Surface Engineering of Carbon Nanotubes with Inorganic-Organic Materials for Electrochemical Capacitors.

Abstract

Composite electrodes comprised of inorganic-organic molecular materials are of great interest in applications such as electrochemical capacitors (ECs). ECs store and release charge via double layer capacitance (EDLC) or pseudocapacitance. The carbon materials used for EDLC has low capacitance, while pseudocapacitive materials such as redox active polymers or metal oxides have long term stability issues. Therefore, anchoring inorganic-organic materials on a carbon substrate can leverage the advantages of both EDLC and pseudocapacitance. Composite electrodes of kegging type polyoxometalates (POM) deposited on polydiallyldimethylammonium chloride (PDDA) via the layer-by-layer assembly has been well established and has shown great promise [1-3] . The issues are the limited operation window and the inert PDDA layer. Suitable alternatives that can complement the redox activity of POMs and enlarge the operating potential window are redox active polymers [4].The objectives of this work are to: 1) develop and optimize an electroactive inorganic-organic composite electrode based on a carbon nanotube (CNT) substrate modified with polyluminol (CpLum) and H5GeMo11VO40.24H2O (GeMo11V); 2) study the electrochemical behavior of the composite electrode with and without PDDA for electrochemical capacitors; and 3) investigate the interactions between each layer.GeMo11V was first deposited onto CNT using PDDA as an anchor. The vanadium atom in the kegging structure provided an additional electrochemical redox peak with improved capacitive behavior on the cyclic voltammogram (CV) as seen in figure 1. First, CpLum, and GeMo11V were applied on CNT (CNT-CpLum-GeMo11V), then co-deposited with PDDA (CNT-CpLum-PDDA-GeMo11V). For both composite electrodes, CpLum and GeMo11V contributed to the redox activity leading to a wider potential window. The CV of CNT-CpLum-GeMo11V showed more reversible redox peaks with a slightly reduced GeMo11V contribution compared to CNT-CpLum-PDDA-GeMo11V that had a smoother CV shape. The decrease in GeMo11V contribution was attributed to the smaller amount of positive charge within the CpLum chain needed to anchor the GeMo11V. Each layer had a distinct role in the inorganic-organic composite electrode. The CpLum polymer added additional redox activity in the higher potential range, while PDDA attracted and held the GeMo11V, which provided redox activity in the lower potential range. All three layers had a synergistic interaction and provided enhanced charge storage properties with good rate capability and cycling stability.REFERENCES 1. M. Genovese, Y. W. Foong and K. Lian, Electrochimica Acta, 199, 261 (2016).2. M. Genovese and K. Lian, Current Opinion in Solid State and Materials Science, 19, 126 (2015).3. M. Genovese, Y. W. Foong and K. Lian, Journal of The Electrochemical Society, 162, A5041 (2015).4. N. Casado, G. Hernández, H. Sardon and D. Mecerreyes, Progress in Polymer Science, 52, 107 (2016). Figure 1