Hydrogen production from water-splitting reaction based on RE-doped ceria–zirconia solid-solutions
Graphical abstract
Highlights
► Effect of RE in CZRE solid-solutions on OSC and hydrogen production activity was investigated. ► Raman analysis showed enhanced oxygen mobility for CZPr sample than others. ► Homogenous distribution of Ce/Zr, enriched Pr on surface and high Ce3+ concentration for CZPr. ► CZPr sample showed high OSC and hydrogen production activity.
Introduction
Hydrogen (H2), the promising energy carrier, is an environmentally attractive and sustainable transportation fuel and has the potential to displace the fossil fuels. In recent years, high-temperature nuclear reactors are being used successfully for H2-production from water with substantially increased efficiency and without consuming fossil fuels, green house gas emissions and other forms of air pollution. Water-splitting reaction for H2-production can be done by high-temperature electrolysis and thermo-chemical processes at high temperatures (>850 °C) in order to achieve competitive efficiencies. High-temperature electrolysis can be done using a Solid oxide electrolysis cell (SOEC) which is a reversely operated Solid oxide fuel cell (SOFC) [1].
A typical SOEC consists of an oxygen ion conducting solid electrolyte sandwiched between H2O-H2-electrode (which is a cathode in SOEC) and O2-electrode (which is an anode in SOEC). In SOECs, it is typically noticed that the degradation rate is much greater than SOFCs. Delamination of the O2-electrode (due to micro-structural changes in bond layer, chromium poisoning and dissociation of bond layer), loss of electrical/ionic conductivity of electrolyte and the adsorption of impurities on the H2-electrode are the main reasons for the degradation of SOECs [1], [2], [3], [4]. Virkar et al. [2] predicted a model for the condition where delamination of the O2-electrode could be avoided and was successful in qualitative comparison for some reports [1], [3], [5], [6]. Hauch et al. [5] demonstrated that the origin of degradation at H2-electrode was due to the segregation of impurities (Si and Al) on the electrode from the sealant due to high partial pressure of steam and this segregation was successfully avoided by using a gold sealant instead of normally used albite glass sealant. From the above observations it can be forecasted that the degradation in SOECs can be minimized or avoided by developing new material systems for O2-electrode, using proper gold sealants and proper SOEC operating conditions. Development of H2-electrode materials is fundamentally very important in order to enhance the H2-production and thereby further decrease the H2-production costs from SOECs than compared to the H2-production costs from other fuel cell systems. Bae et al. [7] reported that the gadolinium doped ceria (GDC) impregnated H2-electrode showed an enhanced performance which was correlated to the oxygen storage capacity (OSC).
Among the ceria–zirconia (CZ) solid-solutions, ceria-rich oxides show high oxygen storage capacity (OSC), reduction properties and phase stability. Along with these properties, the mixed ionic and electronic conductivity of these materials makes them suitable candidates as a catalyst and/or support for SOEC applications [8]. Doping rare earth (RE) with CZ solid-solutions shows further improvements in OSC, redox property, and thermal resistance compared to CZ solid-solutions itself. In this work, the glycine–nitrate process (GNP) has been used to synthesis CZRE (RE = Tb, Pr and La) samples since it is the most suitable synthesis method for producing fairly fine, homogeneous, and complex compositional metal oxide powders [9].
In the present study, the influence of RE dopants on the OSC property that affects the H2-production from the water-splitting reaction has been investigated. The role of RE dopant on the homogeneous distribution of Ce/Zr and surface enrichment of RE in CZRE solid-solutions and its influence in the improvement of the oxygen vacancies was also studied. Characterization of the samples was performed using X-ray diffraction (XRD), Raman spectroscopy (RS), BET surface area, X-ray photoelectron spectroscopy (XPS). The catalytic performance was evaluated for OSC and H2-production from water-splitting reaction.
Section snippets
Experimental
The Ce0.65Zr0.25RE0.1O2 − δ (RE = Pr, Tb and La) powders were successfully prepared by GNP and the synthesis procedure was reported elsewhere [10], [11]. The XRD patterns were obtained by an X-ray generator (Phillips PW 3830) using Ni-filtered Cu Kα radiation. Raman spectra were measured with a Raman spectrometer (BRUKER RFS 100/S FT-Raman Spectrometer). The excitation source was Nd-YAG laser (λ = 1064 nm) and the laser power was 20 mW at the sample point. The calibration for Raman spectroscopy
XRD analysis
The X-ray diffraction patterns of reduced CZRE samples were obtained in order to find out the phase stability and the results were illustrated in Fig.1. For comparison, pure ceria is also included. From Fig. 1(a), single crystalline phases with cubic fluorite structure were noticed for all the samples from the XRD patterns [14]. Fig. 1(b) shows an interesting observation from XRD measurements and was that the XRD peaks of CZ sample were shifted to higher 2θ values with respect to pure ceria and
Conclusion
The effect of rare earth (RE = Tb, Pr and La) dopant on the catalytic performance of CZRE solid-solutions for oxygen storage capacity and hydrogen production has been successfully investigated. The solid-solutions were prepared by glycine–nitrate process. The sustainability of the solid-solutions even after the reduction was confirmed by XRD showing a single crystalline phase with cubic fluorite structure. From Raman spectroscopy measurements, with an excitation laser of 1064 nm, the order of
Acknowledgments
This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and Institutional Research Program of Korea Institute of Science and Technology (KIST) (2E22802). One of the authors (D.H.P.) acknowledges KIST for the award of a STAR Post-Doc Fellowship.
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