| Due to the climate change and the exhaustion of fossil fuel, the development of sustainable energy is of vital importance for human beings. Many advanced technologies for clean energy conversion, such as fuel cells, water electrolysis, metal-air batteries, become the subject of both extensive fundamental and utilitarian studies. The core of these conversion technologies is a series of electrochemical process, which including oxygen reduction reaction(ORR), hydrogen oxidation reaction(HOR), oxygen evolution reaction(OER) and hydrogen evolution reaction(HER). Generally, the kinetics of two-electron transfer in HER and HOR is facile, while a multistep proton-coupled electron transfer in ORR and OER is kinetically sluggish, which determines the comprehensive performance of the electrochemical devices. At present, the best known catalysts are still precious metals, such as Pt-, Ru-, and Ir-based catalysts, but the high cost and scarcity prohibits their large-scale applications for the above mentioned technologies. Therefore, the development of oxygen electrode catalysts with high activity, long-term stability, affordable cost and selectivity is beneficial for the commercialization of clean energy facilities. In this regard, a series of heteroatom-doped mental-free oxygen electrode catalysts which exhibit low cost, high catalytic activity, as well as long durability were controlled synthesized.Firstly, enlightened by the nitrogen-doped graphene nanosheets with good ORR activity, phosphorous-doped graphene(P-TRG) for the first time was facilely synthesized by annealing a homogeneous mixture of graphene oxide and 1-butyl-3-methylimidazolium hexafluorophosphrate(BmimPF6), which is one of the most common phosphorous ionic liquids with nonvolatility and high thermal stability. The results showed that P-TRG still maintains the morphology of graphene and combines the merits of large surface area(497.67 m2 g-1) and relatively high P-doping level(1.16 at. %). Furthermore, the orderly restacking of graphene nanosheets in P-RTG was effectively inhibited by the phosphorous atoms doped into the carbon framework, and then the surface area, pore size and total pore volume were increased remarkably compared with those of the graphene without P-dopants. All these changes are favorable for the exposure of the active sites and the efficient transportation of oxygen gas and electrolyte. The P-TRG is demonstrated to be an effective ORR catalyst with enhanced catalytic activity, long-term stability, excellent tolerance to methanol and CO poisoning in alkaline media, outperforming or at least comparable to the commercial Pt/C catalyst. In addition, we also discussed the catalytic active sites and proposed that the partial oxidized P atoms as a bridge withdraw electrons from the C atoms, creating a net positive charge on the carbon atoms adjacent to the P atom. As a result, the positive charged C atoms become the active sites for ORR.Secondly, using layer-structured Na-MMT as nano-reactor, nitrogen and phosphorous co-doped graphene nanosheets(N, P-G) were obtained by pyrolyzing a homogeneous complex of Na-montmorillonite(Na-MMT), melamine, phosphoric acid and glucose followed by removing the template of Na-MMT. SEM, TEM, XPS and BET were used to scrutinize the catalysts. The results showed that the addition of phosphoric acid has not only made the carbon material with P-dopant, but also promoted N-doping, the formation of hierarchical pore structure, and the increase of surface area. The ORR activity of N, P-G catalyst is better than Pt/C catalyst in alkaline medium, while in acid solution it is better than the peers reported in literatures, although it is still inferior to Pt/C catalyst. In addition, the ORR follows a four-electron reduction pathway on the N, P-G in alkaline and acid medium. The stability and selectivity of N, P-G is also superior to that of Pt/C. Therefor, the N, P-G is suitable for acidic and alkaline fuel cells.Thirdly, inspired by the nitrogen-doped carbon material with good bifunctional electro-catalytic activity and the synergistic effect between dual/multi-doped heteroatoms, we fabricated nitrogen and phosphorous dual-doped graphene/carbon nanosheets(N, P-GCNS) as bifunctional oxygen electrode catalysts, by annealing a dried hydrogel composed of graphene oxide, polyaniline and phytic acid. The obtained N, P-GCNS catalyst possesses large surface area, hierarchical pore and sandwich-like structure. The superior catalytic activity of N, P-GCNS for ORR to Pt/C was confirmed by higher half-wave potential(20 mV) and smaller Tafel slope. OER activity of N, P-GCNS was also evaluated. Its potential corresponding to the current density of 10 mA cm-2(noted as EJ10) was 1.57, which is lower 20 mV than RuO2. Furthermore, N, P-GCNS as a bufunctional oxygen electrode catalyst exhibits the smallest EJ10, OER- EJ-3, ORR with a value of 0.71 V. Therefore, N, P-GCNS is among the best bifunctional oxygen electrode catalysts, showing great promise in the fields of rechargeable metal-air batteries and unitized regenerative fuel cells.Finally, a facile yet cost-effective strategy to fabricate nitrogen and sulfur dual-doped graphene(N, S-RGO) by pyrolysis of L-cysteine and graphene oxide was presented, where L-cysteine was employed as the N and S-containing precursor. The XPS analysis shows that N and S have been successfully incorporated into the carbon matrices and covalently bonded the neighbor carbon atoms with high active bonding configuration(The former is mainly graphite N and pyridine N, the latter is thiophene S). The N, S-G was used as a bifunctional electrocatalysts in alkaline media. For the ORR, the onset potential, half-wave potential and limiting diffusion current density of N, S-RGO catalyst is comparable to commercial Pt/C. When it catalyzed OER, its potential corresponding to the current density of 10 mA cm-2 is similar to RuO2, while the onset potential and current density at high potential is remarkable superior to that of RuO2. Thus, the as prepared N, S-RGO is also a kind of excellent bifunctional oxygen electrode catalysts. |
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