H2 production by thermochemical water splitting with reticulated porous structures of ceria-based mixed oxide materials
Graphical abstract
Introduction
Hydrogen is postulated as a promising energy vector for a future low carbon energy economy [1,2]. However, the use of hydrogen as a clean energy vector must be linked to the development of production techniques based on renewable systems, non-dependent on fossil fuels and free of CO2 emissions. In this way, the availability of solar energy makes water splitting with concentrated solar energy one of the most direct and cleanest method to obtain hydrogen using renewable sources independently of fossil fuels [3]. However, direct thermolysis of water requires temperatures above 2000 °C for a reasonable degree of water dissociation (9% at 1 bar, 25% at 0.05 bar) [4,5], far from the actual temperatures achieved in modern solar thermal systems. Other problems associated to this technology are the huge theoretical radiation losses, proportional to the fourth power of absolute temperature, and the development of appropriate techniques for high-temperature hydrogen and oxygen separation to prevent their recombination and avoiding explosions when cooling the gaseous mixtures [6]. In this way, solar driven thermochemical cycles for water splitting are a promising alternative for hydrogen production, combining lower and technically more manageable temperatures with the production of hydrogen and oxygen in separate steps [5,7].
The simplest thermochemical cycles that decompose water into oxygen and hydrogen follow a two-step process in which a metal oxide is thermally reduced at high temperature, releasing oxygen (R1), and the reduced oxide is finally oxidized with water steam at lower temperatures, producing hydrogen (R2). The overall reaction is the splitting of water in H2 and O2 (1 mol of H2 and ½ mol of O2 per mol of water) [[8], [9], [10]].
There are a large number of two-steps thermochemical cycles reported in literature although only a few of them have been selected for solar hydrogen production, according to different criteria, such as cost, degree of development, environmental risk or energy efficiency [5,[11], [12], [13], [14], [15], [16], [17], [18]]. The metal oxide in R1 can be totally (δ = y) or partially (δ < y) reduced to the metal or a lower metal-valence oxide, respectively depending on the extent of reaction at the thermal reduction temperature in which the reaction was carried out. In any case, redox pairs with high O2 and H2 yields or low reduction temperatures results in high H2/O2 product yields per mass of initial material, as the oxygen released is completely restored with the water steam [15,19].
Over the last decade, ceria (CeO2) has received considerable attention as a redox pair (CeO2/Ce3O4) for two-step water-splitting hydrogen production systems [20,21]. This thermochemical cycle was tested by Abanades and Flamant for the first time in 2006 [22], being the high reactivity of Ce (III) species during the oxidation with water one of the main advantages of this cycle. The temperature required by the complete thermal reduction of CeO2 is close to 2000 °C, with the problems previously mentioned for direct water splitting, and the other potential problems such as partial sublimation and sintering of ceria particles [22]. However, ceria can be partially reduced following reaction R3 at 1500 °C, which is lower than the extreme temperatures necessary for stoichiometric reduction [23]. The partially reduced material can reacts with steam (R4) to produce hydrogen at temperatures between 800 °C and 1100 °C [24,25].
In these redox cycles, the fluorite structure of the cerium oxide phase maintains its structure in a wide range of oxygen stoichiometry (δ) [26]. The oxygen non-stoichiometry and defect chemistry of non-stoichiometric ceria at elevated temperatures lead to the formation of oxygen vacancies that promotes the mobility of the oxygen atoms into the ceria structure, enhancing reduction/oxidation kinetics and stability over many consecutive cycles, as compared to other metal oxides redox pairs [27,28]. Nevertheless, the still high temperatures of the reduction step, around 1500 °C [20], and the low oxygen storage capacity as compared to other redox materials are still a serious drawback [28].
Alternatively, the thermodynamic and kinetic properties of ceria can be altered by doping its fluorite structure with transition metals [28,29] in order to increase the extend of the reduction reaction at lower temperatures. In recent studies with modified-ceria thermochemical cycles, metallic species such as Mn, Fe, Co, Ni, or Cu have been evaluated [19,30,31]. Interestingly, the results suggest that increasing proportions of dopant (>10 mol%) in the ceria structure promotes an enhancement in the oxygen release rate, but accompanied with a decrease in the hydrogen production rate [19]. Consequently, H2/O2 production ratios does not correspond to the stoichiometric decomposition of water into both products, especially for dopants different than Fe and Mn. This fact compromises the long term stability of the material through the subsequent cycles. Nevertheless, efficiency and cyclability of the process are highly dependent on the reduction and oxidation temperatures, being usually found values of 1400–1500 °C and 800–1150 °C for them, respectively [19].
At the same time that basic research is looking for appropriate redox pairs for thermochemical water splitting, the development of reactors capable of perform the process using concentrated solar energy as renewable energy source should be tackled. Several solar reactor concepts have been proposed in order to reach the high temperatures required specially for the reduction step, mainly based on direct irradiation by high fluxes of concentrated solar radiation in the reactor cavity [32]: cavity receivers with rotating or stationary structures [33], glass dome reactors [22], aerosol flow reactors [34] or fluidized bed reactors [35]. The solar cavity reactor is the simplest configuration that allows solar irradiation penetrate into an insulated reaction space, performing the reduction/oxidation steps in a single chamber by switching the atmosphere. Numerous metal oxides structures and supports have been evaluated for solar thermochemical applications in solar reactors [24,32,[34], [35], [36]]. Among them, macroporous structures coated with metallic oxides allow a better penetration and volumetric absorption of solar radiation. However, they suffer problems associated to low loadings of active material or inappropriate mechanical strength. Alternatively, researchers from ETH (Zurich) have developed and demonstrated a solar cavity-receiver containing a reticulated porous ceramic (RPC) sponge structure made of pure CeO2 for solar driven thermochemical cycles [27,37,38]. On one hand, the cavity-receiver configuration allows reaching the temperatures required for the thermochemical water splitting. On the other hand, the ceria-made RPC structure is a completely active material, and consequently combines the advantages of better volumetric radiative absorption, rapid reaction rates, and high mass loading of active material (100 wt%).
This work focuses on the synthesis and evaluation of RPC of Ce0.9Me0.1Oy-based materials for hydrogen production by thermochemical water splitting. In a first step, powder materials (Ce0.9Me0.1Oy, Me = Mn, Fe, Co and Zr) were prepared following a co-precipitation method more simple than other methods reported in literature [19,30]. The mixed oxide with superior performance in terms of H2/O2 productivity and cyclability for continuous production of O2 and H2, was chosen for the development of a RPC following a modification of the replica sponge method proposed for pure ceria [27]. Activity and cyclability of this RPC sponge structure for H2 production were evaluated and compared to the values obtained with the powder Ce0.9Me0.1Oy. Although application of ceria-based materials for thermochemical water splitting has been widely studied in literature, the conformation of these materials as RPC to be used in cavity-receiver reactors has been not proposed yet except for pure ceria [25,27,37,38]. However, obtaining structured materials similar than those RPC obtained with pure ceria, but with significantly lower temperature requirements definitively represents a promising alternative for a future full scale application of this technology for hydrogen production.
Section snippets
Materials
Ce(NO3)3·6H2O (CAS 10294-41-4, purity 99%), Co(NO3)2·6H2O (CAS 10026-22-9, purity ≥ 98%), Mn(NO3)2·4H2O (CAS 20694-39-7, purity ≥ 97%), Fe(NO3)3·9H2O (CAS 7782-61-8, purity ≥ 98%) and ZrO(NO3)2·H2O (CAS 14985-18-3, purity ≥ 99.5%) were purchased from Sigma Aldrich, NH4OH was provided by Fisher (CAS 1335-21-6, purity 35%). Commercial Dolapix CE 64, Optapix PA 4G and Contraspum KWE and polyurethane foams (PUS) used for manufacturing the RPCs were kindly provided by Zschimmer & Schwarz España,
Physicochemical characterization of the Ce-based mixed oxide materials
Table 1 shows the metallic molar composition of the Ce0.9Me0.1Oy samples (Me = Mn, Fe, Co and Zr) prepared by co-precipitation of metallic salts, determined by ICP-AES. As it can be seen, the experimental chemical compositions are close to the theoretical ones. Considering the synthesis method [41,42], the slight differences between the compositions of the materials are due to the different precipitation degree of the metallic species during the process.
Fig. 2 shows the XRD patterns of the
Conclusions
Ceria-Me materials type Ce0.9Me0.1Oy (Me = Fe, Co, Mn, and Zr) have been prepared by co-precipitation and studied for their application in hydrogen production by thermochemical water splitting. Co-precipitation method allows the formation of solid solutions which results in homogeneous materials with lower temperature reduction requirements than the one needed by the pure metal oxides. Among all the materials, the Ce0.9Fe0.1Oy powder oxide shows the best results regarding both the hydrogen
Acknowledgements
The authors thank to “Comunidad de Madrid” and European Structural Funds for their financial support to ALCCONES project (S2013/MAE-2985) and ACES2030-CM project (S2018/EMT-4319). The authors also thanks to Zschimmer & Schwarz España, S.A. for supplying the necessary reagents (Dolapix CE 64, Optapix PA 4G and Contraspum KWE) to make the RPCs.
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