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

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 RuO 2 shows higher pseudocapacitance compared with anhydrous RuO 2 , 3 it is anticipated that hydrous RuO 2 would show higher SAC compared to anhydrous RuO 2 . Here we evaluate and discuss the pseudocapacitance and SAC capability of hydrous and anhydrous RuO 2 -based electrode materials. RuO 2 ·0.5H 2 O or RuO 2 were prepared by heating of RuO 2 · x H 2 O at 200°C or 450°C, respectively. 3 The slurry, prepared by mixing RuO 2 ·0.5H 2 O or RuO 2 , polyvinylidene fluoride and N , N -dimethylformamide, was pasted on a Ti plate and dried at 120 o C 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 RuO 2 ·0.5H 2 O and RuO 2 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 RuO 2 ·0.5H 2 O shows 13.2 times higher capacitance and twice higher SAC. The results suggest that the increase in the specific capacitance by applying RuO 2 ·0.5H 2 O might be attributed to the increase in the SAC value. On the other hand, the specific capacitance of RuO 2 ·0.5H 2 O was higher than that of activated carbon, while the SAC of RuO 2 ·0.5H 2 O 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