Conformation-modulated three-dimensional electrocatalysts for high-performance fuel cell electrodes

Abstract

Unsupported Pt electrocatalysts demonstrate excellent electrochemical stability when used in polymer electrolyte membrane fuel cells; however, their extreme thinness and low porosity result in insufficient surface area and high mass transfer resistance. Here, we introduce three-dimensionally (3D) customized, multiscale Pt nanoarchitectures (PtNAs) composed of dense and narrow (for sufficient active sites) and sparse (for improved mass transfer) nanoscale building blocks. The 3D-multiscale PtNA fabricated by ultrahigh-resolution nanotransfer printing exhibited excellent performance (45% enhanced maximum power density) and high durability (only 5% loss of surface area for 5000 cycles) compared to commercial Pt/C. We also theoretically elucidate the relationship between the 3D structures and cell performance using computational fluid dynamics. We expect that the structure-controlled 3D electrocatalysts will introduce a new pathway to design and fabricate high-performance electrocatalysts for fuel cells, as well as various electrochemical devices that require the precision engineering of reaction surfaces and mass transfer.

Fig. 1. Geometric control of 3D PtNAs via nTP.

(A) A schematic illustration of the fabrication process for the 3D Pt electrocatalysts using the nTP process. Scanning electron micrography images of (B) a monolayer, (C) double layers, and (D) multilayers of PtNAs (left: pitch, 50 nm/width, 20 nm; center: pitch, 200 nm/width, 50 nm; and right: pitch, 1.2 μm/width, 200 nm). (E) Complex PtNAs stacked with a 45°, 30°, and freestanding nanostructure.

Fig. 2. Characterization of fabricated PtNAs by TEM.

(A) A schematic illustration shows dense PtNWs (pitch, 200 nm/width, 50 nm). (B) High-resolution TEM image of the Pt nanowire and corresponding (C) FFT of the whole HRTEM image (yellow box). (D and E) In situ TEM-ASTAR crystal orientation mapping of dense PtNWs along z directions during thermal treatment at 600°C. The inset indicates a color-coded inverse pole figure. The mapping time required to obtain an image is about 40 min, and it was observed in the same sample although not in the same position because of the change in position during the thermal treatment and mapping process. The lower part shows the corresponding crystalline orientation distribution diagram.

Fig. 3. Characterization of fabricated PtNAs by XPS and XAFS.

(A) XPS of Pt (4f) and (B) the ratio of Pt and Pt oxidation products in Pt/C and dense PtNA. (C) Pt L₃-edge x-ray adsorption near-edge structure spectra of Pt/C and dense PtNA. (D) The coordination number for Pt-Pt and (E) Fourier transform magnitude spectra are shown for the Pt L₃ edge of Pt/C and dense PtNA.

Fig. 4. Liquid half-cell test.

(A) The transfer process for fabricated PtNA on glassy carbon or membrane substrates to prepare samples for electrochemical evaluation. (B) The ORR and (C) CV curves of Pt/C and dense PtNA are compared. The inset indicates the corresponding Tafel plot. (D) Mass and specific activities of Pt/C and dense PtNA. The ORR curves during the accelerated degradation test (ADT) (0.6 to 1.1 V, 6000 cycles) for (E) Pt/C and (F) dense PtNA. (G) ECSA retention rate of Pt/C and dense PtNA during the ADT. The ORR curves were acquired in a solution of O₂-saturated 0.1 M HClO₄, and the CV and ADT tests were conducted in a solution of Ar-saturated 0.1 M HClO₄.

Fig. 5. Multiscale PtNA and single-cell test.

(A) A schematic illustration showing a multiscale PtNA. Polarization curves of Pt/C and various PtNA-based MEAs are shown under an atmosphere of H₂/air (B) without outlet pressure and (C) with a total outlet pressure of 150 kPa. (D) Oxygen gains were calculated using the potential difference observed upon exposure to oxygen or air, respectively. (E) The differences in power density between Pt/C and various PtNAs MEAs are shown for a H₂/air atmosphere with a total outlet pressure of 150 kPa. CV curves of (F) Pt/C and (G) multiscale PtNA are shown before and after ADT (1.0 to 1.5 V, 5000 cycles).

Fig. 6. Numerical simulations based on computational fluid dynamics.

(A) Comparison of simulated (lines) and measured (symbols) polarization curves under an operating pressure of 1 atm, (B) individual voltage losses due to ORR kinetic polarization and ohmic polarization across the cathode catalyst layer, (C) the average oxygen concentrations in the cathode catalyst layer as a function of operating current density, and (D) comparison of through-plane liquid saturation profiles for three different cathode catalyst layer designs at 1.0 A cm⁻².