Effect of strontium and zirconium doped barium cerate on the performance of proton ceramic electrolyser cell for syngas production from carbon dioxide and steam
Introduction
Significant amount of carbon dioxide (CO2) has been released from processes involving energy conversion. The emission enhances the green-house effect and contributes to global climate change which could endanger environment and mankind. Consequently, CO2 utilization as a carbon source for the production of fuels and chemicals has gained much interest. Proton ceramic electrolyser cells (PCECs) can convert CO2 and steam into syngas (hydrogen (H2) and carbon monoxide (CO)) which is known as important precursor for industrial chemicals such as higher olefin and alcohols [1], [2], [3], [4], [5]. With CO2 and H2O as a feed, H2 is produced through steam electrolysis reaction (Eqs. (1), (2), (3)). Hydrogen then further reacts with CO2 to form CO. The reverse water gas shift (RWGS) reaction provides syngas (H2 and CO) as a product through Eq. (4).
Electrochemical reaction:
Thermochemical reaction:
The electrolyte of an electrolyser can be oxygen-ion (O2−) conductor or proton (H+) conductor. In theory, proton exhibits much smaller ionic radius comparing to oxygen-ion. Therefore, the mobility is relatively higher, leading to higher conductivity and the cell performance [6], [7], [8], [9]. Proton-conducting electrolytes also permit a significant decrease in operating temperature (400–700 °C), resulting in increasing system robustness and reducing system cost [10], [11].
However, the materials which exhibit high proton conductivity often suffer from poor chemical stability when being exposed to CO2-containing gas. Barium cerate (BaCeO3) is known to exhibit high proton conductivity but low chemical stability against CO2 [9], [12], [13], [14], [15], [16], [17], [18], [19] while barium zirconate (BaZrO3) shows relatively higher chemical stability but lower proton conductivity [14], [15], [20], [21], [22]. Therefore, zirconium doped barium cerate (BCZ) has been used as an electrolyte and composites with metal as an electrode [23], [24], [25], [26]. Doping trivalent such as Y3+, Yb3+, Nd3+ and Gd3+ into BCZ to achieve higher conductivity was also reported [8], [11], [13], [14], [16], [17], [20], [21], [25], [29]. Strontium cerate (SrCeO3) exhibits conductivity and chemical stability against CO2 and the performance lies between BaCeO3 and BaZrO3 [8], [22], [29]. Therefore, strontium doped barium cerate (BSC) and the BSC with trivalent doping have also been investigated [8], [15], [18], [22], [27], [28], [29].
The objective of this study is to optimize both proton conductivity and chemical stability by using a combination of the BaCeO3 (exhibiting high proton conductivity), SrCeO3, and BaZrO3 (exhibiting high chemical stability). The perovskite compositions of strontium doped barium cerate (Ba0.6Sr0.4CeO3-α, BSC), and zirconium doped barium cerate (BaCe0.6Zr0.4O3-α, BCZ) have been synthesized. To achieve higher conductivity, Y3+ was also doped into SCB structure (Ba0.6Sr0.4Ce0.9Y0.1O3-α, BSCY). The electrochemical performance and catalytic activity of the samples were investigated. The catalytic activity of electrode and electrolyte materials toward RWGS reaction was discussed. The chemical stability of prepared samples when being exposed to CO2 was also measured.
Section snippets
Sample preparation
The BC, BSC, BSCY, and BCZ were prepared using solid state reaction method. Stoichiometric amounts of BaCO3 (In house), CeO2 (99.95%, Sigma Aldrich), SrCO3 (99.9%, Sigma Aldrich), ZrO2 (99.0%, Kanto Chemical Company), Y2O3 (99.99%, Sigma Aldrich), were used as precursors. The precursors were mixed by ball milling for 24 h using zirconia balls having 5 mm and 10 mm diameter and ethanol (95%, Sigma Aldrich) as a medium, followed by drying in oven at 105 °C for 24 h and calcination at 1100 °C for
Characterization
The XRD patterns of all prepared electrolyte poweders were shown in Fig. 1. All samples exhibited perovskite structure (JCPDS 82-2425). There was no impurity phase detected and 100% perovskite was obtained in all prepared powder. The crystallite sizes calculated from XRD patterns are also presented in Table 1. They were 37.05, 28.46, 23.65 and 17.46 nm for BC, BSC, BSCY, and BCZ, respectively. The crystallite sizes of the BCZ and the BSC are smaller than that of the BC due to a Zr4+
Conclusion
The BC, BSC, BSCY, and BCZ were successfully synthesized using solid state reaction method with different calcination conditions. The effect of doping Zr4+, Sr3+, and Y3+ in barium cerate structure was investigated in term of electrochemical performance and catalytic activity. Activation energy of conduction of barium cerate decreased when doping with Zr4+ and Y3+. The BSCY exhibited the lowest activation energy of conduction at 64.69 kJ mol−1. Although the BCZ sample presented relatively low
Acknowledgements
The acknowledgement is made to Thailand Research Fund (TRF), grant number: RSA5880040 and RTA5980006. The acknowledgement is also made to NSTDA University Industry Research Collaboration (NUI-RC) for supporting J. Sarabut.
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