Ultrasonic exfoliation of carbon fiber: electroanalytical perspectives

Journal of Applied Electrochemistry (2020) 50:383–394 https://doi.org/10.1007/s10800-019-01379-y RESEARCH ARTICLE Ultrasonic exfoliation of carbon fiber: electroanalytical perspectives Charnete Casimero1 · Catherine Hegarty1 · Ruairi J. McGlynn1 · James Davis1 Received: 7 June 2019 / Accepted: 2 December 2019 / Published online: 30 January 2020 © The Author(s) 2019 Abstract Electrochemical anodisation techniques are regularly used to modify carbon fiber surfaces as a means of improving electrochemical performance. A detailed study of the effects of oxidation (+ 2 V) in alkaline media has been conducted and Raman, XPS and SEM analyses of the modification process have been tallied with the resulting electrochemical properties. The co-application of ultrasound during the oxidative process has also been investigated to determine if the cavitational and mass transport features influence both the physical and chemical nature of the resulting fibers. Marked discrepancies between anodisation with and without ultrasound is evident in the C1s spectra with variations in the relative proportions of the electrogenerated carbon-oxygen functionalities. Mechanisms that could account for the variation in surface species are considered. Graphic abstract Keywords Carbon fiber · Electrode · Surface treatment · Anodisation · Ultrasound · XPS Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10800-019-01379-y) contains supplementary material, which is available to authorized users. * Charnete Casimero casimero‑ 1 School of Engineering, Ulster University, Jordanstown BT37 0QB, Northern Ireland 1 Introduction Carbon is the principal electrode material in a vast number of electrochemical applications and its ubiquity can be attributed to a range of factors: diversity of physical form, rich interfacial chemistry and the relatively inexpensive cost with which the electrodes can be produced [1]. Carbon cloth 13 Vol.:(0123456789) 384 electrodes, in particular, have risen to considerable prominence in recent years as they typically consist of an interpenetrating fiber matrix whose macro porosity and extensive surface area have been found to be highly advantageous in energy applications [2–6] and industrial water treatment [7–10]. Such electrode geometries are also finding favour within the various biosensing communities where the fiber network can serve as a host for bacterial species [4, 11, 12] or a framework for the immobilisation of enzymes [13, 14]. Polyacrylonitrile (PAN) fibers are the most common precursor (> 90%) but there has been considerable interest in other synthetic and bio-based systems [15–18]. Carbonisation of the spun polymer results in the production of fibers and, it is little surprise that both the mechanical and electrochemical properties of the fibers can be highly dependent on the nature of the preparation methods and subsequent processing treatments. It is usual to find that modification of the fibers through either chemical, plasma or electrochemical means is performed in order to improve their performance and, as such, an extensive literature base on surface treatments has arisen in recent years [19, 20]. The modification of carbon fibers has a long history and originated more from the need to improve their structural performance than their electrochemical properties [21, 22]. Carbon fibers are core components within composite matrices intended for high load bearing applications where high tensile strength, stiffness and low density are among the key attributes necessary for such applications [15, 16, 19]. Their reinforcement performance is often dependent upon the interfacial bonding strength between the fibers and the resin matrix [19, 23, 24] however, unmodified carbon fibers tend to possess a relatively featureless surface in terms of chemical functionality and physical morphology, both of which can compromise the mechanical properties of the composite [20, 25]. Surface modification arose largely as a means of addressing these issues through generating a greater range of active functional groups and increasing surface roughness that could more effectively engage in interlocking the fiberresin composite. The influence of various types of surface treatment on the mechanical properties of carbon fiber and their influence on composite performance have been comprehensively reviewed [19, 20]. It must be noted that many of the chemical alterations to the carbon surface can also dramatically improve the performance and versatility of the carbon fibers when applied as electrode materials. While a large range of treatments have been investigated [19, 20], the end outcome is normally the oxidation of the surface to increase the variety of carbonoxygen functional groups and their respective concentrations [8, 26–28]. A summary of the different types of oxygen functionality that can arise as a consequence of surface oxidation is highlighted in Fig. 1 but, it should be noted, these are indicative only of the base functionality and that a 13 Journal of Applied Electrochemistry (2020) 50:383–394 Fig. 1  Summary of the different functional groups present on the surface of carbon fiber (adapted from [29]) spectrum of forms will be generated as the graphitic plane is remodelled [29, 30]. In many respects, the variety of oxygen functionality reported post modification are analogous to those identified in graphene oxide fragments [31] albeit fixed to the body of the fiber structure. An example of nitrogen functionality has also been included in Fig. 1 as there will inevitably be trace components (i.e. pyridinic and pyrrolic) within the graphitic planes. These can be attributed (in the absence of an exogenous nitrogen source) to the incorporation of PAN nitrogen into the graphitic lattice during carbonisation process used to form the fibers [18]. The nitrogen component will be minor in unmodified fibers with s p2 carbon being the predominant species and, it is the oxidative destruction of the latter that gives rise to the carbon-oxygen functionality. A large number of treatment options have been pursued [19], such as electrochemical oxidation (anodisation) which is well established as an effective technique for the generation of oxygen functional groups as well as enabling the etching of the surface [8, 26–28]. It is now commonplace for carbon fiber electrodes, whether discrete fibers, bundles, mats or cloths, to be electrochemically treated prior to further application in order to improve electron transfer kinetics [11, 12, 28]. While it is recognised that the surface functionality increases with increasing charge, the simultaneous application of electrochemical oxidation and ultrasonic activation has yet to be explored. The macro / micro streaming processes and cavitation events arising from the application of ultrasound can significantly influence electrochemical Journal of Applied Electrochemistry (2020) 50:383–394 385 (...truncated)