Development of Highly Stable and Mass Transfer‐Enhanced Cathode Catalysts: Support‐Free Electrospun Intermetallic FePt Nanotubes for Polymer Electrolyte Membrane Fuel Cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) are an alternative clean energy source and they are attracting increased attention. However, several limitations such as degradation of the carbon support and Nafion ionomer in the cathode electrode must be overcome for practical applications of PEMFCs. Support‐free 1D‐ordered intermetallic nanotubes (NTs) are considered as promising candidates for highly active and durable cathode catalysts in PEMFCs. However, 1D nanotubes are difficult to produce at large scale because they have generally been synthesized using a template‐based method that requires multistep synthetic routes. Herein, a simple and scalable method to produce ordered‐intermetallic FePt nanotubes by electrospinning is reported. When tested as cathode catalysts, under the US Department of Energy's reference condition, the activity of face‐centered‐tetragonal (fct) FePt NTs surpasses that of commercial Pt/C. In an accelerated degradation test at 1.4 V for 3 h, the degradation activity rate of fct‐FePt NTs is only 10%, whereas that of commercial Pt/C catalysts is 65%. For practical PEMFCs, this approach would provide simple routes to support‐free intermetallic nanotube structures with superior kinetic activity and higher durability than those of commercial Pt/C catalyst.

Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes

“…14,15 At present, few groups have reported promising durability in PEFCs for spray-coated membrane electrode assemblies (MEAs) prepared from unsupported catalysts. Studies by Tamaki et al 16 (on hollow Pt-Fe nanocapsules) and Lee et al 17 (on FePt nanotubes) showed outstanding retention of electrochemical surface area (ECSA) and performance in H 2 /O 2 polarization (I/E) curves for 10000 potential cycles between 1.0 and 1.5 V RHE , and upon constant exposure to a high potential of 1.4 V RHE for three hours, respectively. However, none of those studies focused on the PEFC-performance of these materials under application-relevant conditions, i.e., using H 2 and air at the anode and cathode inlet feeds, respectively.…”

Nanostructuring Noble Metals as Unsupported Electrocatalysts for Polymer Electrolyte Fuel Cells

“…[65] Another example of unsupported 1D electrocatalysts are the PtFe ordered intermetallic nanotubes that are fabricated by coaxial nozzle electrospinning with separate core and shell solutions. [35] The core silica content in the as-spun nanofibers is subsequently removed by calcination in air, resulting in a recrystallization of the shell Fe/Pt contents into an intermetallic phase with an ordered face-centered tetragonal (fct) structure. The resulting fct PtFe nanotubes show an average outer dia meter of ≈120 nm and 'infinite' length.…”

“…This combination of multiple length scales allows discarding of the carbon support, and the materials' anisotropic morphology is expected to improve mass transport and proton conductivity in the electrode. [35] In another report, surface-modified silica particles were employed to assemble Pt-Fe NPs, followed by dissolution of the silica template. [36] The resulting PtFe catalyst with a network structure consisting of porous and hollow capsule from interconnected nanoparticles exhibits a shell thickness and pore size of ≈10 nm and an outer diameter of ≈400 nm (Figure 6d-e).…”

“…[87,100] Naturally, the transport effectiveness is also a function of the CL thickness, that depends on the loading (mg Pt cm −2 MEA ), Pt concentration (wt% Pt/C) and I/C-ratio, [101] whereby the thicknesses usually amount to around 10 µm. [21,87,90,92,102] Ultimately, O 2 needs to pass, i.e., dissolve and diffuse, through the ionomer film to reach the active site. This step can be described as a local O 2 transport resistance that accounts for up to 50% of the overpotential associated with O 2 transport losses in PEFC cathodes at low loadings (≈0.1 mg Pt cm −2 MEA ) and high current densities >1.5 A cm −2 MEA .…”

“…[19] To realize the elimination of the carbon support, different types of unsupported metallic nanostructures have been developed and tested as electrocatalysts for the ORR in PEFCs. These include: i) noble metal based aerogels with controlled compositions and morphologies that are synthesized by gelation of metal colloidal nanoparticles (NPs), e.g., alloyed Pd x Pt y aerogels, [25] Pt x Ni y aerogels, [26] Pt x Cu y aerogels; [27] ii) multi/bimetallic nano/ meso-structured thin film catalysts with extended surface, tunable composition and controlled morphology that are prepared by physical vapor deposition; [28][29][30][31] iii) template-derived Ptbased nanostructures with controlled morphologies, such as core-shell structures with Pt surfaces (using Pd, [32] Cu, [33] Co [34] nanowires as the template), intermetallic FePt nanotubes (with electrospun Si nanofibers as the template), [35] PtFe hollow nanocapsule structure (silica particles as the template), [36] etc.…”