Efficient Water Oxidation Using CoMnP Nanoparticles

Article abstract of DOI:10.1021/jacs.6b01543

The development of efficient water oxidation catalysts based on inexpensive and Earth-abundant materials is a prerequisite to enabling water splitting as a feasible source of alternative energy. In this work, we report the synthesis of ternary cobalt mang

Full text of DOI:10.1021/jacs.6b01543

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Efficient Water Oxidation Using CoMnP Nanoparticles
Da Li, Habib Baydoun, Claudio N Verani, and Stephanie L. Brock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01543 • Publication Date (Web): 13 Mar 2016
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Efficient Water Oxidation Using CoMnP Nanoparticles
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Da Li, Habib Baydoun, Claudio N. Verani, Stephanie L. Brock*
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
Supporting Information Placeholder
We have previously reported the synthesis of MnP nanopar-
ABSTRACT: The development of efficient water oxidation
catalysts based on inexpensive and Earth-abundant materials
is a prerequisite to enabling water splitting as a feasible
source of alternative energy. In this work, we report the syn-
thesis of ternary cobalt manganese phosphide nanoparticles
from the solution-phase reaction of manganese and cobalt
carbonyl complexes with trioctylphosphine. The CoMnP
nanoparticles (ca. 5 nm in diameter) are nearly monodis-
perse and homogenous in nature. These CoMnP nanoparti-
cles are capable of catalyzing water oxidation at an overpo-
tential of 0.33 V with a 96% Faradaic efficiency when depos-
ited as an ink with carbon black and Nafion. A slight de-
crease in activity is observed after five hundred cycles, which
is ascribed to the etching of P into solution as well as the
oxidation of the surface of the nanoparticles. Manganese-
based ternary phosphides represent a promising new system
to explore for water oxidation catalysis.
ticles.18 However, attempts to use these particles as water
oxidation catalysts revealed that MnP is not stable under
oxidizing conditions, which we attribute to the highly oxoph-
ilic nature of manganese. As shown in Figure S1, the presence
of a broad irreversible preoxidation peak at ~1.4 VRHE is ob-
served in the first oxidation sweep. The following sweeps
revealed a large drop in the current density consistent with
an irreversible oxidative transformation of MnP during water
oxidation. Accordingly, we sought to moderate the activity of
Mn by the inclusion of a second metal, namely, cobalt. In
this paper, we report on a simple and highly reproducible
route for the synthesis of novel cobalt manganese phosphide,
CoMnP, nanoparticles and the investigation of their catalytic
behavior towards water oxidation.
The initial target of our investigation was the phase
Co0.5Mn0.5P, which we expected to be feasibly attained as
discrete nanoparticles based on the facts that (1) MnP and
CoP are isostructural and (2) it is possible to synthesize the
entire solid solution by solid-state methods in the bulk
phase.19-21 Adapting known methods for MnP and CoP nano-
particle synthesis, Co2(CO)8 and Mn2(CO)10 were injected
into a solution of hot 1-octadecene and oleylamine, followed
by injection of trioctylphosphine (TOP) and elevation of the
solution temperature to 350 °C (see Supporting Information,
SI, for details). However, the X-ray diffraction (XRD) pattern
of the product showed that the peaks correspond to ortho-
rhombic CoMnP and not the expected Co0.5Mn0.5P phase
(Figure 1a), and this is confirmed by SAED (Figure S3). Intri-
guingly, we have not been successful preparing the Mn2P
phase (the Mn end-product of Co1-xMnxP); producing only
MnP or no isolable product. We hypothesize that incorporat-
ing Co favors the inclusion of low-valent Mn, thus facilitating
the formation of the “M2P” phase. Transmission electron
microscopy (TEM) images reveal the formation of spherical
nanoparticles with an average diameter of 4.59 ± 0.76 nm
(Figure 1b, S2). The corresponding energy dispersive X-ray
spectrum (EDX, Figure S3) indicates that the atomic ratio of
Co, Mn and P is close to 1:1:1.3. The composition of the nano-
particles was further confirmed from ICP-MS measurements
which show a ratio of Co:Mn:P of 1:1:1.1 (Figure S3). The slight
excess of P is attributed to the presence of residual TOP as
surface binding groups.
Splitting water into hydrogen and oxygen represents an ideal
source of clean renewable energy.1 However, water oxidation
(2H2O → O2 + 4H+ + 4e−), the first step in the overall water
splitting reaction, has a high activation barrier that, coupled
with the need for transfer of four electrons and four protons,
presents a bottleneck in the transformation of water into O2
and H2.
Ruthenium and iridium oxides are well-known water oxida-
tion catalysts.2,3 Nevertheless, the scarcity of Ru and Ir pose
serious limitations to the widespread adoption of water split-
ting as a green approach to renewable energy. In order to
address this problem, research has focused on the synthesis
of novel materials composed of stable, Earth-abundant met-
als capable of efficient catalytic water oxidation. Inspired by
the oxygen-evolving center of Photosystem II, manganese-
based materials (oxides and phosphates) have garnered con-
siderable attention as water oxidation catalysts. 4-8
At the same time, transition metal phosphide nanoparticles
are emerging as a new class of water oxidation catalysts with
reports of high activity in Ni2P,9,10 CoP,11-15 and CoFeP.16,17
While manganese-based phosphides may be natural catalyst
candidates, such materials have yet to be investigated. This
gap is at least in part due to a lack of synthetic methodolo-
gies for manganese phosphide nanoparticles.
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Figure 3b are derived from the polarization curves and they
PDF#42-0932 CoMnP
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show the plots of overpotential vs. the log of the current den-
sity. While the CoMnO2 and the Co2P have a Tafel slope of 95
and 128 mV/dec, respectively; CoMnP has a slope of 61
mV/dec, close to the ideal value of 59 mV/dec (equivalent to
2.3 x RT/F) associated with a one-electron transfer prior to
the rate-limiting step.25,26
(a)
(b)
100 nm
The decrease in overpotential for CoMnP relative to Co2P is
associated with synergism between the two metal centers. It
has been suggested that the insertion of a second metal may
help lower the thermodynamic barrier of a proton-coupled
electron transfer (PCET) pre-equilibrium while facilitating
O-O bond formation, leading to enhanced catalytic activity.25
Most proposed mechanisms for CoOx species suggest the
need the formation of vicinal high-valent oxo species.25,27-29
While the formation of Co=O is energetically demanding and
the product is unstable, the formation of Mn=O species is
relatively facile.30 As such it is expected that the presence of
an Mn center in close proximity to a Co center would lower
the activation barrier needed for catalysis, thus explaining
the decrease in overpotential in CoMnP compared to Co2P.
Likewise, we posit that the high oxophilicity of Mn is moder-
ated by Co, facilitating catalyst turnover. The lower activa-
tion barrier of phosphides over oxides is likely due to the
intrinsic electric conductivity of the phosphides relative to
corresponding oxides. The consequence of these reaction
barrier variations appears to be distinct mechanisms in the
three materials, as reflected in the widely different Tafel
slopes. Further investigation will be necessary to ascertain
the validity of these suggestions.
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2 Theta (degrees)
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Figure 1. Powder XRD (a) and TEM (b) of CoMnP nanoparti-
cles. The reference pattern for CoMnP (PDF # 42-0932) is
shown.
The homogenous nature of the synthesized nanoparticles
was verified by performing scanning transmission electron
microscopy (STEM) measurements combined with elemental
mapping and line scans (Figure 2, S4). The data show that Co,
Mn and P are homogeneously distributed within the nano-
particles, thus suggesting the formation of a solid solution.
(a)
(b)
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(a)
CoMnO2
CoMnP
Co2P
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20
10
0
Figure 2. STEM image and elemental mapping data (a), and
line scan compositional data (b) of CoMnP nanoparticles. Co
is shown in red, Mn in green, and P in blue.
1.3
1.4
1.5
1.6
1.7
1.8
Potential (V) vs. RHE
(b)
Based on prior work with CoFeP, which exhibits higher activ-
ity than the corresponding ternary oxide or the binary Co or
Fe phosphide materials,16 CoMnO2 and Co2P nanoparticles
were prepared for comparison to CoMnP (loading: 0.284
mg/cm2, see SI for details; Figure S5). The catalytic behavior
of the different nanoparticles was determined by preparing
an ink composed of the nanoparticles, carbon black and
Nafion (details in SI). The overpotential, defined as the po-
tential by which the current density reaches 10 mA/cm2, is
commonly used as a figure of merit for heterogeneous water
oxidation catalysts.2,22 For CoMnP the overpotential was 0.33
V at a current density of 10 mA/cm2 (Figure 3, S6). This over-
potential places this CoMnP catalyst among the top tier of
water oxidation catalysts, and on par with iridium oxide (Ta-
ble S1).2,8-14,16,17,23,24 For comparison, Co2P nanoparticles
showed a higher overpotential of 0.37 V and the overpoten-
tial of CoMnO2 was higher still at 0.39 V. The Tafel plots in
Figure 3. (a) Polarization curves for nanoparticles of CoMnP,
CoMnO2 and Co2P in 1.0 M KOH; (b) Tafel plots derived from
the polarization curves.
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The Faradaic efficiency was determined by performing a con-
cies.9,10 Moreover, the P signal after catalytic cycling appears
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trolled potential electrolysis (CPE) experiment using an air-
tight H-type cell, and analyzing the head space gas by gas
chromatography (details in SI, Figure S7). The experimental-
ly determined oxygen quantity was compared to the ex-
pected amount of oxygen based on the charge consumed.
The Faradaic efficiency reached 96 % after ten hours of catal-
ysis.
The stability of the materials under catalytic conditions was
determined by collecting polarization curves between 1.03
and 2.23 V (vs RHE) over 500 cycles. Upon cycling, an in-
crease in the overpotential was observed from 0.33 to 0.37 V
(Figure 4). Interestingly, after intermediate cycling (200 cy-
cles) the overpotential increased to 0.35 V, which was ac-
companied by a nominal increase in the Tafel slope (Figure 4,
inset). However, after continued cycling (500 cycles, Figure
S8) the Tafel slope increased to 76 mV/dec. These observa-
tions suggest that as the catalyst is cycled, the nature of the
catalyst, and consequently the mechanism of operation, is
changing.
very weak (barely discernible above the noise, Figure 5), sug-
gesting that the surface-bound phosphates are being etched.
In order to probe the possibility of leaching during catalysis
ICP-MS analysis was performed on a solution following a
fifteen-hour CPE experiment (Figure S10, Table S2). The re-
sults show the presence of Co, Mn, and P in solution with a
ratio of 1:7:234 thus suggesting that P is leaching into solu-
tion at a significant rate, whereas metal leaching is marginal,
with Mn dominating Co. Because surface-bound phosphates
have been reported to mediate water oxidation via proton-
coupled electron transfer,5 the loss of surface phosphate
combined with metal oxidation may account for the decrease
in activity and shift in the mechanism.
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Figure 5. High-resolution XPS patterns for CoMnP nanopar-
ticles before (top) and after (bottom) electrolysis for 10 h: (a)
Co (2p3/2), (b) Mn (2p3/2), and (c) P (2p3/2).
In conclusion, we have reported on the successful synthesis
of homogenous, nearly monodisperse CoMnP nanoparticles.
This new material is an active and efficient water oxidation
catalyst that can operate at an overpotential of 0.33 V and 96%
Faradaic efficiency. After five hundred cycles, the overpoten-
tial for catalysis increased to 0.37 V, likely due to surface oxi-
dation and phosphorus etching from the nanoparticles. Eval-
uation of the efficacy of phosphate buffers for stabilization,
as well as studies on compositional effects, is underway.
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1st
200th
500th
0
1.3
1.4
1.5
1.6
1.7
Potential (V) vs. RHE
Figure 4. Polarization curves for CoMnP nanoparticles, in 1.0
M KOH initially (black), after 200 (red) and 500 CV sweeps
(blue) vs. RHE. Inset: Tafel plots derived from the cycling
experiments.
ASSOCIATED CONTENT
Supporting Information
In an effort to understand the deactivation, we performed X-
ray photoelectron spectroscopy (XPS) analysis on the sam-
ples before and after a ten-hour CPE experiment (Figure 5,
Figure S9). Before catalysis the high resolution XPS spectrum
of Co (2p3/2) comprises a peak at 777.8 eV assigned to the
binding energy of Co in Co2P,31 the Co2+ peaks at 780.9 eV, as
well as a satellite peak (787.1 eV), corresponding to CoO,
which presumably formed due to surface oxidation. The XPS
spectrum of Mn (2p3/2) exhibited a peak at 641.0 eV, corre-
sponding to oxidized manganese species such as MnO or
Mn2O3. The absence of low-valent Mn peaks in the spectrum
is attributed to the high oxophicility of Mn, resulting in sur-
face oxide formation.32 The XPS spectrum of P (2p3/2) showed
two peaks assigned to phosphide at 129.7 eV and phosphate
or phosphite (POx or P-O species) at 133.4 eV. In contrast, the
high-resolution XPS patterns of Co, Mn, and P after catalytic
cycling show the disappearance of low energy peaks of Co
(777.8 eV) and P (129.7 eV), as well as a shift towards higher
binding energies for Mn (from 641.0 eV to 641.9 eV). These
observations are in accordance with nanoparticle surface
oxidation during the catalytic process to form MPOx and
MOx species and are in line with a recent report in which
Ni2P nanomaterials were oxidized during the course of the
water oxidation reaction to form NiOx and phosphate spe-
Detailed experimental descriptions, material characteriza-
tion, as well as sample calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sbrock@chem.wayne.edu
Author Contributions
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work is supported by the National Science Foundation
under Grant Nos. CHE-1367702 and DMR-1361470 for S.L.B.
and CHE-1500201 for C.N.V. and by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences
under the Grant No. DE-SC0001907 for C.N.V. Both D.L. and
H.B. acknowledge Rumble Fellowship Awards from Wayne
State University. TEM data were acquired on a JEOL 2010
TEM with funds provided by NSF MRI award 0216084. TEM,
ICP-MS and powder X-ray diffraction were acquired in the
Lumigen Instrument Center, Wayne State University. We
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Mendoza-Garcia, A.; Zhu, H. Y.; Yu, Y. S.; Li, Q.;
thank S. Trimpin for use of the RDE, Z. Mei for assistance
with TEM, K. Sun for XPS and Y. Liu for data acquired on the
FEI Titan STEM.
Zhou, L.; Su, D.; Kramer, M. J.; Sun, S. H. Angew. Chem. Int. Ed. 2015,
54, 9642.
(17)
2016, 8, 3244.
(18)
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Mendoza-Garcia, A.; Su, D.; Sun, S. Nanoscale
Gregg, K. A.; Perera, S. C.; Lawes, G.; Shinozaki,
REFERENCES
5
6
7
8
S.; Brock, S. L. Chem. Mater. 2006, 18, 879.
(19)
Fjellvag, H.; Kjekshus, A. Acta Chem. Scand. Ser.
(1)
Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci.
A 1984, 38, 563.
U.S.A. 2007, 104, 20142.
(20)
971.
(21)
Fujii, S.; Ishida, S.; Asano, S. J. Phys. F 1988, 18,
(2)
McCrory, C. C. L.; Jung, S.; Peters, J. C.;
Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977.
9
Grosvenor, A. P.; Cavell, R. G.; Mar, A. J. Solid
(3)
Frame, F. A.; Townsend, T. K.; Chamousis, R. L.;
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
State Chem. 2007, 180, 2702.
Sabio, E. M.; Dittrich, T.; Browning, N. D.; Osterloh, F. E. J. Am.
Chem. Soc. 2011, 133, 7264.
(4)
Nahm, S. H.; Kim, S. H.; Nam, K. T. ACS Catal. 2015, 5, 4624.
(5) Jin, K.; Park, J.; Lee, J.; Yang, K. D.; Pradhan, G.
(22)
Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters,
J. C.; Jaramillo, T. F. J. Mater. Chem. A 2016, 4, 3068.
Jeong, D.; Jin, K.; Jerng, S. E.; Seo, H.; Kim, D.;
(23)
Fominykh, K.; Feckl, J. M.; Sicklinger, J.;
Doblinger, M.; Bocklein, S.; Ziegler, J.; Peter, L.; Rathousky, J.;
Scheidt, E. W.; Bein, T.; Fattakhova-Rohlfing, D. Adv. Funct. Mater.
2014, 24, 3123.
K.; Sim, U.; Jeong, D.; Jang, H. L.; Park, S.; Kim, D.; Sung, N. E.; Kim,
S. H.; Han, S.; Nam, K. T. J. Am. Chem. Soc. 2014, 136, 7435.
(24)
Ryu, J.; Jung, N.; Jang, J. H.; Kim, H. J.; Yoo, S. J.
(6)
Jin, K.; Chu, A.; Park, J.; Jeong, D.; Jerng, S. E.;
ACS Catal. 2015, 5, 4066.
Sim, U.; Jeong, H. Y.; Lee, C. W.; Park, Y. S.; Yang, K. D.; Pradhan, G.
K.; Kim, D.; Sung, N. E.; Kim, S. H.; Nam, K. T. Sci. Rep. 2015, 5.
(25)
Surendranath, Y.; Kanan, M. W.; Nocera, D. G. J.
Am. Chem. Soc. 2010, 132, 16501.
(7)
Kuo, C. H.; Mosa, I. M.; Poyraz, A. S.; Biswas, S.;
(26)
Gileadi, E. Electrode Kinetics for Chemists,
E-Sawy, A. M.; Song, W. Q.; Luo, Z.; Chen, S. Y.; Rusling, J. F.; He, J.;
Suib, S. L. ACS Catal. 2015, 5, 1693.
(8)
Wang, J.; Regier, T.; Dai, H. J. J. Am. Chem. Soc. 2012, 134, 3517.
(9) Han, A.; Chen, H. L.; Sun, Z. J.; Xu, J.; Du, P. W.
Chem. Commun. 2015, 51, 11626.
Chemical Engineers and Materials Scientists; Wiley-VCH: New York,
1993; pp 127-184.
(27)
2014, 6, 362.
(28)
Liang, Y. Y.; Wang, H. L.; Zhou, J. G.; Li, Y. G.;
Zhang, M.; de Respinis, M.; Frei, H. Nat. Chem.
Gerken, J. B.; McAlpin, J. G.; Chen, J. Y. C.;
Rigsby, M. L.; Casey, W. H.; Britt, R. D.; Stahl, S. S. J. Am. Chem. Soc.
2011, 133, 14431.
(10)
Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Energy &
Environmental Science 2015, 8, 2347.
(29)
Kärkäs, M. D.; Verho, O.; Johnston, E. V.;
(11)
Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing,
Åkermark, B. Chem. Rev. 2014, 114, 11863.
W. ACS Catal. 2015, 5, 6874.
(30)
Winkler, J. R.; Gray, H. B. In Molecular
(12)
Wang, P.; Song, F.; Amal, R.; Ng, Y. H.; Hu, X.
Electronic Structures of Transition Metal Complexes I; Mingos, D. M.
P., Day, P., Dahl, J. P., Eds.; Springer: New York, 2012; Vol. 142, p 17.
(31)
G.; Mar, A. Chem. Mater. 2008, 20, 7081.
(32) Colson, A. C.; Whitmire, K. H. Chem. Mater.
2011, 23, 3731.
ChemSusChem 2016, n/a.
(13)
Liu, M.; Li, J. ACS Appl. Mater. Interfaces 2016,
Blanchard, P. E. R.; Grosvenor, A. P.; Cavell, R.
8, 2158.
(14)
Hou, C.-C.; Cao, S.; Fu, W.-F.; Chen, Y. ACS
Appl. Mater. Interfaces 2015, 7, 28412.
(15)
Surówka, J.; Budniok, A.; Bzowski, B. e.;
Warczewski, J. Thin Solid Films 1997, 307, 233.
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