Ceramic carbon electrode-based anodes for use in the Cu-Cl thermochemical cycle☆
Introduction
A thermochemical cycle is a process by which water is decomposed into hydrogen and oxygen through a series of chemical reactions. The chemicals that are used in these reactions are regenerated and recycled during the process. After the principle of the thermochemical cycle was discovered by Funk and co-workers [1], the Gas Research Institute (GRI) funded a long term program to evaluate 200 thermochemical cycles during mid 1980's [2]. They have evaluated these cycles for their kinetics, thermodynamics, material stability and cost analysis and found that 80 cycles were most promising and 15 were feasible [2], [3], [4]. However, all cycles have faced different challenges and are yet truly to progress beyond the laboratory scale.
The Cu–Cl thermochemical cycle has several advantages such as a low operation temperature and a lower demand on the materials of construction compared to other cycles. The reactions in the Cu–Cl thermochemical cycle are shown in the Table 1. There have been several recent studies of the efficiency and economic viability of the cycle as a whole [5], [6], [7], [8]. In addition, there have been several investigation of related to specific steps in the thermochemical cycle, including the electrochemical process [5], thermal decomposition of CuO ∗ CuCl2 [9], and engineering design aspects of drying of CuCl2 (aq) [10] and multiphase reactors scale-up [11]. The Cu–Cl cycle has been the primary focus of Canadian researchers due to the fact that it has an operating temperature of 400–550 °C which allows for it to be linked to Generation IV SCWR (Super-Critical Water Cooled Reactor). The latest advances by the Canadian research team have been reported in a recent review article [12].
One of the main challenges of this process is to achieve high efficiency during the electrolysis of CuCl [13]. The aqueous feed of CuCl must contain chloride ion at a concentration greater than 1 M in order to prevent the precipitation of CuCl(s). In addition, when the chloride ion concentration is greater that 1 M, Cu(I) can form several anionic species (e.g. CuCl2−, CuCl32−) [14]. Thus, the cell reactions can be expressed as follows [15]:
Alternatively, there is a modified version of this cycle where H2(g) is produced electrochemically at the cathode (instead of solid copper) [16]. Because of this, our initial investigations have focused on the electrochemical kinetics at the anode since that reaction is common to both variations.
The choice of electrode material can greatly impact the electrochemical kinetics. Our previous work has shown that the reaction rate for Cu(I) oxidation is approximately three times faster on a Pt electrode compared to a carbon surface [12], [17]. However, carbon black materials have substantially larger specific surface areas which more than compensate for their lower intrinsic activities. This coupled with its low cost, makes carbon the ideal anode electrode materials. Thus, our work has focused on designing electrodes structure with a high surface area and high anion conductivity.
Ceramic carbon electrodes (CCE) possess various desirable electrode properties, including a wide operating potential, high surface area, and high chemical and thermal stability. CCE are prepared by mixing electronically conducting carbon particles with an organosilane monomer that is subsequently polymerized via the sol-gel reaction [18], [19], [20]. CCEs have been reported in the literature by numerous research groups in a wide variety of applications, including fuel cells, lithium ion batteries and supercapacitor applications [20]. For example, Anderson et al., have also reported the use of CCE-based electrodes for direct methanol fuel cells [21]. Eastcott and Easton have recently reported the synthesis and electrochemical studies of CCE materials for H2/air fuel cells [22].
Here we report the fabrication and characterization of CCE materials prepared from Vulcan XC72 carbon black and 3-aminopropyl trimethoxy silane (APTMS). APTMS is polymerized in presence of water to form poly aminopropyl siloxane (PAPS), whose structure is shown in Fig. 1. Under acidic conditions, the resulting PAPS polymer will be protonated and thereby enhance the transport of anionic species. One key advantage of this method is that the ionomer in added in monomer form and subsequently polymerized in the presence of the carbon. Hence the ionomer is evenly distributed throughout the porous catalyst materials. Anode performance has been studied as a function of siloxane loading within the CCE layer.
Section snippets
CCE preparation
CCE materials were prepared by using a base-catalyzed sol-gel method [19], [20]. 400 mg of Vulcan XC72 carbon black (Cabot Corp.) was mixed with the 13.5 mL methanol (Fisher), 2.3 mL of deionized water. 5.2 μL of 6 M ammonium hydroxide was added to the mixture and stirred for 10 min. 0.5–1 mL of APTMS (Aldrich, 97%) was added drop wise using a syringe. The amount APTMS was varied in order to prepare CCEs with varying PAPS loadings. The reactant mixture was then stirred overnight in a covered beaker.
Materials characterization
Figure 3 shows the TGA curves obtained under flowing air for CCE layers with various PAPS loadings. All CCE materials showed a water loss between 80 and 200 °C and underwent combustion between 250–800 °C. Figure 3(b) clearly shows that the combustion process occurs in two distinct steps. The first step began around 250 °C and was attributed to the combustion of the aminopropyl side chain (RNH2). Above 600 °C, all the carbon in CCE material is combusted to CO2. The residue obtained at the end of the
Conclusions
PAPS-based CCE electrodes have been prepared and investigated for use as anode materials in the CuCl electrolysis step of the Cu–Cl thermochemical cycle. Electrochemical results show that these CCE's outperform bare CFP based electrodes. This increased performance was attributed to the higher electrochemically active surface area and the enhanced transport of anionic Cu(I) species within the 3-dimensional electrode layer. Characterization of these CCE materials showed that they have good
Acknowledgements
This work was supported by the Ontario Research Fund (ORF), Atomic Energy Canada Ltd. (AECL), the Natural Sciences and Engineering Research Council (NSERC) of Canada and UOIT.
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Submitted to International Journal of Hydrogen Energy for the Special Issue with select papers from the International Conference on Hydrogen Production Revised Manuscript August 2009