Influence of Nafion-Coating of Ag/Ag2O/C Electrodes on Long-Term Performance of an Electroosmotic Pump

Ag/Ag2O electrodes were fabricated on both carbon fiber paper (CP) and carbon fiber cloth (CC) substrates via galvanostatic electrodeposition to develop an electroosmotic (EO) pump. The results showed that Ag was deposited along the carbon fibers and further grew into Ag particles. The Ag loading on CP was higher than that of CC at the same current density, and the difference increased further with the current density increase. Nafion coating was applied on the as-electrodeposited Ag/Ag2O/CP electrode to prevent the degradation of Ag particles using dip coating, followed by drop coating. The Nafion-coated Ag/Ag2O/CP electrodes showed much better electroosmotic pump performance and long-term stability than the uncoated electrodes, revealing that the Nafion ionomer can enhance the proton conductivity and mitigate the degradation of Ag particles during the electrochemical reactions. The EO pumps built with Nafion-modified Ag/Ag2O/CP electrodes with an active area of 0.28 cm2 generated a maximum pressure of 157 kPa and a maximum flow rate of 28.21 ul min−1. The Postmortem analysis of the pumped working fluids collected from the EO pumps operated at a constant potential for three days was performed to further investigate the effects of Nafion coating on Ag degradation.

Electroosmotic (EO) pumps have been proposed as promising candidates for wearable liquid-drug delivery devices owing to their low power consumption, stationary parts, and ability to provide relatively high pressure and pulsation-free flow. [1][2][3][4][5] An EO pump consists of a membrane electrode assembly structure as a core component, containing two flow-through conductive electrodes and a surface-charged porous membrane. Therefore, EO pump performance strongly depends on electrode type and membrane materials. Silica membranes with a negative surface charge in neutral or basic pH have been mainly used in the EO pump. 5 Advanced electrode materials have been extensively studied for use in EO pumps. [1][2][3][4][5][6][7][8][9][10][11] Most commonly used metal electrodes such as platinum (Pt) and silver (Ag) cause critical problems, such as gas evolution, poor flow control, and metal ion release into the electrolyte. 3,12 A gas-free EO pump using Ag/Ag 2 O electrodes was developed by , with a flow of 5-30 ul min −1 under a potential range of 0.2-0.8 V. 2 When the identical Ag/Ag 2 O electrode was used for the anodes and the cathode, the EO pump could be operated at a potential lower than 1.23 V (where the electrolysis of water occurs) due to a very low oxidation-reduction potential difference (E redox = 0.1 V), thereby preventing the generation of air bubbles during the electrochemical reactions. 2 The utilization of porous Ag/Ag 2 O electrodes can prevent the generation of gas bubbles due to lowvoltage operation, although Ag degradation and the resulting performance decay remain to be addressed. 3 Because the electrochemical reactions must occur on the electrolyte, the metal electrode itself can be either oxidized or reduced, releasing metal ions into the electrolyte (e.g., M → M n+ + ne − ). Therefore, such undesirable electrochemistry is inevitable, causing electrode degradation and in turn, pump performance reduction. Moreover, the rate of degradation might increase proportionally with the increased metal loading, especially when the design of electrodes is incompatible with a microfluidic system. These drawbacks have been solved using the Nafion ionomer, which serves as the binding agent and the protonconducting support in electrochemical systems. 5,[12][13][14][15] Therefore, performance-based structural optimization is necessary to make a more durable and efficient EO pumping system via a thorough systematic study. In this work, we developed an optimal Ag electrode system for an EO pump using Nafion coating and demonstrated its enhanced long-term stability.

Experimental
Preparation of Ag/Ag 2 O/C electrodes.-Two types of carbon support materials were used, commercially available carbon fiber paper (CP) and carbon fiber cloth (CC), which were purchased from JNTG (JNT-30) and Ballard (Avcarb 1071-HCB), respectively. The carbon substrate was firstly treated with air plasma for 1 h to produce a hydrophilic surface before electroplating. The pretreatment of the carbon paper can increase the amount of surface oxygen-containing functional groups, enabling more active sites for Ag ions. 16 Silver electroplating was conducted under galvanic conditions in a twoelectrode cell employing a silver sheet as the cathode substrate. Afterward, the pretreated carbon substrate was immersed in a solution of stirred 0.2 M AgNO 3 , 0.1 M HNO 3 , and 0.015 M citric acid and electroplated at constant current densities of 1.5, 3.0, and 6.0 mA cm −2 for 10, 15, and 20 min, respectively. Next, silver oxides were formed by oxidizing part of the electroplated silver in 0.1 M NaOH solution at 3 mA cm −2 for 30 min. 17 Finally, all the resulting electrodes were thoroughly rinsed with deionized water (18 MΩ cm) and then dried at 60°C for 10 min.
Preparation of Nafion-coated Ag/Ag 2 O/C electrodes.-The electroplated Ag/Ag 2 O/C electrodes were coated with Nafion solution by the dip and drop coating methods followed by a curing process. In the dip coating method, the as-prepared electrodes were dipped into 1 wt% and 5 wt% Nafion solutions (EW = 1100, D521, Dupont) for 1 min. In the drop coating method, the electrodes were drop-coated with 20 ul-120 ul of 5 wt% Nafion solution using a micropipette (model M1000, Gilson). The electrode was then dried in a convection oven at 110°C for 1 h.
Characterizations.-The morphology and elemental composition of the as-fabricated electrodes were analyzed by field-emission scanning electron microscopy (FESEM, Nova NanoSEM 450, FEI) equipped with energy-dispersive X-ray spectroscopy (EDX, Bruker) at an accelerating voltage of 15 kV.
The elemental composition and distribution of the as-fabricated electrodes were analyzed by an electron-probe microanalyzer (EPMA, SX-100) at an operating voltage of 15 kV. The samples were coated with platinum before EPMA observation. z E-mail: kcj@eoflow.com The chemical states and surface composition of the as-prepared electrodes were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA) with a monochromic Al Kα X-ray source (E = 1486.6 eV). The as-prepared electrodes were attached to a sample holder using a small piece of carbon tape. All the spectra were corrected to the C 1 s peak in C-C bonds at a binding energy of 284.6 eV. The surface atomic ratios were calculated from the peak areas normalized by the atomic sensitivity factor of the corresponding element. 18 Postmortem analyses were performed to investigate the degradation of Ag electrodes after 3 days. Inductively coupled plasma-mass spectroscopy (ICP-MS) was performed with an Agilent 7500cx system to analyze the Ag content in the pumped working fluids after 3 days. The microstructural and chemical analyses of the remaining debris in the working fluids were performed by high-resolution transmission electron microscopy (HRTEM) coupled with the selected area electron diffraction (SAED) technique and EDX. The morphologies of the samples were analyzed by an HRTEM using a TECNAI G2 F20 electron microscope (FEI Company, USA) operating at an accelerating voltage of 200 kV. SAED and EDX were utilized to identify the crystallinity and elemental compositions of the samples, respectively. The specimens were prepared by dispersing a small amount of the working fluid in ethanol solution using an ultrasonic generator, putting a droplet of this dispersion onto a standard carbon-coated copper mesh, and drying it in the air overnight.
Pump assembly and performance.-The pump configuration, shown in Fig. 1, consisted of two identical porous electrodes, SUS strips, and polycarbonate (PC) frames sandwiching a porous ceramic membrane. The assembled pump was then sealed with a UV bonding agent (Loctite AA 3943). The membrane (thickness = 1.0 mm) was fabricated by sintering the randomly packed phosphorus (P)-doped silica nanospheres (with an average particle size of 50 nm), as can be seen in Fig. 1b. The pump with an active area of 0.28 cm 2 was preconditioned as reported previously.1 DI water was used as a working fluid. The pump was operated by alternating a constant potential. The pump performance was measured using a microfluidics system, consisting of a digital pressure sensor (MPS3, Elveflow, France) and a flow sensor (MFS, Elveflow, France).1 Pressure and flow rate measurements were performed at 200 Hz using cDAQ 9184 (National Instrument, USA) and an in-house LabVIEW program. 1

Results and Discussion
Two different types of commercially available carbon support materials, i.e., CP and CC, were employed to investigate their effects on the structural properties and morphologies of the electrodeposited Ag electrodes. Figure 2 shows the morphologies and chemical compositions of the pristine CC and CP obtained from FESEM and XPS analyses, respectively. The pristine CC had well-woven carbon fibers with an average diameter of 8.3 um and very smooth surfaces. The pristine CP showed a highly porous (<90%) network of intertwined carbon microfibers with an average diameter of 8.0 um and polytetrafluoroethylene (PTFE) filling in the vacancies. Their surface compositions obtained from XPS analysis are summarized in Table I. The XPS results showed that the pristine CC and CP had similar surface compositions consisting of carbon with a small portion of oxygen and nitrogen. Figure 3 shows the galvanostatic potential-time curve obtained during the Ag electrodeposition onto CP at an applied current density of 3.0 mA cm −2 for 30 min. As can be seen from the graph, the potential rose to a peak value as soon as the current was applied, followed by a slight decrease, finally reaching a steady state. The rapid increase in the potential might be due to the presence of the electrical double layer. A stable potential was 0.37 mV when a current density of 3.0 mA cm −2 was applied. The measured potential of CC (not shown here) was about 0.2 mV higher than that of CP during galvanostatic electrodeposition, indicating that a higher overpotential was required for the CC-based Ag electrodeposition at the same current density due to its higher electrical resistance. The morphology and chemical composition of the Ag electrodeposited on CP (Ag/CP) were characterized by FESEM and EDS, respectively. Figures 4a-4d shows the FESEM images of the Ag/CP electrodes electrodeposited at 3.0 mA cm −2 for 15 min, indicating larger micron-sized spherical particles forming interparticle grain boundaries. A rough globular morphology and numerous aggregated Ag particles (clusters) can be seen on the surface as shown in Fig. 4d. The EDX results in Fig. 2e reveal that only silver and carbon were present at 87.36 at% and 12.64 at%, respectively, and no other elements (impurities) were observed. These results indicate that Ag was homogeneously deposited on the carbon substrate. Figure 5 shows the FESEM images of the Ag/CP electrodes electrodeposited at different deposition times. All electroplating processes were conducted under the same conditions at a constant current density of 3.0 mA cm −2 except the electroplating times ranging from 10 to 20 min. As the electroplating time increased from 10 to 20 min, the particle size and the amount of Ag loading increased. With prolonged electroplating times, the deposited Ag particles tended to aggregate. This might be due to the increased   Figure 6 shows the surface morphology and two-dimensional (2D) elemental mapping images of the Ag/CP electrodes electroplated at different current densities. All electroplating processes were performed under the same electroplating conditions for 30 min, except for the electroplating current densities ranging from 1.5 to 6.0 mA cm −2 . For 1.5 mA cm −2 , the localized spherical Ag particles and agglomerates were observed on the surface of the carbon fibers. When the electroplating current density was more than 3.0 mA cm −2 , the surface of the carbon fibers was covered with Ag. At 6.0 mA cm −2 , the grain boundaries shrunk and disappeared. The elemental mapping images also showed that the carbon fibers were uniformly decorated with a high density of Ag particles and Ag growth proceeded with the increased current density. An increased electroplating current density increased the amount of Ag loading and the thickness of the electrode diameter. As the electroplating current density increased, the movement and nucleation of Ag ions could be faster, leading to increased Ag growth. 20 These results demonstrate that Ag was electrodeposited well on carbon fibers via galvanostatic electrodeposition and that the coating capacity and thickness, and surface morphology can be controlled by  To investigate the influence of the type of the support material on the structural properties of Ag coating, the commercially available conductive CC was employed. Figure 7 shows the EPMA images of  CC-supported Ag electrodes (Ag/CC) prepared under the same electroplating conditions as Ag/CP electrodes. Ag was coated with the surface of the carbon fibers, similar to that observed for CP. However, as can be seen in Fig. 8, the electrodeposition rate was different in that the Ag content and the degree of aggregation on CP were much higher than those on CC. Notably, the difference in Ag loading became more pronounced as the current density increased. These might be attributed to the differences in the intrinsic physical properties of the support materials, such as thickness and porosity, which can affect the rate of electrochemical deposition and thus the nucleation density and growth rate of Ag particles.
For the EO pump built with Ag/Ag 2 O electrodes, its operating voltages were reported to be low below ca. 1.23 V, where electrolysis of water cannot occur. 2 However, the level of performance with Ag or traditional metal electrodes has remained insufficient as an actuator for high pressure and flow rate, agile response, and greater power required in a microfluidic system. 2 Thus, a relatively higher potential is required to increase pump performance, and hence a robust electrode system that can operate at relatively higher potentials must be developed. Furthermore, Ag electrodes are so consumable that their longevity is much more limited at a higher operating potential. Some researchers reported the degradation phenomena of Ag electrodes with a significant performance decay during the electrochemical reactions. 3,5 Figure 9 shows the current behavior of an EO pump built with Ag/Ag 2 O/CP electrodes at a continuous operation of +/− 2.0 V. The graph shows that the current gradually increased up to 12 h and then rose steeply, reaching ca. 300 mA at 16 h. As can be seen in the photographs of the EO pumps before and after operation in Fig. 9, the blackened membrane was observed after the test (16 h) because of Ag degradation due to the dissolution, migration, precipitation, and accumulation of Ag   decreasing the zeta potential of the membrane. This obvious decay of the Ag electrode during electrochemical potential cycling is consistent with the results previously reported. 3,5 The utilization of Nafion solution as a binder and proton conductivity enhancer was performed on the Ag/Ag 2 O/CP electrode system that can be expected to retard Ag degradation during the redox reactions and improve the continuity of the proximal EO flow of ionic liquids. Note that all Nafion tests were conducted using the same Ag/Ag 2 O/CP electrode electroplated at 3.0 mA cm −2 for 15 min, which produced the best EO performance, and its surface composition obtained from XPS analysis is summarized in Table II. The XPS results indicated that the Ag/Ag 2 O/C electrode was composed of Ag, O, and C at 37.48, 28.56, and 33.96 at%, respectively. The effects of Nafion concentration and the coating method were studied to acquire the optimum electrode design for a robust EO pump. Figure 10 shows the 2D elemental mapping images of the Nafion-coated Ag/Ag 2 O/CP electrodes fabricated by different Nafion coating methods and concentrations obtained from the EPMA analysis. The elemental mapping images showed the spatial distribution of the corresponding elements, i.e., F (fluorine), S (sulfur), and Ag, present in the electrodes. F and S elements coexisted on the Ag electrodes after Nafion coating. The corresponding primary elemental compositions are summarized in Table III. The presence of a high level of fluorine and a small portion of sulfur incorporated into the Ag/Ag 2 O/CP electrode after Nafion coating, consequently modified the composition and structure of the electrode surface. The EPMA  results revealed that when a higher concentration of Nafion solution was applied, a larger amount of F element was seen on the electrode surface. However, there was no significant overall difference owing to the coating method when the same concentration of Nafion solution was used. The influence of the Nafion content on EO pump performance was studied, and the results are summarized in Table IV. Note that all performance tests were conducted using only the electrodes to which the drop coating of 5 wt% Nafion solution was applied. The pump performance increased with the increased Nafion content (up to 80 ul), generating a stall pressure of 157 kPa and the maximum flow rate of 28.21 ul min −1 , respectively. There was an optimum Nafion loading of 80 ul for the best pump performance. Consequently, the increase in the Nafion content improved proton transport by the hydrophilic sulfonic acid group (SO 3− ) in Nafion and the adherence of the Ag particles to the carbon fibers. Figure 11 shows FESEM images of 80 ul Nafion-coated Ag/Ag 2 O/CP electrode. Nafion was coated throughout the electrode, especially well bonded between the carbon fibers and Ag particles through proper exposure of the active surface of Ag particles. These structurally-modified features might have beneficial effects not only in the initial performance but also in durability. The current was not measured when the Nafion content was 120 ul or higher. Too much Nafion might lead to poor electrical conductivity and activity of the electrodes by insulating the interfacial mass transport and electrochemically active surface of Ag particles, as shown in Fig. 12. Figure 13 shows the results of the long-term stability tests for the EO pumps built with the Nafion-coated and uncoated Ag/Ag 2 O/CP electrodes during 3 days of a continuous operation at 1.5 V. For the uncoated electrode, the dramatic performance decay was observed on day 1, which reached a nonoperable performance level. On the other hand, the Nafion-coated electrode showed a slight decrease in performance but a stable behavior over 3 days. Note that an apparent difference in membrane color change was observed in the EO pumps using the respective electrodes after 3-day tests, with much less blackening in the Nafion-coated electrode than in the uncoated electrode. These results demonstrate that Nafion coating can effectively reduce the acceleration of irreversible Ag degradation  during the electrochemical potential cycling and enable a more durable electroosmotic pump system with long-term stability.
To investigate the effects of Nafion coating on performance degradation more thoroughly, Ag contents dissolved in the remaining pumped working fluids of the EO pumps assembled with three different electrodes after 3d were analyzed by ICP-MS, respectively, and the results are depicted in Fig. 14. A substantial amount of Ag ions was found in the uncoated electrodes, indicating a considerable Ag degradation during the electrochemical reactions. In the case of the Nafion-coated electrodes, a relatively very small amount of Ag was confirmed regardless of the type of coating, indicating that Nafion coating can effectively mitigate electrode degradation. Nafion coating significantly affects the color of the pumped working fluids collected after 3 days, as shown in the inset of Fig. 14. No significant difference between drop coating and dip coating methods was found, even though the Ag content of the Nafion-dip-coated electrode was slightly lower than that of the dropcoated electrode. Further degradation of Ag particles after 3 days of a continuous operation was obtained based on TEM, SAED, and EDX results. Figure 15 displays TEM images, the corresponding SAED pattern (inset), and EDX spectra of the pumped working fluid for the EO pump with Nafion uncoated after 3 days. The results indicate that the crystalline aggregates of spherical Ag particles were predominantly present in the pumped working fluids after 3 days due to the degradation of Ag particles from the Ag/Ag 2 O/CP electrodes. 21,22 Ag loadings for uncoated and Nafion-coated Ag/Ag 2 O/CP electrodes were evaluated by EPMA analyses before and after 3 days. It was found that Ag loading percentage of the uncoated electrode substantially decreased from 46.70 to 32.54 at% (ca. 30% loss) after 3 days. In the case of Nafion-coated electrodes, no significant changes of Ag loading were observed regardless of the type of coating before and after 3 days. From these various observations and results, we deduced that mechanism of the electrode degradation is induced by the electrochemical potential cycling through the detachment and dissolution of Ag particles, and their migration into the porous membrane, which results in both the abnormal increase in current and the significant performance decay for the EO pump. The application of Nafion coating on Ag-based electrodes through a simple drop or drop-coating method can effectively mitigate electrode degradation to some extent and thus helps to improve pump performance and longevity.

Conclusions
A highly efficient and durable Ag/Ag 2 O/C EO pump was developed using Nafion coating, and its long-term stability was evaluated. Ag was successfully electroplated on carbon substrates via the galvanostatic method and optimized by testing electroplating conditions, such as deposition time and current density, and the type of the carbon support material. The Nafion-coated Ag/Ag 2 O/CP electrodes showed much better electroosmotic pump performance and long-term stability than the uncoated electrodes, revealing that   the Nafion ionomer can enhance the proton conductivity and mitigate Ag degradation during electrochemical potential cycling. The EO pumps built with Nafion-modified Ag/Ag 2 O/CP electrodes with an active area of 0.28 cm 2 generated a maximum pressure of 157 kPa and a maximum flow rate of 28.21 ul min −1 .