Nano Energy (2012) 1, 514–517
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
SHORT COMMUNICATION
Carbon nanomaterials as metal-free catalysts
in next generation fuel cells
Mei Zhang, Liming Dain
Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
Received 5 January 2012; received in revised form 8 February 2012; accepted 24 February 2012
Available online 9 March 2012
KEYWORDS
Abstract
Oxygen reduction
reaction;
Metal-free catalysts;
Fuel cells;
Carbon
nanomaterials;
Doping;
Charge transfer
A new class of carbon nanomaterials has been discovered as metal-free catalysts to dramatically
reduce the cost and increase the efficiency of fuel cells. This article highlights recent progresses
in this emerging research area.
& 2012 Published by Elsevier Ltd.
The rapid increase in the global energy consumption and the
environmental impact of traditional energy resources pose
serious challenges to human health, energy security, and
environmental protection. One promising solution, providing
clean and sustainable power, is fuel cells. Instead of burning
fuel to create heat, fuel cells convert chemical energy
directly into electricity [1]. This energy conversion technology currently receives intensive research and development
focus because of its high energy conversion efficiency
(typically, 40–60% or up to 85% efficiency if waste heat is
captured for use), virtually no pollution, and potential
large-scale applications [2]. By pumping, for example,
hydrogen gas onto the anode, hydrogen is split into its
constituent electrons and protons. The protons diffuse
through the cell toward the cathode while the electrons
n
Corresponding author. Tel.: +1 216 368 4176;
fax: +1 216 368 4202.
E-mail address: liming.dai@case.edu (L. Dai).
2211-2855/$ - see front matter & 2012 Published by Elsevier Ltd.
doi:10.1016/j.nanoen.2012.02.008
flow out of the anode to provide electrical power (Fig. 1a).
Electrons and protons both end up at the cathode to
combine with reduced oxygen to form water. While the very
facile H2 oxidation kinetics greatly reduces the amount of
catalyst (e.g., platinum) at the anode, the slow oxygen
reduction reaction (ORR) on the cathode is a key step to
limit the energy conversion efficiency of a fuel cell and
requires a substantial amount of platinum catalyst (representing at least a quarter of the fuel cell cost). Although
platinum nanoparticles have long been regarded as the best
catalyst for the ORR, the Pt-based electrode suffers multiple
drawbacks, including its time-dependent drift, methanol
crossover, and CO deactivation [2]. This, together with the
high cost of platinum and its scarcity, has made these
catalysts the primary barrier to mass market fuel cells for
commercial applications. Thus, the large-scale practical
application of fuel cells has not been realized, though
alkaline fuel cells with platinum as an ORR electrocatalyst
were developed for the Apollo lunar mission in the 1960s [1].
Carbon nanomaterials as metal-free catalysts
515
Figure 1 (a) Schematic representation of the working principle for a fuel cell. Adapted from Ref. [6]. (b) (left) The calculated
charge density distribution and (right) schematic representations of possible adsorption modes of an oxygen molecule. Adapted from
Ref. [5]. (c) Electronegativity of elements increases along the Y-axis, leading to electron transfer from carbon atom, C, to nitrogen
atom, N, along the gradient. Adapted from Ref. [7].
Even though the amount of platinum needed for the
desired catalytic effect could be reduced using Pt alloys
[3,4], most non-noble metal catalysts still remain too low in
efficiency when compared to Pt or too expensive as
commercial mass production would still require large
amounts of platinum. The large-scale practical application
of fuel cells will be difficult to realize if the expensive
platinum-based electrocatalysts for ORR cannot be replaced
by other efficient, low cost, and stable electrodes.
Along with the intensive research efforts in reducing or
replacing Pt-based electrode in fuel cells, a new class of
carbon nanomaterials has been discovered, which, as
alternative ORR catalysts, could dramatically reduce the
cost and increase the efficiency of fuel cells [5]. For
example, our research has found that vertically-aligned
nitrogen-doped carbon nanotube (VA-NCNT) arrays can act
as a metal-free electrode to catalyze an ORR process free
from CO ‘‘poisoning’’ with a 3-times higher electrocatalytic
activity, much smaller crossover effect, and better longterm operational stability than that of the commercial
platinum-based electrode (C2-20, 20% platinum on Vulcan
XC-72R; E-TEK) in alkaline fuel cells [5].
Based on the experimental observations and quantum
mechanics calculations by B3LYP hybrid density functional
theory [5], we attributed the improved catalytic performance to the electron-accepting ability of the nitrogen
atoms, which creates net positive charge on adjacent
carbon atoms in the nanotube carbon plane of VA-NCNTs
(Fig. 1b) to readily attract electrons from the anode for
facilitating the ORR. The nitrogen-induced charge delocalization can also change the chemisorption mode of O2 from
the usual end-on adsorption (Pauling model) at the nitrogenfree CNT (CCNT) surface (top right, Fig. 1b) to a side-on
adsorption (Yeager model) onto the NCNT electrode (bottom
right, Fig. 1b). The N-induced charge-transfer from adjacent carbon atoms could lower the ORR potential while the
parallel diatomic adsorption could effectively weaken the
O–O bonding, facilitating ORR at the VA-NCNT electrode [5].
Uncovering this new ORR mechanism in the nitrogen-doped
carbon nanotube electrode is significant as that the same
principle could be applied to the development of various
other metal-free efficient ORR catalysts for fuel cell
applications.
Although it is still a challenge to determine the exact
locations of nitrogen atoms in the carbon nanotube
structures and chemical nature of the catalytic sites, recent
research activities carried out in many laboratories, including our own one, have not only confirmed the above findings
but also further proved that the doping-induced charge
transfer has large impact on the design/development of new
metal-free catalytic materials for fuel cell and many other
applications [8]. For instance, Yang et al., [9] have recently
extended the doping atoms to include boron with a
lower electronegativity than that of carbon (Fig. 1c). These
516
authors found that the doping-induced charge redistribution, regardless whether the dopants have a higher (as N) or
lower (as B) electronegativity than that of carbon (Fig. 1c),
could create charged sites (C + or B + ) favorable for O2
adsorption to facilitate the ORR process. This work suggests
further exploration of the metal-free electrocatalysts based
on carbon nanotubes doped by atoms (other than N and B)
with electronegativities different from that of carbon atom
(Fig. 1c). By extension, we have successfully prepared
vertically-aligned BCN (VA-BCN) nanotubes containing both
nitrogen and boron heteroatoms. Due to a synergetic effect
arising from the co-doping of carbon nanotubes with boron
and nitrogen, the resultant VA-BCN nanotubes were demonstrated to show a higher electrocatalytic activity for ORR in
alkaline medium than VA-CNTs doped with either boron or
nitrogen only [10]. The observed superior ORR performance
with a good methanol and CO tolerance and excellent
durability for the VA-BCN nanotube electrode than the commercial Pt/C electrode opens up avenues for the development
of novel efficient metal-free ORR catalysts by co-doping
carbon nanotubes with more than one heteroatom of electronegativities different from that of carbon atom (Fig. 1c).
As a building block for carbon nanotubes, graphene is an
alternative candidate for potential uses as the metal-free ORR
catalyst. Indeed, N-doped graphene (N-graphene) films produced by chemical vapor deposition (CVD) in the presence of
ammonia have recently been demonstrated to show a superb
ORR performance similar to that of VA-NCNTs with the same
nitrogen content in alkaline medium [11]. The ease with which
graphene materials and their N-doped derivatives can be
produced by various low-cost, large-scale methods, ranging
from the CVD to solution exfoliation of graphite [12], suggests
considerable room for cost effective preparation of metal-free
efficient graphene-based catalysts for oxygen reduction. In
addition, Liu et al. [13] have reported the ORR electrocatalytic
performance better than platinum for nitrogen-doped ordered
mesoporous graphitic arrays (NOMGAs) prepared by a metalfree nanocasting technology using a nitrogen-containing
aromatic dyestuff, N,N0 -bis(2,6-diisopropyphenyl)-3,4,9,10perylenetetracarboxylic diimide (PDI), as the carbon precursor.
Owing to the metal-free preparation procedure, the reported
electrocatalytic activity can be attributed exclusively to the
incorporation of nitrogen in PDI-NOMGAs. In a somewhat
related, but independent study, Liu et al. [14] have demonstrated phosphorus-doped graphite layers with high electrocatalytic activity for ORR in an alkaline medium. Metal-free
N-doped MWCNTs or ordered mesoporous carbons (OMCS) have
also been produced through carbonization of a MWCNTsupported polyaniline (PANI) coating [15] or via NH3 activation
[16]. These metal-free N-doped nanocarbons have also been
demonstrated to exhibit high ORR activities even in acidic
electrolytes. Good ORR activities in the acidic media have also
been observed for NCNTs produced by both metal catalyzed
and metal-free nanotube growth processes [17–19]. However,
the catalytic performance of these reported N-doped carbon
nanomaterials in acidic medium still needs to be further
improved to meet the requirement for practical applications.
In addition to the intramolecular charge-transfer that
impart ORR electrocatalytic activities to heteroatom-doped
carbon nanotubes, graphene and graphite described above,
we have recently demonstrated that certain polyelectrolyte
(e.g., poly(diallyldimethylammonium chloride)) adsorbed
M. Zhang, L. Dai
pure carbon CNTs or graphene, either in an aligned or
nonaligned form, could also act as metal-free electrocatalysts for ORR through the intermolecular charge-transfer
from the all-carbon CNTs or graphene to the adsorbed PDDA
[20,21]. It is notable that the PDDA adsorbed verticallyaligned CNT electrode possesses remarkable electrocatalytic
properties for ORR; similar to that of commercially available
Pt/C electrode. These results clearly indicate that the
important role of intermolecular charge-transfer to ORR can
be applied to carbon nanomaterials in general for the
development of various other metal-free efficient ORR
catalysts for fuel cell applications, even new catalytic
materials for applications beyond fuel cells (e.g., metal–air
batteries, electrochemical biosensors). However, further study
on the catalytic mechanism and kinetics is still needed in order
to design and develop functionalized carbon-based catalysts
with a desirable activity and durability. The long-term performance evaluation of these nanocarbon catalysts in actual fuel
cells should also be performed.
We firmly believe that heteroatom-doping of carbon
nanomaterials (e.g., nanotube, graphene, mesoporous carbon)
to induce the intramolecular charge transfer has been shown
to be a promising approach to the development of metal-free,
carbon-based catalysts with even a higher electrocatalytic
activity and better long-term operation stability than that of
commercially available platinum-based electrodes for oxygen
reduction in fuel cells. Furthermore, intermolecular chargetransfer has also been demonstrated to impart ORR activities
to all-carbon carbon nanomaterials for fuel cell applications,
even new catalytic materials for applications beyond fuel
cells. Although the ORR catalytic performance of N-doped
carbon nanomaterials in acidic media is still relatively poor
with respect to that in alkaline electrolytes, judicious
application of the intramolecular (intermolecular) chargetransfer processes with various dopant atoms of different
electronegativities (charged absorption moieties) to different
carbon nanomaterials has opened up the rich field of metalfree ORR electrocatalysts with vast opportunities. Further
development in this exciting field will surely revolutionize the
way in which future energy systems are developed, and should
result in a better fuel economy, a decrease in harmful
emissions, and a reduced reliance on petroleum sources.
Our work was supported financially by the NSF (CMMI1000768, CMMI-1047655, IIP-0924197, DMR-1106160), the
AFOSR (FA2386-10-1–4071, FA 9550-10-1–0546, FA9550-09-10331, FA9550-12-1-0037), the DOD-Army (W911NF-11-10209) and the DOE (DE-SC0003736).
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Mei Zhang is a Research Assistant Professor in the Department of Biomedical
Engineering at Case Western Reserve University (CWRU). She also holds a joint
appointment in University Hospitals-Case
Comprehensive Cancer Center. Zhang’s
expertise ranges across the synthesis and
characterization of polymers and nanomaterials, nanotechnology, tumor immunology, and immunotherapy in the treatment
of cancer. Her primary research interest is to apply polymer
materials to the design and development of novel nanoparticle
systems for multifunctional applications, including gene drug
delivery, biomedical imaging, and bio-energy systems (e.g., biofuel cells).
Liming Dai is the Kent Hale Smith Professor
in the Department of Macromolecular
Science and Engineering at Case Western
Reserve University (CWRU). He is also the
director of the Center of Advanced Science
and Engineering for Carbon (CASE4Carbon).
Before joining the CWRU, he was an
associate professor of polymer engineering
at the University of Akron and the Wright
Brothers Institute Endowed Chair Professor
of Nanomaterials at the University of Dayton. Dr. Dai’s expertise lies
across the synthesis, chemical modification and device fabrication
of conjugated polymers and carbon nanomaterials for energyrelated and biomedical applications.