Electrochimica Acta 117 (2014) 153–158 Contents lists available at ScienceDirect Electrochimica Acta jo u r n al hom ep age: www.elsevier.com/locate/electacta Germanomolybdate (GeMo 12 O 40 4- ) Modified Carbon Nanotube Composites for Electrochemical Capacitors Matthew Genovese, Yee Wei Foong, Keryn Lian ∗ Department of Materials Science and Engineering, University of Toronto Toronto, Ontario, Canada M5S 3E4 a r t i c l e i n f o Article history: Received 2 October 2013 Received in revised form 18 November 2013 Accepted 20 November 2013 Available online 4 December 2013 Keywords: Polyoxometalates Germanomolybdate Layer-by-layer Pseudocapacitance Supercapacitor a b s t r a c t Keggin type germanomolybdate, GeMo 12 O 40 4- (GeMo), was deposited onto multi-walled carbon nano- tubes (MWCNT) via layer-by-layer (LbL) deposition to form composite electrodes for electrochemical capacitors (ECs). The GeMo composite electrode demonstrated charge storage six times greater than that of the bare MWCNT electrode while maintaining excellent conductivity and cycling stability. GeMo also demonstrated charge storage complementary to that of the commercial Keggin type POMs, PMo 12 O 40 3- (PMo) and SiMo 12 O 40 4- (SiMo). Dual-layer coatings superimposing GeMo with either PMo or SiMo showed an additive combination of both active layers, which resulted in cyclic voltammograms (CVs) with overlapping redox features and charge storage twelve times greater than that of the bare MWCNT electrode. Scanning Electron Microscopy (SEM) demonstrated successful single and dual layer coating of POMs on MWCNT with high coverage and uniform surface morphologies. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nano materials such as graphene, onion-like carbon (OLC), and carbon nanotubes (CNT) have emerged as promising electrode materials for electrochemical double layer capacitors (EDLC), due to their large surface area and metal-like conductivity [1–3]. However, storing charge through the electrochemical dou- ble layer alone limits the specific capacitance and energy density of nano-carbon electrodes. One of the most common approaches to improve the energy density of EDLC electrodes is the addition of pseudocapacitive materials. Pseudocapacitive materials allow for reversible Faradaic oxidation/reduction reactions on the electrode surface, and can yield specific capacitance 10 to 100 times greater than EDLC [4]. RuO 2 is one of the most effective pseudocapacitive materials due to its fast reversible redox reactions with overlapping peak potentials: Its high cost, however, proves to be a challenge for commercialization. Polyoxometalates (POMs), a class of large metal oxide clusters, have been investigated as low cost RuO 2 alternatives [5–8]. Keggin-type heteropolyanions are the most well known and widely used class of POMs. The Keggin structure has the general for- mula [XM 12 O 40 ] n- , in which the central heteroatom (i.e., P, Si, or B) is surrounded by twelve addenda atoms (i.e., Mo or W) and forty ∗ Corresponding author. Department of Materials Science and Engineering, Uni- versity of Toronto, 184 College St. Toronto, ON M5S 3E4 Canada. Tel.: +416 978 8631. E-mail address: keryn.lian@utoronto.ca (K. Lian). oxygen atoms. These POMs exhibit multiple, highly stable redox states, which gives them the ability to act as electron reservoirs, an ideal characteristic for electrochemical storage applications [9]. Furthermore, different combinations of hetero and addenda atoms can lead to different electrochemical properties [10], allowing for the ability to tune the electrochemical behaviour of POMs. By combining these POMs with nano-carbon materials, the electrochemical activity of the former can be leveraged with the surface area of the latter to achieve high performance and low cost inorganic-organic composite electrodes. A number of methods can be used to create POM-carbon composites, including simple mixing [7], chemisorption following carbon oxidation [11], elec- trodeposition [10], and layer-by-layer (LbL) self assembly [12–14]. LbL self assembly, achieved through the alternate adsorption of positive and negative layers, is a simple and effective technique to deposit monolayers of oppositely charged species. Martel et al. have reported the LbL deposition of 4 different POM chemistries on smooth glassy carbon, and showed a linear increase in charge storage with the number of layers deposited [12]. Kulesza and his group developed composites of POMs with various conduc- tive substrates [5,8,15]. For Keggin-type POMs, charge storage is often concentrated in certain voltage regions that do not overlap [8,10,12]. In order to combat this limitation, the LbL technique can be used to superimpose layers of different POM chemistries to achieve an electrode which exhibits a collective contribution of each layer. Thus, different POM chemistries with different redox peak potentials can be superimposed to mimic the overlapping redox peaks and ideally capacitive profile of RuO 2 . Our previous 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.114