Manganese-Based Non-Precious Metal Catalyst for Oxygen Reduction in Acidic Media

Abstract

Non-precious metal catalysts (NPMCs) based on manganese (Mn) are prepared by heat treating polyaniline (PANI), manganese acetate and Ketjenblack EC300J (KJ) carbon supports. Using a heat treatment temperature of 950oC, followed by an acid leaching and second heat treatment step, Mn-PANI-KJ catalysts are found to provide onset and half-wave potentials of ca. 0.90 and 0.77 V vs. RHE, respectively, in 0.5 M H2SO4 electrolyte. After 5,000 cycles of electrochemical durability testing, Mn-PANI-KJ demonstrates a half-wave potential loss of only ca. 20 mV, superior to the 80 mV loss for our previously developed iron (Fe)-PANI-KJ catalyst. Increased surface nitrogen concentrations and relative ratios of pyridinic to graphitic nitrogen species were observed at increased Mn-PANI-KJ preparation temperatures, along with the evolution of graphene-like and graphitic nanoshell structures

Full text

Manganese-Based Non-Precious Metal Catalyst for Oxygen Reduction
in Acidic Media
Drew C. Higgins
a,b
, Gang Wu
b
, Hoon Chung
b
, Ulises Martinez
b
, Shuguo Ma
c
,
Zhongwei Chen
a
, and Piotr Zelenay
b
a
Department of Chemical Engineering, Waterloo Institute for Nanotechnology
University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada.
b
Materials Physics and Applications Division, Los Alamos National Laboratory
Los Alamos, New Mexico 87545, USA
c
Department of Chemical Engineering, University of South Carolina
Columbia, South Carolina 29208, USA
Non-precious metal catalysts (NPMCs) based on manganese (Mn)
are prepared by heat treating polyaniline (PANI), manganese
acetate and Ketjenblack EC300J (KJ) carbon supports. Using a
heat treatment temperature of 950
o
C, followed by an acid leaching
and second heat treatment step, Mn-PANI-KJ catalysts are found
to provide onset and half-wave potentials of ca. 0.90 and 0.77 V vs.
RHE, respectively, in 0.5 M H
2
SO
4
electrolyte. After 5,000 cycles
of electrochemical durability testing, Mn-PANI-KJ demonstrates a
half-wave potential loss of only ca. 20 mV, superior to the 80 mV
loss for our previously developed iron (Fe)-PANI-KJ catalyst.
Increased surface nitrogen concentrations and relative ratios of
pyridinic to graphitic nitrogen species were observed at increased
Mn-PANI-KJ preparation temperatures, along with the evolution
of graphene-like and graphitic nanoshell structures.
Introduction
Despite significant progress realized in recent years in the development of polymer
electrolyte membrane fuel cells (PEFCs), widespread commercial success of this
technology is still hindered by several technical challenges, including cost, performance
and durability. The extensive reliance on platinum (Pt) based catalysts to facilitate the
anodic hydrogen oxidation reaction (HOR) and cathodic oxygen reduction reaction
(ORR) has been a major limiting factor, owing to the high cost of this precious metal and
monopolized global distribution (1). Combined with the volatile pricing of this natural
resource, reducing or eliminating the Pt dependence of PEFC technologies is required to
realize sustainable commercial success. As the ORR reaction kinetics are inherently six
order of magnitude slower than the HOR (2), significantly higher Pt loadings at the
cathode are required to facilitate the ORR at rates sufficient for practical devices. This
has inspired significant research activities directed towards the replacement of
conventional Pt catalysts with ORR-active non-precious metal catalysts (NPMCs) (3-9).
Since the 1960s, the majority of NPMC research has focused on the development of
transition metal-nitrogen-carbon complexes (M-N-C), with the most active and stable
catalysts prepared using high-temperature (i.e. 700-1100
o
C) heat treatment, and with iron
(Fe) or cobalt (Co) employed as the transition metal of choice (10-15). While new
10.1149/06131.0035ecst ©The Electrochemical Society
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synthesis strategies have advanced the performance and electrochemical durability of
these catalysts to unprecedented levels (12,14), the current capabilities are still not
adequate for practical deployment. It is therefore of interest to deviate from conventional
approaches to develop and investigate unique NPMC configurations with the potential to
overcome remaining challenges.
Manganese (Mn)-based materials have commonly been employed as ORR active
catalysts in alkaline media (16-19); however their application as ORR catalysts under
acidic conditions, such as those encountered during PEFC operation, remains very
limited (20). Additionally, the recent emergence of many supposedly “metal-free”
graphene-based catalysts has been reported in the literature, with these materials
fabricated using graphene oxide precursors (GO). The GO materials are commonly
prepared by the Hummers method, employing potassium permanganate (KMnO
4
) as a
strong oxidizing agent, and inevitably leading to confounding Mn impurities. These
impurities have been shown to have a significant effect on the resulting ORR activity in
alkaline media (21,22). This work has inspired us to investigate the role of Mn on the
structure, properties and performance of NPMCs operating in acidic electrolytes. It has
been well established that these properties are directly governed by the selection of
transition metal species, and we investigate the effect of using Mn to prepare our
previously developed polyaniline (PANI)-based catalyst technologies (14,23,24).
Experimental Methods
Catalyst Synthesis
In a typical synthesis, 2 mL of aniline was added to 400 mL of 1.5 M HCl, followed
by the addition of 2 g manganese acetate. To polymerize the aniline, 5 g of ammonium
persulfate was added, and the mixture stirred for 3 hours. During this time, 0.4 g of
functionalized Ketjenblack EC300J (KJ) carbon support was dispersed in distilled
deionized water by ultrasonication. The KJ had previously been functionalized by
treatment in 70% HNO
3
at 70
o
C for 8 hours. The entire mixture was stirred at room
temperature for 48 hours, after which the solvent was evaporated at 80
o
C. The resulting
solid was collected, grinded to a fine powder using a mortar and pestle, and then heat
treated under a nitrogen environment at a temperature ranging from 850 to 950
o
C for 1
hour. The heat-treated materials were then subject to acid leaching in 0.5 M H
2
SO
4
at
80
o
C for 8 hours, followed by a second heat treatment at 900
o
C for 3 hours. The resulting
Mn-PANI-KJ catalyst was then collected for subsequent characterization and
electrochemical performance evaluation.
Characterization
Transmission electron microscopy (TEM) images were collected on a FEI Tecnai F30
Analytical TEM system operating at 300 keV. X-ray photoelectron spectroscopy (XPS)
was carried out to investigate surface atomic contents and species identification, done
using a Thermo Scientific ESCALAB 210 and MICROLAB 310D spectrometer
employing Mg K
α
radiation.
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Electrochemical Evaluation
Catalyst inks for all electrochemical evaluations were prepared by sonicating 10 mg
of Mn-PANI-KJ catalyst in 1 mL of isopropanol. The inks were then deposited onto a
glassy carbon disk electrode using a micropipette to achieve a loading of 0.6 mg
cm
-2
.
ORR activity and four-electron selectivity of the Mn-PANI-KJ catalysts were evaluated
using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) testing,
respectively. All measurements were conducted using a CHI electrochemical station and
a standard three-electrode electrochemical glass cell. An Hg/HgSO
4
electrode and a
graphite rod were used as the reference and counter electrodes, respectively. After
measurements, all potentials were converted to the RHE scale. ORR polarization curves
were obtained at room temperature using steady-state staircase voltammetry testing in
oxygen-saturated 0.5 M H
2
SO
4
. The electrode was rotated at 900 rpm, and a step size of
30 mV and time period of 30 s step
-1
was used. The ring electrode was polarized at 1.2 V
vs. RHE for hydrogen peroxide detection, with the method for calculating the selectivity
towards the four-electron reduction of oxygen to water described elsewhere (25).
Electrochemical durability was investigated by repeatedly cycling the electrode potential
between 0.6-1.0 V vs. RHE in oxygen-saturated solution.
Results and Discussion
ORR Performance Evaluation
The ORR performance of Mn-PANI-KJ catalysts prepared using heat-treatment
temperature ranging from 850 to 950
o
C is displayed in Figure 1a. The sample heat
treated at 850
o
C shows an onset potential of ca. 0.84 V vs. RHE, whereas the samples
heat-treated at 900 and 950
o
C display a relatively higher onset potential of ca.
0.89-0.90 V vs. RHE. This near identical onset potential for the samples prepared at 900
and 950
o
C likely indicates a similar active site structure formed in this catalyst system at
temperatures beyond 900
o
C. The Mn-PANI-KJ catalyst prepared at 950
o
C shows the
highest ORR performance, including a half-wave potential of 0.77 V vs. RHE, the best
ever reported for Mn-based catalysts. Upon comparison with Co-PANI-KJ catalyst
(Figure 1b), a significant increase in both onset (70 mV) and half-wave (30 mV)
potential is observed. Additionally, Mn-PANI-KJ demonstrates an onset potential similar
to that of our previously reported high-activity Fe-PANI-KJ catalyst (14), along with a
half-wave potential only ca. 40 mV lower. The H
2
O
2
selectivity of the optimized (950
o
C
treatment) Mn-PANI-KJ catalyst was evaluated by RRDE testing to be ca. 5% over the
broad range of electrode potentials investigated. These results indicate that Mn can act as
an effective transition metal species for preparing NPMC with good ORR performance in
acidic electrolytes.
Electrochemical Durability Investigation
The electrochemical durability of Mn-PANI-KJ catalyst prepared at 950
o
C was
evaluated by repeated electrode potential cycles from 0.6 to 1.0 V vs. RHE under oxygen
saturation (Figure 2a). After 5,000 cycles, Mn-PANI-KJ demonstrates a loss in half-
wave potential of only ca. 20 mV, along with a small loss in the mass-transport limited
current density likely arising from the changes in the electrode microstructure (26). This
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is far superior to our previously prepared Fe-PANI-KJ catalysts, that, despite showing
higher initial ORR performance, demonstrates a ca. 80 mV loss in half-wave potential
after 5,000 cycles (Figure 2b). Additionally, after cycling the Mn-PANI-KJ electrode
15,000 times, the loss of half-wave potential was only ca. 30 mV, indicating the excellent
stability capabilities of this newly developed catalyst system.
Figure 1. (a) ORR polarization curves for Mn-PANI-KJ catalysts prepared at different heat-
treatment temperatures and (b) Mn-PANI-KJ, Co-PANI-KJ and Fe-PANI-KJ catalysts obtained
in oxygen-saturated 0.5 M H
2
SO
4
at 900 rpm.
Figure 2. ORR activity of (a) Mn-PANI-KJ and (b) Fe-PANI-KJ catalysts after electrochemical
durability investigations involving repeated cycling of the electrode potential between 0.6 and
1.0 V vs. RHE in 0.5 M H
2
SO
4
.
Electron Microscopy Studies
To evaluate the effect of heat-treatment temperature on the morphology of the
resulting Mn-PANI-KJ catalysts, TEM analysis was carried out with typical images of
the catalyst materials provided in Figure 3. At temperatures exceeding 900
o
C (Figure
3b-d), the evolution of distinct graphene-like structures is observed in the catalysts,
including the formation of graphitic nanoshells observed at 950
o
C (Figure 3d). Although
the detailed mechanistic pathway of active site formation is still unclear for NPMCs, a
corresponding increase in ORR activity above 900
o
C is observed along with the
formation of these graphene-like and graphitic nanostructures. This may provide more
Potential (V
vs
. RHE)
0.2 0.4 0.6 0.8 1.0
Current density (mAcm
-2
)
-4
-3
-2
-1
0(a)
initial
15K cycles
Potential (V
vs.
RHE)
0.2 0.4 0.6 0.8 1.0
Current density (mAcm
-2
)
-4
-3
-2
-1
0
Initial
5K cycles
(b)
Potential (V
vs
. RHE)
0.2 0.4 0.6 0.8 1.0
Current density (mAcm
-2
)
-4
-3
-2
-1
0
900
o
C
850
o
C
950
o
C
(a)
Potential (V
vs
. RHE)
0.2 0.4 0.6 0.8 1.0
Current density (mAcm
-2
)
-4
-3
-2
-1
0
Mn-PANI-KJ
Co-PANI-KJ
Fe-PANI-KJ
(b)
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edge-plane exposure for oxygen adsorption and subsequent reduction (27-29), and
thereby contribute to the higher activity observed by RDE evaluation.
Figure 3. TEM images of Mn-PANI-KJ catalysts prepared at (a) 850, (b) 900 and (c) 950
o
C.
(d) High-resolution TEM image of a graphitic nanoshell in the sample heat-treated at 950
o
C.
X-ray Photoelectron Spectroscopy Investigation
XPS was used to investigate the surface nitrogen concentrations and relative species
concentrations of the developed catalysts. High-resolution N 1s spectra for Mn-PANI-KJ
catalysts prepared at varying temperatures along with their evaluated nitrogen content are
displayed in Figure 4. Each signal is deconvoluted into three individual peak
contributions arising from pyridinic (ca. 398.6 eV), graphitic (ca. 401.1 eV) and oxidized
(ca. 403.2 eV) nitrogen species. Increased surface nitrogen concentrations are observed
with increasing heat-treatment temperatures, with a maximum of 8.3 at.% of nitrogen
found for the catalyst prepared at 950
o
C. Interestingly, this observation is contradictory to
the traditionally lower nitrogen surface concentrations observed at higher temperatures
for a wide array of Fe and/or Co based NPMCs (10,12,23,30). This provides further
indication that the role of transition metal species during catalyst preparation is unique to
the choice of metal type. Additionally, increased pyridinic to graphitic nitrogen peak
ratios are observed at higher temperatures, with ratios of 0.82, 0.97 at 0.99 determined for
Mn-PANI-KJ catalysts prepared at 850, 900 and 950
o
C, respectively. This again is in
contrast to previous reports on NPMC and nitrogen-doped carbon development, whereby
increased graphitic nitrogen contents are commonly observed at elevated temperatures
(31-33). This suggests that Mn-based species may provide some sort of stabilizing role
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for edge-plane residing pyridinic nitrogen species at elevated temperatures, although this
observation requires further investigation.
Figure 4. High resolution N 1s XPS spectra for Mn-PANI-KJ catalysts prepared at 850, 900 and
950
o
C. Conclusions
A new class of Mn-based NPMCs was prepared by heat treating a mixture of
manganese acetate, polyaniline and a high surface area carbon support. The Mn-PANI-KJ
catalysts prepared at a temperature of 950
o
C provides an ORR onset potential of 0.90 V
vs. RHE, and a half-wave potential of ca. 0.77 V vs. RHE in acidic (0.5 M H
2
SO
4
)
electrolyte. This represents the first report of an Mn-based catalyst with high ORR
activity in acidic electrolyte, approaching that of state-of-the-art Fe-based NPMCs.
Additionally, after 5,000 cycles of electrochemical durability testing, Mn-PANI-KJ
displays a loss in half-wave potential of only ca. 20 mV, a significant improvement over
the 80 mV loss demonstrated by Fe-PANI-KJ. The role of the heat-treatment temperature
employed during catalyst synthesis was investigated, with the formation of graphene-like
structures and graphitic nanoshells observed at a temperature of 900
o
C and higher.
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Higher surface nitrogen concentrations and pyridinic to graphitic nitrogen content ratios
are indicated by XPS for samples prepared at increased temperatures, which is opposite
the trend reported for conventional Fe and Co-based NPMCs.
Acknowledgments
This work was supported by the Natural Science and Engineering Research Council of
Canada (NSERC), the U.S. Department of Energy (DOE) through the Fuel Cell
Technologies Office, and by Los Alamos National Laboratory through the Laboratory-
Directed Research and Development Program.
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