Subscriber access provided by Kaohsiung Medical University
Article
Tuning the Electrochemical Properties of Nitrogen-Doped Carbon Aerogels in a Blend of Ammonia/Nitrogen Gases Tianyu Liu, Tianyi Kou, Daniel Bulmahn, Carlos Ortuno-Quintana, Guoliang Liu, Jennifer Lu, and Yat Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01055 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
Tuning the Electrochemical Properties of NitrogenDoped
Carbon
Aerogels
in
A
Blend
of
Ammonia/Nitrogen Gases Tianyu Liu1,2‡, Tianyi Kou1‡, Daniel Bulmahn1, Carlos Ortuno-Quintana3, Guoliang Liu2, Jennifer Q. Lu3, Yat Li*1 1
Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High
Street, Santa Cruz, California 95064, USA 2
Department of Chemistry, Virginia Polytechnic Institute and State University, 800 West
Campus Drive, Blacksburg, Virginia 24061, USA 3
School of Engineering, University of California, Merced, 5200 North Lake Road, Merced,
California 95343, USA
KEYWORDS: Ammonia; Carbon Aerogels; Supercapacitors; Oxygen Reduction Reaction; Catalysts
ACS Paragon Plus Environment
1
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 32
ABSTRACT Nitrogen-doped carbons (NCs) are emerging as high-performance and inexpensive materials for electrochemical energy storage and conversion. The combined merits of carbon and nitrogen dopants allow NCs to possess the advantages of carbon as well as the unique functionalities of N-moieties. Conventionally, NCs are produced by pyrolysis of nitrogen-rich organic precursors such as naturally abundant bio-polymers. However, these NCs generally exhibit poor electrochemical performance due to their limited surface area and the loss of N-moieties at elevated temperatures. In this work, we modified the widely-practiced pyrolysis protocol by blending 20 vol.% of ammonia gas into nitrogen atmosphere at the early stage of pyrolysis. The carbonization of chitosan, a naturally abundant biopolymer, in the N2/NH3(20 vol.%) gas mixture led to N-doped carbon aerogels (NCAs) with roughened surface, increased surface area, augmented micropore volume and high content (up to 11.3%) of nitrogen-containing functionalities (pyrrolic-N, pyridinic-N and graphitic-N). These features improve the performance of NCs as both binder-free supercapacitor electrodes and noble-metal-free oxygen reduction reaction catalysts. NCAs prepared by this method achieved an outstanding gravimetric capacitance approaching 400 F g-1 at 1 A g-1, and rate capability of 75.3% (from 1 A g-1 to 100 A g-1). They are also potent oxygen reduction reaction catalysts. We anticipate that the strategy demonstrated herein can be extended to other biomasses to produce NCs with tailored surface area, pore volume and contents of nitrogen functionalities for a spectrum of electrochemical applications including supercapacitors, rechargeable batteries, catalysts, and fuel cells.
ACS Paragon Plus Environment
2
Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
1. INTRODUCTION Nitrogen-doped carbons (NCs) have become rising stars for electrochemical applications including electrochemical capacitors, battery electrodes and electrocatalysts due to their improved performance demonstrated over their non-doped carbon counterparts. NCs inherit the advantages of carbon, i.e., excellent electrical conductivity, structural tunability, chemical stability and versatility of morphology, while also possessing unique functionalities arising from doped nitrogen atoms.1 For instance, as electrode materials for supercapacitors, NCs reportedly exhibit improved charge storage capability because of the pseudo-capacitance contributed from pyrrolic-N atoms and pyridinic-N atoms.2 As oxygen reduction reaction (ORR) catalysts, NCs exhibited excellent activity that in some cases rivals that of the benchmark noble-metal-based catalysts.3,4 The N-moieties can also be coordinated with transition metals (e.g., Fe and Co), which can further improve the ORR activity through synergistic effects between N atoms and the metal atoms.3,5,6 These outstanding performances clearly showcase NCs' potential for substituting conventional activated carbon (as supercapacitor electrodes) and noble metals (as ORR catalysts) for large-scale electrochemical applications. At laboratory scale, the production of NCs is mainly grouped into two categories based on the type of the precursors. The first category deploys synthetic materials with high N contents as the precursors. Typical examples include metal organic frameworks,7, covalent organic frameworks,8 ionic liquids9 and certain polymers (e.g., polyacrylonitrile 10,11, polypyrrole12,13 and Schiff-base networks14,15). Other materials with no presence of N element such as graphene oxide can also lead to NCs but additional nitrogen sources must be accompanied.16,17 It is possible to retain the unique structural or compositional properties of these precursors in the resulting NCs; however, the synthesis of these precursors is often extremely time-consuming.
ACS Paragon Plus Environment
3
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 32
This inevitably compromises the merits of time-efficiency and low fabrication cost. The second category utilizes inherently N-rich biomasses as NC precursor candidates. Some of the most popular options are biological macromolecules (chitosan, plant fibers, proteins, spent coffee powder, etc.),18,19 and N-containing organic compounds (e.g., thiamine20). The abundance of these natural products enables this synthetic approach to be widely practiced in both laboratory and in industrial settings for large-scale production of NCs. Despite the simplicity of synthesis, the preparation of NCs from the natural products still faces challenges. First, the content of N-containing groups of NCs is typically lower than 5 at.%. Although being intrinsically high in some precursors (e.g., chitosan), the N concentration is eventually decreased after pyrolysis at high temperatures in chemically inert atmospheres, and it is further lowered if activation methods, e.g., potassium hydroxide (KOH) activation, are involved.2,21 Low-temperature annealing can keep relatively high N content, but it is not preferred for electrochemical applications since the resulting products are often incompletely carbonized and have poor electrical conductivity.12 In general, maintaining N-moieties and improving electrical conductivity of the resultant carbons are mutually exclusive for the conventional pyrolysis strategies. The second obstacle relates to the structural properties of the carbon products. Specifically, carbons derived from biomasses without physical or chemical activations tend to have limited surface area (
