SUPPORTING INFORMATION
The Influence of Solution pH on Pt Anode Catalyst in Direct Formic Acid Fuel Cells
Jiyong Joo,†,# Myounghoon Choun,‡,# Jahoon Jeong,‡ and Jaeyoung Lee*,†,‡ †
Ertl Center for Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea ‡ School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea * Corresponding author. E-mail:
[email protected]
1
Experimental detail 0.2 M phosphate buffer solutions ranging from pH 2.0 to 12.0, prepared from H3PO4, NaH2PO4, Na2HPO4, and/or Na3PO4, were used as electrolyte solutions. In the absence of the phosphate buffer solution, pH was adjusted by controlling the molar ratio of HCOOH and HCOONa.1-2 All solutions were obtained using 18 MΩcm D.I water. The solutions were saturated with N2 before each experiment. CV were evaluated using a standard three-electrode glass cell and a potentiostat (Biologic, VSP). Catalyst ink was prepared by dispersing 10 mg of 46.8 % Pt/C (Tanaka) in a mixture of 10 µL of 10 wt% Nafion solution (Sigma-Aldrich), 2.5 mL of D.I water, and 2.5 mL of isopropyl alcohol. After the catalyst ink was sonicated for 15 min, an aliquot of the suspension was dropped onto the rotating glassy carbon disk electrode (0.196 cm2). Ag/AgCl (3M KCl) and Pt coil were used as the reference and counter electrodes, respectively. To evaluate the apparent activation energy, CVs were conducted at temperature of 298, 308, and 318 K. The temperature was controlled by circulation of ethylene glycol flowing into a water jacket of the electrochemical cell. In addition to CVs, chronoamperograms (CAs) at voltages of 0.19 V (vs onset potential of hydrogen adsorption (OHA)) were obtained at 298 K. In single cell measurements, Pt/C catalyst ink was coated on a gas diffusion layer/carbon paper (Toray TGP-H-060) with a cell geometric area of 9.0 cm2. Further, 46.8 % Pt/C (Tanaka) of 1 mg cm-2 was used as anode and cathode catalyst. To explain the influence of pH effect on HCOOH/HCOO- oxidation, we used cation exchange membranes to conduct Na+ from the anode to the cathode.3 The cathode and anode were placed on both sides of the cation exchange membrane and were hot pressed at a temperature 140 oC and a pressure of 3 MPa for 300 s. A flow rat of 4 ml/min was used to feed the blended fuel into the anode flow channel and an acid solution (H2SO4) containing H2O2 into the cathode flow channel. The 4M H2O2 and 1M H2SO4 concentration, exhibiting the highest performance, were selected for 2
further examination.4 The operating temperature was set at 60oC. Electrochemical impedance spectra were measured with a two-electrode configuration (Zahner Zennium, Zahner Elektrik GmbH & Co.). The frequency range investigated ranged from 10 kHz to 632 mHz examined at 18 points.5 The amplitude of the sinusoidal voltage was adjusted to 5% of the steady-state voltage.
Long-term poisoning rate To investigate the effect of pH on COads coverage on the catalyst surface, we conducted chronoamperometric measurements to determine the long-term poisoning rate (Figure S1).6 The poisoning rate was calculated following the equation below. σ=
100 × (1)
As shown in Figure S1, the poisoning rate decreases with increasing pH and the lowest poisoning rate appeared at pH of ~7. This implies that formation of CO from indirect oxidation of formic acid or formate is suppressed at pH of ~7 compared to that at pH of < 4. This strongly supports the finding that the highest current density is exhibited at pH of ~7.
Poisoning rate / % s-1
0.020
0.016
0.012
0.008
0.004 2
3
4
5
6
7
pH
Figure S1. Poisoning rate at constant potential of 0.19 V vs onset potential of hydrogen
3
adsorption in various pH electrolyte.
Apparent activation energy We conducted cyclic voltammetry of various pH solution at various temperature to obtain the apparent activation energy values at various potential values (Figure S2). We calculated the apparent activation energy from Arrhenius equation as follows.
k = A (2) j = nFk (3) ln # = ln($% ) −
'( (4) )*
As shown in Figure S2, pH 6.9 solution exhibited the lowest activation energy over the entire voltage region from 0.24 to 0.64 V. The pH 3.62 solution exhibits a similar activation energy to that of pH 2.27 up to 0.34 V and a smaller activation energy than that of pH 2.27 solution, at 0.44 V. The high activation energy of pH 2.27 and 3.62 solutions can be attributed to indirect oxidation of formic acid because indirect oxidation kinetics are slow compared with the kinetics of direct oxidation. Therefore, the lowest activation energy of pH 6.9 reflects the fact that formic acid is predominantly oxidized through the direct oxidation pathway at pH 6.9. This result is in good agreement with the low poisoning rate of the pH 6.9 solutions shown in Figure S2.
4
Activation energy / kJ mol-1
40 35
pH 2.27 pH 3.62 pH 6.9
30 25 20 15 0.2
0.3
0.4
0.5
0.6
0.7
Voltage vs OHA / V
Figure S2. Apparent activation energy of different pH solutions at various potential values. Voltage iss expressed versus onset potential of hydrogen adsorption (OHA).
References 1. Gao, Y.-Y.; Tan, C.-H.; Ye-Ping, L. I.; Guo, J.; Zhang, S.-Y. J. Hydrogen Energy 2012, 37, 3433-3437. 2. Guo, J.; Gao, Y. Y.; Tan, C. H.; Li, Y. P.; Zhao, S. L.; Bai, L. Z.; Zhang, S. Y. Fuel Cells 2013, 13, 167-172. 3. An, L.; Zhao, T. S. Energy Environ. Sci. 2011, 4, 2213-2217. 4. An, L.; Zhao, T. S. J. Hydrogen Energy 2011, 36, 9994-9999. 5. Kang, S.; Lee, J.; Lee, J. K.; Chung, S.-Y.; Tak, Y. J. Phys. Chem. B 2006, 110, 7270-7274. 6. Jiang, J.; Kucernak, A. J. Electroanal. Chem. 2003, 543, 187–199.
5
