Preparation And Electrochemical Properties Of Heteroatom-Codoped Nanoporous Carbons Derived From Metal-Organic Frameworks

The fast consumption of traditional energies will not meet the requirement of the rapid development of society and economy. As a new energy conversion device, fuel cell has received wide attention because of its high-efficiency and environment friendly. Fuel cell is a highly possible technology to convert the chemical energy of fuel to electricity in a high-efficiency one-step way, in which a major limiting factor is the extremely slow oxygen reduction reaction (ORR) at the cathode. As we know, platinum-based materials are the most active catalysts for ORRs up to now. Unfortunately, the high cost, scarcity, and weak long-term durability of platinum-based catalysts significantly hinder its large-scale industrial applications. Non-precious-metal catalysts (NPMCs) with outstanding electrocatalytic performances for ORRs have been investigated extensively. Carbon-based materials, especially heteroatom-doped carbons, have been considered as efficient NPMCs owing to their high methanol-CO tolerance and good durability compared to commercial Pt catalysts. In addition, hydrogen is an economic and environment-friendly fuel used in electrochemical cell for powering vehicles or electric devices to solve the energy crisis. Developing effective electrocatalysts for the hydrogen evolution reaction (HER), which is the most important process in producing hydrogen, is another important topic currently.In this work, we used the P-modified MOFs as the template, and produced the P-N codoped nanoporous carbons for ORR and HER. This study opens a new route for bifunctional catalysts for ORR and HER. The main context of this work is as following.1) UiO-66-NH2, a MOF containing nitrogen, has been carefully selected as the initial precursor. It also plays the role of nitrogen and carbon sources. The P-N-rich precursor with homogeneous phosphorous distribution in framework is obtained by using a suitable P source, glyphosine, to react with the -NH2 group in UiO-66-NH2. Subsequently, the N-P-carbons were obtained via pyrolysis process. The effects of carbonization temperature on the structure and ORR performance of the samples were studied. The sample obtained at T=950? shows the best ORR performance. By direct pyrolysis of the UiO-66 and UiO-66-NH2, we explored the impact of the P and N on ORR performance. The results show that N and P play the synergistic effect for facilitate the catalytic active site.2) These newly developed UiO-66-NH2 based P-N-Carbons exhibit high special surface area up to 1095 m2/g and considerable catalytic activity. In 0.1 M KOH, P-N-Carbon-950 exhibits the high ORR activity close to the Pt/C catalysts, the onset and halfwave potentials are 0.98V and 0.80V, and the limited diffusion current density is 4.83mA/cm2, while the electron transfer number is 3.83 at 0.6V. Moreover, P-N-Carbon-950 shows no methanol poisoning phenomenon along with superior long-term stability. After 6 hours i-t chronoamperometric response test, the current of P-N-Carbon-950 remains 95% while that of 20% Pt/C drops to 67%.3) We choose MIL-101-NH2(Fe) instead of UiO-66-NH2 as the initial precursor to obtain P-N co-doped nanoporous carbon materials. The final product can be used as the bifunctional catalyst for ORR and HER. In 0.1M KOH media, the sample obtained from second pyrolysis under 950?, Fe-NH2+P-900-2, shows excellent ORR catalytic activity (the onset and half-wave potentials are 0.99 and 0.82V vs. RHE, respectively) and nearly four electron selectivity (the electron transfer number is 3.9 at 0.6 V), and the limited diffusion current density is 5.5mA/cm2, which is outperforms the 20% Pt/C. In 0.5 H2SO4 media, Fe-NH2+P-900-2 show considerable HER catalytic activity, the HER onset potential is -0.15V, the potential at 10mA/cm2 is-0.255V.