Comment on “Carbon nanotube/graphene composite for enhanced capacitive deionization performance” by Y. Wimalasiri and L. Zou

In a recent study, Wimalasiri and Zou [1] have reported the use and performance of composite electrodes of carbon nanotubes (CNT) and graphene for application as porous electrodes in capacitive deionization (CDI). While CDI is emerging as an attractive technology for water desalination, and novel electrode materials and composites are important contributions to the advancement of the field, there are several issues in this study that we must comment on. We first address the capacitive deionization (CDI) performance reported by Wimalasiri and Zou [1], namely an adsorption of NaCl in the composite electrodes of 26.42 mg/g at a cell voltage of Vcell = 2.0 V. This value is approximately one order of magnitude higher than what has been reported for this kind of composite material in 2012 by Zhang et al. [2], namely 1.4 mg/g at Vcell = 2.0 V, and very recently by Li et al. [3], namely 0.9 mg/g at Vcell = 1.6 V. The difference in performance is in contrast to the very similar synthesis route of all three studies (i.e., reduction of graphene oxide/CNT composite) and the comparable pore characteristics of the composite electrode (Zhang et al.: 480 m2/g; Li. et al.: up to 450 m2/g; Wimalasiri and Zou: 391 m2/g). As these two examples show, in previous literature, values for salt adsorption in electrodes composed of graphene, CNTs, or composites thereof, are all very close and in the low range 1–3 mg/g, defined per g of both electrodes combined, see Table 1. Such moderate performance is in line with expectations, because the specific surface area of these materials is moderate (i.e., a few hundred m2/g). In contrast, the much higher salt adsorption capacity in CDI known for microporous carbons occurs in conjunction with high specific surface areas in excess of 1000 m2/g [4] and [5]. Thus, it remains unclear how the material reported by Wimalasiri and Zou with a specific surface area below 400 m2/g and pores in the range of 3–4 nm for graphene and 7–8 nm for CNT/graphene composites would provide such a high salt adsorption capacity and high capacitance (220 F/g = 0.56 F/m2). We like to note that the stated salt adsorption capacity of 26 mg/g is per gram of CNT/graphene, and not per gram of total electrode, which would reduce the number to 19 mg/g because 28 mass% of the total electrode is composed of binder and graphite.