Simple Graphite and Thermoplastic Mixtures for Complicated Electrochemical Systems
Authors: Klunder Kevin jay , Henry Charles S 
Published: February 2020

A new carbon composite electrode will be presented. The advantages of composite electrodes include low cost, ease of catalyst incorporation, and facile fabrication. Persistent common problems with composite electrodes include low conductivity, sluggish electron transfer, difficulty in patterning, and complications with incorporation into advanced sensors such as microfluidics. To overcome these issues a solvent assisted process was developed to fabricate thermoplastic electrodes (TPEs) using polymethylmethacrylate (PMMA) combined with graphite.

After fabrication, we tested the ability to pattern these electrodes using common manufacturing processes. It was discovered that these materials maintained the properties of the thermoplastic binder, enabling the electrodes to be patterned with channels and pillars of micron dimensions using hot embossing. The “chewing gum” like consistency of the semi-dried electrode material lends itself to simple templating techniques, were the shape and arrangement of the electrode is fully customizable. In this work, a CO2 laser used for templating electrode dimensions of ~150 µm, the resolution limit of the laser. The CO2 laser allowed for easy fabrication of wall-jet, band, and individually addressable microelectrode arrays. The entire process is easily scaled up and can create tens to hundreds of working electrodes in a single batch, or very large centimeter to meter sized electrodes.

Electrode characterization was performed with electrochemistry, conductivity measurements, and various forms of spectroscopy (XPS, SEM/EDS, Raman, XRD). Emphasis was placed on understanding the relationship between particle loading and particle size. The particle loadings ranged from 20 to 85 mass % graphite, using graphite particle sizes of 0.5 to 100 µm. Interestingly, adjusting these simple parameters had an enormous impact on both electrochemical performance and conductivity. High conductivities of ~800 S/m were achieved with 11µm particles, as well as stable and enhanced electrode kinetics, while 0.5 and 100 mm particles generated lower conductivity and somewhat slower electron transfer.

Initial experiments with the Fe(CN)6
3-/4- redox couple using impedance spectroscopy and cyclic voltammetry show that the TPE electrode out performs a DropSens commercial screen printed electrode as well as glassy carbon. The low charge transfer resistance of the TPE (<5 Ωcm2) more resembles the kinetics of a platinum electrode. The capacitance and solvent window were also measured, as they are critical parameters of the performance of electrochemical sensors. Both properties were highly dependent on the particle size, graphite purity, and graphite mass loading.

An in-depth electrochemical investigation of the electrode surface following the redox mediator flow chart of McCreery was performed to give insight into the chemical composition of the electrode surface. Evidence for surface oxides (quinones), metallic impurities, and both basal and edge character from the graphite lattice were observed. The enhanced electrochemical reactivity of TPEs towards common analytes of dopamine, ascorbic acid, urate, NADH, thiols, and hydrazine is similar to that of carbon nanotubes and graphene. A major finding of this work is that electrochemical activity of the TPE can be tuned when the surface is activated through sanding, polishing or plasma cleaning.

Finally, as a proof of concept, electrochemical cells were fabricated to analyze the oxidative load of aerosol particles through the use of the dithiothreitol(DTT) assay. The TPE devices enable ultralow volume sampling (~300 µL) by utilizing a microelectrode array. The high activity of the electrodes realized electrochemical sensing without the need for the commonly used cobalt phthalocyanine catalyst. This unique TPE sensing system achieved unprecedented resolution of the DTT Assay, allowing for measurements to be taken at ~10 second intervals. The rugged, reusable, complex sensor required less than one man hour to fabricate, with a materials cost of <$1.

Figure 1