Salt Adsorption Capacity of Hydrous and Anhydrous Ruthenium Oxides for Capacitive Deionization

Abstract

Capacitive deionization (CDI) is a promising technology for desalination of seawater and recovery of mineral resources. By applying a voltage to a pair of electrodes within the potential of water electrolysis, ions such as Li+ and Na+ can be recovered by electrosorption. In order to increase the salt adsorption capacity (SAC) of the electrodes, various metal oxides have been proposed.1 For example, manganese oxide, titanium oxide and ruthenium oxide show higher SAC than activated carbon, owing to the contribution from both electric double-layer capacitance and pseudocapacitance. In particular, the SAC of ruthenium oxide electrodeposited on activated carbon was ~4 times higher than that of pristine activated carbon due to the additional pseudocapacitance.2 Since hydrous RuO2 shows higher pseudocapacitance compared with anhydrous RuO2,3 it is anticipated that hydrous RuO2 would show higher SAC compared to anhydrous RuO2. Here we evaluate and discuss the pseudocapacitance and SAC capability of hydrous and anhydrous RuO2-based electrode materials. RuO2·0.5H2O or RuO2 were prepared by heating of RuO2·xH2O at 200°C or 450°C, respectively.3 The slurry, prepared by mixing RuO2·0.5H2O or RuO2, polyvinylidene fluoride and N, N-dimethylformamide, was pasted on a Ti plate and dried at 120oC to prepare electrodes (10-11 mg cm-2) for batch-mode CDI experiments. Activated carbon (MSP-20) electrode was prepared as a control sample. The CDI experiment was carried out with a voltage of 1.2 or 0 V, by circulating 100 mL NaCl feed solution (initial concentration: 5 mM NaCl) at a flow rate of 9.8 mL min-1 per electrode. The SAC was determined by measuring the change in the conductivity on the negative electrode side, using a calibration curve of conductivity and concentration of NaCl solution.The specific capacitance of RuO2·0.5H2O and RuO2 were 172 F g-1 and 13 F g-1, while the SAC was 7.4 mg g-1 and 4.0 mg g-1, respectively. This means that RuO2·0.5H2O shows 13.2 times higher capacitance and twice higher SAC. The results suggest that the increase in the specific capacitance by applying RuO2·0.5H2O might be attributed to the increase in the SAC value. On the other hand, the specific capacitance of RuO2·0.5H2O was higher than that of activated carbon, while the SAC of RuO2·0.5H2O was lower than that of activated carbon. There seems to be a difference in the effect of pseudocapacitance and electric double layer capacitance on the SAC value.This work was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Challenging Research (Exploratory) Grant Number 17K19174. References K. Singh, S. Porada, H. D. de Gier, P. M. Biesheuvel and L. C. P. M. de Smet, Desalination, 455, 115 (2019).X. Ma, Y. Chen, K. Zhou, P. Wu and C. Hou, Electrochim. Acta, 295, 769 (2019).W. Sugimoto, H. Iwata, K. Yokoshima, Y. Murakami and Y. Takasu, J. Phys. Chem. B, 109, 7330 (2005). Figure 1