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Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle

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Science  02 Aug 2013:
Vol. 341, Issue 6145, pp. 540-542
DOI: 10.1126/science.1239454

Isothermal Water Splitting

Solar concentrators can create extremely high temperatures that can drive chemical reactions, including the thermal splitting of water to provide hydrogen. A metal oxide catalyst is needed that is usually cycled between hotter conditions where it is reduced and cooler conditions where it is reoxidized by water. This cycling can limit catalyst lifetime, which can be costly. Muhich et al. (p. 540; see the Perspective by Roeb and Sattler) developed an approach that allowed the redox cycle to be driven isothermally, using pressure swings.

Abstract

Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS’s overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the “hercynite cycle” exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Hydrogen is an attractive fuel because it produces only water when burned and can be used in highly efficient fuel cells (1, 2), but it must be derived from some other chemical source, preferably a renewable one such as water (3). Splitting H2O into H2 and O2 can be achieved by direct thermolysis (46)—for example, by solar thermal water splitting (STWS), which stores solar energy as H2 at high theoretical maximum efficiencies (7, 8). Unfortunately, direct thermolysis requires temperatures exceeding 2700°C that are impractical for industrial processes (9). However, two-step thermochemical H2O splitting based on metal oxide reduction and oxidation (redox) cycles produces appreciable amounts of H2 at the more technically feasible reduction temperatures Tred of 1200° to 1500°C (4, 10).

In traditional two-step temperature-swing water splitting (TSWS), O2 is generated by the reduction of a metal oxide during a high-temperature step according toEmbedded Image (1)where Tred ≈ 1200° to 1500°C. The second step involves lowering the temperature and exposing the reduced metal oxide to water, which reoxidizes the metal oxide and produces H2 according toEmbedded Image (2)where oxidation temperature Tox < Tred. One major advantage of this approach is that although the complete cycle produces H2 and O2 from H2O, they are produced in two separate steps, thus simplifying separation. Traditional thermodynamic analysis treats the process as a closed system and suggests that a substantial temperature difference between the oxidation and reduction steps is necessary to split water (1013). However, large temperature swings between the reduction and oxidation steps cause thermodynamic inefficiencies from the irreversible heat losses incurred upon cooling of the active material to the oxidation temperature and the heat required solely to reheat the active material to its reduction temperature. Additionally, thermal stresses arising from the rapid thermal cycling of the system over large temperature differences present engineering and materials challenges in the design of high-temperature redox systems. However, contrary to widely accepted redox gas-splitting theory (11), we demonstrate that a change in temperature between reduction and oxidation steps is unnecessary, and that isothermal water splitting (ITWS) driven by swings in steam partial pressure not only produces H2, but outperforms traditional TSWS and provides an opportunity for expanding renewable H2 generation.

We used the “hercynite cycle” to examine TSWS and ITWS. Unlike more conventional nonvolatile metal oxide redox chemistries, in which reduction results in either O vacancy formation (e.g., cycles involving fluorite- or perovskite-type crystal structures such as CeO2) (14) or the formation of solid solutions (e.g., ferrites) (1518), reduction occurs via reaction between MFe2O4 and Al2O3 to form stable aluminates (19, 20). During oxidation by H2O, the M-ferrite spinel (MFe2O4) and alumina (Al2O3) reform, liberating H2. For cobalt, the governing reactions areEmbedded Image (3)Embedded Image (4)Reduction via reaction 3 begins at temperatures as low as 940°C, ~150°C below where ferrites and ceria start to reduce because the formation of the stable aluminates is more thermodynamically favorable than the formation of solid solutions or vacancies. Hence, the hercynite cycle is selected to demonstrate ITWS.

We used a hercynite cycle–active material composed of 19.8 weight percent CoFe2O4 on Al2O3 (21, 22) in a stagnation flow reactor (23) to establish the baseline for comparison of TSWS to ITWS, as outlined in the supplementary materials and shown in fig. S1. In TSWS, the active materials are reduced (reaction 3) under inert gas flow (He) at 1350°C and 101.3 kPa for 60 min, then cooled to 1000°C for water reoxidation (reaction 4). A gas stream of 50 volume percent (vol%) steam in inert He was used to oxidize the reduced materials for 25 min, producing 31.4 ± 2.3 μmol of H2 per gram of total material (Table 1). The extent of reduction and water-splitting capacity depends on the reduction temperature. Higher temperatures lead to a larger fraction of reduced Fe2+ ions. The number of available Fe2+ ions dictates the total H2-generating capacity of the active material, with a maximum ratio of one H2 molecule produced to every two Fe2+ cations. In our experiment, oxidation at 1000°C was characterized by a slow rate of reaction (a peak H2 generation rate of 0.06 ± 0.04 μmol g–1 s–1) and was likely surface reaction–limited (20). Because the oxidation reaction is slow, it is unlikely that most of the Fe2+ ions are reoxidized to Fe3+ during the 25 min, which limits the total H2 production capacity of the 1350°/1000°C TSWS cycle.

Table 1 H2 production capacities and peak rates.

A comparison of the solar thermal water splitting cycle capabilities is shown for both temperature swing and isothermal operation.

View this table:

Higher TSWS reduction and oxidation temperatures result in increased H2 and O2 production capacity and water-splitting rates. Active material cycled between 1500° and 1200°C produced 93.7 ± 19.2 μmol H2 g–1 with a peak rate of 0.32 μmol H2 g–1 s–1 (Table 1). The higher reduction temperature resulted in more complete reduction of the active material and a larger thermodynamic driving force for H2O splitting. The higher rates of H2 generation at 1500°/1200°C resulted from two factors: (i) the intrinsically higher reaction rates that arise from carrying out a kinetically limited process at a higher temperature, and (ii) the larger thermodynamic driving force for oxidation, which comes from the larger extent of reduction. The ability to perform water oxidation at temperatures above the onset of reduction suggests that ITWS is possible (24) and that the current understanding of the thermodynamics of STWS is incomplete.

Hercynite cycle–active materials split water in a two-step isothermal redox cycle, as exemplified by the 1350°C ITWS results shown in Fig. 1. H2 generation under ITWS conditions was achieved at a temperature above the minimum reduction temperature by first flowing an inert gas through the reactor to sweep away O2 generated during the reduction step and subsequently injecting steam during the oxidation step to produce H2 (fig. S3). For 1350°C ITWS, the active particles produced 45.1 ± 7.6 μmol O2 g–1 during 1 hour of reduction and 102 ± 18 μmol H2 g–1 during 25 min of exposure to 50 vol% steam in He. During reduction, the initial O2 plateaus arise from incomplete H2O removal from the system. Once all steam is swept from the reactor, the O2 peak occurs as the materials reduce, followed by the normal exponential decay as the reaction goes to completion. 1350°C ITWS produces substantially more O2 and H2 than 1350°/1000°C TSWS and slightly more O2 and H2 than 1500°/1200°C TSWS. The slight deviation of the 2.26:1 H2/O2 ratio from the expected 2:1 ratio, although within error, likely arises from slight differences in the sensitivities and response times of the O2 analyzer and of the mass spectrometer used for measuring H2. The higher gas-splitting production capacity of ITWS was accompanied by a higher H2 production rate of 0.55 ± 0.16 μmol H2 g–1 s–1. ITWS’s higher rate of H2 generation and its lack of irreversible heat losses associated with the change in temperature between oxidation and reduction steps (which is required for TSWS) results in ITWS having a higher theoretical efficiency than TSWS, with the conditions examined in this study (see the supplementary materials for a comparison of ITWS and TSWS efficiencies, in particular table S1).

Fig. 1 Isothermal water splitting at 1350°C.

The H2 and O2 generation rates are shown in red and blue, respectively.

Traditional TSWS thermodynamic theory uses a closed-system model in which an oxidation temperature lower than the reduction temperature raises the chemical potential of the oxidizing gas and reduced metal oxide above that of the product gases and oxidized material in order to drive the oxidation reaction. In contrast, ITWS in an open system uses swings in the partial pressure of the oxidizing gas to produce the chemical potential differences that drive the oxidation and reduction steps. To form a two-step water-splitting closed-cycle system, a process for the recombination of H2 and O2 must be included, such as an electrolyzer (fig. S4) (5). In the ITWS process, the partial pressure of generated gaseous reduction and oxidation products remains low by sweeping them from the reactor. The low chemical potential of O2 during reduction and high H2O and low H2 chemical potentials during oxidation drive the respective reactions forward to enable ITWS. Furthermore, sweeping the product gases away from the metal oxide prevents reverse reactions from occurring.

Not only are redox reactors open systems, but the reactions take place between gases adsorbed on the surface and the solid phase. Thus, the chemical potential and the coverage of gas molecules adsorbed on the surface are the relevant quantities to consider for analyzing redox reaction thermodynamics. Because the coverage of adsorbates, and therefore their chemical potentials, are related to partial pressure, the oxidation reaction is driven by increasing the H2O partial pressure and maintaining a low H2 partial pressure. Therefore, ITWS relies on a chemical potential difference derived from readily adjustable gas-phase partial pressures, rather than having to depend on the problematic large temperature changes of TSWS.

This analysis suggests that by increasing the partial pressure of the oxidizing gas, we can drive the water-splitting reaction further toward the products. Indeed, an increased partial pressure of water resulted in higher H2 production capacities at higher rates (Fig. 2). For 1350°C ITWS, steam concentrations in He of 33%, 43%, and 50% (overall pressure held at 101.3 kPa) correspond to H2 production capacities of 40 ± 9, 72 ± 8, and 102 ± 18 μmol H2 g–1 and peak production rates of 0.06 ± 0.02, 0.15 ± 0.07, and 0.55 ± 0.16 μmol H2 g–1 s–1, respectively. The increased peak production rates correspond to shorter oxidation times, as expected. The higher steam pressure results in a higher water chemical potential on the active material’s surface, which provides a higher thermodynamic driving force for oxidation. Additionally, the higher steam concentrations increase the overall rate of reaction by increasing the reactant concentration. This result suggests that decreases in the time required for the oxidation step and possible increases in the ITWS temperature can be achieved by increasing the partial pressure of the oxidizing gas. Furthermore, relative to the 1350°/1000°C ceria redox cycle (which is considered the current state-of-the-art material for TSWS redox processing), the ITWS hercynite cycle produces >6 times as much H2 on a total-mass basis and >12 times as much H2 on an active-materials basis (47% active) (Fig. 3 and Table 1). In addition to ITWS’s favorable kinetics and thermodynamics (Fig. 3 and table S1), ITWS reduces both irreversible heat losses and thermal shock concerns that limit the efficiency and operations of traditional TSWS.

Fig. 2 The effect of steam pressure on 1350°C ITWS.

From left to right: H2O partial pressures of 33.7 kPa, 43.4 kPa, and 50.6 kPa.

Fig. 3 A comparison of STWS.

H2 production curves are shown for hercynite cycle–based 1350°/1000°C TSWS (solid line, shown at twice its experimentally found value), 1500°/1200°C TSWS (long dashed line), 1350°C ITWS (short dashed line), and ceria-based 1350°/1000°C TSWS (dotted line).

Supplementary Materials

www.sciencemag.org/cgi/content/full/341/6145/540/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S4

Table S1

Reference (25)

References and Notes

  1. Acknowledgments: Supported by NSF grant CBET 0966201 and by the U.S. Department of Energy Fuel Cell Technologies Office through the Solar Thermochemical Hydrogen (STCH) directive. We thank V. Aston, A. Sagastegui, and C. Herradón. Data used in this study will be made freely available upon request.
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