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Analysis of Material and Charge Transfer in the Fiber Catalyst Layer for Polymer Electrode Fuel Cells
In recent years, nanofiber electrocatalyst for polymer electrode fuel cells has been proposed [1]. Generally, researches on catalyst layer were focused on particle catalyst layers [2]. However, material and charge transfer in the fiber catalyst layer should be different from that of the particle catalyst layer. In our previous work, a power density of fuel cell was increased with catalyst layer of longer fibers. However, transport situation inside fiber catalyst layer is still unclear. Information on relationship between structure and rate process for fiber catalyst layer are required to design new catalyst layers. In this work, we evaluated material and charge transfer in the fiber catalyst layer by using rotating disk electrode. Carbon alloy nanofiber was examined as a cathode catalyst.
Experimental
Nanofiber catalyst was prepared by electrospinning. Polyacrylonitrile, dimethylformamide, and iron acetylacetonato were mixed. The mixture was electrospun with potential gradient of 1.33 kV cm−1 and flow rate of 50 mL min−1. Obtained polyacrylonitrile nanofiber was stabilized at 250 °C under air atmosphere, and carbonized at 900 °C under nitrogen atmosphere. Nanofiber catalyst was crushed mechanically. We obtained three kinds of nanofibers with mean fiber length of 0.32 (S), 3.5 (M), and 6.9 mm (L).
The relationship between amount of catalyst loading and mass/charge transfer rate was evaluated by using rotating disk electrode. Oxygen reduction current was measured under oxygen flow in 0.5 M H2SO4 solution. Linear sweep voltammetry was performed from 0 V vs. NHE to 1.2 V vs. NHE with scan speed of 10 mV s−1. Rotating frequency was from 400 rpm to 2500 rpm. The effect of Levich current, diffusion limited current through the solution boundary layer, was removed by Koutecky–Levich plot. Consequently, we obtained a current density which reflects rate process inside catalyst layer (iI). We assumed void fraction and active site distribution inside catalyst layer are uniform. We evaluated contributions of kinetic current density (iK) and diffusion limited current density (iS), respectively by Bard−Andrieux's method [3−5].
Results and Discussion
Radii of obtained fibers were around 200–300 nm. A relationship between catalyst loading and iIfor each fiber catalyst layer is shown in Figure 1. The transport situation was discontinuity changed at higher loading amount of the fiber catalyst for M and L. The discontinuity occurred at lower amount of catalyst loading for L compared with that of M. The mass transfer rate and charge transfer rate was increased at higher loading amount of longer catalyst. This situation is explained by the difference of layer structure between the fiber catalyst layer and the particle-like catalyst layer. At higher loading amount of longer catalyst, the catalyst layer is considered to show a high void fraction. Fiber catalyst is thought to be stood on glassy carbon electrode. Hence, charge transfer was promoted and active site was survived without clogging. Also catalyst layer with high porosity is considered to promote mass transfer rate compared with “packed” catalyst layers.
Conclusion
The mass transfer and charge transfer rate was increased at higher catalyst loading with longer nanofibers. In these conditions, the catalyst layer is considered to show a high void fraction, and this structure has advantages to increase mass transfer and charge transfer rate.
Acknowledgement
A part of this study was supported by element innovation project from ministry of education, culture, sports, science and technology of Japan.
References
[1] Y. Ito et al., J. Power Sources, 242, 280−288 (2013)
[2] E. Higuchi et al., J. Electroanal. Chem., 583, 69−76 (2005)
[3] A. Bard and L. Faulkner, Electrochemical methods Fundamentals and Applications, second ed., John Wiley & Sons (2001)
[4] C. Andrieux et al., J. Electroanal. Chem., 131, 1–35 (1982)
[5] J. Leddy et al., J. Electroanal. Chem., 187, 205–227 (1985)