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N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells.

Shui J, Wang M, Du F, Dai L - Sci Adv (2015)

Bottom Line: Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes.However, their large-scale practical application in more popular acidic polymer electrolyte membrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test.We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best NPMC in acidic PEM cells.

View Article: PubMed Central - PubMed

Affiliation: Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science Engineering, Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA.

ABSTRACT
The availability of low-cost, efficient, and durable catalysts for oxygen reduction reaction (ORR) is a prerequisite for commercialization of the fuel cell technology. Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes. However, their large-scale practical application in more popular acidic polymer electrolyte membrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test. We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best NPMC in acidic PEM cells. This work represents a major breakthrough in removing the bottlenecks to translate low-cost, metal-free, carbon-based ORR catalysts to commercial reality, and opens avenues for clean energy generation from affordable and durable fuel cells.

No MeSH data available.


Fabrication of MEA of VA-NCNT arrays and its performance in a PEM fuel cell.(A) Schematic drawings for the fabrication of MEA from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photo image of the used MEA after durability test with the cross-section SEM images shown in the inserts. (D) Polarization curves as the function of the areal current density after accelerated degradation by repeatedly scanning the cell from OCV to 0.1 V at the rate of 10 mA s−1. (E) Polarization and power density as the function of the gravimetric current density. Cathode catalyst loading 0.16 mg cm−2, Nafion/VA-NCNT = 1/1. H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.
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Figure 1: Fabrication of MEA of VA-NCNT arrays and its performance in a PEM fuel cell.(A) Schematic drawings for the fabrication of MEA from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photo image of the used MEA after durability test with the cross-section SEM images shown in the inserts. (D) Polarization curves as the function of the areal current density after accelerated degradation by repeatedly scanning the cell from OCV to 0.1 V at the rate of 10 mA s−1. (E) Polarization and power density as the function of the gravimetric current density. Cathode catalyst loading 0.16 mg cm−2, Nafion/VA-NCNT = 1/1. H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.

Mentions: VA-NCNT arrays have been previously reported to show excellent ORR performance (1), even superior to the commercially available Pt/C electrodes, in electrochemical half-cells with alkaline electrolytes, as also confirmed by the VA-NCNTs used in this study (figs. S1 to S3). To carry out the performance evaluation of VA-NCNTs in PEM fuel cells, we made the VA-NCNT arrays (80 μm in height, a surface packing density of 0.16 mg cm−2) into a membrane electrode assembly (MEA) at the highest allowable catalyst loading of 0.16 mg cm−2. Figure 1 schematically shows procedures for the MEA preparation (Fig. 1A), along with a typical scanning electron microscopic (SEM) image of the starting VA-NCNT array (Fig. 1B) and a photographic image of the newly developed MEA (Fig. 1C), whereas the MEA fabrication details are given in the Supplementary Materials. Briefly, we first performed the electrochemical oxidation in H2SO4 to remove Fe residue, if any, in the VA-NCNTs made from pyrolysis of iron(II) phthalocyanine (1), followed by etching off the purified VA-NCNT array from the Si wafer substrate in aqueous hydrogen fluoride [10 weight percent (wt %)], rinsing it copiously with deionized water, transferring it onto a gas diffusion layer [GDL; Carbon Micro-porous Layer (CMPL), ElectroChem Inc.], and drop-coating with a sulfonated tetrafluoroethylene-based ionomer “Nafion” (DuPont) as binder and electrolyte, which was then assembled with a Pt/C-coated GDL as the anode and an intermediate layer of proton-conductive membrane (Nafion N211, DuPont) as the separator (see the Supplementary Materials for detailed preparation and fig. S4 for the MEA cross-section images). As can be seen in Fig. 1 (A to C) and fig. S4, the NCNT ORR catalyst within the MEA thus produced largely retained its vertical alignment.


N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells.

Shui J, Wang M, Du F, Dai L - Sci Adv (2015)

Fabrication of MEA of VA-NCNT arrays and its performance in a PEM fuel cell.(A) Schematic drawings for the fabrication of MEA from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photo image of the used MEA after durability test with the cross-section SEM images shown in the inserts. (D) Polarization curves as the function of the areal current density after accelerated degradation by repeatedly scanning the cell from OCV to 0.1 V at the rate of 10 mA s−1. (E) Polarization and power density as the function of the gravimetric current density. Cathode catalyst loading 0.16 mg cm−2, Nafion/VA-NCNT = 1/1. H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.
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Figure 1: Fabrication of MEA of VA-NCNT arrays and its performance in a PEM fuel cell.(A) Schematic drawings for the fabrication of MEA from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photo image of the used MEA after durability test with the cross-section SEM images shown in the inserts. (D) Polarization curves as the function of the areal current density after accelerated degradation by repeatedly scanning the cell from OCV to 0.1 V at the rate of 10 mA s−1. (E) Polarization and power density as the function of the gravimetric current density. Cathode catalyst loading 0.16 mg cm−2, Nafion/VA-NCNT = 1/1. H2/O2: 80°C, 100% relative humidity, 2-bar back pressure.
Mentions: VA-NCNT arrays have been previously reported to show excellent ORR performance (1), even superior to the commercially available Pt/C electrodes, in electrochemical half-cells with alkaline electrolytes, as also confirmed by the VA-NCNTs used in this study (figs. S1 to S3). To carry out the performance evaluation of VA-NCNTs in PEM fuel cells, we made the VA-NCNT arrays (80 μm in height, a surface packing density of 0.16 mg cm−2) into a membrane electrode assembly (MEA) at the highest allowable catalyst loading of 0.16 mg cm−2. Figure 1 schematically shows procedures for the MEA preparation (Fig. 1A), along with a typical scanning electron microscopic (SEM) image of the starting VA-NCNT array (Fig. 1B) and a photographic image of the newly developed MEA (Fig. 1C), whereas the MEA fabrication details are given in the Supplementary Materials. Briefly, we first performed the electrochemical oxidation in H2SO4 to remove Fe residue, if any, in the VA-NCNTs made from pyrolysis of iron(II) phthalocyanine (1), followed by etching off the purified VA-NCNT array from the Si wafer substrate in aqueous hydrogen fluoride [10 weight percent (wt %)], rinsing it copiously with deionized water, transferring it onto a gas diffusion layer [GDL; Carbon Micro-porous Layer (CMPL), ElectroChem Inc.], and drop-coating with a sulfonated tetrafluoroethylene-based ionomer “Nafion” (DuPont) as binder and electrolyte, which was then assembled with a Pt/C-coated GDL as the anode and an intermediate layer of proton-conductive membrane (Nafion N211, DuPont) as the separator (see the Supplementary Materials for detailed preparation and fig. S4 for the MEA cross-section images). As can be seen in Fig. 1 (A to C) and fig. S4, the NCNT ORR catalyst within the MEA thus produced largely retained its vertical alignment.

Bottom Line: Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes.However, their large-scale practical application in more popular acidic polymer electrolyte membrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test.We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best NPMC in acidic PEM cells.

View Article: PubMed Central - PubMed

Affiliation: Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science Engineering, Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA.

ABSTRACT
The availability of low-cost, efficient, and durable catalysts for oxygen reduction reaction (ORR) is a prerequisite for commercialization of the fuel cell technology. Along with intensive research efforts of more than half a century in developing nonprecious metal catalysts (NPMCs) to replace the expensive and scarce platinum-based catalysts, a new class of carbon-based, low-cost, metal-free ORR catalysts was demonstrated to show superior ORR performance to commercial platinum catalysts, particularly in alkaline electrolytes. However, their large-scale practical application in more popular acidic polymer electrolyte membrane (PEM) fuel cells remained elusive because they are often found to be less effective in acidic electrolytes, and no attempt has been made for a single PEM cell test. We demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibited significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best NPMC in acidic PEM cells. This work represents a major breakthrough in removing the bottlenecks to translate low-cost, metal-free, carbon-based ORR catalysts to commercial reality, and opens avenues for clean energy generation from affordable and durable fuel cells.

No MeSH data available.