자기가습형 PEMFC를 위한 Pt-membrane 제조와 성능 비교

안성진, 이한규, 김도영, 이태희

화학공학과, 연세대학교

Comparision and Development of Pt-membranes Reformed by Two kinds of Methods For Self-humidification PEMFC

S. J. An, H. K. Lee, D. Y. Kim and T. H. Lee

Dept. of Chem. Eng., Yonsei University

Introduction

Water management in electrolytes is one of the complicated problems to be overcome.  So far, water content in PEMs has been managed indirectly by humidifying reactant gas.  External humidification of the reactant gas is a burden for the fuel cell system. It requires a gas humidification subsystem that adds to the weight.  In state-of-the-art commercial stacks with stack integrated gas humidification systems (DeNora, Italy, 5 kW stack and Ballard Power Systems, Canada, 5kW stacks), the humidification section accounts for about 20% of the stack weight.  Furthermore, the humidified reactant gas transports latent heat caused by water vaporization into the cell, which has to be removed by the cooling system.  Therefore, this method makes the system complicate and lower the cell's energy efficiency because of the large latent heat. This also has difficulty responding to a quick start-up due to a large heat capacity of water reservoir.

In this study, our goal is to manufacture Pt-membrane for operating fuel cells without external humidification.  So, we manufacture two different reformed Pt-membranes, first by cycling the membrane between the reduction agent and Pt solution, second by dipping it in reduction agent after Pt ions are equilibrium in it.  We changed the reaction time both at cycling and dipping and the cell temperature for proper operating conditions.

Theory

We researched new PEMs that did not require external humidification subsystems.  The Pt particles in the membrane produced water by the crossover of hydrogen and oxygen.  These gases lowered cell performance. We made new PEMs by applying different methods and demonstrated the characteristics of self-humidification under the various operating temperatures.

In the cycling method study, the Impregnation reduction reaction is illustrated schematically in figure 1. The solution of the reduction agent, , and metal ions, Pt+2, were separated by a membrane. The two components diffused into the membrane and simultaneously reacted with each other to form reduced Pt in the membrane.

In the dipping method study, equilibrium impregnation-reduction is also illustrated schematically in figure 1. But the procedure is different from the cycling method. Unlike the cycling method, which is simultaneous reaction, the membrane was equilibrated for 24 hr at 30 0C with a solution of Pt(NH3)4Cl2 in the impregnation step.  After which, the membrane was exposed to solution of NaBH4.

Experimental

In this experiment, E-Tek electrodes were used.  The average Pt loading in the electrode is 0.4 mg/cm2.  Prior to preparation of the membrane-electrode assembly, the electrodes were impregnated with Nafion solution.  The membrane was Nafion 115 (thickness: 127m) and Nafion 112 (thickness: 50□m).  These were cleaned by being immersed in boiling 3% H2O2 for 1 hour, and then in boiled 1 mole H2SO4 for 1 hour.  The membrane was then rinsed in boiling de-ionized water for 1 hour and the procedure was repeated at least twice to remove the sulfuric acid completely.

In the cycling method study, Pt was loaded with a reaction time of 15 minutes, cycled 4 times.   The concentration of 0.5 mM Pt(NH3)4Cl2 and 1.0 M NaBH4 at pH 13 (NaOH) were used as a platinum solution and a reduction agent, respectively.  The side of the membrane facing the platinum solution was turned over to the reduction agent solution and this process was repeated for 4 times.

In the dipping method study, the membrane was equilibrated for 24 hr with a solution of 0.5 mM Pt(NH3)4Cl2.  After impregnation, the membrane was exposed to a solution of NaBH4 at pH 13 (NaOH) that varied from 10 min to 20 min.  After reaction, the membrane was boiled in a solution of 1M H2SO4 and in distilled water to exchange sodium ions with hydrogen ions in the membrane.  After completing the membrane and the electrode treatment, they were assembled and hot pressed at 1200C and 3 Metric ton for 3 minutes. The operating temperature was 30, 50, and 60 oC and the pressure of cell was 1 atm.

Results and Discussion

At first, we examined the influence of the Pt loading method to control the equal distribution of Pt particles.  The operating cell temperature was 50 0C.  As the cell temperature rose, the performance data of cycling method was very unstable in some cases (we couldn't get the data from cycling 30 min. 2 times and cycling 30 min. 4 times) so we fixed the temperature to compare the effect of different loading methods.   Cycling for 15 minutes 4 times was the best cell performance we could get.  We assumed that reaction time played in an important role at forming water in the membrane when cell operated.  The performance using the dipping method was also very unstable over 50 0C and the performance depended on reduction time.  When reduction time was over 10 minutes, the performance was poor.  Figure 2 shows the performance of the Pt-membrane reformed by cycling 15 min. 4 times and by 10 minutes dipping for reduction.  We got better results from the dipping method than that from the cycling method.

We examined the dependence of the cell operation temperature and the dependence of reduction time using dipping method on cell performance.  Figure 3 and figure 4 shows the Effect of reduction time using dipping method and cell temperature.  Reduction for 10 minutes caused the best performance.  When reduction time was longer than 10 minutes, Pt particles gathered together and grew bigger within the membrane.  This meant the active area of the Pt particles decreased.  The result was that membrane dried during cell operation without subsystems of humidifier because poor activities of Pt particles in the membrane did not generate enough water not to dry the membrane.  Cell temperature was also an important factor.  Cell performance was increased by 50 0C but not at 600C.  We assumed that cell temperature over 50 0C was too high to keep hydrating the membrane without external water support.  Putting the cell temperature at 50 0C was much more profitable than that at 30 0C for reacting diffusion gas with Pt particles.  Figure 5 shows the cell performance with reformed Nafion 115 and Nafion 112.  Temperature and rate of gas flow were fixed at 50 0C and 25 ml/min, respectively.  When the flow rate went past 25 ml/min., we could not get data because the high rate of gas flow took water away from the surface of the membrane.  As figure 5 shows, reformed Nafion 112 performed better than reformed Nafion 115.  This is because hydrogen ion permeability of normal Nafion 112 is higher than that of normal Nafion 115.  As well, Nafion 112 is thinner than Nafion 115.  Therefore, less amount of water was required in Nafion 112 and it had low ohmic overpotential.  Table 1 shows physicochemical properties of Nafion membranes.

Conclusion

We developed and compared Pt-membranes that were reformed by dipping and cycling methods.  The results were :

1.	Performance of fuel cells with Pt-membranes indicated that PEM fuel cell could be operated without subsystem of humidification.

2.	The dipping method performed better than the cycling method.

3.	Proper reduction time of dipping method was 10 minutes.

4.	Proper cell temperature was 50 0C.

Reference

1. Raymond Liu, Wei-Haw Her, and Peter S. Fedkiw, J.Electrochem.Soc., 139, 1(1992)

2. Masahiro Watanabe, Hiroyuki Uchida, yasuhiro Seki, and Masaomi Emori, J.Electrochem.Soc.,143, 12(1996)

3. Masahiro Watanabe, Hiroyuki Uchida, yasuhiro Seki, and Masaomi Emori, J.Electrochem.Soc.,5, 4(1998)

Figure 1. The mechanism of Pt particle formation

Table 1. Physicochemical properties of Nafion membrane
Property	Nafion 112	Nafion115
Thickness (□m)	50	127
Hydrogen ion permeability (mol/m2 h)	21.50□10-2	9.62□10-2
Amount of water (unit-area based)	0.314	0.381