Researchers at Dalian Institute of Chemical Physics (China) have synthesized an advanced catalytic layer in the membrane electroide assembly (MEA) for proton exchange membrane fuel cells (PEMFCs) using vertically aligned polymer–polypyrrole (PPy) nanowire arrays as ordered catalyst supports.

In a paper published in the Journal of Power Sources, they report that a single cell fitted with their MEA yields a maximum performance of 762.1 mW cm^-2 with a low Pt loading (0.241 mg Pt cm^-2, anode + cathode). The advanced catalyst layer indicates better mass transfer in high current density than that of commercial Pt/C-based electrode. The mass activity is 1.08-fold greater than that of US Department of Energy (DOE) 2017 target.

Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most promising alternative energy sources for stationary and transportation applications because of their high power density, quick start-up, zero or low emission, and low operating temperature. However, the cost of the PEMFCs must be reduced for wide adoption. Over the past decades, significant research has been devoted to decrease the cost of cell components without sacrificing their performance and durability and to decrease the amount of platinum (Pt). Many studies have focused on (1) the development of non-Pt-group metal catalysts and (2) the utilization of other metals (Pd, Fe, Co, Ni, and Cu) with Pt to form a core-shell structure or alloy, or the improvement of Pt utilization efficiency with an ordered electrode structure. Of these efforts, the development of advanced catalytic layer architecture is of importance to obtain an efficient membrane electrode assembly (MEA), which is the core part of PEMFCs and has significantly influenced the performance and durability of fuel cells.

In this study, a novel method of synthesizing advanced catalytic layer in MEA for PEMFCs is presented with vertically aligned PPy nanowire arrays as PtPd catalyst supports.

—Jiang et al.

The team investigated PtPd alloy catalysts with various Pt loadings formed on PPy nanowire arrays in the anode or cathode (PtPd-PPy). The arrays were hot-pressed on both sides of a Nafion membrane to construct a membrane electrode assembly (without additional ionomer).

The ordered thin catalyst layer (approximately 1.1 μm) was applied in a single cell as the anode and the cathode without additional Nafion ionomer. Since there is no additional ionomer on the catalyst layer of the proposed electrode, the water film acts as the proton-conducting pathway.

FESEM of (a) PtPd-PPy-203 nanowires arrays, (b) PtPd-PPy-402 nanowire arrays, (c) cross-section of PtPd-PPy-402 nanowire arrays on Nafion-membrane after hot-press (scale bar: 1 mm), and (d) the cross-section of PtPd-PPy-402 nanowires on Nafion membrane with high magnification. (Image by Jiang et al.)

They concluded that the high performance attained is due to the ordered electrode structure with a high Pt utilization and the improvement of the mass transport of the reactant and products in high current density.

Broadly, the benefits of the approach are:

1. removal of ionomers or binders for better gas transport;

2. elimination of carbon supports;

3. thin-film, extended-surface PtPd catalysts; and

4. low tortuosity pathways for transportation.

However, cell performance needs further improvement through structure optimization. The work provides a novel idea for the fabrication of a core-shell structure catalyst in the microscale and a new method to prepare a thin-film electrode. Furthermore, the approach developed in this work is of good scalability and is beneficial to the development of other fuel cells. (Green Car Congress)