Performance of Direct Formate Fuel Cell Using Non-Precious Metal Cathode Catalyst
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(2) Overall:. 1 HCOO− + O2 + OH − → CO23− + H2O, 2 E 0 =1.45V (vs. SHE). (3). Several studies have been done on the direct formate fuel cell (DFFC), as summarized in Table 1. The effect of different operating parameters on the DFFC performance were investigated such as formate salt or electrolyte (KOH) concentration, types of formate salt (HCOOK or HCOONa) and operating temperature. Bartrom and Haan (2012) are the first who reported on direct formate fuel cell (DFFC) operation. The effect of different formate salts (HCOOK or HCOONa) on DFFC performance was studied. DFFC with HCOOK in KOH produces a slightly higher power density than that achieved by HCOONa in KOH as anode fuel. Next, they found that the DFFC still produces competitive performance when KOH was removed from the fuel stream (Bartrom et al., 2013). Nguyen et al. (2013) also reported the significant performance of DFFC operation without addition of KOH as compared with the other alkaline direct alcohol fuel cell. In these works, the membrane electrode assemblies (MEA) used were composed of palladium black and platinum black for the anode and cathode catalyst, respectively. Besides, there are also studies done using non-platinum cathode (non-precious metal; NPM) catalyst as one of the efforts to develop a low cost DFFC. A promising DFFC using carbon-supported palladium (Pd/C) anode and carbon-supported silver (Ag/C) (Jiang and Wieckowski, 2012), non-precious Fe–Co (Zeng et al., 2014) and Fe–Co/C cathode catalyst (Wang et al., 2016) have been reported, as shown in Table 1. These studies used relatively high concentrations of fuel comparing to the case of Pt cathode and they showed high power density. These facts revealed that DFFCs with the NPM cathode catalyst can achieve promising and comparable performance with high concentration fuel as compared to the Pt-based cathode catalyst. The high performance using NPM is supposed to be caused by not only high ORR activity of NPM catalyst in alkaline condition, but. also high fuel tolerance of NPM catalyst, which means inactive to fuel due to the crossover. However, the effect of the fuel tolerance of NPM and Pt under alkaline condition was not investigated in previous studies. Therefore, we present here a DFFC with the transition metal-nitrogen doped carbon nanotube catalyst (Fe-NCNT and Co-NCNT) at the cathode which has not yet been reported for DFFC operation. We have previously reported that the Co-NCNT catalyst demonstrated the highest performance, which is comparable to the direct formic acid fuel cell (DFAFC) with Pt/C cathode, due to its high fuel tolerance in acidic medium (Abd Lah Halim et al., 2019). However, the performance under alkaline condition is still unclear. Therefore, the ORR activity and the formate tolerance, which is an ORR activity containing the fuel, under alkaline condition was studied. The synthesized catalysts were applied as cathode catalyst and the effect of different HCOONa concentrations on the DFFC performance was studied. The performance obtained under the alkaline condition was then compared with the acidic condition reported in our previous study.. 1. Experimental Method 1.1 Materials Multi-walled carbon nanotubes (MWCNTs) and dicyandiamide (DCDA) were purchased from Tokyo Chemical Industry Co. Ltd., which acted as the carbon support and nitrogen precursor, respectively. The metal sources were iron chloride (FeCl3) and cobalt chloride (CoCl2), and the polyvinylpyrrolidone (PVP) were supplied by Wako Pure Chemical Industries, Ltd. Potassium hydroxide (KOH) and sodium formate (HCOONa) were also from Wako Pure Chemical Industries, Ltd. 1.2 Catalyst preparation Iron- and cobalt-nitrogen doped-carbon nanotubes (FeNCNT and Co-NCNT) catalysts were prepared based on a previous work (Ratso et al., 2016a). Anhydrous metal chlo-. Table 1 Summary of direct formate fuel cell (DFFC) studies Anode catalyst. Cathode catalyst. Operating temperature. Bartrom and Haan (2012). Pd black. Pt black. 60°C. Bartrom and Haan (2012). Pd black. Pt black. 60°C. Bartrom et al. (2013). Pd black. Pt black. 60°C. Nguyen et al., (2013). Pd black. Pt black. 50°C. Jiang and Wieckowski (2012). Pd/C. Ag/C. 120°C. Zeng et al. (2014). Pd/C. Fe-Co. 80°C. Wang et al. (2016). Pd/C. Fe-Co/C. 60°C. Reference. Vol. 54 No. 5 2021. Fuel and oxidant 1 M HCOOK+2 M KOH Oxygen 1 M HCOONa+2 M KOH Oxygen 1 M HCOOK+2 M KOH Oxygen 1 M HCOOK Oxygen 6 M HCOOK+2 M KOH Dry oxygen 5 M HCOOK+1 M KOH Dry oxygen 4 M HCOOK+4M KOH Oxygen. Max. power density [mW cm−2] 144 125 267 106 160 250 258. 233.
(3) ride (FeCl3 or CoCl2), dicyandiamide (DCDA) and multiwalled carbon nanotubes (MWCNTs) were used as metal, nitrogen and carbon precursor, respectively. MWCNTs were mixed in ethanol by sonication for 30 min. After a homogeneous solution was obtained, anhydrous metal salt (FeCl3 or CoCl2) was added. The amounts of Fe and Co added were 2.5% and 5% of the MWCNTs weight, respectively. A nitrogen precursor, DCDA with a weight ratio to MWCNTs of 20 : 1, was added. The mixture was sonicated for 2 h and dried in a vacuum oven at 70°C. The dried powder obtained was pyrolyzed in N2 flowing tube furnace at 800°C for 2 h. Catalysts obtained were designated as Fe-NCNT and CoNCNT. 1.3 Electrochemical characterization The catalyst ink for the oxygen reduction reaction (ORR) activity measurement was prepared by mixing the catalyst powder with 2-propanol, water and alkaline ionomer solution (Sustainion XB-7). The mixture was sonicated for 30 min. The catalyst ink was dropped on the glassy carbon (GC) electrode with the catalyst loading of 17.3 µg/cm2. The catalyst coated GC electrode was dried at 60°C for 15 min. The electrochemical measurement was done in a conventional three-electrode electrochemical cell configuration. The cell consists of catalyst ink coated glassy carbon (GC) disk as working electrode, Pt wire as the counter electrode and reversible hydrogen electrode (RHE) as reference electrode. As electrolyte, 0.1 M KOH solution was purged with N2 gas for 30 min before measurement. Then, cyclic voltammetry (CV) was performed for 20 cycles in a potential range from 0 to 1.2 V (vs. RHE) at a scan rate of 10 mVs−1. Then, the oxygen reduction reaction (ORR) polarization curve was recorded by using the rotating disk electrode (RDE) technique in O2-saturated 0.1 M KOH solution using RRDE-3A Rotating Ring Disk Electrode Apparatus (ALS Co., Ltd.) with measurement error of less than 1.0%. The catalyst tolerance toward the formate salt was also conducted in 0.1 M KOH with the absence and presence of 1 M of HCOONa. These RDE measurements were also carried out for commercial Pt/C (TEC10E50E, Tanaka Holdings Co., Ltd.) for comparison. 1.4 Single cell fabrication and measurement Membrane electrode assemblies (MEA) were prepared using commercial Pd/C (Ishifuku Metal Industry Co., Ltd.) as the anode catalyst and Fe-NCNT or Co-NCNT as the cathode catalyst. Catalyst ink was prepared by mixing the catalyst powder with 5 wt% alkaline ionomer solution (Sustainion XB-7), 2-propanol and water. A homogeneous ink solution was obtained after 30 min of sonication. Catalyst layers were ultrasonically sprayed on the carbon cloth (CC plain, Etek) and carbon paper (Toray TGP-H-060) for the anode and cathode, respectively, as reported in other study (Zainoodin et al., 2018). Pd/C with Pd loading of 2.0 mg cm−2 was prepared for the anode. For the cathode catalyst, the Fe-NCNT and Co-NCNT loading obtained was 2.9 mg cm−2 and 2.3 mg cm−2, respectively. For Pt/C cathode catalyst, 2.0 mg cm−2 of Pt loading was also pre234. pared for comparison. The ionomer content for both anode and cathode catalyst is 50 wt% of the catalyst loading. Before the MEA fabrication, a Fumapem FAA-3-50 membrane (FuMA-Tech) as an anion exchange membrane (AEM) was pre-treated in 0.5 M KOH solution overnight. The MEA was fabricated by pressing the AEM in between the anode and cathode electrodes at room temperature, 5 MPa for 5 min. The MEA was placed in a single cell (FC-05-02-H2R, ElectroChem, Inc.) that consisted of current collectors, graphite block containing flow field channels and rubber sheets which act as gasket. The single cell was pre-treated by 2 M KOH containing 2 M HCOONa supplied to the anode and dry oxygen supplied to the cathode side at 60°C. The current–time (i–t) and current–voltage (i–V) were repeatedly measured for several times until stable performance was achieved. After completing the pre-treatment, the single cell was tested with 2 M KOH containing different HCOONa concentrations (2, 4 and 6 M) and 500 mL min−1 dry oxygen at 60°C. The single-cell measurement was conducted using a potentiostat (HZ-7000, Hokuto Denko Corp.) with a potential measurement accuracy of ±0.05% of reading ±1 mV and a current measurement accuracy of ±0.2% of full-scale range.. 2. Results and Discussion 2.1 Electrochemical characterization Based on the physical characterization by XRD and XPS which were explained in our previous work, nitrogen and metal (Fe and Co) were incorporated into the MWCNT carbon structure for both synthesized catalysts. The nitrogen binding configuration existed in the Fe-NCNT and Co-NCNT are pyridinic-N and pyrrolic-N species (Abd Lah Halim et al., 2019). These nitrogen species were proved as contributing to the ORR activity in both acidic and alkaline condition (Osmieri et al., 2016). Therefore, the ORR activity of the Fe-NCNT and Co-NCNT catalyst in alkaline condition were conducted. The reduction curve on the ORR activity of the Fe-NCNT and Co-NCNT catalyst synthesized in this study and the commercial Pt/C catalyst. Fig. 1 The oxygen reduction reaction (ORR) curve for commercial Pt/C, Fe-NCNT and Co-NCNT catalyst in the O2-saturated 0.1 M KOH solution; rotation rate is 1,900 rpm Journal of Chemical Engineering of Japan.
(4) in alkaline medium is shown in Figure 1. These reduction curves are obtained by subtracting the capacitance current measured in the N2-saturated electrolyte from the reduction current in the O2-saturated electrolyte. Based on the reduction curve, the ORR catalytic activity was determined by the onset potential when the reduction current is first observed (−0.1 mA cm−2) above the background capacitance current (Wei et al., 2000). It was found that the commercial Pt/C catalyst gave the highest onset potential in alkaline medium, which is 0.95 V. The onset potential reduced to 0.87 V for both Fe-NCNT and Co-NCNT catalyst. The small difference (80 mV) between of the onset potential between the commercial Pt/C catalyst and the NPM catalysts synthesized in this study was also shown in previous studies reported on. Fig. 2 The oxygen reduction reaction (ORR) curve for (a) commercial Pt/C, (b) Fe-NCNT and (c) Co-NCNT catalyst in the absence and presence of 1 M HCOONa in O2-saturated 0.1 M KOH solution; otation rate is 1,900 rpm Vol. 54 No. 5 2021. the non-precious metal N-doped carbon based catalyst for which the ORR activity was comparable with the commercial Pt/C catalyst in alkaline medium (Yao et al., 2015; Zhao et al., 2016; Ratso et al., 2016b). In addition, the onset potential difference between these NPM catalysts with the Pt/C catalyst is less under the alkaline condition as compared with the acidic condition reported in a previous work (Abd Lah Halim et al., 2019). For the single DFFC application, it is important to determine the tolerance of cathode catalyst toward the formate. To the best of our knowledge, the formate tolerance on the NPM catalysts synthesized in this study has not been investigated elsewhere. Therefore, the catalyst tolerance toward sodium formate (HCOONa) was investigated for the FeNCNT, Co-NCNT catalyst. The reduction curve for ORR activity of the Fe-NCNT, Co-NCNT and commercial Pt/C catalyst in the absence and presence of HCOONa are presented in Figure 2. For the commercial Pt/C catalyst, there is an obvious formate oxidation peak observed which indicates that the Pt/C catalyst has low tolerance toward HCOONa in alkaline medium. In addition, the onset potential is reduced to 0.4 V in the presence of HCOONa. Meanwhile, both Fe- and CoNCNT catalyst did not show formate oxidation peak in the presence of HCOONa, indicating high formate tolerance in the alkaline medium. Fe-NCNT catalyst was found to be slightly affected by the presence of HCOONa for which the onset potential shifted from 0.87 to 0.86 V; whereas, CoNCNT recorded similar onset potential in the absence and presence of HCOONa, indicating high tolerance toward formate. It can be concluded that these NPM catalysts have better formate tolerance as compared with the commercial Pt/C catalyst in alkaline medium. As compared with the previous study, the NPM catalysts exhibited better fuel tolerance than the commercial Pt/C catalyst under acidic condition, which leads to high performance when applied to the cathode in DFAFC operation. Thus, the Fe-NCNT and Co-NCNT catalyst were applied as the cathode catalyst for single-cell measurement in DFFC operation. 2.2 Single direct formate fuel cell (DFFC) measurement Single DFFC measurement was conducted by using different cathode catalysts which are commercial Pt/C, FeNCNT and Co-NCNT. At first, the performance was measured and compared at 60°C, 2 M HCOONa with 2 M KOH as the anode and 500 mL min−1 dry oxygen as the cathode feed. The results are presented in Figure 3 and the data are summarized in Table 2. From the measurement, it was shown that the highest performance was achieved by the DFFC using commercial Pt/C as the cathode catalyst. The maximum power density is 103.9 mW cm−2. This value is comparable with the other DFFCs reported in literatures (Bartrom and Haan, 2012; Nguyen et al., 2013). The maximum power density achieved by DFFC with Fe- NCNT and Co-NCNT are lower than the Pt/C. To increase the performance for the DFFC with non-precious metal cathode catalyst, the usage of fuel can be increased as they showed 235.
(5) Fig. 3 Polarization and power density curve for DFFC using Pt/C, Fe-NCNT and Co-NCNT catalyst as the cathode with 2 M HCOONa and 2 M KOH as the fuel at 60°C of operating temperature Table 2 Single-cell performance result for DFFC at 60°C Cathode catalyst. Open circuit voltage, OCV [V]. Cell resistance, R [mΩ]. Maximum power density [mW cm−2]. Pt/C Fe-NCNT Co-NCNT. 0.92 0.90 0.87. 72 65 65. 103.9 32.2 88.7. high fuel tolerance. Therefore, the single cell was further tested in different fuel concentration which discussed in the next section. 2.2.1 Effect of different formate (HCOONa) concentration on DFFC performance The effect of the HCOONa concentration on the DFFC performance of the Pt/C, FeNCNT and Co-NCNT cathode catalyst are presented in Figure 4 and the results are summarized in Table 3. The anode fuel was maintained at 2 M KOH with 2, 4 and 6 M HCOONa. It is shown that the highest maximum power density achieved by the DFFC utilizing Pt/C as cathode catalyst at 2 M HCOONa. As the concentration of HCOONa increases higher than 2 M, the performance of the DFFC is significantly decreased. The performance degradation at higher HCOONa concentration would be due to the crossover of formate and alkali from the anode to the cathode through the membrane. Since the Pt/C catalyst showed very low tolerance toward HCOONa, the presence of HCOONa at the cathode leads to the formate oxidation reaction on the Pt surface and thus, reduced cell performance. In addition, the effect of the crossover can be verified from the drop of open circuit voltage (OCV) as the HCOONa concentration increases from 2 to 4 M (Su et al., 2019). For the Fe- and Co-NCNT cathode catalyst, the optimum concentration is at 4 M HCOONa, resulting a maximum power density of 39.7 mW cm−2 and 89.8 mW cm−2 for Fe-NCNT and CoNCNT, respectively. The performance decreases for both catalysts as the HCOONa concentration increased more than 4 M. This similar trend was found in another study reported (Zeng et al., 2014); a DFFC employing non-precious metal catalyst at the cathode performed the best at an optimum fuel concentration (5 M HCOOK with 1 M KOH). 236. Fig. 4 Polarization and power density curve for DFFC using CoNCNT catalyst as the cathode with 2 M KOH and (2–6 M HCOONa) as the fuel at 60°C and dry oxygen as the oxidant Table 3 Summary of single-cell measurement in DFFC operation at different fuel concentration Cathode catalyst Pt/C. Fe-NCNT. Co-NCNT. Open circuit Formate voltage, concentration OCV [V] [M] 2 4 6 2 4 6 2 4 6. 0.92 0.87 0.85 0.90 0.97 0.83 0.88 0.88 0.91. Maximum Cell resistance, power density [mW cm−2] R [mΩ] 72 86 98 65 64 64 64 74 76. 103.9 66.1 52.9 32.2 39.7 19.9 88.7 89.8 76.9. Journal of Chemical Engineering of Japan.
(6) Based on these results, both NPM catalysts showed lower maximum power density than the Pt/C catalyst in DFFC at 2 M HCOONa. However, as the HCOONa increased to 4 and 6 M, the Co-NCNT catalyst showed higher maximum power density than the Pt/C catalyst. This is due to the low formate tolerance of Pt/C catalyst, thus reducing the DFFC performance at higher formate concentration. Nevertheless, as the formate concentration increases from 4 to 6 M, the maximum power density decreases for both NPM catalysts. For Fe-NCNT catalyst, the OCV decreases at 6 M HCOONa, indicating the fuel crossover effect on the Fe-NCNT catalyst which is also proved in Figure 2(b) in that there is slight decrease in the onset potential in the presence of formate. Meanwhile, it can be observed that the OCV increases at 6 M HCOONa for Co-NCNT catalyst, indicating the high formate tolerance of the Co-NCNT catalyst. The decrease in the maximum power density can be ascribed by the high cell resistance at high fuel concentration, which leads to the decreased of membrane conductivity due to the insufficient water to hydrate the membrane (Wang et al., 2016). For the alkaline condition, it can be concluded that the maximum power densities of DFFC with both NPM catalysts are lower than that achieved by Pt/C catalyst at optimal formate concentration. Comparing with the acidic condition in our previous work (Abd Lah Halim et al., 2019), the maximum power density achieved by DFAFC with the NPM cathode catalyst is higher than that of the Pt/C cathode catalyst at optimal fuel concentration. Therefore, the NPM catalysts studied are preferable for the acidic condition over the alkaline condition. In addition, the fuel crossover under acidic condition is more significant than the alkaline condition, which is indicated by the decay in OCV value. For instance, the Pt/C cathode catalyst experienced a large decrease in OCV, which is 100 mV in DFAFC operation; whereas, the OCV reduction is less (50 mV) in the DFFC operation, indicating the less severe fuel crossover in the DFFC operation as compared with the DFAFC operation. Since the DFAFC with NPM cathode catalyst experienced less decay of OCV than the Pt/C catalyst, it is suggested that the application of the high fuel tolerant NPM catalyst at the cathode is needed and more effective for achieving high performance in DFAFC operation.. Conclusions In summary, iron- and cobalt nitrogen-doped carbon nanotubes (Fe-NCNT and Co-NCNT) were synthesized and applied as the cathode catalyst for DFFC. From the electrochemical measurement in alkaline medium, the prepared catalysts showed lower ORR activity as compared with the commercial Pt/C catalyst. However, both Fe-NCNT and Co-NCNT catalyst exhibited better formate tolerance than the Pt/C catalyst. For the single-cell performance test, the effect of different fuel concentration on the DFFC performance was investigated. DFFC with commercial Pt/C catalyst showed the highest maximum power density of 103.9 mW cm−2 with 2 M KOH containing 2 M HCOONa. Vol. 54 No. 5 2021. DFFC with Fe-NCNT and Co-NCNT cathode catalyst gives the highest maximum power density of 39.7 mW cm−2 and 89.8 mW cm−2, respectively, at 60°C with 2 M KOH containing 4 M HCOONa. DFFC with the NPM catalysts showed lower performance than Pt/C catalyst at 2 M HCOONa, but Co-NCNT catalyst exhibited better performance than the Pt/C catalyst at 4 M and 6 M HCOONa, indicating that the NPM catalyst is available for high concentration operation of DFFC. Comparing with our previous work, the NPM catalysts are more effective as cathode catalyst in DFAFC than the DFFC operation, and thus, these NPM catalysts are more preferable for the acidic condition than the alkaline condition. Acknowledgement This work was supported by JSPS KAKENHI 18H01774.. Literature Cited Abd Lah Halim, F., T. Tsujiguchi, Y. Osaka and A. Kodama; “Performance of Direct Formic Acid Fuel Cell Using Transition MetalNitrogen-Doped Carbon Nanotubes as Cathode Catalysts,” Int. J. Energy Res., 43, 8070–8084 (2019) An, L. and R. Chen; “Direct Formate Fuel Cells: A Review,” J. Power Sources, 320, 127–139 (2016) Bartrom, A. M. and J. L. Haan; “The Direct Formate Fuel Cell with an Alkaline Anion Exchange Membrane,” J. Power Sources, 214, 68–74 (2012) Bartrom, A. M., J. Ta, T. Q. Nguyen, J. Her, A. Donovan and J. L. Haan; “Optimization of an Anode Fabrication Method for the Alkaline Direct Formate Fuel Cell,” J. Power Sources, 229, 234–238 (2013) Jiang, J. and A. Wieckowski; “Prospective Direct Formate Fuel Cell,” Electrochem. Commun., 18, 41–43 (2012) Nguyen, T. Q., A. M. Bartrom, K. Tran and J. L. 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(7) Carbon Nanotubes with Different Morphologies from MelamineFormaldehyde Resin,” ACS Appl. Mater. Interfaces, 7, 7413–7420 (2015) Zainoodin, A. M., T. Tsujiguchi, M. S. Masdar, S. K. Kamarudin, Y. Osaka and A. Kodama; “Performance of a Direct Formic Acid Fuel Cell Fabricated by Ultrasonic Spraying,” Int. J. Hydrogen Energy, 43, 6413–6420 (2018). 238. Zeng, L., Z. K. Tang and T. S. Zhao; “A High-Performance Alkaline Exchange Membrane Direct Formate Fuel Cell,” Appl. Energy, 115, 405–410 (2014) Zhao, Q., Q. Ma, F. Pan, J. Guo and J. Zhang; “Facile Synthesis of N-Doped Carbon Nanosheet-Encased Cobalt Nanoparticles as Efficient Oxygen Reduction Catalysts in Alkaline and Acidic Media,” Ionics, 22, 2203–2212 (2016). Journal of Chemical Engineering of Japan.
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