Hollow cobalt phosphide octahedral pre-catalysts with exceptionally high intrinsic catalytic activity for electro-oxidation of water and methanol

Abstract

Microstructural engineering is an effective approach to improving electrocatalytic activity of a catalyst. Shape-controlled hollow nanostructures represent a class of interesting architectures for use in electrocatalysis, given that they may offer large surface area, preferably exposed active sites, reduced diffusion pathways for both charge and mass transport, as well as enhanced catalytic activity due to the nano-cavity effect. Herein, we for the first time report the synthesis of hollow cobalt phosphide nanoparticles with well-defined octahedral shape (CoP OCHs) via multi-step reactions. When used as pre-catalysts for the oxygen evolution reaction (OER) in alkaline solution, the as-obtained CoP OCHs not only show high apparent catalytic activity requiring an overpotential of only 240 mV to deliver the benchmark current density of 10 mA cm-2, but also exhibit an exceptionally high catalyst surface-area-based turnover frequency (TOF) of 17.6 s-1 and a catalyst mass-based TOF of 0.072 s-1 at a low overpotential of 300 mV, demonstrating excellent intrinsic catalytic activity. Moreover, the CoP OCHs also show high electrocatalytic activity for the methanol oxidation reaction (MOR), outperforming most metal phosphide based MOR catalysts reported so far. The synthetic strategy reported here can be readily extended to prepare other hollow shape-controlled metal phosphide catalysts.

Full text

Hollow cobalt phosphide octahedral pre-catalysts
with exceptionally high intrinsic catalytic activity
for electro-oxidation of water and methanol†
Junyuan Xu,
a
Yuefeng Liu,
b
Junjie Li,
a
Isilda Amorim,
a
Bingsen Zhang,
c
Dehua Xiong,
d
Nan Zhang,
a
Sitaramanjaneya Mouli Thalluri,
a
Juliana P. S. Sousa
a
and Lifeng Liu *
a
Microstructural engineering is an effective approach to improving
electrocatalytic activity of a catalyst. Shape-controlled hollow nano-
structures represent a class of interesting architectures for use in
electrocatalysis, given that they may offer large surface area, prefer-
ably exposed active sites, reduced diffusion pathways for both charge
and mass transport, as well as enhanced catalytic activity due to the
nano-cavity effect. Herein, we for the first time report the synthesis of
hollow cobalt phosphide nanoparticles with a well-defined octahedral
shape (CoP OCHs) via multi-step reactions. When used as pre-
catalysts for the oxygen evolution reaction (OER) in an alkaline
solution, the as-obtained CoP OCHs not only show high apparent
catalytic activity requiring an overpotential of only 240 mV to deliver
the benchmark current density of 10 mA cm
2
, but also exhibit an
exceptionally high catalyst surface-area-based turnover frequency
(TOF) of 17.6 s
1
and a catalyst mass-based TOF of 0.072 s
1
at a low
overpotential of 300 mV, demonstrating excellent intrinsic catalytic
activity. Moreover, the CoP OCHs also show high electrocatalytic
activity for the methanol oxidation reaction (MOR), outperforming
most metal phosphide based MOR catalysts reported so far. The
synthetic strategy reported here can be readily extended to prepare
other hollow shape-controlled metal phosphide catalysts.
Introduction
Electrochemical oxidation of water or alcoholic molecules is
a key reaction in many energy conversion devices (e.g. water
electrolyzers,
1
fuel cells,
2
and metal–air batteries
3
) and indus-
trial processes.
4
Conventionally, platinum group metal (PGM)
catalysts are needed for water and alcohol oxidation to achieve
practically high rates. However, the prohibitive cost and low
natural abundance of PGMs substantially limit the widespread
deployment of the above-mentioned energy conversion devices.
In this context, considerable effort has recently been devoted to
developing inexpensive and earth-abundant electrocatalysts for
the oxygen evolution reaction (OER)
1
and alcohol electro-
oxidation reaction (EOR),
5–8
respectively, aiming to achieve
electrochemical performance comparable to or exceeding that
of PGM catalysts. In particular, transition metal phosphides
(TMPs) have emerged as promising alternative candidates to
PGMs, owing to their metalloid characteristics, high intrinsic
catalytic activity and reasonably good chemical stability, which
are of great benet for electrocatalysis.
9,10
Given that the activity of electrocatalysts is largely dependent
on their surface properties, a lot of research studies have
focused on structural engineering of catalysts to preferably
expose as many catalytically active sites as possible. Specically,
TMPs with various nano-architectures including nano-
particles,
10–13
nanowires,
14–18
nanotubes,
19–21
nanorods,
6,22–24
nanosheets,
25–28
sea-urchin,
29
polyhedra,
30,31
core–shell,
32,33
porous
7,34,35
and hollow structures
36–40
have been reported, and
they mostly demonstrated good electrocatalytic performance for
the OER
10,12–23,25–27,29–40
and EOR.
5–7
Hollow structured catalysts,
among others, are of particular interest for electrocatalysis, in
that they can offer large surface area, abundant catalytically
active sites, reduced diffusion lengths for both mass and charge
transport, and are able to effectively prevent agglomeration and
surface area loss during the reaction.
41
Moreover, it has been
demonstrated that electrocatalytic reactions can be enhanced in
the nanoscale cavities of hollow catalysts due to the conne-
ment effect.
42,43
However, hollow TMP-based OER and EOR
electrocatalysts, especially those having a well-dened shape,
have been rarely studied so far.
30,36–40
In this work, we report the synthesis of hollow cobalt phos-
phide (CoP) nanoparticles (NPs) with a well-dened octahedral
shape. This involves hydrothermal synthesis of cobalt
monoxide (CoO) nano-octahedra (OCHs), followed by surface
a
International Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga,
4715-330 Braga, Portugal. E-mail: lifeng.liu@inl.int
b
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 116023, Dalian, China
c
Shenyang National Laboratory for Materials Science and Institute of Metal Research,
Chinese Academy of Sciences, 110016, Shenyang, China
d
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of
Technology, 430070, Wuhan, China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta07958g
Cite this: DOI: 10.1039/c8ta07958g
Received 16th August 2018
Accepted 2nd October 2018
DOI: 10.1039/c8ta07958g
rsc.li/materials-a
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oxidation and phosphorization treatment and a subsequent
chemical etching process. We demonstrate that the as-obtained
CoP OCHs can serve as highly efficient pre-catalysts for the OER
in an alkaline electrolyte, exhibiting outstanding apparent
activity with an overpotential of h
10
¼240 mV to deliver
10 mA cm
2
, exceptionally high surface-specic turnover
frequency (TOF) of 17.6 s
1
at h¼300 mV, and very good
catalytic stability. Furthermore, the CoP OCHs are also capable
of catalyzing the methanol oxidation reaction (MOR), showing
catalytic performance better than that of other TMP-based MOR
catalysts reported before.
5–7
Results and discussion
The synthesis of hollow CoP OCHs is schematically illustrated
in Scheme 1. CoO NPs with a uniform size and a well-dened
octahedral shape were rst obtained through a simple hydro-
thermal method. The collected CoO OCHs were then annealed
at 300 C in air for 2 h, which resulted in the formation of
a CoO@Co
3
O
4
core/shell nanostructure. Subsequently, the
resulting CoO@Co
3
O
4
OCHs were phosphorized at the same
temperature using sodium hypophosphite (NaH
2
PO
2
) as the
phosphorus source and high-purity nitrogen as the carrier gas,
during which the surface oxide layer was converted to a phos-
phide shell (denoted as CoO@CoP). In addition, during phos-
phorization voids will be created due to unequal diffusion rates
of CoP and CoO at their interface (i.e. the Kirkendall effect), and
some cobalt phosphate would form because of the inter-
diffusion of phosphor and oxygen elements. In the nal
etching step, hydrochloric acid (HCl, 0.5 M) can penetrate into
the OCHs through either the voids or porous channels gener-
ated by the dissolution of cobalt phosphate, leaching the CoO
cores out and eventually leading to the formation of hollow
porous CoP OCHs. It is worth mentioning that surface oxidation
of CoO OCHs is a necessary step to obtain well-dened hollow
CoP OCHs. Direct phosphorization of CoO would lead to the
formation of non-uniform CoO@CoP particles with an ill-
dened shape and to signicant agglomeration, as shown in
Fig. S1 (ESI).†It is believed that the Co
3
O
4
layer plays a crucial
role in stabilizing the octahedral structure during the phos-
phorization treatment. The post-annealing temperature
(i.e. 300 C) of CoO OCHs was chosen according to our
thermogravimetric-differential scanning calorimetry in air
(TG-DSC, Fig. S2, ESI†), where we found that CoO starts to get
oxidized above 250 C. An attempt to perform surface oxidation
at a higher temperature, e.g., 350 C, was also made. However,
no phase-pure CoP OCHs could be obtained in this case, and
a certain amount of Co
3
O
4
still remained aer the nal
chemical etching process in 0.5 M HCl, albeit the nal product
also exhibited a hollow octahedral morphology (Fig. S3, ESI†).
The initial CoO and nal hollow CoP OCHs as well as the two
intermediate core/shell nanostructures were comprehensively
characterized and analyzed by X-ray diffractometry (XRD),
scanning and transmission electron microscopy (SEM & TEM)
equipped with energy-dispersive X-ray spectroscopy (EDX), and
X-ray photoelectron spectroscopy (XPS).
XRD examination conrmed that the octahedral NPs ob-
tained upon the hydrothermal synthesis consist exclusively of
cubic phase CoO (ICDD no. 00-048-1719, Fig. S4, ESI†). Surface
oxidation at 300 C resulted in the formation of a cubic Co
3
O
4
(ICDD no. 00-043-1003) surface layer, and the surface layer was
completely converted to orthorhombic CoP (ICDD no. 00-029-
0497) aer phosphorization, as unambiguously illustrated in
the XRD patterns of CoO@Co
3
O
4
and CoO@CoP (Fig. S4, ESI†).
Aer chemical etching, all diffraction peaks resulting from CoO
disappeared and the nal product can be assigned to phase-
pure CoP.
The morphology, microstructure and composition of CoO,
CoO@Co
3
O
4
, CoO@CoP and hollow CoP OCHs were further
examined by SEM and TEM. SEM observations revealed that
CoO NPs with an average size (vertex to vertex) of 170 nm and
a well-dened octahedral shape can be obtained on a large scale
(Fig. S5a, ESI†). These octahedral NPs are single-crystalline, as
conrmed by the electron diffraction (ED) pattern and the high-
resolution TEM (HRTEM) image shown in Fig. 1b and c,
respectively. TEM elemental analysis illustrated that both Co
and O are evenly distributed over a single OCH (Fig. 1d). The
surface oxidation and subsequent phosphorization treatment
do not markedly change the morphology and bulk crystallinity
of the octahedral NPs (Fig. S5b, S5c, S6 and S7, ESI†), as
Scheme 1 Schematic illustration of the synthesis procedure of hollow CoP OCH pre-catalysts.
Fig. 1 TEM characterization of CoO OCHs. (a) TEM image, (b) SAED
pattern, (c) HRTEM image, and (d) HAADF image and elemental
mappings of Co, O and their overlap.
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evidenced by the fact that the diffractions resulting from CoO
still dominate in the ED pattern acquired from a single
CoO@Co
3
O
4
or CoO@CoP (Fig. S6b and S7b, ESI†). However,
HRTEM imaging indeed showed the formation of a 3–4nm
Co
3
O
4
layer aer post-annealing of CoO NPs in air and the
conversion of Co
3
O
4
to a porous layer aer the phosphorization
(Fig. S6c and S7c, ESI†). The latter was also unambiguously
conrmed by the TEM elemental mapping (Fig. S7d, ESI†).
Moreover, according to HRTEM imaging (Fig. S7c, ESI†) and
EDX line scan (Fig. S8, ESI†) analyses of CoO@CoP, there is an
amorphous phase between the CoO core and the formed CoP
layer, which likely consists of cobalt phosphate that results from
the outward diffusion of oxygen and inward diffusion of phos-
phorus during the phosphorization treatment.
19
Hollow CoP inheriting the well-dened octahedral shape of
the initial CoO OCHs was obtained aer chemical etching of
CoO@CoP. Both SEM and TEM observations conrmed that the
CoO cores were completely gone aer etching (Fig. S5d†and
2a). ED and HRTEM examinations showed that hollow
OCHs comprise exclusively orthorhombic CoP (Fig. 2b and c),
consistent with the XRD result. EDX analyses revealed that aer
chemical etching the P signal was substantially increased while
the O signal signicantly reduced, indicating that pure CoP was
obtained (Fig. S9, ESI†). The weak O peak might result from the
surface oxidation upon exposing the sample to air.
Furthermore, we prepared porous CoP nanospheres (NSs)
comprising randomly oriented ne crystallites and used them
as a control pre-catalyst when assessing the electrocatalytic
performance of hollow CoP OCHs, in order to scrutinize the
inuence of the microstructure on the catalytic activity. These
porous CoP NSs were synthesized by phosphorization of the
Co-glycorate precursor (Fig. S10 and S11, ESI†).
44
They have an
average diameter of ca. 240 nm and consist exclusively of
orthorhombic CoP, the same as that of hollow CoP OCHs.
Moreover, the porous CoP NSs have similar crystallinity to that
of hollow CoP OCHs (1.06% vs. 1.15%), according to our
quantitative analysis using Highscore soware (Fig. S12, ESI†).
In addition, XPS analyses revealed that both hollow CoP OCHs
and porous CoP NSs have essentially similar surface chemistry
(Fig. S13, ESI†).
The OER activity of hollow CoP OCHs and porous CoP NSs
was investigated in 1.0 M KOH electrolyte using cyclic voltam-
metry (CV), electrochemical impedance spectroscopy (EIS) and
chronopotentiometry (CP). For comparison, the electrocatalytic
performance of CoO OCHs, CoO@Co
3
O
4
, CoO@CoP and
commercial RuO
2
NP control catalysts was also measured under
the same conditions. All catalysts were loaded on carbon paper
(CP) substrates, and the loading density was optimized to be
0.5 mg cm
2
(Fig. S14, ESI†). Prior to the catalytic test, pre-
activation was carried out by repetitive CV scans at 5 mV s
1
in the potential range of 1.0–1.7 V vs. reversible hydrogen
electrode (RHE) until a steady state CV curve was obtained
(Fig. S15, ESI†). Fig. 3a shows the cathodic branches of iR-cor-
rected CV curves of all samples. A bare CP substrate only
generates negligible current density, suggesting that it's not
catalytically active towards the OER. The overpotential (h
10
)
needed to deliver the benchmark current density of 10 mA cm
2
is broadly used as an indicator to compare the apparent cata-
lytic activity. The hollow CoP OCHs only need a h
10
of 240 mV to
deliver 10 mA cm
2
, substantially lower than that of porous CoP
NSs (h
10
¼280 mV) and commercial RuO
2
NPs (h
10
¼310 mV);
moreover, they can afford a high current density of
290 mA cm
2
at h¼320 mV. In addition, the hollow CoP OCHs
show OER activity signicantly better than that of CoO@CoP,
CoO and CoO@Co
3
O
4
control catalysts (Fig. S15, ESI†), indi-
cating that phosphorization and microstructure engineering
indeed substantially improve the catalytic performance.
Furthermore, the apparent OER activity of hollow CoP OCHs
also outperforms that of many other mono-metallic phosphide
catalysts reported recently in the literature, such as CoP (h
10
¼
248,
14
290,
45
320 mV
46
), Ni
2
P(h
10
¼290
15
or 280 mV
47
) and FeP
(h
10
¼280,
19
288
20
or 350 mV
25
), and remarkably it is even
superior to that of some bi- or tri-metallic phosphides that are
supposed to have better performance than mono-metallic
phosphides due to the synergy between transition
metals.
12,13,17,21,23,25,29–32,34–40,48–52
A detailed comparison between
the hollow CoP OCHs and other catalysts is summarized in
Table S1 (ESI).†
While the apparent OER activity is heavily dependent on the
loading mass, mass activity and specic activity, which are
obtained through normalizing the catalytic current by the mass
of catalysts and the real catalytically-active surface area,
respectively, can better reect the utilization of catalysts and the
intrinsic activity of catalytic materials. To this end, the mass
activities of hollow CoP OCHs and control catalysts are
compared (Fig. 3b), and hollow CoP OCHs are found to exhibit
a mass activity of 581 A g
CoP
1
at h¼320 mV, substantially
higher than those of porous CoP NSs (91 A g
CoP
1
) and RuO
2
NPs (31 A g
RuO
2
1
) at the same overpotential. Furthermore,
the specic activities of all catalysts were calculated upon
normalizing the catalytic current by their corresponding
electrochemically-accessible surface area (i.e. ECSA, see
Experimental details, Fig. S16, ESI†).
53,54
Hollow CoP OCHs
exhibit an ECSA of 67.5 cm
catalyst2
, similar to that of RuO
2
NPs
(75.0 cm
catalyst2
) but remarkably higher than that of porous CoP
NSs (40.0 cm
catalyst2
). Nevertheless, aer normalization, hollow
CoP OCHs still show the best specic activity, able to deliver
Fig. 2 TEM characterization of hollow CoP OCHs. (a) TEM image, (b)
SAED pattern, (c) HRTEM image, and (d) HAADF-STEM image of CoP
OCHs and elemental mappings of Co, P and their overlap.
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4.3 mA cm
catalyst
2
at h¼320 mV (Fig. 3c), ca. 4 and 20 times
more active than porous CoP NSs and RuO
2
NPs, respectively. It
is worth noting that hollow CoP OCHs and porous CoP NSs have
essentially the same crystal phase (Fig. S12, ESI†) and surface
chemistry (Fig. S13, ESI†), and similar crystallinity and electrical
conductivity (as illustrated by the equivalent series resistance R
s
of 0.45 Ucm
2
for hollow CoP OCHs vs. 0.52 Ucm
2
for porous
CoP NSs, Fig. S17, ESI†), and therefore it is assumed that the
high intrinsic (specic) OER activity of hollow CoP OCHs results
from their unique microstructure where more catalytically
active sites are likely preferably exposed and better mass
transport can be achieved.
The OER activity of all catalysts was further assessed using
the turnover frequency values calculated based on the catalyst
mass (TOF
mass
) and surface active sites (TOF
surface
)ath¼260,
280, 300 and 320 mV, respectively. The details about the
calculation are presented in the ESI.†Assuming that all metal
species are catalytically active (i.e. the lower limit), the TOF
mass
value of hollow CoP OCHs is 0.072 s
1
at h¼300 mV, signi-
cantly higher than that of many non-precious OER catalysts
including oxides,
55,56
(oxy)hydroxides,
57,58
carbides,
59,60
chalco-
genides,
61
nitrides
62
and phosphides
12,13,15,21,22,29–32,35,36,38–40,50–52
(Table S1, ESI†). If only surface active sites are taken into
account (Fig. S18, ESI†),
63,64
the TOF
surface
value of hollow CoP
OCHs can be even higher, reaching 17.6 s
1
at h¼300 mV
(Fig. 3e), substantially outperforming both porous CoP NSs and
RuO
2
NPs and indicating that the hollow CoP OCHs are
intrinsically more active for the OER.
The OER kinetics of all catalysts was investigated by Tafel
analysis. As shown in Fig. 3f, the Tafel slope of hollow CoP OCHs
is only 38 mV dec
1
, smaller than that of both porous CoP NSs
(45 mV dec
1
) and commercial RuO
2
NPs (53 mV dec
1
), sug-
gesting a more favorable OER rate at the hollow CoP OCH pre-
catalysts. The fast OER kinetics of hollow CoP OCHs was also
veried by EIS analysis (Fig. S17, ESI†), where the charge transfer
resistance (R
ct
¼0.80 Ucm
2
) of hollow CoP OCHs measured at
h¼260 mV is remarkably lower than that of porous CoP NSs
(6.4 Ucm
2
) and commercial RuO
2
NPs (10.6 Ucm
2
).
Furthermore, the catalytic stability as a crucial performance
indicator was assessed using chronopotentiometry at a constant
Fig. 3 OER performance of hollow CoP OCH pre-catalysts measured in 1.0 M KOH electrolyte at room temperature. The OER performance of
porous CoP NSs and commercial RuO
2
NPs is given for comparison (a) iR-corrected polarization curves (i.e. the cathodic branches of the CV
curves shown in Fig. S14†) recorded at a scan rate of 5 mV s
1
in the potential range of 1.0–1.7 V vs. RHE. (b) Mass activity. (c) Specific activity. (d)
Mass-based and (e) surface-charge-based TOF values calculated at h¼260, 280, 300 and 320 mV. (f) Tafel analysis. (g) Chronopotentiometric
curves recorded at a constant current density of 10 mA cm
2
.
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current density of 10 mA cm
2
(Fig. 3g). Both hollow CoP OCHs
and porous CoP NSs showed very good stability, and could
sustain at 10 mA cm
2
with little degradation for at least 60 h. In
contrast, the overpotential needed for RuO
2
NPs to maintain
10 mA cm
2
was increased by 33 mV in 60 h, exhibiting gradual
degradation.
Besides the OER, the catalytic performance of hollow CoP
OCH pre-catalysts towards the MOR was also investigated and
compared to that of other non-noble metal based catalysts.
5–8
Fig. 4a shows apparent and mass-specic MOR activities of
hollow CoP OCHs and porous CoP NSs measured in a mixed
solution of 1.0 M KOH and 1.0 M CH
3
OH (Fig. S19, ESI†). A
remarkable decrease in the onset potential (dened as the
potential at 1 mA cm
2
)ofca. 170 mV was observed for both
OCH and NS pre-catalysts, suggesting that they are catalytically
active for the MOR.
5–7
The mass activity of hollow CoP OCHs is
206.2 A g
CoP
1
at 0.4 V vs. saturated calomel electrode (SCE),
much higher than that of porous CoP NSs (69.7 A g
CoP
1
) and
other CoP catalysts reported previously.
5–7
Furthermore, hollow
CoP OCHs show a specic activity of 1.53 mA cm
2
at 0.4 V vs.
SCE, almost two times higher than that of porous CoP NSs
(0.87 mA cm
2
), implying that the hollow octahedral CoP is
intrinsically more active for the MOR. A detailed comparison
between hollow CoP OCHs and other non-precious MOR cata-
lysts reported in the literature is summarized in Table S2 (ESI).†
As far as the MOR kinetics is concerned, the Tafel slope of
hollow OCHs is smaller than that of porous NSs, showing
a favorable reaction rate (Fig. 4c). This was also corroborated by
the EIS analysis (Fig. 4d), where the R
ct
of hollow OCHs
(1.2 Ucm
2
)isca. 3 times smaller than that of porous NSs
(3.1 Ucm
2
), suggesting that the MOR occurs faster on OCH
pre-catalysts. The catalytic stability of hollow CoP OCHs and
porous CoP NSs was investigated by continuous CV scans in the
potential range of 0–0.35 V vs. SCE at a scan rate of 100 mV s
1
.
Upon a given number of CV cycles, the catalytic current density
at 0.44 V vs. SCE and the R
ct
value extracted from the EIS Nyquist
plot (Fig. S20, ESI†) were compared to the initial values (Fig. 4a
and d). As shown in Fig. 4e and f, almost no decay in the current
density and R
ct
aer 2500 cycles are observed for both hollow
CoP OCHs and porous CoP NSs, demonstrating very good
stability of these catalysts. A further increase in cycle number to
4000 results in a decrease in the current density (to 70 and 72%
of its initial value for OCHs and NSs, respectively) and an
increase in R
ct
; however, this can be recovered to a large extent
by replenishing the electrolyte (Fig. S20, ESI†), indicating that
the observed performance decay may be mostly due to the
consumption of methanol.
The composition and morphology of hollow CoP OCHs
subjected to extended OER and MOR stability tests were
examined (Fig. S21–S24†). The P 2p XPS spectrum shows that
the binding energy (BE) peak originating from phosphide
disappears, and only a weak P–O peak remains. In the Co 2p
3/2
spectrum, a predominant peak located at around 780 eV is
observed, which can be assigned to the BE of Co in
CoOOH.
13,53,65
Moreover, the Co–O and O–H components
appearing in the deconvoluted O 1s XPS spectrum represent the
oxygen bonds in CoOOH.
13,53,65
The XPS analyses conrm that
the initial CoP has been mostly converted to CoOOH upon the
extended OER and MOR electrolysis. This agrees well with
previous reports on phosphide-based OER catalysts where the
Fig. 4 MOR performance of the CoP pre-catalysts. (a) Apparent and mass activities of hollow CoP OCHs (orange) and porous CoP NSs (olive)
recorded at a scan rate of 5 mV s
1
in 1.0 M KOH + 1.0 M methanol (solid lines) and 1.0 M KOH (dotted lines), respectively. (b) Specific activity. (c)
Tafel analysis. (d) Nyquist plots measured at 0.35 V vs. SCE. The scattered open circles are experimental data and the dotted lines are fitting
curves. The inset shows the equivalent circuit model. The catalytic current densities recorded at 0.44 V vs. SCE and R
ct
values as a function of the
number of CV scans (0 to 0.35 V vs. SCE at 100 mV s
1
) for (e) hollow CoP OCH and (f) porous CoP NS pre-catalysts.
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progressive in situ transformation of phosphide to corre-
sponding (oxy)hydroxide under alkaline OER conditions had
been observed by many groups.
10,14,15,45–52
Our SEM-EDX anal-
yses further corroborate the leach of P and substantial increase
of the O signal for the tested catalysts. Although the octahedral
shape is not visible anymore, the catalysts remain separate
without signicant aggregation and interwoven layered ne
structures are found to form. The composition and morphology
changes were also observed for porous CoP NS pre-catalysts, as
shown in Fig. S25–S28 (ESI).†
It is worth stressing that notwithstanding the composition
and morphology transformation upon extended electro-
oxidation, the long-term electrocatalytic performance is
predominantly governed by the initial performance of TMP
“pre-catalysts”. Namely, once the “pre-catalyst”demonstrates
outstanding catalytic activity, this catalytic performance will be
retained during the subsequent long-term electro-oxidation
process, even if the morphology, crystal structure and compo-
sition of “pre-catalysts”will signicantly change in this process.
This phenomenon has already been widely observed for non-
oxide based OER catalysts,
10,14,15,44–49,53,66–71
though how the
catalytic activity is balanced during the very complex morpho-
logical, structural and compositional transformation has been
unclear so far (A compensation mechanism might exist). In this
sense, developing high-performance “pre-catalysts”, like the
hollow CoP OCHs presented in this work, remains signicant.
Conclusions
In summary, we have for the rst time synthesized hollow cobalt
phosphide octahedral nanoparticles and utilized them as pre-
catalysts for electro-oxidation of water and methanol.
Beneting from the unique structural features, the hollow cobalt
phosphide nano-octahedron pre-catalysts show both high
apparent and high intrinsic catalytic activities for the oxygen
evolution and methanol oxidation reactions, in comparison to
cobalt phosphide nanospheres having the same crystal structure
and surface chemistry as well as similar feature sizes, crystallinity
and electrical conductivity. Although the morphology, structure
and composition of the hollow cobalt phosphide octahedra
signicantly changed upon the extended stability test, the
outstanding catalytic performance was inherited which endows
the transformed catalysts to drive the oxygen evolution and
methanol oxidation reactions for a long time without remarkable
degradation. The hollow cobalt phosphide nano-octahedra hold
a substantial promise for efficiently catalyzing the electro-
oxidation of water and methanol in water electrolyzers and
direct methanol fuel cells, respectively.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the European Horizon
2020 project “CritCat”under the grant agreement number
686053. L. F. L. acknowledges the nancial support from the
Portuguese Foundation of Science and Technology (FCT) under
the projects “IF/2014/01595”and “PTDC/CTM-ENE/2349/2014”
(grant agreement No. 016660).
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