Investigation of Zr, Gd/Zr, and Pr/Zr – doped ceria for the redox splitting of water

Highlights

• H₂O splitting of reduced Zr, Gd/Zr, and Pr/Zr doped ceria is surface reaction limited.

• Pr and Gd addition induce non-redox active sites, detrimental to H₂ production.

• Surface H₂ formation is rate limiting, activation barriers > bulk O₂ diffusion.

• Steam oxidation is best described by a deceleratory power law model (F-model).

Abstract

There is a renewed interest in CeO₂ for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO₂ is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (Zr_xCe_1-xO₂, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M_0.1Zr_0.25Ce_0.65O₂, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O₂ cycle capacity and H₂ production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H₂ production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H₂O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO₂. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H₂ production, in agreement with experimental work. The calculated surface H₂ formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.

Introduction

One of the most difficult challenges facing the 21st century is building a sustainable energy economy to reduce our over-reliance on fossil fuel. Two-step solar thermochemical processes utilizing non-volatile metal oxide cycling are a promising method to capture and utilize solar energy to produce renewable hydrogen. The process can be carbon neutral and does not require a difficult or expensive separation process. When compared to other processes, such as biomass production and processing, artificial photosynthesis, and photovoltaic-driven electrolysis, two-step solar thermochemical processing can operate at higher thermal efficiencies and requires less land and water to operate [[1], [2], [3], [4], [5]].

A generic two-step cycle based on ceria is described by reactions (1) and (2). Heat from concentrating solar energy thermally reduces the metal oxide (CeO₂) to a sub-stoichiometric oxide (CeO_2-δ, where δ represents the extent of oxygen non-stoichiometry in the solid) at temperatures (T_H) between 1450 and 1550 °C, producing O₂. The sub-stoichiometric ceria is then taken off sun and oxidized by exposure to steam at some lower temperature (T_L, typically ≤ 1100 °C), thus producing H₂ and completing the cycle.

$$\frac{1}{\text{δ}}{\text{CeO}}_2\stackrel{+\Delta }{\to }\frac{1}{\text{δ}}{\text{CeO}}_{2-\text{δ}}+\frac{1}{2}{\text{O}}_2$$

$$\frac{1}{\text{δ}}{\text{CeO}}_{2-\text{δ}}+{\text{H}}_2\text{O }\stackrel{-\Delta }{\to }\frac{1}{\text{δ}}{\text{CeO}}_2+{\text{H}}_2$$

This same cycle can be used to perform carbon dioxide splitting to produce CO, which can be used as a feedstock for synthetic fuel production [2,4,5].

There are numerous types of metal oxide chemistries for two-step solar thermochemical cycling. Metal oxide of non-volatile ferrite is the prototypical cycle, in which a solid solution of ferrite spinels (M_xFe_3-xO₄ where M is typically Fe, Mn, Co, Ni) is redoxed [6,7]. While this chemistry also has the potential for large cycle capacity, sintering and formation of a molten phase often lead to irreversible deactivation. To improve cyclability and mechanical integrity, these ferrites are often combined with refractory materials such as ZrO₂, YSZ [[8], [9], [10]], or Al₂O₃ [11]. The mixing of ferrite spinels with Al₂O₃ led to the development of the “hercynite cycle” where hercynite (FeAl₂O₄) can be reduced at lower temperatures, but is hampered by slower kinetics for oxidation at the typical 1000 °C oxidation temperature [12], although faster oxidation has been demonstrated at 1350 °C, where the redox “hercynite cycle” has been operated isothermally [[12], [13], [14], [15]].

The final type of material for solar thermochemical processing is the non-stoichiometric oxide, such as perovskite [[16], [17], [18]] or ceria. The ceria is the prototypical oxide cycle, and has been extensively investigated [19,20]. It has fast redox kinetics [21] but requires high temperatures (T_H = 1450–1550 °C) to achieve the high extent of oxygen deficiency for efficient fuel production [22]. This non-stoichiometric ceria cycle is in contrast to the stoichiometric reduction of ceria to Ce₂O₃ that requires temperatures in excess of 2000 °C and is plagued with sublimation of the oxide [23]. Currently, cerium oxide is a favored material for the two-step solar thermochemical water splitting because of its high ion conductivity [[24], [25], [26]], rapid exchange kinetics [27], and excellent thermal stability [2].

In an effort to increase cycle capacity and reduce the thermal reduction temperature, substitution and doping of ceria are often employed to introduce lattice defects and create additional oxygen vacancies [[28], [29], [30], [31], [32], [33], [34], [35], [36]]. Taking a page from three way catalysts (TWCs) and heterogeneous catalysis [[37], [38], [39], [40]], Le Gal et al. were the first to examine the effects of incorporating Zr⁴⁺ into the lattice of ceria for two-step thermochemical applications. They have shown that Zr substitution can increase the extent of thermal reduction, leading to higher redox performance [[41], [42], [43]], by significantly decreasing reduction enthalpy [44,45]. In addition to zirconium, the introduction of reducible dopants such as praseodymium (Pr) [34] and trivalent cations such as gadolinium (Gd) has also been reported to increase oxygen storage capacity (OSC) and enhance reducibility of ceria based materials [31,[46], [47], [48]]. These authors have also studied the kinetics of water (WS) and carbon dioxide splitting (CDS) of Zr-substituted ceria. They reported that CDS of Zr_0.25Ce_0.75O₂ is limited by diffusion, with activation energies ranging from 83 to 103 kJ/mol depending on the synthesis method [42], and 51 kJ/mol for WS, where surface reaction followed by diffusion is attributed as the rate controlling mechanism [41]. The diffusion model only fits the TGA data collected at later points in the oxidation process, and morphological/crystallographical changes appear to occur in their materials, as evidenced by the cycle-to-cycle diminishing fuel production. It is critical to properly assess the effects of heat and mass transfer limitations when performing kinetics studies [49,50].

This study evaluates the thermochemical water splitting (WS) of binary (Zr_0.1Ce_0.9O₂, Zr_0.15Ce_0.85O₂, and Zr_0.25Ce_0.75O₂) and ternary oxides (Pr_0.1Zr_0.15Ce_0.75O₂, Pr_0.1Zr_0.25Ce_0.65O₂, and Gd_0.1Zr_0.25Ce_0.65O_1.95). In contrast to the work by Le Gal et al., we evaluate the redox capacity of thermally equilibrated materials that are compositionally and crystallographically stable [51]. In addition, we use quantum simulations to understand and explain the doping effects on both thermodynamics and kinetics.