Chemistry Top Ten: Cheap Cobalt Catalyst Challenges Pricey Platinum Performance
Two adjacent papers in the current Hot Ten, #6 and #7, deal with the chemistry of fuel cells. According to citations recorded in brand-new papers indexed in Web of Science during a recent two-month period, these reports rank among the most-cited chemistry papers published in the last two years. One day, the technology described in the papers might provide the hydrogen-fuelled alternative to gasoline-powered motors.
What’s Hot in Chemistry
|Rank||Paper||Citations This Period (Mar-Apr 13)||Rank Last Period (Jan-Feb 13)|
Y.W. Zhu, et al., “Carbon-based supercapacitors produced by activation of graphene,” Science, 332(6037): 1537-41, 24 June 2011. [U. Texas, Austin; Quantachrome Insts., Boynton Beach, FL; Brookhaven Natl. Lab., Upton, NY
Z.C. He, et al., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells,” Adv. Materials, 23(40): 4636, 25 October 2011. [South China U. Sci., Quangzhou; Chinese Acad. Sci., Suzhou; Hong Kong Baptist U.]
Y.M. Sun, et al., “Solution-processed small-molecule solar cells with 6.7% efficiency, Nature Materials, 11(1): 44-8, January 2012. [U. Calif., Santa Barbara]
Z.C. He, et al., “Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure,” Nature Photonics, 6(9): 591-5, September 2012. [S. China U. Technol., Guangzhou, China]
Y. Umena, et al., “Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å,” Nature, 473(7345): 55-60, 5 May 2011. [Osaka City U., Japan; Okayama U., Japan]
G. Wu, et al., “High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt,” Science, 332(6028): 443-7, 22 April 2011. [Los Alamos Natl. Lab., NM; Oak Ridge Natl. Lab., TN]
Y.Y. Liang, et al., “Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction,” Nature Materials, 10(10): 780-6, October 2011. [Stanford U., CA; Canadian Light Source, Saskatoon, Canada]
T.Y. Chu, et al., “Bulk heterojunction solar cells using thieno[3,4-c]pyrrole-4,6-dione and diethieno[3,2-b:2’,3’-d]silole copolymer with a power conversion efficiency of 7.3%,” J. Am. Chem. Soc., 133(12): 4250-3, 30 March 2011. [Natl. Res. Council of Canada, Ottawa, Ontario; U. Laval, Quebec, Canada]
Q. Li, et al., “Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets,” J. Am. Chem. Soc., 133(28): 10878-84, 20 July 2011. [Wuhan U. Technol., China; Natl. Ctr. Nanosci. & Technol., Beijing, China;
S.A. Freunberger, et al., “Reactions in the rechargeable lithium-O2 battery with alkyl carbonate electrolytes,” J. Am. Chem. Soc., 133(20): 8040-7, 25 May 2011. [U. St. Andrews, Fife, UK; Toyota Motor Europe, Zaventem, Belgium; Paul Scherrer Electrochem. Lab., Villigan, Switzerland]
|SOURCE: Thomson Reuters Web of Science
NB. Only papers indexed by Thomson Reuters since May 2011 are tracked. The “+” sign indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings
Hydrogen as a transport fuel may have been greatly over-hyped in the 1990s and 2000s, but fuel cells using it offer a future of pollution-free vehicles because the only waste product they emit is water vapor. What makes fuel cells expensive, however, is their current reliance on platinum. This metal sells at $45 per gram, and the amount needed for a fuel-cell driven motor adds around $1,800 to the cost of the vehicle.
Cobalt costs 20 cents a gram, iron even less. These could replace platinum thanks to researchers at Los Alamos National Laboratory (LANL) in New Mexico. This is one of the most active centers of fuel cell research; one highly cited report featuring LANL authors is a 2007 reviewthat has gathered more than 700 citations (see R. Borup, et al., “Scientific aspects of polymer electrolyte fuel cell durability and degradation,” Chem. Rev., 107: 3904-51, 2007). Another paper from LANL, on non-platinum catalysts for fuel cells, has also attracted wide attention, with 550 citations (see R. Bashyam, P. Zelenay, “A class of non-precious metal composite catalysts for fuel cells,” Nature, 443: 63-6, 2006). Now at #6 in the current Hot Ten is their 2011 paper.
Polymer electrolyte fuel cells (PEFC) consist of two electrodes: at one, hydrogen is oxidized, at the other, oxygen is reduced. Both processes need a catalyst, and the best one is platinum or a platinum-rich alloy. The oxygen reduction reaction (ORR) electrode requires more catalyst than the hydrogen oxidation electrode.
There are three requirements of the ORR electrode: it has to be efficient, durable, and able to promote the four-electron reduction of oxygen to water and not the two-electron to hydrogen peroxide. This chemical has the potential to damage a fuel cell by attacking the polymer membranes that are an essential component.
COBALT AND IRON
In paper #6, Piotr Zelenay, and his colleagues Gang Wu and Christina Johnston, describe an ORR cathode in which the catalyst is iron/cobalt fixed in a carbonaceous material derived from aniline. Co-author Karren More of Oak Ridge National Laboratory used micrographic imaging to probe the mechanism of how the new catalyst works.
As Zelenay tells ScienceWatch: "We have found a catalyst with good durability and life-cycle relative to platinum-based catalysts. To all intents and purposes, this is a zero-cost catalyst in comparison to platinum, so it directly addresses one of the main barriers to hydrogen fuel cells."
The new catalyst delivers high power efficiently and can be switched on and off many times without affecting performance. And it does not produce hydrogen peroxide except in minute amounts. In this respect it performs better than platinum catalyst fuel cells. Not surprisingly, the team’s invention is being patented.
The new catalysts were constructed by means of a high-temperature method using iron and cobalt salts and polyaniline as the precursor of the carbon and nitrogen components, both of which are necessary. The process starts with commercial carbon particles which are mixed with aniline oligomers and iron(III) and cobalt(II) nitrates. Ammonium persulfate is added to fully polymerize the aniline followed by heat treatment of between 400 and 1000oC under an atmosphere of nitrogen. (The material produced at 900oC performed best of all.) The product was then left in contact with dilute sulfuric acid for 8 hours to remove unwanted material.
The resulting polyaniline-metal-carbon catalyst (PANI-M-C) was shown to have remarkable activity, and its structure examined by Fourier transform IR and scanning tunneling microscopy revealed graphene-like components. A more detailed analysis of the reduction catalyst has now been published in the Journal of Physical Chemistry C, (M. Ferrandon, et al., “Multitechnique characterization of a polyaniline-iron-carbon oxygen reduction catalyst,” 116; 16001-13, 2012). PANI-M-C was subjected to a 700-hour fuel test and was calculated to lose only a few percent of performance after 30,000 cycles of operation.
The Zelenay group’s most recent paper reports no loss of performance of a hydrogen fuel cell with a polyaniline/iron catalyst on multi-walled carbon nanotubes (G. Wu, et al., “A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability,” Chem. Commun., 49: 3291-3, 2013). It too performed consistently for 500 hours of operation, albeit at a low fuel cell voltage of 0.4 V. At higher voltages it was not quite so good.
COBALT BY ITSELF
Carbon in the form of graphene is the catalyst support material in paper #7, which is also concerned with fuel cell technology. It comes from a group at Stanford University and has Hongjie Dai as the lead author. The catalyst it describes is nano-crystalline cobalt oxide (Co3O4) on reduced graphene oxide, and it too works well, and not only for oxygen reduction but for oxygen evolution as well. This could be part of a so-called regenerative fuel cell which can operate either as a fuel cell producing electricity, or as a generator producing the hydrogen and oxygen by the electrolysis of water.
The Stanford catalyst was synthesized using using mildly oxidized graphene sheets in an ethanol/water solvent containing cobalt acetate, and heated at 80oC. This was followed by heating at 150oC with ammonium hydroxide which caused the cobalt oxide crystals to form and reduced the oxidized graphene. The result was a hybrid material which gave a high performance comparable to platinum, and was even more stable.
Neither Co3O4 nor graphene individually showed catalytic activity for oxygen reduction but their combination did extremely well, and it was even improved by doping the graphene with nitrogen. The Stanford authors attribute the unusual catalytic activity to a synergetic chemical coupling between Co3O4 and graphene.
Although the idea of a hydrogen-based economy seems somewhat quaint, now that abundant supplies of fossil methane are being exploited by fracking, it must surely come one day. When that happens, cheap and efficient fuel cells will be needed, and there is every indication that they will be ready and waiting.
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.
The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.