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Graphene quantum dot engineered nickel‑cobaltphosphide as highly efficient bifunctional catalystfor overall water splitting

Tian, Jingqi; Chen, Jie; Liu, Jiyang; Tian, Qinghua; Chen, Peng

2018

Tian, J., Chen, J., Liu, J., Tian, Q., & Chen, P. (2018). Graphene quantum dot engineerednickel‑cobalt phosphide as highly efficient bifunctional catalyst for overall water splitting.Nano Energy, 48, 284‑291. doi:10.1016/j.nanoen.2018.03.063

https://hdl.handle.net/10356/137063

https://doi.org/10.1016/j.nanoen.2018.03.063

© 2018 Elsevier Ltd. All rights reserved. This paper was published in Nano Energy and ismade available with permission of Elsevier Ltd.

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1

Graphene quantum dot engineered nickel-cobalt

phosphide as highly efficient bifunctional catalyst

for overall water splitting

Jingqi Tiana, Jie Chena, Jiyang Liub, Qinghua Tianb, Peng Chen a,*

aSchool of Chemical and Biomedical Engineering Nanyang Technological University, 637457,

Singapore

bDepartment of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second

Avenue, Xiasha Higher Education Zone, Hangzhou 310018, PR China

ABSTRACT: Graphene quantum dot (GQD) is the most recent addition to the nanocarbon

materials family which promises a wide spectrum of novel applications. On the other hand,

bimetallic phosphides are emerging for their unique potentials for electrocatalysis. Herein, we

have demonstrated the fabrication of heterostructured nanosheet arrays of ternary nickel -cobalt

phosphide (NiCo2P2) and GQD hybrid (NCP/G NSs) and the use as bifunctional catalysts for

overall water splitting in alkaline medium. NCP/G NSs exhibit excellent electrocatalytic activity

towards hydrogen evolution reaction (reaching 100 mA cm-2 at an extremely low overpotential

of 119 mV), superior to any other non-noble metal catalyst. Furthermore, an electrolyzer

equipped with two identical NCP/G NS electrodes at an exceptionally small amount of catalyst

loading (0.31 mg cm-2) is able to achieve efficient overall water splitting (10 mA cm-2 at 1.61 V)

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with high stability. The careful comparison with NiCo2P2 nanowires (NCP NWs) synthesized

under the same conditions without GQDs (in terms of electrocatalytic performance, atomic and

electronic structures, and electrochemical properties) reveals the mechanistic roles of GQDs in

morphology control and performance enhancement. In addition, the performance comparison

with ternary nickel-cobalt oxide (NiCo2O4) and GQD hybrid (NCO/G NSs) suggests the

advantage of bimetallic phosphides over oxide counterparts.

Keywords: Water splitting, Electrocatalysis, Graphene quantum dot, Ternary transition metal

phosphide

1. Introduction

Electrochemical water splitting offers a promising solution for the clean and renewable

hydrogen fuel supply [1]. But currently the required operating voltage for overall water splitting

is still much larger than the theoretical thermodynamic potential window (~1.23 V) because the

current catalysts for the involved two half reactions, hydrogen evolution reaction (HER) and

oxygen evolution reaction (OER), still demand a large overpotential [2,3]. The implement of

noble-metal-based electrocatalysts can effectively reduce the overpotentials to make the process

energy-efficient, such as Pt-group metals [2], Ru- or Ir-based compounds [4,5] as state-of-the-art

electrocatalysts for HER and OER, respectively. To realize practical and sustainable application,

a great deal of effort has been devoted to devise nonprecious yet efficient catalysts, among which

the most promising are transition metal derivatives, typically transition metal oxide/hydroxides

for OER [6], and transition metal chalcogenides [7], carbides [8], nitrides [9] and phosphides

[10-15] for HER. Practically industrial water electrolyzers usually require the operating

compatibility of both anode and cathode in the same alkaline medium, therefore requiring a

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common bifunctional catalyst effective for both HER and OER. But it still remains as the great

challenge to develop earth-abundant bifunctional electrocatalysts for overall water splitting with

comparable catalytic properties as the precious counterparts [16-20].

The key to overcome the challenge relies on designing hybridized catalysts through

interface engineering to achieve desired nanostructures and synergistic effects [16,21,23,24]. In

line with this vision, various forms of nanocarbon materials have been coupled with

electrocatalysts to boost catalytic performance [25]. Graphene quantum dots (GQDs), which are

atomically-thin and nanometer-wide planar carbon structures, are the most recent addition to the

nanocarbon materials family [26]. They offer new opportunities to design novel catalysts for

water splitting owing to their unique properties including 0D planar structure, large fraction of

active edge sites and defect sites, various functionalities conferred by grafted chemical moieties

or doped heteroatoms, excellent electron transfer ability, and high dispersibility [27-29].

Although a number of studies have explored the use of GQDs for electrocatalytic oxygen/carbon

dioxide reduction reactions [29-32], there are only a few attempts to employ GQDs for

electrocatalytic water splitting [33-35]. Luo et al. synthesized GQD-Au hybrid as acidic HER

electrocatalyst [33]. The enhanced catalytic activity can be ascribed to the fact that the strong

coupling between the two components prevents aggregation of Au nanoparticles thus leading to

exposure of more active sites. In another work, GQD was used to enhance the HER performance

of MoS2 catalyst by modulating the electronic structure to achieve better electron transport [34].

More recently, Lv et al. incorporated nitrogen-doped GQD and Ni3S2 into a synergistic

electrocatalyst for overall water splitting [35]. But its HER overpotential (218 mV) to achieve 10

mA cm-2 is much larger than Pt/C (~49 mV) even with a large catalyst loading.

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As compared to the largely explored transition metal oxides as non-noble catalysts,

transition metal phosphides are attracting increasing research interests because of their unique

properties such as good chemical and thermal stability, relatively high electrical conductivity,

and strong mechanical strength [36,37]. But the HER activity of binary compounds such as CoP

[10], NixP [11], FeP [12] and Cu3P [14] is much poorer than Pt. Conceivably, ternary transition

metal compounds (metal alloys/oxide/chalcogenides, etc) shall provide opportunities to create

more active sites, enhance electrical conductivity or lower the hydrogen adsorption free energy,

whereby enabling improved catalytic activity and stability [38].

Herein, we demonstrated a novel heterostructure consisting of hybridized nanosheets of

ternary nickel-cobalt phosphide (NiCo2P2) and GQD hybrid (NCP/G NSs) supported on titanium

mesh (TiM), which serves as high performance bifunctional electrocatlyst for overall water

splitting. The achieved HER overpotential of 119 mV is the record low. In the alkaline medium

and with a current density of 10 mA cm-2, a low operating voltage of 1.61 V is attained for

overall water splitting, outperforms the current industrial standard Pt/C-RuO2 couple (1.73 V).

2. Experimental Section

2.1. Materials

All chemicals were purchased from Sigma-Aldrich Pte Ltd (Singapore) and used as

received without further purification, including Co(NO3)2·6H2O, Ni(NO3)2·6H2O, CO(NH2)2,

NH4F, NaH2PO2, KOH, HNO3, Pt/C (20 wt% Pt on Vulcan XC-72R), RuO2. Ti mesh (wire

diameter: 0.35 mm) was purchased from Wintek Technology Pte Ltd. The water used throughout

all experiments was purified using a Millipore system.

2.2. Synthesis of the graphene quantum dot (GQD)

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GQDs were prepared by refluxing carbon black powders (0.2 g) with nitric acid (50 mL, 6

M) for 24 h. After centrifugation (5000 rpm for 10 min) to remove large pieces and aggregates,

the sample was heat-dried to give reddish-brown powders and was subsequently re-suspended in

water. The obtained suspension was then ultrafiltered twice (Amicon Ultra-4, Millipore) to retain

the particles between 3 and 10 kDa. Then the suspension was freeze-dried overnight to obtain

GQD powder.

2.3. Synthesis of the nickel-cobalt hydroxide/GQD nanosheet arrays (NCOH/G NSs) and nickel-

cobalt oxide/GQD nanosheet arrays (NCO/G NSs)

Co(NO3)2·6H2O (2 mmol), Ni(NO3)2·6H2O (1 mmol), CO(NH2)2 (50 mmol), NH4F (2

mmol), and 10 mg GQD powder were dissolved in 40 mL water under vigorous stirring for 20

min. The mixture was then transferred into a Teflon-lined stainless autoclave, together with a

piece of Ti mesh (2cm × 3cm) which was cleaned by sonication sequentially in acetone, water

and ethanol for 10 min each. The autoclave was sealed and maintained at 120 °C for 6 h in an

electric oven. After the autoclave cooled down to room temperature, Ti mesh supported

NCOH/G NSs were taken out and washed with water thoroughly before drying. For comparison,

nickel-cobalt hydroxide nanowire arrays (NCOH NWs) were prepared under the conditions

except the addition of GQDs. For investigation of the GQD effect on the morphology control, the

amount of GQD was reduced to 5.2 mg or increased to 30 mg. To produce NCO/G NSs, the as-

prepared NiCo-hydroxide/G NSs were placed in a tube furnace and heated in Ar to 350 °C for 2

h with a ramping rate of 2 °C min-1.

2.4. Synthesis of NCP/G NSs

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In the tube furnace and under a slow argon flow, NaH2PO2 (upstream) and Ti mesh

supported NCO/G NSs (downstream) were placed in a porcelain boat, with the molar ratio of Co

to P being 1:6. At a ramping rate of 1 °C min-1, the samples were heated to 350 °C and

maintained at this temperature for 60 min, and then naturally cooled to ambient temperature

under Ar. NCP NWs and phosphidation of GQD (G) were prepared following the same

procedure.

2.5.Characterizations

X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance with Cu Kα

radiation (λ=1.5406 Å). The morphology and structure of the samples were determined by a

field-emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and transmission

electron microscopy (TEM, Tecnai G2 F30 S-TWIN operated at 300 kV). The nitrogen

adsorption-desorption isotherm was characterized with a Quantachrome Autosorb AS-6B system

at liquid N2 temperature. Fourier transform infrared spectroscopy (FTIR) was obtained from a

Bruker spectrometer (Vertex 70). Raman spectra were recorded in Renishaw InVia Reflex

Raman with a 514 nm excitation wavelength.

2.6. Electrochemical measurements

Electrochemical measurements were performed with a CHI 660D electrochemical analyzer

(CH Instruments) equipped with a conventional three-electrode cell (The electrochemical cell is

one compartment.), using NCP/G NSs or NCP NWs or NCO/G NSs on TiM, a saturated calomel

electrode (SCE), and a graphite plate as the working electrode, reference electrode and counter

electrode respectively. Potentials were referenced to a reversible hydrogen electrode (RHE)

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following the equation:ERHE = ESCE+ 0.059pH + 0.242. All electrochemical data was shown

with iR correction. Pt/C, RuO2 and GQD inks were prepared by dispersing 2 mg Pt/C, RuO2 or

GQD in 500 μL EtOH/H2O solution (V/V is 1:1) with 17.5 μL of 5 wt% Nafion solution. After

sonication for 1 h, 78 μL of the ink was loaded onto Ti mesh and air-dried at room temperature.

Electrochemical impedance spectroscopy (EIS) was recorded with frequency range of 0.1-100

kHz. For full water splitting measurement, NCP/G NSs on TiM were directly used as both anode

and cathode in a standard two-electrode system. There are no activation processes for all

electrochemical tests.

3. Results and discussion

As previously reported, graphene quantum dots (GQDs) were readily synthesized from low-

cost carbon black [39]. The obtained GQDs show narrow size distribution (5.2 +/- 0.9 nm, n =

106; Fig. S1A) with a lattice spacing of 0.240 nm corresponding to (1120) lattice fringe of

graphene (Fig. S1B) [39]. Both Fourier transform infrared spectroscopy (FTIR) and X-ray

photoelectron spectroscopy (XPS) reveal the presence of oxygen functional groups on GQD (Fig.

S1C-D), which can act as ion binding and nucleation sites for the formation of nanosheets. The

fabrication of nickel-cobalt phosphide (NiCo2P2) and GQD hybrid nanosheets (NCP/G NSs) on

titanium mesh (TiM: aperture ~0.35 mm, wire diameter ~0.16 mm) was accomplished by three-

step reactions as illustrated in Fig. 1A. In the first step, GQD was added during the

hydrothermally formation of NiCo-hydroxide (NCOH/G) on TiM. As characterized by field

emission scanning electron microscopy (FESEM), the entire surface of TiM is uniformly coated

with the interconnected wrinkled NCOH/G nanosheets with lateral sizes of 100-200 nm (Fig. S2).

Subsequent calcination at 350 °C in Ar atmosphere converts the precursor into NiCo2O4/GQD

nanosheet (NCO/G NS), which is confirmed by X-ray diffraction (XRD) characterization

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(matching JCPDS 20-0781 [40], Fig. S3A). The original solid nanosheets are transformed into

porous nanosheets likely caused by gas release during decomposition of the precursor (Fig. S3B-

D).

Transition electron microscopy (TEM, Fig. S3E-F) further revealed that NCO/G NS is

composed of many infinitesimal interconnected nanoparticles (diameter of 5-10 nm) with the

lattice spacing of 0.245 nm and 0.468 nm corresponding to the (111) and (311) planes of

NiCo2O4. It is worth noting that adjacent to NiCo2O4 nanoparticles are GQDs with characteristic

lattice spacing of 0.240 nm.

The subsequent phosphidation of NCO/G NSs under Ar atmosphere at 350 °C using

NaH2PO2 as the phosphorus source leads to appearance of the sharp characteristic diffraction

peaks corresponding to hexagonal NiCoP (JCPDS 71-2336) [13] and orthorhombic CoP (JCPDS

29-0497) [10] in XRD pattern (Fig. 1B). The presence of GQD is evidenced by a broad and weak

peak around 24°, which is in good accordance with that from pure GQD (Fig. S1E). FESEM

image shows that the two-dimensional nanosheet morphology is preserved after phosphidation,

resulting in the formation of vertically oriented and interconnected porous nanosheets (Fig. 1C).

The TEM image of nickel-cobalt phosphide and GQD hybrid nanosheets (NCP/G NSs) reveals

the interconnected nanoparticles with diameter of 5-10 nm (Fig. 1D). In the high-resolution TEM

(HRTEM) image of NCP/G nanosheet, the well-resolved lattice fringes with interplanar

distances of 0.283 nm and 0.187 nm are ascribed to (011) and (202) planes of CoP, while

interplanar distances of 0.220 nm can be indexed to (111) planes of NiCoP (Fig. 1E). Both XRD

and TEM characterizations suggest that phosphidation results in the formation of a new kind of

solid solution [41], for which one NiCo2P2 nanoparticle is composed of distinct crystal domains

of NiCoP and CoP. The TEM image also shows an adjacent GQD with characteristic lattice

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spacing of 0.241 nm. Fig. 1F presents the scanning TEM (STEM) image and the corresponding

energy-dispersive X-ray (EDX) elemental mapping images of Co, Ni, P, C, and O for a single

NCP/G NS, manifesting that it is porous and all the elements are homogeneously distributed.

EDX spectroscopy uncovers the 1:2:2 atomic ratio between Ni, Co and P in NCP/G NSs and

5.2:1 mass ratio between GQD and NiCo2P2 (Fig. S4). Nitrogen adsorption/desorption isotherm

of the NCP/G NSs (Fig. S5) indicates a Brunauer−Emmett−Teller (BET) surface area of about

119 m2 g−1. The Barrett−Joyner−Halenda (BJH) pore-size distribution curve shows a broad peak

ranging from 5 to 100 nm. Conceivably, the small pores (<10 nm) are on the nanosheets while

the larger pores are the inter-spacings between the nanosheets. Such high specific surface area

and porosity ensure exposure of abundant active sites, efficient electrolyte transport / charge

transfer, and readily release of generated gases.

The abundant negatively-charged oxygen functional groups (carboxyl and hydroxyl groups)

on GQD provide binding sites to metal ions. Such interaction facilitates the formation of NiCo-

hydroxide nuclei, which in turn grow along the two-dimensional basal plane of GQD and

eventually form of hybrid layer structure of NiCo-hydroxide and GQD. Raman spectroscopy

shows that G band of GQD red-shifts after hybridization, suggesting a strong interaction and

electron transfer from GQD to NCP (Fig. S6) [42]. In addition, D band to G band ratio increases

after hybridization indicating the increase of GQD defects [43]. In comparison, without GQDs

the same synthesis procedure produces NCP nanowires on TiM, implying the critical role of

GQDs in morphology control (Fig. S7).

NCP/G NSs on TiM was then employed as the self-supported cathode for HER in alkaline

solution. Fig. 2A presents the polarization curves of NCP/G NS, NCP NW, NCO/G NS, G and

Pt/C on TiM in 1.0 M KOH with a scan rate of 2 mV s-1. NCP/G NS exhibits excellent HER

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activity as compared to NCP NW and NCO/G NS, and even more it is greatly superior to the

HER benchmark catalyst (Pt/C) beyond -100 mV. The measured overpotential (ηHER) to achieve

a current density of 100 mA cm-2 (needed for practical electrolysis) for NCP/G NS is only 119

mV, which is much smaller than that for Pt/C (180 mV) and NCP NW (208 mV, Fig. 2B). To

achieve a current density of 200 mA cm-2, Pt/C requires a large overpotential of 323 mV whereas

NCP/G NS only require 138 mV. Actually to the best of our knowledge, NCP/G NS is the best

non-precious HER catalyst thus far (comparison Table S1).The corresponding Tafel plots for

these electrodes are shown in Fig. 2C. NCP/G NSs exhibit a comparable Tafel slope (62.3 mV

dec-1) to Pt/C (62.8 mV dec-1), which is much smaller than that of NCP NWs (69.8 mV dec-1)

and NCO/G NSs (138.6 mV dec-1), indicating a Volmer-Heyrovsky-mechanism driven fast

reaction kinetics [44]. The excellent HER performance of NCP/G NSs is also originated from the

fast charge transfer and transport process as evidenced by the electrochemical impedance spectra

(Fig. 2D). Specifically, NCP/G NSs exhibit the lowest intrinsic resistance as indicated by the

steepest slope in the low frequency region and smallest charge transfer resistance indicated by

the smallest semicircle in the high frequency region. In addition, based on electrical double-layer

capacitance (EDLC) measurements (Fig. S8), the estimated electrochemical active surface area

(EASA) of NCP/G NSs (539 cm2 mg-1) is much greater than that of NCP NWs (139 cm2 mg-1).

Moreover, NCP/G NS exhibits a stable HER performance with negligible current loss after

1000 cycles (Fig. 2E). The chronoamperometry measurement further demonstrates that NCP/G

NS almost fully maintains the working efficiency after 20 h operation even at a high current

density of 100 mA cm-2 (Fig. 2F). In all the experiments, NCP/G NSs were synthesized with an

optimal loading of GQDs. Too much or too less amount of GQDs compromises the performance

(Fig. S9). Conceivably, increase of GQDs means more coverage on the active sites of NCPs

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while decrease of GQDs means compromise of GQD’s facilitating roles in charge

transfer/transport and reaction kinetics.

As confirmed by X-ray photoelectron spectrocopy (XPS), Ni, Co, P, C, and O elements are

present in NCP/G NSs (Fig. S10A). The high-resolution Ni 2p3/2 spectrum (Fig. 3A) can be

deconvoved into three peaks corresponding to Ni(δ+) at 853.3 eV and Ni2+ at 856.7 eV (main

peak) and 861.7 eV (satellite peak) from NiCo2P2,40 while the three resolved peaks in Co 2p3/2

spectrum can be assigned to Co(δ+) at 778.5 eV, oxidized Co species at 781.8 eV, and satellite

peak at 785.4 eV [10,15] (Fig. 3B). The deconvolved peaks in P 2p spectrum can be assigned to

P(δ-) in phosphides at 129.3 eV, oxidized P species at 134.0 eV [10,45], and P-C bonding at

132.0 eV [46] (Fig. 3C). High-resolution C 1s spectrum (Fig. S10B ) presents the peaks located

at 284.7 eV (C=C/C-C), 286.3 eV (C-OH/C-P), 286.7 eV (C-O-C), and 289.1 eV (C=O) [47].

The increase of C-OH/C-P peak and decrease of C-O-C and C=O peaks as compared with GQD

(Fig. S1D) further confirm the formation of C-P bonds upon phosphidation. The four

deconvolved peaks in O 1s spectrum are originated from M-O at 529.8 eV, O-C=O/P-O/oxygen

vacancies at 531.3 eV, C=O at 532.3 eV, C-OH at 533.2 eV, and chemisorbed oxygen at 536.3

eV (Fig. S10C) [48,49]. Compared with NCP NWs (Fig. 3A-C), the peak location for Ni(δ+),

Co(δ+), and P(δ-) are negatively shifted (by 0.3, 0.3, and 0.5 eV respectively) in NCP/G NSs,

which is likely due to charge transfer from GQD to phosphide via non-covalent interaction. All

these observations suggest that there are both covalent and non-covalent interactions between

GQDs and phosphides.

The tafel slope of NCP/G NS (62.3 mV s -1) indicates that the HER reaction undertakes the

two-step Volmer-Heyrovsky pathway, which consists of water adsorption and dissociation to

form OH- and adsorbed H (Hads) (H2O + e-→ Hads + OH-), and subsequent OH- desorption and

12

recombination of Hads to yield H2 (Hads + H2O + e-→ H2 + OH-) [44,50]. In Volmer step (rate

determining step), it is commonly accepted that water molecules are adsorbed on the catalyst

surface via noncovalent interaction between positively charged metallic centers M(δ+) and O

atom [16,51], whereas hydrogen bonding between H of H2O and P in phosphides is usually

impossible to form owing to the similar electronegativaties of the two elements. But in our

system, GQD induced charge transfer leads to more negative P(δ-) centers as compared with P in

the ordinary metal phosphides, which therefore could serve as hydrogen-bond acceptors to

strengthen interaction between water and phosphide [52]. According to Subbaraman's work [51],

H2O and adsorbed OH- compete with each other for non-covalent interaction with alkaline metal

centers (here, Co, Ni in NCP) to form OH --M+(H2O)x at a certain equillibrium. The reactivity of

Volmer step increases if adsorption of H2O is enhanced or adsorption OH- is reduced. Desirably,

the electron-rich domains of NCP/G NSs as revealed by Raman and XPS analyses favor H2O

adsroption while repel OH- anions. Furthermore, the lattice mismatch between GQD and

phosphide can cause defects in phosphide at the GQD/phosphide interface, which are particularly

active for dissociative adsorption of water molecules [53,54]. The adsorbed water molecule

dissociates into OH- and H atom in situ upon receiving electrons from the cathodem. Thus-

formed H atom then moves to neighboring unoccupied M(δ+) to form Hads (Fig. 3D). The

following Heyrovsky step includes combination of two Hads to produce H2 and detachment of

OH- from M(δ+), which is facilitated by the negatively charged domain of NCP/G NSs. As

shown in Fig. S11 in SI, after HER experiment, the peak locations of Ni(δ+), Co(δ+), and P(δ-) in

NCP/G NSs do not shift, indicating that the non-covalent interaction between GQD and

phosphides is well preserved during HER process. The estimation of turnover frequency (TOF),

which is the H2 generation rate at each active site, reflects the intrinsic activity of the catalysts

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[55]. TOF of NCP/G NSs is calculated to be 0.051 s-1 at the overpotential of 100 mV, which

triples that of NCP NWs (0.017 s-1) (Fig. S12). This implies that GQD incorporation largely

improves the surface water dissociation rate.

OER prefers alkaline condition whereas traditional HER electrocatalysts are often only

effective in acidic condition. Therefore, bifunctional electrocatalysts for both HER and OER in

the same alkaline electrolyte is highly desirable for industrial realization of full water splitting.

Here, we further demonstrate that in the same alkaline solution, NCP/G NSs are not only

extraordinarily efficient for HER (Fig. 2) but also effective for OER (Fig. 4). Polarization curves

in Fig. 4A show that to achieve a current density of 100 mA cm-2, NCP/G NSs require a much

smaller overpotential (400 mV) as compared to that of RuO2 (473 mV, industrial standard), NCP

NWs (485 mV) and NCO/G NSs (539 mV). Note that phosphidized of GQD (G) exhibits

negligible OER activity, suggesting that P-doping itself is not a major factor contributing to the

high performance of NCP/G NSs. This low overpotential is among the best as compared with

other high-performance OER electrocatalysts (Table S2). Also, OER activity of NCP/G NSs has

a fast kinetics as evidenced by that fact that the Tafel slope of NCP/G NSs is also smaller than

that of RuO2, NCP NWs and NCO/G NSs (Fig. 4B). This is attributable, at least in part, to the

fast charge transfer and transport process as evidenced by the small semicircle in the high

frequency range and steep slope in the low frequency range in the EIS spectrum, respectively

(Fig. S13). After 20-h continuous electrolysis, The OER activity of NCP/G NSs well sustains

without obvious decay, manifesting its high stability (Fig. 4C).

To understand the actual active species on NCP/G NS for OER, XPS characterization was

performed. The XPS survey spectrum (Fig. S14) reveals that oxygen abundance is increased

significantly after OER, suggesting that the surface of NCP/G NSs has been heavily oxidized

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during OER. This observation is consistent with the finding by the recent studies, i.e., the in-situ

generated metal oxide/hydroxide is the real OER active species on transition metal phosphides

[56]. The formation of active oxygen species is further supported by the oxidation peaks (1.2–

1.4V) characteristic to Ni2+→Ni3+ and Co2+→Co3+ in LSV curve in Fig. 4A, as well as

disappearance of M(δ+) peak and inreased metal oxide peak after OER in XPS spectrum (Fig.

S14B-D). However, the transition metal oxide counterpart (NCO/G NS) has inferior OER

performance because of its poor conductivity (Fig. 4A). It implies that superior OER activity of

NCP/G NS benefits from the more conductive phosphide core. Finally, the Faradaic efficiency of

NCP/G NS is measured to be 99.1% for HER and 98.6% for OER (Fig. S15).

Taken together, the excellent catalytic performance and durability of NCP/G NSs for both

HER and OER could be explained by the following: 1) GQDs promote the formation of

nanosheet structure of the catalyst which is believed to be better than nanowire or other

nanostructures in terms of exposure of active sites, ion diffusion, charge transfer and transport; 2)

incorporated GQDs facilitate charge transfer and transport; 3) charge transfer from GQDs

induces electron-rich domains on NCP whereby promoting H2O adsorption and release of H2 and

OH-; 4) the defects caused by lattice mismatching at the GQD/phosphide interface are

particularly active for dissociative adsorption of water molecules; 5) defects on GQDs are

catalytically active; 6) NCP has high catalytic activities and good conductivity; 7) both GQD and

NCP are chemically stable; 8) peeling off catalysts from the substrate due to generation of gas

bubbles is not observed because the nanosheet array is not favorable to bubble adhesion and the

spacings between the vertically aligned nanosheets provide channels for quick gas release [57].

Encouraged by the outstanding performance for both OER and HER in alkaline medium,

NCP/G NS on TiM was further used as the bifunctional electrodes in an alkaline electrolyzer

15

(Fig. 4D). Impressively (Fig. 4E), the overall water splitting performance of such NCP/G NSs ‖

NCP/G NSs full cell (cell voltage of 1.61 V to achieve 10 mA cm-2) is much superior to RuO2 ‖

Pt/C couple (1.73 V) and NCP NWs ‖ NCP NWs couple (1.77 V). Such performance is among

the best as compared to other non-noble bifunctional catalysts for overall water splitting (Table

S3). Although a few bifunctional catalysts offer a lower cell voltage than ours at the same current

density of 10 mA cm-2 [122,58,59], but they all require several folds higher catalyst loading. In

other words, our electrolyzer is not only highly efficient but also cost effective. The main

challenge to realize bifunctional catalysis is often that most HER catalysts lose activity in

alkaline condition. As NCP/G NSs exhibit the best HER performance in alkaline solution thus

far, its combination with the best OER catalyst (e.g., Ni3S2 nanorods/Ni foam [60], FeNiP

nanoplate arrays [61]) promise the best overall performance for water splitting. Furthermore,

when the applied voltage is set at 1.61 V, a constant current density of 10 mA cm-2 can be well

maintained for 10 h with drastic bubbling observed at both electrodes (Fig. 4F), demonstrating

the good stability of our electrolyzer. We further investigated the water splitting performance of

NCP/G NSs in neutral media. NCP/G NSs was first employed for HER in 1.0 M PBS (pH 7). It

shows a tafel slope of 100.6 mV dec-1 with excellent durability and fast charge transfer and

transport process (Fig. S16A-C). The overpotential (ηHER) to achieve a current density of 20 mA

cm-2 for NCP/G NS is 248 mV, which outperforms many recently developed neutral HER

catalysts [56-59]. The OER performance of NCP/G NSs has also been investigated (Fig. S16 D-

F). The Tafel slope (271 mV dec-1) indicates that the OER follows a Volmer-Heyrovsky pathway.

NCP/G NSs requires overpotential of 703 mV for 10 mA cm-2, which is superior to the recent

OER electrocatalysts in neutral media [56, 60, 61].

4. Conclusion

16

In summary, we have demonstrated the fabrication of heterostructured nanosheet arrays of

ternary nickel-cobalt phosphide (NiCo2P2) and graphene quantum dots (GQD) hybrid supported

on titanium mesh (NCP/G NSs on TiM), which can serve as the bifunctional catalysts for both

HER and OER. To the best of our knowledge, the HER performance (with an extremely low

overpotential of 119 mV to achieve the current density of 100 mA cm-2) is currently matchless.

And its OER performance is also among the best. The overall water splitting electrolyzer

equipped with a pair of such electrodes (NCP/G NSs ‖ NCP/G NSs) in the alkaline medium is

able to attain the current density of 10 mA cm-2 with a low cell voltage of 1.61 V, outperforming

the current industrial standard Pt/C ‖ RuO2 couple (1.73 V). The potential of its practical use is

corroborated not only by its high efficiency but also its high stability and cost effectiveness (due

to low catalyst loading and scalable synthesis process). NCP/G NSs are superior to ternary

nickel-cobalt oxide (NiCo2O4) and GQD hybrid (NCO/G NSs) and NiCo2P2 nanowires (NCP

NWs) synthesized under the same conditions without GQDs. The superior performance of

NCP/G NSs are attributed to the critical roles of GQDs in morphology control, enhancing charge

transfer and transport, and improving the catalytic kinetics. This study highlights the unique

potential of GQDs as the enhancer for electrocatalysis.

Acknowledgements

This research is supported by an AcRF tier 2 grant (MOE2017-T2-2-005) from Ministry of

Education (Singapore).

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version.

17

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Fig. 1. (A) Schematic illustration of the fabrication procedure of NCP/G NSs on TiM. (B) XRD

pattern of NCP/G NSs. (C) SEM image of NCP/G NSs. (D) TEM and (E) High-resolution TEM

images of NCP/G nanosheet. (F) STEM image and EDX elemental mapping of Co, Ni, P, C, and

O on a NCP/G NS.

24

Fig. 2. (A) Polarization curves of NCP/G NSs, NCP NWs, NCO/G NSs, P-doped GQD (G) and

Pt/C with a scan rate of 2 mVs-1 in 1.0 M KOH. (B) Comparison of the overptotential (η)

required to achieve current density of 100 mA cm-2 for NCP/G NSs, NCP NWs, and Pt/C. (C)

The corresponding Tafel plots. (D) EIS spectra of NCO/G NSs, NCP NWs and NCP/G NSs

recorded at a constant potential of -0.2 V vs. RHE. (E) Polarization curves of NCP/G NSs

initially and after 1000 CV scanning between +0.1 and -0.2 V vs. RHE. (F) Time-dependent

current density curve for NCP/G NSs under static overpotential of 120 mV for 20 h.

25

Fig. 3. High-resolution XPS spectra of (A) Ni 2p, (B) Co 2p, (C) P 2p in NCP/G NSs and NCP

NWs. (D) The schematic of probable electrocatalytic mechanism of NCP/G NSs for HER and

OER. Blue and yellow regions represent electron depletion and accumulation, respectively.

26

Fig. 4. (A) Polarization curves of NCP/G NSs, NCP NWs, NCO NSs, P-doped GQD (G) and

RuO2 with a scan rate of 2 mV s-1 in 1.0 M KOH. (B) The corresponding Tafel plots. (C) Time-

dependent current density curve for NCP/G NSs under a constant potential of 1.61 V for 20 h.

(D) Polarization curves of NCP/G NSs and NCP NWs with a scan rate of 2 mV s-1 in 1.0 M

KOH. (E) Two-electrode polarization curves of NCP/G NSs ‖ NCP/G NSs, NCP NWs ‖ NCP

NWs, and RuO2 ‖ Pt/C couples. (F) Time-dependent current density curve for NCP/G NSs ‖

NCP/G NSs couple under a constant potential of 1.61 V for 10 h. The inset shows the

photograph of generated gas bubbles on both electrodes.