Fuel cells are clean and efficient power sources which can convert chemical energy of hydrogen and oxygen into electricity. The Gibbs free energy of hydrogen fuel cells can be as high as -273.13 kJ mol-1, and the theoretical energy conversion efficiency is high as well. However, the cathode oxygen reduction reaction (so-called ORR reaction) is kinetically slow and regarded as bottleneck of fuel cell technology. Thus extensive efforts have been spent on developing efficient catalysts where oxygen molecules can be reduced to water efficient and fast. In this thesis, the research efforts are focused on developing various nanostructured materials as efficient ORR catalysts. In detail, the research mainly focuses on two directions: first, develop Pt-based ORR catalysts with improved ORR activities compared to commercial Pt/C catalysts; second, develop non-precious but highly-efficient ORR catalysts.
Commercial Pt/C catalysts are one of the best ORR catalysts due to their good balance of activities and stabilities. However, the price of Pt is high and its reserve is limited, thus efforts should be spent in improving specific activity of Pt-based ORR catalysts aiming to decrease usage of Pt and the cost on catalysts. Recently it’s reported that deliberate manipulation chemical functionalization of metal nanoparticles can significantly increase their electrocatalytic activities. Inspired by those work, acetylene derivatives were adopted to functionalize surface of Pt nanoparticles in the first project, in order to explore different metal-ligand interfacial bonding interactions on the nanoparticle electrocatalytic activity. It’s revealed that specific activity of Pt nanoparticles after ligand functionalization was all markedly better than that of commercial Pt/C catalysts, which was accounted for by the deliberate manipulation of the electronic structure of the Pt nanoparticles.
Besides organic ligand functionalization, graphene-based materials were reported capable of decreasing surface electron density of Pt, which is believed beneficial for the electrocatalytic activity. In the second project, Pt nanoparticles were deposited on graphene quantum dots, a relatively new member of graphene-based materials, producing Pt/GQD composite as evidenced by TEM and XRD measurements. In comparison with commercial Pt/C catalysts, the Pt/GQD composite showed markedly enhanced catalytic activity in oxygen reduction reaction, with an onset potential 70 mV more positive than that of Pt/C, a specific activity that was almost nine times higher, and excellent long-term stability.
Although it’s proven effective to improve ORR activity of Pt-based materials by ligand functionalization and introducing interactions between graphene quantum dots, the high price of Pt still drives efforts spent in developing non-precious efficient ORR catalysts. Compared with Pt, copper is inexpensive candidate materials with good conductivity, and polycrystalline copper electrode is reported to exhibit apparent electrocatalytic activity in oxygen reduction reaction. It’s also suggested that partial oxidation of copper surfaces might be crucial for ORR activities, so alkyne-capped copper nanoparticles were prepared hoping to realize control oxidation on the surface and their catalytic activities were evaluated. Electrochemical studies showed that the as-prepared alkyne-capped copper nanoparticles exhibited much better activity in oxygen reduction than those poly- or single-crystalline copper electrodes, yet low-efficient 2e pathway is dominant for those copper-based materials.
To further improve ORR activity of copper-based material, graphene quantum dots, which contain both significant portion of sp2 carbon and wealthy structural defects, were introduced to support copper nanoparticles. Cu/GQD nanocomposites with varied defect concentrations exhibited significantly electrocatalytic activities towards oxygen reduction reaction, where highly-efficient 4e pathway is dominant for those composite catalysts. It’s also figured out that there is an optimal defect concentration where both metal-sp2C interactions and surface defect mediations contribute most to the ORR activities.
For non-precious ORR catalysts, their long-term stabilities are more concerned than the catalytic activities. Although metal-sp2C interactions are reported able to stabilize the overall structure, carbon-based supports can be oxidized at high potentials, hampering the stability under long-time cycle tests. TiO2 is quite stable in extreme conditions (strong acidic or alkaline solutions) and can resist high oxidizing potentials. It’s also reported that there are strong metal-support interactions between TiO2 and metal centers. Herein, copper nanoparticles were directly grown on surface of TiO2 and selected organic ligands were also attached on surface of copper nanoparticles, producing both Cu/TiO2 and CuHC10/TiO2 nanoparticles. Electrochemical studies demonstrated that both Cu/TiO2 and CuHC10/TiO2 are able to catalyze oxygen reduction reactions following an almost four-electron reduction pathway. CuHC10/TiO2 exhibited the best ORR activity with a high current density and long-term stability after 4000 cycles at high potentials.