Comparison of Six Different Printed Ag Inks for Coulometric Removal of Chloride Ions from Seawater: Towards an Integrated Microfluidic Platform for Desalination

Abstract

This paper is the first report on the electrochemical characterization and comparison of printable silver (Ag) inks for the fabrication of planar Ag electrodes for integration in a microfluidic platform for seawater desalination. Compared to previously reported devices [1], [2], our platform was fully fabricated using microfabrication techniques to enable miniaturization of the components and exploit scaling for the improvement of the device electrochemical performance. Screen printing and inkjet printing were investigated to overcome limitations of more conventional microfabrication techniques for Ag deposition such as sputtering and evaporation. For the electrochemical devices reported here, the standard deposition and in particular electrodeposition techniques have adhesion and uniformity issues when performed on a large area resistive substrate due to stress for layer thicknesses above 2 \U0001d707\U0001d45a [3]. Our microfluidic platform is shown in Figure 1 together with a schematic cross section. It consists of the following components: a glass substrate with a patterned Ag electrode serving as working electrode (WE), a laser cut shallow channel (40 \U0001d707\U0001d45a thick) made of pressure sensitive adhesive (PSA) serving as fluidic compartment for seawater, a Nafion cation-exchange membrane, a 5 \U0001d45a\U0001d45a deep polydimethylsiloxane (PDMS) chamber serving as supporting electrolyte reservoir, a second glass substrate with patterned Ag/AgCl electrode serving as reference electrode (RE). Desalination of seawater can be achieved through the electrolysis of seawater [1]: Cl- ions are removed by chloridation of the Ag WE by applying a positive potential vs. the RE, while Na+ions are removed by diffusion through the Nafion membrane to the supporting electrolyte; the Ag electrodes are regenerated by applying a negative potential. Six Ag inks with different formulations in terms of solvent, particles size and solid content, were selected among the wide range of inks available on the market. The Novacentrix HPS-021LV and Dupont 5064H and PE825 inks were screen printed, and the Suntronic U5603, Novacentrix JS-B40G and PV Nanocell SicrysTM I50TM-119 inks were ink-jet printed. A common design was used (4 cm2 square) and all the inks were thermally cured. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were used to characterize the electrodes morphology and composition; SEM pictures are shown in Figure 2. The electrochemical behavior of the electrodes was investigated in order to define the most suitable ink for coulometric Cl-removal. Cyclic voltammetry was carried out in a 0.6 M NaCl synthetic solution by using a three electrode set-up: the oxidation and reduction current peaks of the Ag inks were found at different potentials vs. Ag/AgCl/3 M KCl RE as shown in Figure 3a. Two-steps chronoamperometric measurements (60s of oxidation to AgCl and 120 s of regeneration to Ag) were carried out for each ink at different potentials allowing the quantification of their oxidation ability by registering the oxidation charge. For all the inks the charge increased with the applied potential and Figure 3b shows an example of chronoamperogram registered at +0.9 V and -1.0 V. Repetitive chronoamperometric measurements were performed in order to qualitatively observe the adhesion properties of the different inks and predict their lifetime. Table 1 reports some properties of the investigated inks as well as the two key factors defining their performance: oxidation charge and adhesion. The most suitable ink for desalination is the ink PE825 which not only showed the best oxidation ability, i.e. a charge of 3.97 C (9.93 mC/mm2 vs. 15.5 mC/mm2of an Ag wire) as the ink 5064H, but also offered the best adhesion to glass withstanding more than 60 oxidation/regeneration cycles against the 25 cycles of the latter. This ink was therefore chosen for fabricating the electrodes of the microfluidic platform with the further advantage that screen printing is a technique cheaper and less time-consuming than inkjet printing. The electrochemical properties of the microfluidic platform are currently under investigation: charge and conductivity measurements are used for the quantification of the NaCl concentration of the desalted sample.