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The objective of this project is to use electrospinning to create a novel type of protonic-electronic mixed-conducting nanofiber electrodes for sustainable energy conversion. This ACS PRF fund provided financial support for the PI to establish an active research group to carry out research on developing these novel mixed-conducting nanofiber fuel cell electrodes. The fund mainly supported 58% of a graduate student, 50% of a post-doctoral research associate, and 5% of two additional post-doctoral research associates. All of them got excellent training in materials and fuel cell research, which could help the U.S. stay in a lead position in these strategic fields. The work supported by this fund has scientific and economic impacts. Foundation for fabricating multicomponent nanofibers with controllable nanostructures and functionalities has been established. Research of fuel cell technologies, which can improve air quality, reduce greenhouse gas emissions, and promote alternative energy usage, has also been advanced. In addition to fuel cells, the processing-structure-performance relationships obtained in this work are being used to guide the design of advanced mixed-conducting nanofiber materials for many other applications including batteries, solar cells, biocatalysts, and chemical and biological protection.

Fuel cells are considered promising candidates for generating power in a clean manner because they provide electricity without combustion and pollutants associated with burning fossil fuels. The operating principal for fuel cells involves the oxidation of hydrogen and the reduction of oxygen over precious metal catalysts in two electrodes (i.e., anode and cathode):

Anode: H₂ à 2H⁺ + 2e^-

Cathode: ½O₂ + 2H⁺ + 2e^- à H₂O

Overall Reaction: H₂ + ½O₂ à H₂O

A significant barrier to widespread commercial use of fuel cells is, therefore, the cost of these precious metal catalysts (e.g., Pt). To effectively use expensive catalyst, Pt in electrodes must have simultaneous access to reactants, protons, and electrons. In the first year of the project, we synthesized carbon nanofiber-supported Pt, in which the Pt catalyst has simultaneous access to reactants, electrons, and protons, thereby resulting in high catalyst utilization and effective power generation. Carbon nanofibers were synthesized by combining the electrospinning and carbonization techniques. Pt/carbon composite nanofibers were prepared by using two different approaches, i.e., electrodeposition and chemical deposition, respectively. In the second year of the project, the work was focused on the characterization and improvement of the electrochemical performance of those Pt/carbon composite nanofibers.

Figure 1. CV responses in 0.2 M H₂SO₄ for carbon and Pt/carbon nanofibers prepared using different deposition durations (a: 0.5, b: 1.0, c: 2.0, and d: 3.0 hr). Scanning rate: 50.0 mV·s^-1.

Figure 2. CV responses in 5.0 mM K₄[Fe(CN)₆] + 0.1 M KCl for carbon and Pt/carbon nanofibers prepared using different deposition durations (a: 0.5, b: 1.0, c: 2.0, and d:. 3.0 hr). Scanning rate: 50.0 mV·s^-1.

In the first approach, Pt/carbon nanofibers were synthesized by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers, and the morphology and size of Pt nanoparticles were controlled by the surface treatment of carbon nanofibers and electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions, which are shown in Figures 1 and 2, respectively. The electrocatalytic activities of Pt/carbon composite nanofibers were further measured by the oxidation of methanol, which is shown in Figure 3, and the electrochemical characteristics of Pt/carbon nanofibers with different deposition durations are summarized in Table 1. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm^-2 has the highest activity.

Figure 3. Current-potential curves of carbon (a) and Pt/carbon (b) in 0.125 M CH₃OH + 0.20 M H₂SO₄ at 5.0 mV·s^-1. Deposition time: 1.0 hr.

Table 1. Electrochemical characteristics of Pt/carbon nanofibers with different deposition durations. Solution: 0.125 M CH₃OH + 0.20 M H₂SO₄. Scanning rate: 5.0 mV·s^-1.

In the second approach, Pt/carbon composite nanofibers were prepared by chemically depositing Pt nanoparticles directly onto electrospun carbon nanofibers using a polyol processing technique. The morphology and size of Pt nanoparticles were controlled by pre-treating carbon nanofibers using acid oxidation or 1-aminopyrene functionalization. The resultant Pt/carbon composite nanofibers were characterized by running cyclic voltammogram in 0.5 M H₂SO₄ and 0.125 M CH₃OH + 0.2 M H₂SO₄ solutions, which are shown in Figures 4 and 5, respectively. In order to study the efficiency of methanol oxidation, the electrochemical characteristic data of Pt/carbon composite nanofibers treated by acid oxidation (Pt/AO-CNFs) and 1-aminopyrene functionalization (Pt/1-AP-CNFs) are also summarized in the Table 2. Results show that Pt/1-aminopyrene functionalized carbon nanofibers possess the properties of high active surface area and improved performance towards the electrocatalytic oxidation of methanol.

Figure 4. Current-potential curves of Pt/AO-CNFs (a) and Pt/1-AP-CNFs (b) in 0.5 M H₂SO₄ at 50 mV·s^-1.

Figure 5. Current-potential curves of Pt/AO-CNFs (a) and Pt/1-AP-CNFs (b) in 0.125 M CH₃OH + 0.2 M H₂SO₄ at 5 mV·s^-1.

Table 2. Electrochemical characteristics of Pt/AO-CNFs and Pt/1-AP-CNFs. Solution: 0.5 M H₂SO₄ for EASA and 0.125 M CH₃OH + 0.2 M H₂SO₄ for other characteristics. Scanning rate: 5 mV·s^-1.

In conclusion, Pt/carbon nanofibers prepared by both electrodeposition and chemical deposition approaches possess the properties of highly active surface area and good performance towards the electrocatalytic oxidation of methanol, which can be directly used as anode electrodes in fuel cells.

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