Effects of Nafion ionomer content on the performance of membraneless direct formate fuel cells

Herein, the performance of alkaline, membraneless, and direct formate fuel cells (DFFCs) in a particular configuration was measured and investigated at various fuel concentrations ranging from 1 to 5 M and fuel flow rates ranging from 1 to 5 mL/min, with anode Nafion ionomer contents of 3.73, 2.49, and 1.87 mg/cm2, at cell temperatures ranging from room temperature to 60℃. An anolyte solution containing potassium formate (HCOOK) and potassium hydroxide (KOH) was fed into the flow field sandwiched between the anode and cathode catalyst layers, while air was forced into the airflow field at a 100‐sccm flow rate as the oxidant. Both Pd and Pt are used as the anode and cathode catalysts, respectively, of the DFFCs with 2‐mg/cm2 identical loading. The results showed that both the open circuit voltage and the maximum power density of the DFFCs were enhanced by reducing the Nafion ionomer content. Besides, feeding the DFFCs with the anolyte containing 3‐M HCOOK in 3‐M KOH yielded the highest cell output. As the DFFCs were fed with the anolyte containing 3‐M HCOOK in 3‐M KOH at 3 mL/min with anode Nafion ionomer content of 1.87 mg/cm2, the peak power density values of the fuel cells at room temperature and 60℃ were 66.24 and 85.60 mW/cm2, respectively.


| INTRODUCTION
Direct liquid fuel cells (DLFCs) whose anode are fed with aqueous reactants are currently studied intensively because of their numerous advantages, such as instant recharging, ease of transport and storage, and high energy density, 1 in contrast with H 2 -fed PEMFCs. Although DLFCs use a polymer membrane to separate the anodic and cathodic reactions and transport the ion, fuel crossover from the anode to the cathode degrades the cell performance and remains a challenge. 1 Using a liquid electrolyte stream by DLFCs to replace the proton exchange membrane might be a promising architecture for DLFCs, 2-12 since the electrolyte flow between electrodes can prevent the fuel from approaching the cathode, enhance the ionic conductivity, and reduce the fuel cell cost. Membraneless fuel cells (MFCs) are generally achieved using a one-pass, single microchannel sandwiched between two face-to-face catalyst layers, and in which two laminar streams, including the anolyte and an electrolyte, flow in parallel. Since the electrolyte stream flows between the fuel and the cathode catalyst layer, the fuel molecules diffused into the electrolyte stream can be carried downstream 13 and then discharged, leading to negligible fuel crossover. Besides commonly used liquid fuels, such as formic acid, 2,3,5,9 methanol, 6,7 and alcohol, 10 there are still numerous chemicals that can be used as liquid fuel, such as the H 2 O 2 , 4 NaBO 3 ⋅4H 2 O, 8 NaBH 4 , 11 and HCOONa, 12 in acidic or alkaline media. Note that direct formate fuel cells (DFFCs) have recently attracted significant attention and are becoming an emerging energy technology, primarily because they use carbon-neutral fuel with low-cost electrocatalytic and membrane materials. 14-20 Among those studies, the low catalytic activity of Pt/C for the oxidation of potassium formate was also found in. 18 Furthermore, although numerous MFC studies have been published, only a few studies 13,14,21,22 attempted to enlarge the fuel flow field of the MFCs with more complicated fuel cell configuration to produce a higher current density.
Concerning the DLFC performance, besides reducing fuel crossover, the gas diffusion electrode (GDE) for the anode must be specially treated whether the polymer membrane is used or not, because, first, the fuel is required to wet the catalyst layer of the GDE. Second, adequate room in the electrode is necessary to permit the gaseous product (CO 2 ) removal for the case of methanol and formic acid oxidation. Finally, optimal dispersion of the polymer binder must be presented within the catalyst layer to keep the discrete catalyst particle retained and highly porous on the GDE.
Li et al 23 tested alkaline direct ethanol fuel cells (DEFCs) with either an anion-conducting ionomer A3 or a neutral polymer PTFE, presented in the anode catalyst layer. The results showed that the cell performance depended on the A3 ionomer content, when feeding the C 2 H 5 OH solution as fuel, whereas the PTFE binder yielded better performance than the A3 binder when feeding the C 2 H 5 OH-KOH solution as fuel. Kang et al 24 investigated the effect of Nafion ionomer aggregation state, which can be tuned by heat treatment of Nafion solution, within the anode catalytic layer for a direct formic acid fuel cell. The results showed that the Nafion aggregation decrement within the catalyst ink increased the Nafion ionomer and catalyst utilization. Once the Nafion solution was preheated before coating on the diffusion layer, the Nafion loading within the anode catalyst layer became a minor effect on the cell performance.
An et al synthesized a novel agar chemical hydrogel (ACH) electrode binder to prepare the fuel cell cathode for alkaline-acid DEFC. 25 The results revealed that using the ACH-based cathode in the fuel cell fed with a fuelelectrolyte solution improved the cell performance than using conventional Nafion ionomer-based electrodes because of the hydrophilic nature and water retention characteristic of agar caused superior mass/charge transport in the fuel cell. Sun and Li 26 proposed a concept of preparing anion-ionomer-free electrodes for anion-exchange membrane (QAPS membrane) DFFCs (AEM DFFCs) using Pd as the catalyst for both electrodes. The catalyst layers on both electrodes contained either QAPS (an anion-conducting material) or 10 wt% PTFE as a binder. The results showed that the AEM DFFC containing PTFE in the catalyst layers yielded higher peak power density than the other containing QAPS because the adhesion of PTFE on Pd/C results in a highly porous catalyst layer, and the hydroxide ion-containing formate solution facilitates mass and charge transport, thereby enlarging the triplephase boundary for both anodic formate oxidation and cathodic oxygen reduction reactions. Pan et al 27 compared the PVDF-HFP electrode binder with conventional Nafion and PTFE electrode binders in terms of the electrode morphology and performance of alkaline-acid direct ethylene glycol fuel cells possessing Nafion 211 membrane. They found that the fuel cell using the PVDF-HFP-based anode exhibited the highest peak power density because of the higher electrochemical surface area for its intrinsic porous property, followed by the fuel cell using Nafion-based electrode, and the worst was the fuel cell using PTFE-based electrode.
From the abovementioned literature, an ionomer is necessary for fabricating the DLFC anode, and both the ionomer type and content are important in fuel cell performance. Generally, for conducting anions, fuel cells having A3-and QAPS-based electrodes have been tested, whereas fuel cells having ACH-and Nafion-based electrodes have been tested for conducting cations. Furthermore, some neutral polymers such as PTFE and PVDF-HEP were also used as ionomers in the fuel cell electrode. Herein, alkaline, membraneless, DFFCs with an in-house fabricated anode are fabricated with a single serpentine channel as a fuel flow field over a large-scale electrode, compared to traditional MFCs. The anode and cathode catalyst layers will directly contact the HCOOK solution based on the finding of a previous study. 18 Such design greatly simplifies the membraneless DFFC configuration with only one liquid stream needed for electricity generation. The effect of the contents of the easily accessible and cationconducting Nafion ionomer on the performance of the anion-exchange, membraneless DFFCs will be tested and investigated.

| Fabrication of MFC
Herein, membraneless DFFCs with different amounts of Nafion ionomer added in the anode catalyst layer as a binder were tested at various concentrations and fuel flow rates at a 100-sccm airflow rate. Each MFC tested herein comprised two stainless steel plates as end plates of the fuel cells, PTFE gaskets to avoid leakage, gold-plated copper foils as electric collectors for both electrodes, composite carbon plate having a serpentine channel as airflow field plate, and a fuel/electrolyte flow field plate that was milled to achieve a through serpentine PEEK-made channel. The red and green arrows in the schematic of the membraneless DFFCs ( Figure 1A), respectively, indicate the anolyte and airflow path from the inlet to the outlet. Note that the anode and cathode GDEs were placed on both sides of the fuel/electrolyte flow filed plate with the catalyst layer facing the fuel/electrolyte stream ( Figure 1B). Several holes were drilled on the sidewalls of both end plates to install cartridge heaters and T-Type thermocouples, respectively, to maintain the fuel cells' temperature fixed at a specific value using a temperature control unit.
The thickness of the fuel/electrolyte and airflow field plates are 1.0 and 2.0 mm, respectively. The width, pitch, and depth of the serpentine microchannel fabricated on the airflow field plate are 0.8, 0.8, and 1.0 mm, respectively (Figure 2A), whereas the width and pitch of the through serpentine microchannel on the fuel/electrolyte flow field plate are both 0.8 mm ( Figure 2B). Since the serpentine microchannel on the fuel/electrolyte flow field plate is through, the fuel/electrolyte flow could touch the catalyst layers on both electrodes. Figure 3A shows the arrangement of each component of the membraneless DFFCs, and the exterior dimensions of the actual membraneless DFFCs in Figure 3B are ~60 mm ×60 mm ×37 mm.

| Electrode preparation
The anode GDE was self-fabricated with different amounts of Nafion content. First, the Pd/C catalyst powder (30 wt%, Sigma-Aldrich 407 305) was ultrasonically dispersed in a solution containing deionized water, n-propyl alcohol, and an appropriate amount of Nafion ® PFSA polymer dispersion (20 wt%, DE2020, DuPont™). The average diameter of Pd particles was 5.5 nm. Subsequently, the suspension was pipetted onto a carbon paper (Toray TGP-H-090). The carbon paper coated with catalyst slurry was finally dried in a vacuum oven at 80℃ for 20 min to obtain the anode GDE with a fixed 2-mg/cm 2 Pd loading and desired amount of the Nafion content. Three different levels of the Nafion content in the anode catalyst layer were prepared and tested (Table 1). A commercially available GDE prepared using commercially available carbon-supported Pt catalyst (70 wt%, Alfa Aesar HiSPEC TM 13100) was employed as the cathode GDE. The resulting Pt loading on the cathode GDE was 2 mg/cm 2 . Figure 4 shows the experimental setup schematic comprising several major components, including a tested fuel cell, a fluid delivery system, an air-pumping system, an electronic load for cell performance measurement, and a cell temperature control unit. During the experiments, both electric collectors of the fuel cells were connected to an electronic load (KIKUSUI PLZ-70UA) to measure the polarization curve at ambient pressure under stepwise potentiostatic control. The voltage and current resolution for this electronic load are 0.1 mV and 0.01 mA, respectively. All polarization curves herein were measured with a 50-mV potential step from 1.0 to 0 V. Each potential's duration was 15 seconds, and data were recorded once every 0.5 second using a personal computer. Afterward, the mean of all values recorded at each potential was calculated. The current density i and power density p of this MFC are estimated based on the measured cell current J, cell voltage V, and effective electrode area A = 4.0 cm 2 as follows.

| Experimental setup
A peristaltic pump and its accessories (YOTEC PF103 and FL15) were employed to achieve the circulation between the fuel/electrolyte mixture solution tank and the tested fuel cell connected with silicone tubing at desired volumetric flow rates. Besides, to feed the fuel cells' cathode with atmospheric air, an air-pumping system comprising an air compressor (LIDA JW-1510N), a filter to prevent submicron particle and lubricant mist from entering the gas diffusion layer of the cathode, a pressure regulator to maintain a favorable air pressure before entering the flow meter, and a flow meter (NEW-FLOW TLFC-09-A-1-W-2-A-1-1) to control the flow rate into the cathode, was also used. The temperature control unit comprising a power supply, heaters, temperature sensors, and PID controllers was also employed to maintain a given temperature for the fuel cell and fuel/electrolyte mixture. A mixture of potassium formate (HCOOK) and potassium hydroxide (KOH) solutions was used as the fuel and supporting electrolyte while testing the fuel cells. Table 1 presents the HCOOK-to-KOH The membraneless DFFCs having three different Nafion ionomer contents in the catalyst layer in the fuel cell anode were tested at various concentrations and

| RESULTS AND DISCUSSION
The oxidation and reduction half-reaction of potassium formate and oxygen in alkaline media have been reported 14 as follows: Combining the aforementioned equations, the overall reaction of the alkaline DFFC can be expressed as, Evidently, the theoretical open circuit voltage, which will later be denoted as OCV, shown above, is higher than that of hydrogen and direct methanol fuel cells. Figure 5 shows the membraneless DFFC performance measured using 1-M formate mixed with KOH of various concentrations at various volumetric flow rates, as the Nafion ionomer content in the anode was 3.73 mg/cm 2 . (3) The fuel cell performance increases as the fuel volumetric flow rate increases regardless of the KOH concentration. Besides, the KOH concentration increment at a given formate concentration can improve the fuel cell performance by reducing the cell internal resistance at the expense of the polarization curve retraction in the high current region of the V-I curves. Since the KOH added in the HCOOK solution of the membraneless DFFCs is served as the liquid electrolyte that helps the hydroxide ions to conduct through the anolyte solution to the anode ( Figure 1B), and the ionic conductivity of the anolyte solution usually increases as the liquid electrolyte concentration increases, the cell internal resistance could be reduced as the KOH concentration increases. The maximum power density in Since the performance of this membraneless DFFCs always peaks at a 3.0-mL/min volumetric flow rate, irrespective of the anolyte concentration, the results measured at the 3.0-mL/min flow rate will be discussed. Figure 6 plots the polarization curves measured at various HCOOK concentrations. The OCV decreases as the HCOOK concentration increases because of the cathode mixed potential, and such a drop is significant at low KOH concentration. Also, the membraneless DFFC tested with the anolyte solution containing 1-M KOH revealed relatively high ohmic overpotential, as mentioned earlier. The highest maximum power density in Figure 6 was ~54 mW/ cm 2 when feeding the membraneless DFFC with the anolyte containing 3-M formate and 5-M KOH.
As the Nafion ionomer content in the anode decreases, the maximum power density and OCV increase at a given concentration ( Figure 7A and B). In contrast to the results shown in Figure 6, the membraneless DFFC tested with 3-M HCOOK in 3-M KOH yielded the highest maximum power density in both Figure 7A and B. Figure  the highest power density, the maximum power density of the membraneless DFFC measured at both anolyte compositions at different Nafion ionomer contents is plotted in Figure 9. The maximum power density is increased monotonously as the flow rate increases. Besides, the lower the Nafion ionomer content in the anode, the higher the maximum power density herein. The curves in Figures 6  and 7 show that the maximum power density of the membraneless DFFC tested with the anolyte composition, other than those discussed in Figure 9, is significantly lower. The highest maximum power density reached approximately 66 mW/cm 2 , as the anode of the membraneless DFFC contained 1.87-mg/cm 2 Nafion ionomer.
To discuss the effect of the Nafion ionomer content in the anode catalyst layer on the fuel cell performance, SEM images of the anodic electrode containing different Nafion ionomer contents are presented in Figure 10. Instead of showing the electrode's clear carbon fibers in Figure 10A,  the rest of the SEM images in Figure 10 show numerous flakes of different sizes. It seems that the more the Nafion ionomer is added as a binder, the more the Pd/C powder agglomerates. After drying in the vacuum oven, the Pd/C powder agglomeration on the anode surface is likely to become structurally flaky ( Figure 10B-D). However, the white, fluffy structures on the flakes, like the site where a pink arrow indicates in each SEM image, could be the highly porous catalyst sites because they look like the well-dispersed Pd/C powder, but not solid agglomerates. By observing Figure 10B and C, it seems that the fluffy structure on the anode surface can be uniformly spread as the Nafion ionomer content is few. This trend could explain why less Nafion ionomer content in the anode catalyst layer leads to higher OCV and maximum power density of the membraneless DFFC herein. Since the highest cell output for the membraneless DFFC herein occurred when the membraneless DFFC having Nafion ionomer content of 1.87 mg/cm 2 in the anode with the anolyte solution containing 3-M HCOOK in 3-M KOH at a flow rate of 3 mL/min was fed, the polarization curves of the membraneless DFFC tested under the abovementioned conditions at a specific temperature ranging from 30℃ to 60℃ are shown in Figure 11. As expected, the maximum power density of the membraneless DFFC increases as the cell temperature increases. The highest power density of the membraneless DFFC reached 81 and 85.6 mW/cm 2 at 50℃ and 60℃, respectively. However, as the cell temperature increased from 50℃ to 60℃, the gas bubble was exhausted from the liquid outlet during the experiment because the forced air invades the liquid passage for the incomparable thermal expansion of each fuel cell component. Furthermore, the aforementioned gas bubbles might also be CO 2 , which has been reported 16,18 as a possible side-product during the formate oxidation reaction in alkaline media. Based on the discussion in, 28 it can be realized that CO 2 bubbles can be formed once the current produced is high enough. Therefore, the membraneless DFFC test is limited to 60℃ herein.
Finally, Figure 12 compares the membraneless DFFC performance, herein, with that of the MFCs published in the literature under similar conditions. The detailed conditions under that the fuel cells were measured are listed below. First, the OCV value of this membraneless DFFC exceeds that of most fuel cells, except those tested by Pramanik and Rathoure 11 and Liu et al, 12 in Figure 12. This trend occurs because the theoretical OCVs of the fuel cells in both studies 11,12 are as high as 1.64 and 2.826 V, respectively, for the particular reactants used. However, the actual OCVs of most MFCs in Figure 12 are much lower than the theoretical OCVs, indicating a severe crossover effect and high mixed potential of such MFCs. To alleviate the crossover effect, Liu et al 12 designed a particular fuel cell architecture by introducing an additional electrolyte stream between the fuel and oxidant, possibly leading to a notable ohmic overpotential and a steep polarization curve ( Figure 12). The low mixed potential of the membraneless DFFC with its cathodic catalyst in contact with the fuel is responsible for its high OCV. Furthermore, since the fuel crossover does not significantly affect the performance of this membraneless DFFC, the performance of the membraneless DFFC could be further enhanced by fabricating the fuel flow field plate in Figure 2B to be extremely thin, and widening the channel in the fuel flow field to acquire larger active electrode area.
Second, the polarization curve of the present fuel cell seems to drop more rapidly than the curve measured by Yu

| CONCLUSION
Herein, alkaline, membraneless DFFCs possessing a particular electrode arrangement were proposed and fabricated, and the resulting alkaline, membraneless DFFCs were tested with varying Nafion ionomer contents ranging from 1. 87 to 3.73 mg/cm 2 in the anode at various fuel flow rates, anolyte concentrations, and temperatures at a given airflow rate, 100 sccm. The catalyst in the anode and cathode are Pd and Pt, respectively, with 2-mg/cm 2 identical loading. The results are summarized below.
1. As the Nafion ionomer content decreases from 3.73 to 1.87 mg/cm 2 in the anode, the maximum power density and OCV of the DFFCs increase at a given anolyte concentration. 2. The OCV of the DFFCs was particularly low as the HCOOK concentration was much higher than the KOH concentration. However, the OCV can be increased to 0.92 V when HCOOK and KOH are of comparable concentrations, slightly depending on the Nafion ionomer content and flow rate. 3. The DFFC performance increased as either the operating temperature or fuel volumetric flow rate increased. However, gas bubbles are exhausted through the fuel outlet while testing the DFFCs at 60℃. The highest peak power density of the membraneless DFFCs reached 66.25 and 85.6 mW/cm 2 at room temperature and 60℃, respectively, when feeding the DFFCs containing anode Nafion ionomer of 1.87 mg/cm 2 with 3-M HCOOK in 3-M KOH at a flow rate of 3 mL/min. 4. Even with the arrangement of sandwiching anolyte stream between the anode and cathode catalyst layer, it was found that the mixed potential of this DFFC is insignificant compared to that of published MFCs utilizing aqueous fuel other than HCOOK. It is expected that the performance of the membraneless DFFC could be further enhanced by designing the fuel flow field plate to be extremely thin to reduce the internal resistance, as reported in, 2 and widening the channel in the fuel flow field to make the electrode more useful with oxygen fed as the oxidant.