Carbon-based metal-free catalysts for electrocatalytic energy conversion reactions

Abstract

Among various means for energy conversion, fuel cells and water splitting are of great importance because they can directly convert chemical energy to electricity or fuel with much higher efficiency and lower greenhouse gases emissions than other well-established technologies based on conventional fossil fuel combustion and petroleum industry. However, such promising devices always require noble metal catalysts, such as Pt, to facilitate low overpotential and fast kinetics of their sluggish half-cell reactions (such as oxygen reduction reaction, ORR and hydrogen evolution reaction, HER) for practical applications. It is a tremendous challenge to find new generation precious metal alternatives with reduced cost, enhanced stability, and environmentally friendly properties for sustainable clean energy generation. Concerning these issues, this thesis aims to design and develop cost-effective carbon-based metal-free catalysts (doped graphenes and graphitic carbon nitride etc.) for some key energy conversion reactions with high performances and high stability. The first aspect in this thesis is to investigate a series of metal-free electrocatalysts to replace precious Pt catalyst for cathodic oxygen reduction reaction (ORR) in fuel cells. Based on theoretical prediction, a g-C3N4@carbon hybrid was designed and synthesized by uniform incorporation of graphitic carbon nitride (g-C3N4) into a mesoporous carbon (CMK-3) to enhance the electron transfer efficiency to g-C3N4. The resulting g-C3N4@carbon electrocatalyst exhibited competitive catalytic activity and superior methanol tolerance compared to a commercial Pt/C catalyst in 0.1 M KOH solution. Furthermore, it demonstrated significantly higher catalytic efficiency (nearly 100% of four-electron ORR process selectivity) than a Pt/C catalyst. Another group of carbon-based metal-free materials (i.e. two dimensional graphene) were also evaluated as the potential alternatives for Pt for highly efficient ORR. It is well known that the engineering of pristine graphene by chemical substitution of some carbon atoms with heteroatoms is an effective way to tailor its electronic structure and (electro)chemical properties. Therefore, to study the effect of different dopants on the ORR activity of graphene, nitrogen (N), boron (B), oxygen (O), sulphur (S), and phosphorus (P) singly doped graphenes were synthesized by typical chemical doping methods. The trends of ORR properties (includes onset potentials, exchange current densities, four-electron selectivity and kinetics current densities) on these doped graphenes was experimentally measured and evaluated in the 0.1 M KOH solution. The best performance of a doped graphene material, which will be even better than that of commercial Pt/C in the same condition, was also predicted by the corroboration of Density Functional Theory (DFT) calculation based on the electrochemical experiments results. Besides single dopant, a boron and nitrogen co-doped graphene (B,N-graphene) was further synthesized by sequential incorporation of N and B heteroatoms into the selective sites of graphene. The specific B–C–N binding configuration induced a synergistically enhanced reactivity in B,N-graphene, which consequently exhibited an onset potential closer to that of commercial Pt/C and much higher electrocatalytic activity and efficiency than those of singly doped graphenes. Origin of the synergistic performance enhancement on this material was then unveiled by DFT calculations, and the role of B and N heteroatoms in ORR reactivity was also clarified. Hydrogen evolution reaction (HER), as a fundamental step of electrochemical water splitting, is also a key energy conversion reaction and normally requires a favourable catalyst to achieve fast kinetics for practical applications. It is discovered that a metal-free catalyst C3N4@NG, by coupling g-C3N4 with nitrogen doped graphene (N-graphene; NG) possessed unique molecular structure and electronic properties for electrocatalytic HER application. This metal-free hybrid showed comparable or even better electrocatalytic HER activity than the existing well-developed metallic catalysts, such as nanostructured MoS2 materials. Electrochemical measurements in combination with DFT calculations reveal that its unusual electrocatalytic properties originate from a synergistic effect of this hybrid nanostructure, in which g-C3N4 provides highly active hydrogen adsorption sites while N-graphene facilitates the electron-transfer process for the proton reduction. Like the case of ORR, we also presented our attempt to design a co-doped graphene catalyst with a synergistically enhanced HER activity. First, a computational screening procedure was performed by DFT calculations to explore a wide variety of non-metallic elements-doped graphene models, followed by selecting one couple of heteroatoms (N and P) as co-dopants with remarkable contrast in their charge population in graphene matrix. The proof-of-concept studies were carried out by simultaneously incorporating N and P heteroatoms into graphene matrix to induce the aforementioned synergistic effect. The resultant N,P-graphene catalyst showed much lower HER overpotential and higher exchange current density than those of all investigated pure and doped graphene samples. With comparable electrocatalytic activity to some of the traditional metals, the newly developed metal-free catalyst showed robust stability and applicability in a wide range of pH values. These findings in this thesis provided clear evidence that, similar to precious metals, the well-designed metal-free counterparts have also a great potential for highly efficient electrocatalytic energy conversion reactions, thus opening a new avenue towards replacing noble metals by broader alternatives in a wide variety of applications.