conduction benefits the fuel cell performance resulting in the high outputs of the current and power densities. The hydride ions, H− , on the other hand, are always influences, e.g., lowering the fuel cell voltages and hindering proton transport since the hydride ions may act as the trapper against proton transport. These effects become more significant at high temperatures due to increasing the mobility of the hydride ions. The excellent performances for the hydro-type electrolyte fuel cells, e.g., the short circuit current density close to 1000 mA/cm2 and the peak power density of 160 mW/cm2 at 500 mA/cm2 (0.32 V) for the LiF-CaH2 -Al2 O3 fuel cell at 800 ◦ C, and 180 mW/cm2 at 300 mA/cm2 (0.6 V) for the LiF-BaF2 -CaH2 -Al2 O3 fuel cell at 750 ◦ C, demonstrate new opportunities for development of the practical and innovative technology for the advanced ceramic fuel cells. Proton conduction in these hydro-halides (hydrochlorides and hydrofluorides) opens also a completely new academic area with

significant importance for both fundamental and applied research. Acknowledgment This work is supported in the Swedish side by NUTEK (Swedish National Board for Industrial and Technical Development) and the Swedish Research Council for Engineering Sciences (TFR); in the Chinese side by the National Nature Science Foundation of China (NSFC) and Ministry of Science and Technology of China (MSTC) through the co-operative research in University of Science and Technology of China. References 1. B . Z H U , I . A L B I N S S O N , B . - E . M E L L A N D E R and G .- Y . M E N G , in 9th International Conference on Solid State Protonic Conductors, Bled, Slovenia, August 17–21, 1998, to be also published in Solid State Ionics. 2. B . Z H U , G . - Y . M E N G and B . - E . M E L L A N D E R , J. Power Sources, in press. 3. B . Z H U , J. Mater. Sci. Lett., in press. 4. B . Z H U , J. Power Sources, in press. 5. B . Z H U , Mat. Res. Bull. 35(1), in press. 6. V . P . G R E L O V and S . P A L ’G U E V , Soviet Electrochem. 28 (1992) 1294. 7. M . K A M A T A , A . M A T S U M O T O and T . E S A K A , Denki Kagaku 66 (1998) 443. 8. R . L E V E Q U E , M . Z A N N E , D . V E R G A T - G R A N D J E A N and J . F . B R I C E , J. Solid State Chem. 33 (1980) 233. 9. M . D E S O U Z A , A . D . G O´ N G O R A , M . A E G E R T E R and ¨ T Y , Phys. Rev. Lett. 25 (1970) 1426. F. LU

Received 26 May and accepted 15 July 1999

1809