DOI:10.30919/es8d516

Received: 23 Mar 2019
Revised: 11 May 2019
Accepted: 11 May 2019
Published online: 30 May 2019

Nitrogen Doped Coal with High Electrocatalytic Activity for Oxygen Reduction Reaction

Chi Zhang1, Yunchao Xie1, Heng Deng1, Cheng Zhang1, Jheng-Wun Su1 and Jian Lin1*

1Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, Missouri 65211, USA

*To whom correspondence should be addressed. E-mail: linjian@missouri.edu


Abstract

It is of a great challenge to develop high efficient nonprecious electrocatalysts to replace Pt-based catalysts for oxygen reduction reaction (ORR). This study introduces a cost-effective and environmentally friendly ORR electrocatalyst based on nitrogen doped coal (NOC) via a pre-oxidization of raw coal followed by a urea assisted annealing process. The obtained materials show great electrocatalytic activity with the onset potential, half-wave potential, and diffusion-limited current density comparable to those of a commercial Pt/C catalyst. In addition, it follows a four-electron pathway and has relatively low peroxide yield. Finally, it exhibits a good stability and strong tolerance to methanol poisoning. A systematic characterizations illustrate that these great performance could arise from high graphitic N and pyridinic N contents, existence of trace metal elements, and porous structures in the synthesized NOC. The demonstrated high performance makes NOC a promising catalyst for applications in metal air batteries and alkaline fuel cells.


Table of Content

This research reported a cost-effective and environmentally-friendly method to convert coal into a high active electrocatalyst towards oxygen reduction reaction.

 

 

Keywords: Coal; Nitrogen doping; Oxygen reduction reaction (ORR); Electrocatalyst


1. Introduction

Oxygen reduction reaction (ORR) is a key reaction in a polymer electrolyte membrane fuel cell (PEMFC), an advanced sustainable energy conversion technology with high energy conversion efficiency and environmentally friendly features.1-2 However, as a sluggish process, the rate of the cathodic ORR is much slower than that of the anodic hydrogen oxidation reaction (HOR). Conventionally, precious metal group (PMG) based materials are used as the efficient catalysts for ORR. But high cost of the PMG severely restricts the widespread applications of PEMFC.3 Thus, it is of great importance to develop nonprecious alternatives. In the past years, doping heteroatoms (e.g., N, B, P, F and S) into synthetic carbon framework has been proved as an efficient strategy.4-6 For instance, Zhang et al. adopted a simple annealing method to synthesize phosphorus doped graphene which showed good catalytic activity for ORR as a metal-free catalyst.7 Very recently, co-doping of nitrogen and trace amount of transition metal (e.g. Fe and Co) in the form of transition metal-nitrogen-carbon (M-N-C) has shown superior performance to PMG.8-9

Coal, one of the most abundant carbon sources in the earth, would offer a promising potential as a starting material for producing high-efficiency ORR catalysts. However, its main usage is direct combustion to produce heat and electricity. By this way, the value of coal is not fully realized. Its exploration as a material for some high-value applications such as electronics, energy storage, and catalysts has been limited. In the past decades, although coal has been explored as a carbon source to produce high-value added synthetic carbonaceous materials such as carbon nanotubes (CNTs), activated carbon (AC), and graphene,10-12 these processes all require harsh reaction conditions and complicated experimental procedures, leading to high fabrication cost. Recently, a pioneering work was reported by Ye et al. who developed a facile one-step wet chemistry method to extract and purify graphene quantum dots from coal.13 Since then, a variety of follow-up works have been demonstrated. For instance, Keller et al. developed a solution-based method to prepare a coal based thin film with tunable electrical properties for a Joule heating device.14 This directly utilization of coal in its pristine form has greatly broadened its applications.

Neverlethess, most of the developed processes involve a purification step, which intent to remove impurity elements such as S, N, Fe, Mn, Al and Mg from the coal. Though it is necessary for certain applications, it requires intensive time and results in low production yields. Moreover, it also scarifies possible benefits from these elements, as they have been proved to improve the catalytic activity of ORR.15 In the past few years, although some effort has been made to use coal as a precursor for producing electrocatalysts for ORR16-19 all of the associated synthesis methods adopted ammonia, an expensive and corrosive gas as the nitrogen source. Moreover, the utilization of the ammonia in the process is quite low. Finally, the performance of the resulting coal based electrocatalysts, some of which are even incorporated by Fe element, are still much lower than that of PMG. As a result, it is still quite imperative to develop a cost-effective and facile manner to synthesize high efficient coal based electrocatalysts. Herein, we report synthesis of nitrogen doped oxidized coal (NOC) which is synthesized by an oxidization process by strong acids. Without further purification, the oxidized coal is pyrolyzed in a nitrogen environment by using urea as the nitrogen source. The resulting materials display outstanding catalytic performance toward ORR in terms of onset potential, half-wave potential, and diffusion-limited current density, long stability, and methanol tolerance. Systematic characterizations show that the high catalytic activities could be attributed to high graphitic N and pyridinic N contents, existence of trace metal elements, and porous structures in the synthesized NOC.  


2. Experimental details

Anthracite (Fisher Scientific, catalogue number S98806), H2SO4 (95–98 %, Sigma-Aldrich), HNO3 (70 %, Sigma-Aldrich), and urea (Sigma-Aldrich) were used as received unless otherwise specified. Firstly, the coal was ball-milled for 72 h and then sieved with 200 mesh sieve. 0.3 g of the obtained fine black powder was added into a three-neck glass flask containing 60 m L of concentrated H2SO4 (98 %) and 20 m L HNO3 (63 %), and followed by sonication for 2 h. The mixture was stirred overnight and diluted with deionized (DI) water. The obtained oxidized coal (OC) was washed and separated by centrifugation (10000 rpm, 10 min) several times until the pH value approached 7.0. Then, OC was dried at 80 oC in an oven. 25 mg of the dry OC was mixed with 0.5 g urea and grinded in an agate mortar. Finally, the mixture was pyrolyzed at 1000 oC for 60 min in an Argon atmosphere to produce nitrogen doped OC (NOC).

2.2. Material characterization

A Rigaku X-ray diffractometer (XRD) equipped with Cu K-α X-rays was used to explore the structures of the samples at a scan rate of 5 °/min. A FEI Quanta 600F environmental scanning electron microscopy (SEM) equipped with Bruker Energy Dispersive X-ray spectroscopy (EDS) at a working voltage of 5 kV was adopted to observe the surface morphology and do the elemental mapping. HRTEM was performed on a FEI Tecnai F30 Twins. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a KRATOS AXIS 165 X-ray photoelectron spectrometer that is equipped with a monochromatic Al Kα source. Raman spectra were collected at 514 nm excitation using a Renishaw inVia Raman spectroscopy. Fourier transform infrared spectroscopy (FTIR) was performed using a Thermo Nicolet Avatar 360 FT-IR spectrometer (Thermo Electron Inc. USA). Nitrogen adsorption–desorption measurements were carried out with a Beckman Coulter SA 3100 Surface Area and Pore Size Analyzer.

2.3. Electrochemical measurements

The electrocatalytic performance of the catalysts was assessed by a CHI 708E electrochemical analyzer (CH Instruments, Inc.,) in a typical three-electrode setup. 4 mg of the NOC was dispersed in a solution containing 760 μL of DI water, 200 μL of ethanol and 40 μL of 5 wt% Nafion. Then the obtained ink was ultra-sonicated for 2 hours to get homogeneous dispersion. A 10 μL ink was dropped on the surface of a glassy carbon rotating ring-disk electrode (RRDE, 4 mm in diameter) and dried in air. The RRDE deposited by ink with a mass loading of 0.31 mg/cm2 was used as a working electrode. A platinum wire and Ag/AgCl served as the counter and reference electrodes, respectively. Before each measurement, the electrolyte was saturated by O2 or N2 for 30 minutes.

The electron transfer number n from RDE was calculated by Koutecky-Levich (K-L) equation based on the linear scan voltammetry (LSV) curves under different rotating speeds:

where J is the measured current density; JL and JK are the limiting current density and kinetic current density, respectively; w stands for the angular rotating speed of the disk; F is Faraday constant (96485 C/mol), C0 is the bulk concentration of O2 in 0.1 M KOH (1.2 ´ 10-6 mol/cm3); D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 ´ 10-5 cm2/s); and v is the kinematic viscosity of 0.1 M KOH (0.01 cm2/s).

The peroxide yield (H2O2 %) and the electron transfer number n were calculated by the following equations:

where Ir is the ring current density and Id is the disk current density; N is the ring current collection efficiency and it is equal to 0.424.


3. Results and Discussion

Surface morphology and microstructures of the raw coal (RC), OC, and the as-prepared NOC were first explored by scanning electron microscopy (SEM). As shown in Fig. 1a, after the ball milling, the raw coal exhibits an irregular surface as a bulk material. After oxidization, surface morphology of OC does not change much (Fig. 1b). After further pyrolysis, the resulting NOC exhibits highly porous microstructures (Fig. 1c-d), which can be attributed to the release of volatile matter in the coal during the high temperature thermal treatment. The energy-dispersive X-ray spectroscopy (EDS) elemental analysis was further performed on the area shown in Fig. 1d. As shown in Fig. 1e, besides the main element C, NOC also contains N, O, Si, Ca, Fe, Mg, Al, Si, S, and Ti. It should be noted that incorporation of these elements including Fe, Al, N, Si and S has been proved as an efficient way to boost the catalytic activity of the carbon based catalysts.8 Meanwhile, a detailed EDS spectrum corresponding to Fig. 1d shows that N elements are uniformly distributed among the NOC matrix (Fig. 1f). High resolution transmission electron microscopy (HRTEM) was further performed to explore the structure. Fig S1 shows a typical amorphous carbon structure. The BET surface areas of the RC, NRC and NOC were also acquired by nitrogen adsorption–desorption measurements. As summarized in Table S1, NOC has a much larger surface area than NOC and RC. Such high surface area of NOC could contribute to fast mass transfer in electrocatalysis.