MIXED IRON OXIDES FOR SOLAR HYDROGEN PRODUCTION THROUGH TWO-STEP WATER SPLITTING THERMOCHEMICAL CYCLES R. Fernández-Saavedra, F. Fresno, M. Sánchez, M.B. Gómez-Mancebo, R. Fernández-Martínez and A. Vidal Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT). Avenida Complutense, 22. 28040 Madrid, Spain. e-mail:
[email protected]
Abstract Different mixed iron oxides (ferrites) were examined for solar hydrogen production from water by two-step thermochemical cycles. Mn-ferrites partially substituted with different metals were prepared by a polymeric precursor method based on the Pechini process, giving compounds in the form M 0.25 Mn 0.75 Fe 2 O 4 (M = Mn, Co, Ni and Cu), which were chemical and structurally characterised by ICP-AES and XRD. The ability of these materials for hydrogen production through thermochemical cycles was also tested. In the first step of the cycle (activation), ferrites were heated at 1373 K under an inert atmosphere, resulting in oxygen-deficient structures. In this step, oxygen loss and activation temperature were also studied by thermal analyses techniques (TG-DTA). In the second step (hydrolysis), activated ferrites were reacted with water vapour at 1350 K to produce hydrogen. Among the studied materials, Ni 0.25 Mn 0.75 Fe 2 O 4 leads to the highest hydrogen evolution per gram of sample, being also the closest to stoichiometric H 2 production. Keywords: hydrogen, solar chemistry, thermochemical cycles, ferrites, solar fuels. 1
Introduction
The increasing concern about global warm and fossil fuels reserves has led to the search of alternative, clean and sustainable energy systems. In this respect, great attention has been paid in the recent past to hydrogen as a clean fuel.1 However, about 96 % of the total H 2 production is nowadays carried out from fossil sources,2 which means no immediate solution to the above mentioned concerns. Consequently, great effort is being dedicated to the development of alternative routes to obtain truly clean hydrogen for future applications. The most abundant, and virtually endless, source of hydrogen in nature is water, which can be thermally (among other ways) decomposed into oxygen and hydrogen: H2O Æ H2 + ½ O2
(1)
Unfortunately, reaction (1) is not as simple in the practice as it conceptually is. On the one hand, it requires temperatures higher than 2500 K to achieve a reasonable H 2 yield and, on the other hand, efficient separation of O 2 and H 2 must be undertaken as, otherwise, they would react back to H 2 O at these temperatures. The use of thermochemical cycles, consisting of a series of reduction – oxidation steps which net result is reaction (1), is a feasible way to avoid the problems inherent to thermal water dissociation, as they can be carried out at lower temperatures and permit the formation of O 2 and H 2 in different steps. Obviously, to obtain clean hydrogen, the necessary energy for these processes must be supplied from a renewable source. Although thermochemical cycles were originally planned to be carried out with nuclear heat, the temperatures required for many cycles extended the research in this field to concentrated solar energy.3 Among the great amount of cycles that have been proposed, those based on metal oxides are considered as the most appropriate candidates for their utilization with concentrated solar radiation, taking into account their simplicity and the ability of solar concentration technologies to achieve the required temperatures.4,5 These cycles involve the reduction (activation) of the metallic oxide as the endothermic step and a subsequent exothermic re-oxidation with water vapour (hydrolysis) to regenerate the former oxide and produce hydrogen, as illustrated by the following reactions: (2) Activation: MO x Æ MO x-δ + δ/2 O 2
Hydrolysis:
MO x-δ + δ H 2 O Æ MO x + δ H 2
(3)
One of the metal/oxide cycles proposed for solar water splitting is based on the redox pair Fe 3 O 4 /FeO.6 High theoretical efficiencies were reported for this cycle,7 and the hydrolysis reaction of solar-produced FeO to yield hydrogen has been demonstrated.8 However, the activation temperature of Fe 3 O 4 is still high (ca. 2500 K).5 Structurally connected to Fe 3 O 4 , mixed iron oxides with general formula MFe 2 O 4 (M: metal), spinel ferrites, are promising materials for solar hydrogen production, since they can be partially reduced at lower temperatures than Fe 3 O 4 . The partially reduced oxide can still react with water to yield hydrogen and regenerate the pristine ferrite to complete the thermochemical cycle.9,10 Ferrites with different compositions have been tested for this purpose, among which those containing manganese have received much attention. Manganese and manganese – nickel ferrites were originally proposed for water splitting with solar energy.9,11 Laboratory tests with the second ones gave as a result the reduction of the Ni 0.5 Mn 0.5 Fe 2 O 4 ferrite in Ar at 1373 K with oxygen evolution and the reaction of the reduced oxide with water at 973 K to produce hydrogen.9 Combinations of manganese and other metals, like Zn and Cu, in the spinel structure have been also assayed.12-14 In addition, cycles that combine Mn ferrites with CaO or Na 2 CO 3 in the activation step have been studied.15,16 Based on MnFe 2 O 4 , and with the aims of investigating the possible improvement of its performance for solar hydrogen production and identifying the most suitable materials for further studies, we recently reported on the preparation and thermal behaviour of Ni x Mn 1-x Fe 2 O 4 17,18 and M 0.25 Mn 0.75 Fe 2 O 4 (M: metal) spinels.19 In this work, we have extended the study of M 0.25 Mn 0.75 Fe 2 O 4 (M: Mn, Co, Ni or Cu) to hydrogen production in the water splitting reaction of the reduced oxide, in order to compare the effect of the different substituted cations on the behaviour of manganese ferrite. 2
2.1
Experimental
Materials preparation and characterisation
M 0.25 Mn 0.75 Fe 2 O 4 (M: Mn, Co, Ni, Cu; samples named hereafter as 1Mn, 0.25Co, 0.25Ni and 0.25Cu, respectively) ferrites were prepared by a polymeric precursor process, detailed elsewhere17 and based on the Pechini method. This method utilizes the ability of certain alpha-hydroxycarboxylic acids to form polybasic metallic chelates. Such chelates can undergo polyesterification to form a resin that after ignition forms the final oxide.20 Chemical analyses of the samples dissolved in aqua-regia were carried out by ICP-AES with a Varian 735-ES spectrometer, using several wavelengths for each element. Powder X-ray diffraction (XRD) patterns were recorded by means of a PANalytical X´Pert PRO diffractometer operating in θ-θ configuration, with Cu Kα radiation at 45 KV and 40 mA and equipped with a curved graphite monochromator to improve the peak-tobackground ratio for samples containing significant amounts of iron or manganese. Thermogravimetric analyses (TGA) were carried out in a Seiko TG/DTA 6300 thermobalance, with 10-15 mg of sample in a Pt crucible and adding Al 2 O 3 powder to avoid sintering. 2.2
Activation and water splitting tests
Two-step water splitting cycles with the prepared ferrites were carried out in the laboratory with a versatile reaction system, a scheme of which is shown in figure 1. Ar was used as the carrier gas for both activation and hydrolysis reactions. The gas flow, measured and controlled by a mass flow controller and indicator (Alicat Scientific), was passed, either directly (activation) or after saturation with water vapour (hydrolysis), through a fixed bed reactor placed inside an electric furnace. After the elimination of the water excess when necessary, the outlet gas was analyzed with a micro – gas chromatograph (Varian CP4900) equipped with a molecular sieve column and a TCD detector. The Ar flow employed for the reactions was 100 sccm and the temperature of the saturation unit (see figure 1) for the hydrolysis reaction was 323 K, which means a water mole fraction of 0.12 assuming an atmospheric pressure of 760 torr. The ferrite powder was mixed with 2 mm diameter Al 2 O 3 spheres to avoid sintering, placed inside a non-porous alumina tubular reactor and fixed with refractory fibre. The final temperatures were 1373 K for the activation step and 1223 K for the water splitting reaction, with a heating ramp of 20 K / min.
Figure 1. Scheme of the experimental set-up for the activation and water splitting reaction tests.
3
3.1
Results and Discussion
Characterisation of the materials
Table 1 shows the obtained metal contents of the studied materials, expressed as weight percentage and compared to the values calculated for the M 0.25 Mn 0.75 Fe 2 O 4 stoichiometry, to which the starting amount of metals were adjusted for the synthesis. The manganese and dopant metal contents are in good accordance with the calculated values, while the iron contents are slightly lower than those expected. Weight % Calculated Obtained a M Mn Fe M Mn Fe 23.8 48.4 23.2±0.4 46.4±1.0 1Mn 6.4 17.8 48.2 6.4±0.3 17.4±0.3 45.7±1.0 0.25Co 6.3 17.8 48.2 6.0±0.3 17.1±0.4 44.5±0.9 0.25Ni 6.8 17.7 48.0 6.8±0.2 17.5±0.4 45.5±0.9 0.25Cu Table 1. Metal contents as determined by ICP-AES. a Experimental errors were calculated with duplicated analysis using three different wavelengths. Sample
The XRD patterns of the prepared materials showed that the Ni-doped manganese ferrite (0.25Ni) is the only one constituted exclusively by the spinel ferrite phase. The other prepared ferrites present secondary phases, i.e., Fe 2 O 3 (hematite) in the case of 0.25Co, 0.25Cu and 1Mn ferrites and manganese (III) oxide only in this last material. As it was already reported,17 the pure hematite phase does not generate an oxygen deficient structure during the activation step at the employed temperature. However, in the present materials, this phase may be doped with some of the existing cations, which may lower its reduction temperature. Therefore, some activity for the studied reactions of the hematite phase present in these samples must not be discarded. Thermogravimetric analyses (TGA) of the different samples were carried out as a preliminary study of their capacity for partial reduction in an inert atmosphere, and hence of their potential for water splitting. Figure 2 shows the TGA curves obtained under a nitrogen flow, normalized to the weight of the samples at 873 K, after the elimination of surface water and hydroxyl groups. The weight of the 1Mn sample diminishes in the 923 – 1273 K, with a maximum rate at 1143 K, resulting in a final weight loss of 2.8 %. An experiment with a gas chromatograph coupled to the exhaust gas of the thermobalance confirmed that this weight loss corresponded to oxygen evolution. Partial substitution of Co, Ni or Cu for Mn results in lower weight losses but also lower reduction temperature, as it can be observed from the curves in figure 3. Thus, the nickel containing sample exhibits a 1.1 % weight loss, in spite of being the only pure spinel phase, with a maximum rate at approximately
923 K. Cobalt and copper – doped manganese ferrites display similar behaviours, with 1.5 % weight losses starting at a temperature similar to that of 0.25Ni, but with a maximum rate at about 1163 K. These results would mean that partial substitution of the different cations for Mn would result in a lower amount of hydrogen produced, but also in a lower reduction temperature. The first effect could be considered as negative from the point of view of net hydrogen production, but the second one could imply a lower energy input for the cycle to be carried out, which could compensate for the lower net production in terms of energetic efficiency. Thus, the thermal characterisation data reveal an interesting potential improvement of the performance of manganese ferrites for hydrogen production that, nevertheless, must be evaluated from the results of water splitting tests presented in the next section.
Figure 2. TGA curves of the ferrites: (a) 1Mn, (b) 0.25Co, (c) 0.25Ni, (d) 0.25Cu.
3.2
Water splitting tests
Figure 3 displays the gases evolution in the activation (O 2 ) and hydrolysis (H 2 ) steps obtained with the 0.25Ni sample. In the activation step, oxygen starts evolving at a temperature around 823 K, slightly lower than that observed in the TGA. In the water splitting step, the reaction starts at a slightly lower temperature as indicated by the observed hydrogen evolution. Table 2 summarises the total amount of gases evolved in the activation and water splitting reaction tests with the different ferrites, obtained from the integration of the molar flow – time curves. The weight losses and expected evolved oxygen from the thermogravimetric analyses are included for comparison. The amounts of O 2 formed per gram of ferrite in the activation step fit reasonably well the weight losses observed in the TGA, although some deviation is observed in the cases of 0.25Co and 0.25Ni, which must not be surprising considering the different conditions of the two types of experiments, in terms of sample mass and contact between the gas and solid phases. As it was observed in the TGA curves, Mn ferrite gives rise to the highest oxygen evolution, with partial substitution of Co, Ni, and Cu resulting in a lower amount of O 2 per gram of material. Table 2. TGA and water dissociation results obtained with the studied ferrites. TGA Reaction tests Sample Estimated δ in Yield name Weight loss (%) Estimated O 2 a O2 a H2 a (%) b Me 0.25 Mn 0.75 Fe 2 O 4-δ 1Mn 2.8 0.87 0.40 0.90 0.02 1 0.25Co 1.5 0.47 0.22 0.37 0.02 3 0.25Ni 1.1 0.35 0.16 0.46 0.06 7 0.25Cu 1.5 0.47 0.22 0.48 0.006 0.6 a mmol / g ferrite b yield = (mol H 2 ) / (2 mol O 2 )
Figure 3. Temperature ramps and gas evolutions during the activation (O 2 ) and hydrolysis (H 2 ) steps with the 0.25Ni sample. The hydrogen productions in the hydrolysis reactions are in all cases lower than those expected from the evolved amounts of oxygen in the activation step, considering the stoichiometry of reaction (1). This is reflected in the yield indicated in table 2, which has been considered as the relation between the obtained amount of hydrogen and that expected from the formed oxygen taking into account the stoichiometry of water dissociation. Indeed, the highest amount of hydrogen, as well as the highest yield is obtained with the 0.25Ni sample despite the fact that this is the material leading to the lowest weight loss in the TGA experiments. A XRD study of the samples after the activation and hydrolysis steps was carried out to follow the evolution of the ferrites during the thermochemical cycle. The XRD patterns are displayed in Figure 4 together with those of the original ferrites. As it can be observed, in the case of the samples that contain more than one crystalline phase (i.e., 1Mn, 0.25Co and 0.25Cu), the XRD patterns after the activation step reveal the spinel ferrite as the only crystalline phase. Therefore, a solid state reaction is occurring during the activation step. Thus, the observed oxygen evolution may result from this solid state reaction rather than from the formation of a reduced species, leading to the low hydrogen evolution observed in the subsequent water-splitting reaction. The spinel phase remains as the only one present after the hydrolysis step, except for the case of the 0.25Cu sample, which pattern reveals the presence of a small amount of delafossite, CuFeO 2 . Delafossite had been previously reported to appear in the case of Cu-containing ferrites.14,21 In the case of the 0.25Ni sample, the one leading to the highest amount of H 2 and the only one composed uniquely by the spinel phase, the XRD patterns indicate that this phase remains as the only one present after the activation step, with no formation of a reduced phase such as the doped wustite phases that had been detected in previous reports about Ni and Co ferrites.22,23 The lattice parameter a 0 of the cubic spinel phase in 0.25Ni was calculated from the XRD patterns. As it had been previously reported,9 the lattice parameter was increased from 8.40 (± 0.02) Å in the original sample to 8.47 (± 0.02) Å in the activated one due to the formation of an oxygen-deficient structure. The value of a 0 after the hydrolysis reaction, 8.46 (± 0.02) Å, is nearly equal to that of the activated ferrite, in accordance with the 7 % yield in hydrogen formation that suggests that only a part of the activated ferrites is being re-oxidised in the second step of the cycle. Table 3. Comparison of the results obtained with the 0.25Ni sample to those reported in the literature for the first cycle with related ferrites. Activation H2 Hydrolysis yield (%)a Reference Ferrite (mmol / g ferrite) Temperature (K) This work Ni 0.25 Mn 0.75 Fe 2 O 4 0.06 7 1373 10 Ni 0.5 Mn 0.5 Fe 2 O 4 0.23 quantitative 1273 NiFe 2 O 4 0.02 28 1473 14 Ni 0.5 Mn 0.5 Fe 2 O 4 0.05 1473 2.9 × 10-4 22 NiFe 2 O 4 0.12 1673 a yield = (mol H 2 ) / (2 mol O 2 )
Table 3 displays a comparison of the amount of hydrogen obtained with this sample and the results reported in the literature for the first cycle with related ferrites. The total amount of hydrogen reported here is higher than that obtained by Han et al.14 in the first cycle with a NiFe 2 O 4 ferrite at higher temperatures, although they reported a higher yield with respect to stoichiometry. In the case of Ni 0.5 Mn 0.5 Fe 2 O 4 , a very low production of hydrogen was reported in that work.14 However, the results reported here are far from those reported by Tamaura et al. with Ni 0.5 Mn 0.5 Fe 2 O 4 , in terms of yield and net hydrogen production, and with a somewhat lower activation temperature.10 The H 2 production reported by Kodama et al. with nickel ferrite is also higher than the one obtained with 0.25Ni, although their activation temperature was higher and they did not report the amount of oxygen evolved in the activation step.22
Figure 4. XRD patterns of (A) 1Mn, (B) 0.25Co, (C) 0.25Ni and (D) 0.25Cu; (a) original sample, (b) after activation step and (c) after hydrolysis step. The symbols indicate the phases to which the peaks correspond: (*) spinel ferrite, (+) Fe 2 O 3 (hematite), (x) Mn 2 O 3 , (~) CuFeO 2 (delafossite). Taking into account that the Ni-containing ferrite was the one leading to the highest hydrogen production, a second cycle was carried out with this material to assess the ciclability of the process. The results of the two cycles are compared in Table 4. The second activation leads to a lower amount of evolved oxygen, which may be expected considering that only a part of the ferrite activated in the first cycle was re-oxidised in the first hydrolysis step. The amount of hydrogen obtained in the second hydrolysis is consequently lower than that evolved in the first one, but with a similar (somewhat higher) yield with respect to water dissociation stoichiometry. Thus, the second cycle proceeds in a similar way compared to the first one. However, the low yields of the hydrolysis step would mean a long-term deactivation of the ferrite. Consequently, efforts must be dedicated to study the origin of this low yields and increase the hydrolysis effectiveness to obtain a fully functional material for the thermochemical cycle.
Table 4. Results of the first and second water splitting cycles with the Ni 0.25 Mn 0.75 Fe 2 O 4 material. Hydrolysis Cycle no. O2a H2a yield (%)b 1 0.46 0.06 7 2 0.22 0.04 9 a mmol / g ferrite b yield = (mol H 2 ) / (2 mol O 2 )
4
Conclusions
Metal-doped ferrites with formula M 0.25 Mn 0.75 Fe 2 O 4 (M: Mn, Co, Ni, Cu) have been studied for solar production of hydrogen from water via two-step thermochemical cycles. Thermogravimetric analyses under inert atmosphere indicate the capability of the materials to loose oxygen as the first (activation) step. Manganese ferrite displays the highest weight loss in TGA, with metal doping resulting in lower weight losses but also lower activation temperatures. The amount of oxygen released in the activation step closely matches the loss of weight observed in the thermobalance. The activated ferrites lead, however, to a hydrogen production lower than that expected from the water splitting stoichiometry. Among the studied materials, Ni 0.25 Mn 0.75 Fe 2 O 4 gives rise to the highest hydrogen evolution per gram of sample, being also the closest to stoichiometric H 2 production. XRD studies indicate that the Ni-containing ferrite is the only one composed uniquely by a spinel phase, while the other samples contain additional crystalline species. A solid state reaction occurs in the activation step with these additional phases, whereas the Ni 25 Mn 0.75 Fe 2 O 4 material maintains the spinel phase all over the thermochemical cycle. The second cycle with the Ni-containing sample leads to a similar yield in the H 2 generation reaction.
Acknowledgements This work has received financial support from the Comunidad de Madrid, through the PHISICO2 program. Thanks are indebted to Drs. A. J. Quejido, I. Rucandio, T. Hernández and P. Galán for helpful assistance and comments.
References and Notes 1. 2. 3. 4. 5. 6. 7. 8. 9.
Lattin, W.C., Utgikar, V.P., 2007, “Transition to hydrogen economy in the United States: A 2006 status report”, Int. J. Hydrogen Energy 32, pp. 3230-3237. Ewan, B.C.R., Allen, R.W.K., 2005, “A Figure of Merit Assessment of the Routes to Hydrogen”, Int. J. Hydrogen Energy, 30, pp. 809-819. Kodama, T., Gokon, N., 2007, “Thermochemical cycles for high-temperature solar hydrogen production”, Chem. Rev. 107, pp. 4048-4077. Perkins C., Weimer A.W., 2004, “Likely near-term solar-thermal water splitting technologies”, Int. J. Hydrogen Energy 29, 1587-1599. Abanades, S., Charvin, P., Flamant, G., Neveu, P., 2006, “Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy”, Energy 31, pp. 28052822. Nakamura, T., 1977, “Hydrogen production from water utilizing solar heat at high temperatures”, Sol. Energy 19, pp. 467-475. Steinfeld A., Sanders S., Palumbo R., 1999, “Design aspects of solar thermochemical engineering – a case study: two-step water-splitting cycle using the Fe 3 O 4 /FeO redox system”, Sol. Energy 65, pp. 43-53. Charvin P., Abanades S., Flamant G., Lemort F., 2007, “Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production”, Energy 32, pp. 1124-1133. Tamaura, Y., Steinfeld, A., Kuhn, P. and Ehrensberger, K., 1995, “Production of solar hydrogen by a novel, 2-step, water splitting thermochemical cycle”, Energy 20, pp. 325-330.
10. Tamaura, Y., Kojima, M., Sano, T., Ueda, Y., Hasegawa, N., Tsuji, M., 1998, “Thermodinamic evaluation of water splitting by a cation excessive (Ni, Mn) ferrite”, Int. J. Hydrogen Energy 23, pp. 1185-1191. 11. Ehrensberger, K., Frei, A., Kuhn, P., Oswald, H. R., Hug, P., 1995, “Comparative experimental investigations of the water splitting reaction with iron oxide Fe 1-y O and iron manganese oxides (Fe 1-x Mn x ) 1y O”, Solid State Ionics 78, pp. 151-160. 12. Agrafiotis, C., Roeb, M., Konstandopoulos, A.G., Nalbandian, L., Zaspalis, V. T., Sattler, C., Stobbe, P., Steele, A.M., 2005, “Solar water splitting for hydrogen production with monolithic reactors”, Sol. Energy 79, pp. 409-421. 13. Ishihara, H., Hasegawa, N., Aoki, H., Kaneko, H., Suzuki, A., Tamaura, Y., 2004, “Two-step water splitting for solar H 2 production with ZnII-MnII,III-FeIII spinel structure using concentrated solar heat”, Solid State Ionics 172, pp. 117-119. 14. Han, S.B., Kang, T.B., Joo, O.S., Jung, K.D., 2007, “Water splitting for hydrogen production with ferrites”, Sol. Energy 81, pp. 623-628. 15. Tamaura, Y, Hasegawa, N., Kojima, M., Ueda, Y., Amano, H., Tsuji, M., 1998, “Water splitting with the Mn(II)-ferrite – CaO – H 2 O system at 1273 K”, Energy 23, pp. 879-886. 16. Alvani, C., La Barbera, A., Ennas, G., Padella, F., Varsano, F., 2006, “Hydrogen production by using manganese ferrite: Evidences and benefits of a multi-step reaction mechanism”, Int. J. Hydrogen Energy 31, pp. 2217-2222. 17. Marcos, M.J., Hernández, T., Sánchez, M., Quejido, A.J., Romero, M., 2007, “Preparation of mixed iron oxides for solar hydrogen production by means of the Pechini technique”, Proc. ASME Energy Sustainability ES-2007 Conference, Long Beach, California. 18. Sánchez, M., Fernández, M., Marcos, M.J., Hernández, T., Quejido, A.J., Romero, M., 2007, “Thermal behaviour of Mn-Ni substituted ferrites for their application to solar hydrogen production”, Proc. 2nd International Congress of Energy and Environment Engineering and Management, Badajoz, Spain. 19. Sánchez, M., Hernández, T., Marcos, M.J., Fernández, M., Gómez, M.B., Quejido, A.J., Romero, M., 2007, “El papel de los metales de transición en espinleas de hierro y su aplicación a la generación de H 2 solar”, Proc. I Congreso Ibérico de Refrigeración e Hidrógeno con Energías Renovables REYER 2007, Burgos, Spain. 20. Pechini, M.P., 1967, U.S. Patent No. 3330697. 21. Kaneko, H., Yokoyama, T., Fuse, A., Ishihara, H., Hasegawa, N., Tamaura, Y, 2006, “Synthesis of new ferrite, Al–Cu ferrite, and its oxygen deficiency for solar H 2 generation from H 2 O”, Int. J. Hydrogen Energy 31, pp. 2256-2265. 22. Kodama, T., Gokon, N., Yamamoto, R., 2008, “Thermochemical two-step water splitting by ZrO 2 supported Ni x Fe 3-x O 4 for solar hydrogen production”, Sol. Energy 78, pp. 73-79. 23. Kodama, T., Kondoh, Y., Yamamoto, R., Andou, H., and Satoh, N., 2005, “Thermochemical Hydrogen production by a redox system of ZrO 2 -supported Co(II)-ferrite”, Sol. Energy 78, pp. 623-631.
