In situ grown cobalt phosphide (CoP) on perovskite nanofibers as an optimized trifunctional electrocatalyst for Zn-Air batteries and overall water splitting

Article abstract of DOI:10.1039/c9ta08936e

Developing highly efficient and stable multi-functional electrocatalysts is of prime importance for renewable energy technologies. Herein, a trifunctional electrocatalyst featuring CoP socketed on an oxygen deficient layered perovskite is synthesized through an in situ exsolution and an intriguing "post-growth" approach. As revealed by experimental characterization and density functional theory (DFT) simulation, the simultaneous introduction of CoP and oxygen vacancies modifies the electronic configuration of the catalysts and the reactants adsorbed on their surface. The intimate interpenetration of these two components yields synergistically active sites, which results in excellent performances towards the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution reaction (HER). The CoP-PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (CoP-PBSCF) electrode demonstrates a high power density (138.0 mW cm^-2), a competitive charge/discharge polarization, and a superior stability to commercial Pt/Ru/C in Zn-Air batteries. Moreover, it shows a low overpotential (0.440 V) in overall water splitting, surpassing most reported electrocatalysts. The proposed protocol offers an attractive strategy for designing perovskite-based electrocatalysts for versatile devices.

Full text of DOI:10.1039/c9ta08936e

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Materials Chemistry A
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PAPER
In situ grown cobalt phosphide (CoP) on perovskite
nanofibers as an optimized trifunctional
electrocatalyst for Zn–air batteries and overall
water splitting†
Cite this: J. Mater. Chem. A, 2019, 7,
26607
Ya-Qian Zhang,
and Jing-Li Luo
Hong-Biao Tao,
*
Zhou Chen, Meng Li,
Yi-Fei Sun, Bin Hua
Developing highly efficient and stable multi-functional electrocatalysts is of prime importance for
renewable energy technologies. Herein, a trifunctional electrocatalyst featuring CoP socketed on an
oxygen deficient layered perovskite is synthesized through an in situ exsolution and an intriguing “post-
growth” approach. As revealed by experimental characterization and density functional theory (DFT)
simulation, the simultaneous introduction of CoP and oxygen vacancies modifies the electronic
configuration of the catalysts and the reactants adsorbed on their surface. The intimate interpenetration
of these two components yields synergistically active sites, which results in excellent performances
towards the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution reaction (HER).
The CoP-PrBa0.5Sr0.5Co1.5Fe0.5O5+d (CoP-PBSCF) electrode demonstrates a high power density (138.0
mW cmꢀ2), a competitive charge/discharge polarization, and a superior stability to commercial Pt/Ru/C
in Zn–air batteries. Moreover, it shows a low overpotential (0.440 V) in overall water splitting, surpassing
most reported electrocatalysts. The proposed protocol offers an attractive strategy for designing
perovskite-based electrocatalysts for versatile devices.
Received 15th August 2019
Accepted 6th November 2019
DOI: 10.1039/c9ta08936e
rsc.li/materials-a
proved to be highly active towards the OER. Despite the fact that
considerable catalytic activities are achieved on precious
1. Introduction
metals, the high cost and limited availability have inevitably
impeded their large-scale application in various electro-
chemical devices.
The rapid growth of global energy demand, associated with the
accompanying concern over climate change, has sparked
intense interest in the exploration of renewable energy. Energy
conversion and storage technologies, with an emphasis on
electrochemical devices, hold great potential to secure full
deployment of sustainable energy by mitigating the uctuation
of energy demand and the intermittency of renewable energy
generation. Of particular interest among these advanced elec-
trochemical technologies have been fuel cells and metal–air
batteries. While metal–air batteries and regenerative fuel cells
are extremely attractive due to their high energy density, their
practical application is oen hampered by the sluggish kinetics
and large overpotentials in the oxygen reduction reaction
(ORR), oxygen evolution reaction (OER),1,2 and hydrogen
evolution reaction (HER).3,4 By far, noble-metal-based catalysts
have been exclusively used as state-of-the-art electrocatalysts,
among which Pt has been explored as an efficient electrocatalyst
towards the ORR and HER, while RuO2 and IrO2 have been
Developing catalytically active and cost-effective electro-
catalysts for oxygen electrocatalysis (ORR/OER) as well as the
HER is, thus, signicantly needed.5,6 In the past few decades,
electrocatalysts, such as carbonaceous materials, transition
metal oxides (e.g., spinel and perovskite oxides), transition
metal phosphides, selenides, suldes and carbides, have been
widely studied for the ORR, OER and HER. Among the afore-
mentioned noble-metal-free alternatives, perovskite oxides
(ABO3) offer competitive advantages in terms of cost, natural
reserves, intrinsic activity and stability in alkaline solution.7,8
Their unique electronic structure and chemical defect proper-
ties enable them to be an attractive alternative for the ORR/
OER.8 Extensive research conducted on perovskites has
demonstrated that the intrinsic catalytic activity for the ORR
and OER is strongly correlated with the electron occupancy in
anti-bonding orbitals of B sites (eg) and B–O covalency.9,10
Perovskite oxides, with an eg occupancy close to unity and
a strong B–O covalency, can give rise to maximum intrinsic ORR
and OER activities. To this end, strategies such as partial cation
substitution and oxygen-deciency formation have been
Department of Chemical and Materials Engineering, University of Alberta, Edmonton,
Alberta T6G 1H9, Canada. E-mail: jingli.luo@ualberta.ca
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta08936e
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successfully employed to tune the catalytic activity towards the alkaline environment. A strong interfacial charge transfer
ORR and OER; electrocatalysts like La0.3(Ba0.5Sr0.5 0.7 between CoP and perovskite was veried by X-ray photoelectron
)
-
Co0.8Fe0.2O3ꢀd (LBSCF) and PrBa0.5Sr0.5Co1.5Fe0.5O5+d (PBSCF) spectroscopy (XPS) and density functional theory (DFT). The
have been developed as bifunctional electrocatalysts with intercalation of CoP into perovskite oxides yields a substantial
enhanced activity for the ORR and OER.11,12 Besides, the appli- number of synergistically active sites, thus affording enhanced
cation of perovskites to catalyze the HER in alkaline media has catalytic activity and an extended functionality. More impor-
also drawn attention most recently.13–15 Among others, NdBa2- tantly, the excellent trifunctional electrocatalytic activities of
Mn2O5.5+d (NBM) and Ba0.5Sr0.5Co0.8Fe0.2O3ꢀd (BSCF) based CoP-PBSCF enable its widespread application in rechargeable
materials have been explored as HER electrocatalysts in alkaline Zn–air batteries and water splitting systems. The design strategy
media, which enables the potential application of perovskites in described here offers a new approach to the design and
overall water splitting systems.14,15 Nevertheless, their catalytic synthesis of multifunctional hybrid electrocatalysts with various
activities towards the HER are still inferior to those of many applications in electrochemical storage and conversion devices.
electrocatalysts like precious metals, transition metal suldes/
phosphides, etc., and substantial effort is still needed to
further improve their catalytic activity for the HER.
2. Results and discussion
Since the oxygen catalytic reaction and hydrogen catalytic The preparation procedures for CoP-PBSCF are depicted in
reaction occur under very different potentials, with various Fig. 1a. Initially, an A-site decient perovskite oxide with
intermediate species (e.g. OHꢀ, O2ꢀ, OOHꢀ, H2O, H* and H2) controlled non-stoichiometry, (PrBa0.5Sr0.5 0.95
(Co1.5Fe0.5)0.95-
)
being generated within these reactions, it is challenging to Co0.05O5+d (A-PBSCF), was fabricated through electrospinning,
develop an electrocatalyst with a single component that has followed by calcination at 900 ꢁC for 2 h. On account of the
favorable adsorption/desorption energies for all these species versatility and size controllability of electrospinning, a nano-
within the context of multipurpose electrocatalysis.6 A rational brous perovskite (A-PBSCF) with controllable morphology and
design of electrocatalysts that features multiple active compo- composition was obtained in this step. Thereaer, upon
nents and sufficient catalytic sites is expected to deliver an reduction, the excess Co cations in A-PBSCF were spontaneously
enhanced catalytic activity in versatile electrochemical devices. released from the perovskite oxide lattice and emerged on the
Heterostructure engineering is an important approach to perovskite surface to form a metallic Co NP decorated perov-
achieving strong and stable interfacial contact between each of skite, Co-PrBa0.5Sr0.5Co1.5Fe0.5O5+d (denoted as H-PBSCF).
the components.16,17 Importantly, our recent work has exem- Finally, the exsolved Co NPs were transformed into CoP NPs
plied that the introduction of a secondary phase through in through a phosphatization process. The combination of an in
situ exsolution grants the possibility to create a strong coupled situ exsolution and a post-phosphatization process exquisitely
heterostructure and simultaneously generate a signicant leads to a uniform formation of CoP on the perovskite surface.
amount of surface oxygen vacancies, which consequently
The X-ray diffraction (XRD) patterns in Fig. 1b show the
enables extended functionalities.16,18 Recent studies conducted formation of single phase A-PBSCF aer annealing at 900 C,
by Shao et al. have veried that synergistically catalytically active suggesting that Co cations were successfully dispersed into the
sites can arise from the perovskite/metal nanoparticle (NP) crystalline lattice without segregation. The Rietveld renement
interface, where the perovskite oxide can facilitate water XRD pattern in Fig. S1† shows a tetragonal structure of A-PBSCF
dissociation and the in situ exsolved Ni NPs play an important with space group P4/mmm (Table S1;† Rietveld renement: a ¼
ꢁ
role in H* adsorption.13 The in situ exsolution approach offers b ¼ 3.845 A, c ¼ 7.715 A), in good agreement with the high-
the possibility for perovskites to be utilized as multifunctional resolution transmission electron microscopy (HRTEM) results.
electrocatalysts for overall water splitting/rechargeable fuel Aer the phosphatization post-treatment, an emergence of
˚
˚
cells.
additional diffraction peaks was found in the XRD pattern of
Inspired by the in situ exsolution approach, further investi- CoP-PBSCF at 31.6ꢁ and 48.1ꢁ, which signied the phase
gation was conducted in this direction for developing multi- formation of CoP. Moreover, no observable peaks derived from
functional perovskite catalysts. In general, transition metal the possible destruction of perovskite or impurities such as Co
phosphides have shown excellent electrocatalytic activity for the and CoOx were detected in the XRD pattern of the CoP-PBSCF,
HER in alkaline electrolyte. Since P atoms possess a stronger suggesting that the phosphatization process successfully
electronegativity, they tend to draw electrons from transition transformed the exsolved Co NPs into ultrane CoP NPs without
metals, thus favoring proton adsorption on their surface.19,20 causing the structure of the parent perovskite to deteriorate.
This leads to a signicantly enhanced HER activity over Due to the Co exsolution, the A-site decient perovskite (A-
transition-metal based electrocatalysts.
PBSCF) reverted to a stable stoichiometry.21 As a result, the
Herein, we demonstrate a facile heterostructure approach to diffraction peaks of the perovskite experience a slight shi in
tuning a single-phase perovskite into an active dual-component CoP-PBSCF, indicative of an increased lattice parameter (Table
˚
˚
electrocatalyst. We show that CoP can be integrated with S1;† Rietveld renement: a ¼ b ¼ 3.891 A, c ¼ 7.739 A).
perovskite through an in situ exsolution and phosphatization The eld-emission scanning electron microscopy (FESEM)
process to form the CoP-PrBa0.5Sr0.5Co1.5Fe0.50O5+d (CoP-PBSCF) images in Fig. S2e and f,† along with the scanning transmission
hybrid, which simultaneously serves as a multifunctional elec- electron microscopy bright-eld (STEM-BF) image in Fig. 1c,
trocatalyst towards catalyzing the ORR, OER and HER in an show the morphology and structure of the CoP-PBSCF. The CoP-
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Fig. 1 (a) Schematic illustration of the preparation procedure for CoP-PBSCF. (b) XRD patterns of A-PBCCF and CoP-PBSCF, showing that the
CoP species are present after in situ exsolution and phosphatization processes (inset: standard XRD pattern of CoP, PDF#29-0497); (c) STEM-
bright field image of the CoP-PBSCF; (d) HAADF-STEM image of the CoP-PBSCF, the corresponding elemental mapping images and overlay map
of Co/Fe/P elements; (e) HRTEM of the CoP-PBSCF; (f) HRTEM image and simulation of CoP NPs; (g) HRTEM image of the perovskite oxide in
CoP-PBSCF.
PBSCF demonstrates a reticulate nanobrous architecture with on the perovskite surface. The crystal fringe in Fig. 1d shows d-
˚
the ber diameters ranging from 150 to 250 nm. A rough surface spacings of 2.533 and 2.470 A, which match well with the (200)
was found on CoP-PBSCF due to the post-growth of CoP NPs on and (11–1) planes of CoP, respectively. Fig. 1g shows the HRTEM
the perovskite surface. The N2 adsorption and desorption image along the [110] zone axis of the CoP-PBSCF together with
isotherm curves of CoP-PBSCF, H-PBSCF and A-PBSCF are its diffractogram patterns. The lattice fringes show d-spacings of
˚
shown in Fig. S3;† the Brunauer–Emmett–Teller (BET) analysis 3.871 and 2.752 A, in accordance with the (002) and (ꢀ110) planes
reveals a specic surface area of 15.87 m2 gꢀ1 for CoP-PBSCF, of the PBSCF. The additional reection spot circled in the dif-
similar to that of the H-PBSCF (15.43 m2 gꢀ1) and A-PBSCF fractogram image (Fig. 1g) corresponds to the tetragonal super-
(16.20 m2 gꢀ1). The BJH pore size distribution curve inserted lattice along the c-direction. The HRTEM image along with the
in Fig. S3† veries the mesoporous structure of CoP-PBSCF. The diffractogram suggests a double perovskite structure, where Pr
mesoporous brous structure is expected to benet the surface and Ba (Sr) are ordered in the A site, forming a –[PrOx]–[Co(Fe)
active sites and the oxygen/hydrogen diffusion pathway.
O2]–[Ba(Sr)O]–[Co(Fe)O2]–[PrOx]– layered structure.22 The pristine
High angle annular dark eld imaging (HAADF) together with A-PBSCF displays a similar structure with a slight volume
HRTEM convincingly veries the interpenetration of CoP shrinkage (Fig. S4†), while a uniform distribution of all the
nanoparticles (NPs) and perovskite oxides. The STEM-energy elements is detected within the perovskite bers (Fig. S5†). The
dispersive X-ray (EDX) maps in Fig. 1d clearly reveal that the HRTEM (Fig. S6†) and STEM-EDX (Fig. S7†) images of H-PBSCF
perovskite matrix is coupled with CoP NPs aer the phosphati- reveal that aer the heat-treatment in a 5% H2/N2 atmosphere,
zation process. A close observation of Fig. 1e and f shows that the Co NPs are exsolved from the perovskite lattice and socketed into
uniformly dispersed NPs with diameters of 5–7 nm are socketed the perovskite matrix.
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The surface chemistry of the electrocatalysts was probed coupling effects and micro/mesoporous properties, thus
using XPS. Fig. 2a shows the XPS O 1s spectra of A-PBSCF, H- enabling an enhanced catalytic activity and a strong durability.
PBSCF and CoP-PBSCF, where the O 1s can be deconvoluted The electrocatalytic activities for the ORR, OER and HER were
into four different subpeaks, corresponding to lattice oxygen evaluated in an alkaline environment (0.1 or 1.0 M KOH) using
(O2ꢀ), highly oxidative oxygen species (O22ꢀ/Oꢀ), hydroxyl a rotating disk electrode (RDE). Fig. 3a shows the OER activities
groups/adsorbed oxygen (–OH/O2) and adsorbed molecular of CoP-PBSCF, pristine PBSCF, H-PBSCF, CoP and commercial
water (H2O).23 An increased surface adsorbed H2O and surface IrO2 catalyst, in 0.1 M KOH solution. The CoP-PBSCF shows
adsorbed oxygen/hydroxyl was found on the surface of the H- a remarkably enhanced OER activity, with a small onset
PBSCF and CoP-PBSCF, suggesting
a strong hydrophilic potential of 1.570 V (vs. RHE), and a minimum overpotential of
nature. The strong hydrophilic nature and the surface oxygen 378 mV at a current density of 10 mA cmꢀ2 among all the
vacancies are expected to play important roles in accelerating studied catalysts (Table S2†). The OER catalytic activity of CoP-
the adsorption of H2O and OHꢀ groups on the catalysts and PBSCF is considerably higher in 1.0 M KOH (Fig. 3b), coupled
enhancing catalytic activity.16 Fig. 2b shows the Co 2p/Ba3d with overpotentials of 340 mV and 520 mV at current densities
spectra of A-PBSCF, H-PBSCF and CoP-PBSCF. The peak of 10 and 100 mA cmꢀ2, respectively. By contrast, signicantly
located at 777.69 eV in A-PBSCF is attributed to the Ba–O higher overpotentials of 640 and 600 mV at a current density of
interrelation, evidence for the structural stress caused by the A- 100 mA cmꢀ2 are needed for the pristine PBSCF and CoP NPs,
site deciency.24 The Ba–O interrelation peak vanished aer the respectively. Moreover, the CoP-PBSCF demonstrates a Tafel
Co exsolution, together with the emergence of an additional slope of 81.5 mV decꢀ1, much lower than those (108.5 and
weak peak at 778.16 eV (2p 3/2 peak of metallic Co). This is 103.6 mV decꢀ1) for pristine A-PBSCF and CoP NPs. The supe-
because the perovskite reverted to a more stable structure, while rior OER activity of CoP-PBSCF could be ascribed to the syner-
metallic Co was generated at the same time.17 The CoP binding gistic catalytic effect between CoP and PBSCF perovskite in
energy in CoP-PBSCF is in close proximity to the metallic Co. a heterostructured CoP-PBSCF. It is also noteworthy that, with
Due to the charge transfer between Co and P, the Co 2p 3/2 peak the partial exsolution of Co cations, the H-PBSCF demonstrates
shied slightly from 778.16 to 778.20 eV.25 The Fe 2p core an enhanced OER activity as compared to the A-PBSCF, showing
spectra are shown in Fig. S8,† where a signicantly increased a smaller overpotential of 570 mV at a current density of 100 mA
Fe4+/Fe3+ ratio is found on the Fe 2p prole of CoP-PBSCF in cmꢀ2 and a Tafel slope of 99.0 mV decꢀ1 (Fig. 3c).
comparison to the A-PBSCF. Provided that the high oxidation
The LSV curves in Fig. 3d conrm the ORR electrocatalytic
state of Fe4+ features an electronic conguration of t32g e1g, an activity of CoP-PBSCF. Apparently, the CoP-PBSCF demonstrates
optimized orbital occupancy is achieved on CoP-PBSCF.23 In a more positive onset potential in comparison with A-PBSCF and
this regard, it is suggested that the strong coupling effect CoP. Meanwhile, a half-wave potential of 0.752 V was achieved
between the emerged CoP NPs and the perovskite matrix can over CoP-PBSCF, which is only 110 mV lower than that of Pt/C,
lead to a modied electronic structure within the perovskites, surpassing many of the best perovskite electrocatalysts reported
which improves the catalytic efficiency for the ORR/OER. In the (Table S3†). In contrast, the A-PBSCF and CoP show half-wave
case of P (Fig. 2c), the P 2p spectrum of CoP-PBSCF shows the potentials of 0.709 and 0.623 V, respectively. In addition, the H-
binding energy of the phosphates (PO43ꢀ or PO3ꢀ or P2O5) and PBSCF also demonstrates an enhanced ORR performance in
phosphide.26,27 The presence of phosphates is presumably due comparison with the A-PBSCF, showing a half-wave potential of
to the surface oxidation of CoP, yet some literature suggests that 0.733 V in 0.1 M KOH. The electron transfer number of the CoP-
the coexistence of a cobalt oxide based compound would PBSCF for the ORR was determined from the LSV curves
weaken the collected XPS signal of CoP.25
measured at different rotation speeds (400, 900, 1600, and 2500
The as-prepared hybrid with intimate contact between CoP rpm). The Koutechy–Levich (K–L) plots inserted in Fig. 3e
and PBSCF is expected to take full advantage of its strong demonstrate a linear relationship between jLꢀ1 and uꢀ1/2 (where
Fig. 2 XPS spectra showing the (a) O 1s spectra of A-PBSCF, H-PBSCF and CoP-PBSCF; (b) Co 2p/Ba 3d spectra of A-PBSCF, H-PBSCF and CoP-
PBSCF; and (c) P 2p spectra of CoP-PBSCF.
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Fig. 3 LSV curves for the OER on A-PBSCF, H-PBSCF, CoP-PBSCF, CoP and IrO2/C in (a) 0.1 M KOH and (b) 1 M KOH at 1600 rpm and (c) the
corresponding Tafel curves in O2 saturated 1 M KOH; (d) ORR LSV curves of A-PBSCF, H-PBSCF, CoP-PBSCF, CoP, commercial Pt/C and (e) CoP-
PBSCF in 0.1 M KOH at different rotation speeds (400, 900, 1600, and 2500 rpm); inset: K–L plots at various potentials. (f) Tafel curves of A-
PBSCF, H-PBSCF, CoP-PBSCF, CoP, and commercial Pt/C in 0.1 M KOH at 1600 rpm.
jL is the measured limited current density and u is the electrode indicating an inefficient electron transfer on the CoP electro-
rotating rate) at various potentials. The electron transfer number catalyst. In fact, transition metal phosphides like CoP generally
n was determined to be close to 4.0 for the perovskite oxide based exhibit poor ORR catalytic activities; however, these interstitial
electrocatalysts (Fig. 3e, S9 and S10†), suggesting a four-electron alloys could play a positive role as a conductivity modier in
transfer pathway over the perovskite. In contrast, the electron hybrid catalysts owing to their superior electronic conduc-
transfer number was measured to be ꢂ2.0 for CoP (Fig. S11†), tivity.28,29 Thus, the incorporation of CoP into PBSCF perovskite
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presumably leads to an improvement in ORR activity. The rechargeable Zn–air batteries fabricated with the CoP-PBSCF air
enhanced electrocatalytic activity for the ORR was also conrmed electrode are competitive to those with the most active noble
by the cyclic voltammetry (CV) curves (Fig. S12†) and Tafel slopes metal catalysts of Pt/Ru/C, and even outperform them when
(Fig. 3f). The peak potential shied from 0.708 V (A-PBSCF) to polarized at a relatively higher current region.
0.744 V (H-PBSCF) and 0.768 V (CoP-PBSCF) vs. RHE. The CoP-
As shown in Fig. 4b, the peak discharging power density of
PBSCF shows a Tafel slope of 85.7 mV decꢀ1, signicantly CoP-PBSCF reaches 138.0 mW cmꢀ2, which is comparable to
smaller than those of A-PBSCF (122.7 mV decꢀ1), H-PBSCF that of the commercial Pt/Ru/C electrocatalyst (142.3 mW
(99.8 mV decꢀ1) and CoP (142.2 mV decꢀ1), which indicates cmꢀ2). Moreover, the CoP-PBSCF demonstrates an exceptional
a faster kinetics on CoP-PBSCF.
rechargeability in Zn–air batteries. As shown in Fig. 4c, when
In light of the superior electrochemical activities for both the cycled at a constant current density of 10 mA cmꢀ2, no obvious
OER and ORR, the CoP-PBSCF was expected to show an excel- deterioration is detected on CoP-PBSCF during the operation
lent bifunctional electrocatalytic activity in rechargeable Zn–air period. The rechargeable Zn–air battery with the CoP-PBSCF
batteries. We then integrated the bifunctional oxygen electro- retains a voltaic efficiency of 58.3% even aer 100 cyclic
catalysts in Zn–air batteries and evaluated their bifunctional charging–discharging tests. The socketed structure resulting
applicability. Fig. 4a shows the charging and discharging from the exsolution and phosphatization effectively prevents
polarization curves of the rechargeable Zn–air batteries based the loss of catalytically active NPs in the cyclic test, thus
on A-PBSCF, CoP-PBSCF and Pt/Ru/C air electrodes. As antici- contributing to the superior electrocatalytic activity and good
pated, the Zn–air battery with the CoP-PBSCF electrode stability. Conversely, the battery constructed with the Pt/Ru/C
demonstrates a smaller voltage gap (0.840 V at 10 mA cmꢀ2 and air electrode shows obvious degradation, with a dramatically
1.148 V at 50 mA cmꢀ2) in comparison with the A-PBSCF increased overpotential aer 20 h of operation, which could be
(0.951 V at 10 mA cmꢀ2 and 1.320 V at 50 mA cmꢀ2), suggest- ascribed to the oxidative potential induced catalyst passivation
ing its excellent bifunctional activities. Notably, the and/or catalyst detachment.
Fig. 4 Electrochemical performance of the rechargeable Zn–air batteries using A-PBSCF, CoP-PBSCF and Pt/Ru/C air electrodes. (a) Charging
and discharging polarization curves of rechargeable zinc–air batteries. (b) Discharging polarization curves along with the corresponding power
density. (c) Galvanostatic charge–discharge curves of A-PBSCF, CoP-PBSCF and Pt/Ru/C air electrodes at a constant current density of 10 mA
cmꢀ2
.
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Besides the optimized ORR/OER performances, the CoP- (97.6 mV decꢀ1), conrming a more efficient HER on CoP-
PBSCF also exhibits superior electrocatalytic activity towards PBSCF as compared to its counterparts.
the HER, which is comparable to that of many other famous
The CoP-PBSCF electrocatalyst shows promising OER and HER
non-noble-metal-based catalysts (Table S4†). As shown in activities in RDE tests, which suggests that the CoP-PBSCF may
Fig. 5a, the HER polarization curve of CoP-PBSCF was recorded simultaneously serve as an anode and cathode electrocatalyst in
in Ar-saturated 1.0 M KOH solution. For comparison, the HER overall water splitting systems. To demonstrate its practical
LSV curves of A-PBSCF, H-PBSCF, CoP and commercial Pt/C application, we further applied the CoP-PBSCF catalyst onto Ni-
were also recorded. As anticipated, commercial Pt/C is highly foam substrates and evaluated the overall water splitting perfor-
active towards the HER, with an onset potential of almost 0 mV mance in 1.0 M KOH solution. Fig. 5c shows the measurements of
(vs. RHE) and an overpotential of 33 mV at a current density of water electrolysis in a two-electrode conguration with CoP-PBSCF
10 mA cmꢀ2. Note that the CoP electrocatalyst also delivers serving as both the anode (OER) and the cathode (HER). Expect-
excellent activity, showing
a positive onset potential of edly, the CoP-PBSCF shows a remarkable electrochemical perfor-
ꢀ0.196 V, and a small overpotential of ꢀ0.209 V at a current mance, achieving a current density of 10 mA cmꢀ2 at a low
density of 10 mA cmꢀ2. Beneting from the catalytically active overpotential of 440 mV, close to that of the Pt/C//IrO2/C (h ¼ 370
sites of CoP, the CoP-PBSCF displays a high activity towards the mV). More importantly, the CoP-PBSCF shows a strong durability
HER, achieving a current density of 10 mA cmꢀ2 at an over- in the water electrolyzer. As shown in Fig. 5d, when polarized at
potential of 240 mV. In contrast, a larger overpotential of a current density of 10 mA cmꢀ2, the potential remains at 1.69 V
373 mV is required to reach a current density of 10 mA cmꢀ2 on during a 12 h chronopotentiometric test.
the A-PBSCF, which suggests an inferior catalytic activity on the
To explore the origin of the signicantly enhanced trifunc-
single component perovskite. Moreover, as demonstrated in tional catalytic activity, we compared the electrochemically
Fig. 5b, the CoP-PBSCF exhibits a lower Tafel slope of 93.8 mV active surface area (ECSA) of the three perovskite electro-
decꢀ1 relative to A-PBSCF (114.5 mV decꢀ1) and H-PBSCF catalysts. The ECSA of each catalyst is estimated from the as-
Fig. 5 (a) HER polarization curves and (b) the corresponding Tafel curves of A-PBSCF, H-PBSCF, CoP-PBSCF, CoP, and commercial Pt/C in 1 M
KOH. (c) Polarization curves of the overall water splitting reaction on the CoP-PBSCFkCoP-PBSCF, Pt/CkIrO2/C, and bare Ni foamkNi foam
electrodes in 1 M KOH solution (the electrocatalysts were loaded on Ni foam electrodes with a mass loading of 2 mg cmꢀ2). (d) Chro-
nopotentiometric test of overall electrochemical water splitting on CoP-PBSCFkCoP-PBSCF at a current density of 10 mA cmꢀ2
.
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Fig. 6 (a) HER free energy diagrams of Co-sites on CoP, and Co-sites on the perovskite surface of CoP-PBSCF and PBSCF. (b) Calculated density
of states of O for CoP-PBSCF and pristine PBSCF. (c) Calculated density of states of Co for CoP-PBSCF and pristine PBSCF.
measured double-layer capacitance according to the equation environment, including the Volmer reaction (M + H2O + eꢀ /
ECSA ¼ Cdl/Cs, while the double layer capacitance (Cdl) is MHads + OHꢀ), and the accompanying step of converting Had to
calculated from the current density difference (DJ/2 ¼ (Ja ꢀ Jc)/2) H2 through the Heyrovsk´y reaction or Tafel reaction.30,31
at different scan rates within a non-faradaic potential range.13 Previous research has veried that the transition metal oxide
As presented in Fig. S13,† the Cdl of the A-PBSCF, H-PBSCF, and with its surface rich in hydroxyl groups can accelerate the
CoP-PBSCF are measured to be 1.01 mF cmꢀ2, 0.98 mF cmꢀ2 dissociation of H2O in the Volmer reaction.13,25 Meanwhile, the
and 1.14 mF cmꢀ2. Although these electrocatalysts possess OHꢀ generated during the Volmer reaction could be preferably
nearly identical ECSA, the CoP-PBSCF demonstrates signi- adsorbed on the surface of the perovskite owing to the high
cantly higher catalytic activity compared to A-PBSCF and H- oxophilicity and strong electrostatic affinity of positively
PBSCF, which rules out the possibility of ECSA being the charged Co and Fe species.25,32 This helps to prevent the
origin of the catalytic activity enhancement in CoP-PBSCF. The blockage of active sites on CoP. As mentioned above, owing to
signicantly enhanced activity of CoP-PBSCF is most likely the desired DGH*, the produced Had could be efficiently con-
associated with its high intrinsic activity.
verted into H2 on the CoP surface in the following step. As
To gain further insight into the enhanced HER activity, DFT a result, a synergy is created between the PBSCF and CoP
simulations were applied to calculate the hydrogen adsorption components to boost the HER activity, while the intimate
Gibbs free energy (DGH*), given that DGH* is oen used as contact and strong coupling of the CoP and PBSCF matrix
a descriptor for the HER. Our computational results in Fig. 6a further promote the cooperative effect. The reduced over-
have revealed that H* can be favorably adsorbed on the surface of potential and Tafel slope of CoP-PBSCF conrm that the phos-
CoP, which is in good agreement with the experimental results. phatization process effectively improves the electrocatalytic
The DGH* value on the Co site of CoP is found to be ꢀ0.14 eV for activity of the HER.
the Co–H* state, close to the optimal hydrogen adsorption
Besides the enhanced HER activity, DFT calculation was also
energy. In contrast, the pristine PBSCF has DGH* values of 0.82 employed to investigate the origin of the enhanced oxygen elec-
and 1.02 eV for the Co–H* state and Fe–H* state on the B sites of trocatalytic activity. As shown in Fig. 6b and c, the incorporation
perovskites, respectively, indicating a weak hydrogen bonding of CoP on the perovskite oxide surface results in a change in the
and inferior adsorption strength on the perovskite surface. With density of states within perovskite oxides. The surface O p-band
regard to CoP-PBSCF, the emergence of CoP on the perovskite center relative to the Fermi level is calculated to be 2.70 eV in
surface leads to a strong interaction between the exsolved CoP CoP-PBSCF, lower than that of pristine PBSCF (2.74 eV). The
NPs and the perovskite surface, resulting in a favorable modi- oxygen 2p character relative to the Fermi level was reported to be
cation in the electronic structure. The electron transfer between strongly correlated with both oxygen incorporation and evolution
the exsolved CoP and perovskite yields reduced DGH* values (0.76 during electrochemical reactions.33 Moving the metal d states
and 0.98 eV for the Co–H* state and Fe–H* state) and an closer in energy to the O 2p states facilitates the metal–oxygen
enhanced H* adsorption on perovskite surface. The modication hybridization, B–O bond covalency and oxygen exchange kinetics,
of the electronic structure contributes to an enhanced intrinsic thus promoting the oxygen electrocatalysis on the perovskite.34,35
HER activity on the perovskite by facilitating H* adsorption and The enhanced ORR/OER activity is likely to be attributed to the
accelerating the conversion into H2.
optimal electronic properties of CoP-PBSCF.
In addition to the enhanced intrinsic activity on the perov-
skite surface, the strong coupling between perovskite oxides
and the exsolved CoP NPs is expected to contribute to a syner-
gistic electrocatalytic effect. It has been well documented that
there are two different steps involved in the HER in an alkaline
3. Conclusions
In summary, we have reported a novel and effective approach to
developing
a heterostructured CoP-PBSCF electrocatalyst,
whereby the ultrane CoP NPs are socketed on the perovskite
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Journal of Materials Chemistry A
oxide surface. The as-obtained electrocatalyst demonstrates Ar ow, the CoP-PBSCF powder was collected. The collected
excellent trifunctional electrocatalytic activities towards the CoP-PBSCF was washed with water and ethanol, and then dried
ꢁ
ORR/OER/HER in an alkaline environment. Remarkably, the under vacuum at 60 C overnight.
CoP-PBSCF has been successfully employed in the practical
application of rechargeable Zn–air batteries and water splitting
systems, showing competitive activities to those of precious
4.4 Synthesis of CoP
CoP nanoparticles were synthesized based on phosphatization
metal catalysts with a strong durability. The signicantly
of Co(OH)2, according to a previous report.36 The synthesis
enhanced multifunctional activity suggests that the integration
procedure is briey described below: 200 mg of Co(NO3)2$6H2O
of CoP contributes to synergistic catalytic sites, while the strong
and 55 mg of C6H5Na3O7$2H2O were rst dissolved in 100 mL of
coupling effect and intimate contact between CoP NPs and
deionized water. Then 1.0 M NaOH solution was added drop-
PBSCF facilitates ion migration within the reaction. The
wise to the metal nitrate solution under vigorous stirring. The
homogeneous incorporation of metallic CoP is also expected to
solid product Co(OH)2 was precipitated from the solution aer
an addition of 3.5 mL NaOH solution. The solid product from
provide an optimized electron transfer path and an improved
conductivity. Moreover, the proposed strategy shows potential
this stage was immediately centrifuged and then dried at 60 ꢁC
for designing and synthesizing various nanostructured mate-
for 5 h under vacuum. The synthesis of CoP nanoparticles was
rials with extended functionalities, which broadens the appli-
similar to the phosphatization process described above. 50 mg
cation of perovskite oxide based electrocatalysts in diverse
Co(OH)2 and 250 mg NaH2PO2$H2O were placed in two cruci-
electrochemical devices.
bles in a tube furnace on the upstream and downstream sides,
respectivꢀe1ly. The tube furnace was heated to 300 ꢁC at a rate of
2 ꢁC min . The temperature was maintained at 300 ꢁC for 2 h in
an Ar ow (50 mL minꢀ1) to yield the CoP nanoparticles. Aer
being washed with water and ethanol, the CoP product was
4. Experimental section
4.1 Synthesis of the A-PBSCF catalyst
Ba(NO3)2 (assay 100.0%), Sr(NO3)2 (assay $99%), and
Fe(NO3)3$9H2O (assay $99.9%) were purchased from Fisher
Scientic Corporation. Pr(NO3)3$6H2O (assay $99%),
Co(NO3)2$6H2O (assay $99%), and poly(vinylpyrrolidone) (PVP,
assay 12 to 12.8% nitrogen, average MW 1 300 000) were
purchased from Acros Organics. To prepare A-PBSCF, stoi-
chiometric amounts of metal nitrates (Pr(NO3)3$6H2O,
Ba(NO3)2, Sr(NO3)2, Co(NO3)2$6H2O, and Fe(NO3)3$9H2O) were
rstly dissolved in N,N-dimethylformamide (DMF) solvent at
a total metal cation concentration of 0.60 M, followed by stirring
for 12 h to form a clear solution. The as-obtained precursor was
labelled as precursor A. The poly(vinylpyrrolidone) (PVP) was
slowly dissolved in DMF solvent in a weight ratio of 30 : 100
(PVP/DMF), and labelled as precursor B. Precursors A and B
were mixed at a volume ratio of 1 : 1 to achieve a desired
viscosity. The as-obtained precursor was electrospun into
brous materials at an applied voltage of 20 kV and a distance of
15 cm between the needle and an aluminium foil collector. The
syringe pump was set at a ow rate of 0.3 mL hꢀ1. The resulting
nanobers were peeled from the collected material and then
ꢁ
collected, and then dried under vacuum at 60 C. The FESEM
image and XRD pattern of the as-synthesized CoP are shown in
Fig. S14.†
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the Natural Sciences and
Engineering Research Council of Canada (Discovery Grant
RGPIN-2016-05494) and the AITF Graduate Student
Scholarship.
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