Supporting Information

Nanosized LiNi1-xFexPO4 Embedded in Mesoporous Carbon

Matrix for High-performance Electrochemical Water

Splitting

Shaojun Ma,ab‡ Qing Zhu,ab‡ Zhi Zheng,ab Wenlou Wang*ab and Dongming Chena

a Department of Chemical Physics, University of Science and Technology of China,

Hefei, Anhui 230026, P. R. China

b Nano Science and Technology Institute, University of Science and Technology of

China, Collaborative Innovation Center of Suzhou Nano Science and Technology,

Suzhou, Jiangsu, 215123, P. R. China

*Correspondence to: wlwang@ustc.edu.cn

Electronic Supplementary Material (ESI) for Chemical Communications.This journal is © The Royal Society of Chemistry 2015

Experimental Sections

Materials and Preparation of the catalysts

LiNi1-xFexPO4@C (0 ≤ x ≤ 1) was prepared via a spray dry process as follows: Firstly,

LiOH and NH4H2PO4 (Aldrich, 99.9%) were dissolved in deionized water for 5 min at

25 oC with the molar ratio of 1.005:1. Simultaneously, Ni(CH3COO)2•4H2O,

FeC2O4•2H2O and citric acid (with the same molar amount of NH4H2PO4) were

dissolved in deionized water and small amounts ethylene glycol was dropped into

solution. Then the two solutions were mixed together and reacted at 80 oC until

forming well-dispersed suspension under the protection of nitrogen. After that, the

suspension was carried out spray drying and the inlet and outlet temperatures were

220 oC and 120 oC, respectively. Finally, the precursor was calcined at 400 oC for 2h

and 650 oC for 8h under a nitrogen atmosphere. With changing the molar ratio of

Ni(CH3COO)2•4H2O and FeC2O4•2H2O, a serious of LiNi1-xFexPO4@C products

were obtained.

Characterizations

Powder X-ray diffraction patterns were collected using a Rigaku/Max-3A X-ray

diffractometer with Cu Kα radiation (λ = 1.54178 Å). The SEM images were taken

using a field-emission scanning electron microscope (JSM-6701F, JEOL) operated at

an accelerating voltage of 5 kV. The transmission electron microscopy (TEM) images

were taken on a Hitachi Model H-7650 transmission electron microscope with a LaB6

source operated at a 100 kV accelerating voltage. Energy-dispersive X-ray

spectroscopy (EDS) analysis was performed on a JEOL-2010 microscope with an

accelerating voltage of 200 kV. All the samples were prepared by depositing a drop of

diluted suspensions in ethanol on a carbon-film-coated copper grid. Raman spectra

were measured with a Perkin-Elmer 400F Raman Spectrometer using a 514.5 nm

laser beam. X-Ray photoelectron spectroscopy was performed with a Perkin–Elmer

RBD upgraded PHI-5000C ESCA system. Nitrogen adsorption measurements were

performed at 77 K using a Micromeritics ASAP 2020 system utilizing Brunauer–

Emmett–Teller (BET) calculations for surface area.

Electrode preparation

Electrochemical experiments were analyzed using a traditional three-electrode

configuration cell, which was connected to a CHI660D electrochemical analyzer

(Shanghai Chenhua Limited, China). A platinum wire was used as the auxiliary

electrode and a double-junction Ag/AgCl (KCl saturated) electrode was used as the

reference electrode with 1 M KOH as electrolyte. Both the counter and reference

electrodes were rinsed with distilled water and dried with compressed air prior to the

measurements. The glassy carbon electrodes (5.0 mm in diameter) loaded with

catalysts were used as the working electrodes. The working electrodes were prepared

as follows: the catalysts (4 mg) were suspended in 2 ml of ethanol and 20 μL Nafion

(5%), the resulting solution was referred to as the “catalyst ink” and was sonicated

until excellent dispersion was achieved. Then a 30 μL aliquot of the ink was dropped

onto the glassy carbon rotating disk electrode, yielding an approximate loading of 300

μg cm-2. After the ink was dried, the electrode was visually inspected to ensure

uniform film formation. Carbon cloths (CC) were cleaned carefully with acetone, and

then washed in succession with ethanol and deionized water for three times. The

catalyst-loaded carbon cloth anode was prepared by mixing the prepared sample (90

wt.%) and conductive adhesive (10 wt.%) slurry coated onto 1 cm × 2 cm carbon

cloths with a mass loading ~0.8 mg cm−2 and dried under ambient conditions. The

catalyst’s activity towards OER was tested in O2-saturated electrolyte solution by

linear sweep voltammetry (LSV) from 0 to 0.8 V versus Ag/AgCl, which was

performed at 5 mV s-1 after purging the electrolyte with O2 gas for 30 min at room

temperature. The working electrode was rotated at 1600 rpm during testing. The

accelerated stability tests were performed in O2-saturated 1 M KOH at room

temperature by potential cycling between 0 and 0.55 V versus Ag/AgCl at a sweep

rate of 100 mV/s for 1000 cycles. At the end of the cycles, the resulting electrodes

were used for LSV at a sweep rate of 5 mV/s. For samples loaded on CC,

chronoamperometric measurements were done at η = 300 mV. ECSA was determined

by measuring the capacitive current associated with double-layer charging from the

scan-rate dependence of CVs. The potential window of CVs was -0.08 to 0.08 V

versus Ag/AgCl (1M KOH). The double-layer capacitance (Cdl) was estimated by

plotting the ΔJ (ja − jc) at 0 V versus Ag/AgCl (1 M KOH) against the scan rate.

RHE calibration.

In all measurements, we used Ag/AgCl (KCl saturated) as the reference electrode. It

was noted that the current density was normalized to the geometrical area and the

measured potentials vs Ag/AgCl were calibrated with respect to a reversible hydrogen

electrode (RHE) scale according to the Nernst equation (ERHE = EAg/AgCl + 0.059×pH

+ 0.197); the overpotential (η) was calculated according to the following formula: η

(V) = ERHE −1.23 V.

Scheme S1. Schematic Illustration of the preparation process of LiNi1-xFexPO4@C

and photograph of spray drying devices.

Fig. S1 SEM images of LiNi1-xFexPO4 prepared without citric acid (a) and LiNi1-

xFexPO4@C samples obtained with different ratios of Ni and Fe reactants: (b) r = 1:1,

(c) r = 2:1, (d) r = 3:1, (e) r = 4:1, (f) r = 5:1. The scale bars are 200 nm.

Fig. S2 XRD patterns of LiNi1-xFexPO4@C NPs with different compositions.

Fig. S3 TGA curves of LiNi1-xFexPO4@C (3:1) in air atmosphere.

Fig. S4 (a) Linear-sweep voltammograms showing the electrocatalysis of water

oxidation by LiNi1-xFexPO4 and (b) corresponding Tafel plots obtained in O2-saturated

KOH (1 M) at a scan rate of 5 mV s-1 at rotation speed of 1600 rpm.

Fig. S5 XPS spectra of (a) Ni 2p, (b) Fe 2p of the mesoporous LiNi1-xFexPO4@C (3:1)

composite.

X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental

composition of LiNi1-xFexPO4@C (Ni : Fe = 3 : 1). As shown in Fig. S5a, the two

typical binding energy at 856.5 and 874.5 eV are correspond to Ni0 2p3/2 and Ni0 2p1/2,

respectively. And the peaks appeared at 861.7 and 881.2 eV are attributed to the Ni2+

2p3/2 and Ni2+ 2p1/2, respectively. It could be seen from Fig. S5b that the Fe 2p

spectrum split into 2p1/2 and 2p3/2 due to the spin-orbit coupling. The Fe 2p spectra are

split in two parts and each part consists of a main peak and “shake-up” satellite peaks,

respectively. Here, the two distinct BEs peaks at 710.8 and 724.0 eV are attributed to

the characteristic of the valence of the Fe2+ state in the LiFePO4 olivine-structure as

reported previously.S1, S2 The appearance of satellite peaks at 713.8 and 727.1 eV is

typical characteristic feature of transition metal ions with partially filled d-orbits. S3, S4

The core level photoelectron peak positions are very close to previous reports,

indicating the presence of Ni2+ and Fe2+ in as obtained LiNi1-xFexPO4@C NPs.

Moreover, the Ni/Fe atomic ratio (2.48:1) at the surface of the materials is lower than

that of the feeding molar ratio, indicating a minor enrichment in Fe at the surface of

the products.

Table S1. Comparison of OER performance in alkaline media of with other non-

noble metal electrocatalysts (a Catalysts directly grown on current collectors).

Catalyst Tafel Slope (mV/dec)

η(@10 mA/cm2) Electrolyte Ref.

LiFexNi1-xPO4@C/GC 78 0.311 1 M KOH This work

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Amorphous Ni-Co Oxide/Au-filmsa 39 0.325 1 M NaOH ACS Nano, 2014, 8, 9815.[S26]

CoO/NG/GC 71 0.34 1 M KOH Energy Environ. Sci., 2014, 7, 609.[S27]

Ni-Co LDH/CPa 40 0.367 1 M KOH Nano Lett., 2015, 15, 1421.[S28]

ZnxCo3-xO4 NWs/Tia 51 0.32 1 M KOH Chem. Mater., 2014, 26, 1889.[S29]

Ni30Fe7Co20Ce43Ox/GC 70 0.41 1 M NaOH Energy Environ. Sci. 2014, 7, 682.[S30]

Ni–Co Hydroxide/GC 65 0.46 0.1 M KOH

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Annealed C-Co NPs/GC N.A. 0.39 0.1 M KOH

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Fig. S6 Cyclic voltammograms of LiFexNi1-xPO4@C (a) and LiFexNi1-xPO4 (b)

composite measured at different scanning rates in 1 M KOH. (c) Corresponding

charging current density differences (ΔJ = ja − jc) plotted against scan rates.

It is generally supposed that the amount of active sites in OER is directly proportional

to electrochemically active surface area. The electrochemically active surface area

was assessed by capacitance measurements as it is presumed to be linearly

proportional to the double layer capacitance (Cdl). The linear slope of capacitive

current versus scan rate, equivalent to twice of the double-layer capacity Cdl, was used

to represent the ECSA.S33, S34 Therefore, the relative value of electrochemically active

surface area can be obtained by normalization of Cdl. It can be seen from Fig. S6 that

the slope of LiFexNi1-xPO4@C is almost 43-fold higher than that of LiFexNi1-xPO4,

clearly demonstrating that with the presence of carbon layer, the LiFexNi1-xPO4@C

composite has much larger amount of active sites in OER.

Fig. S7 (a) LSV curves of carbon cloth (CC) and LiFexNi1-xPO4@C/CC. (b)

Chronopotentiometry of LiFexNi1-xPO4@C/CC at η = 300 mV. All the measurements

were performed in O2-saturated 1 M KOH solution.

Considering that three-dimensional electrode could bring a larger active surface area

in a compact volume and thus achieves a greater geometrical current density favored

by practical applications, the mesoporous LiFexNi1-xPO4@C grown on carbon cloth

(CC) electrode has also been prepared. As shown in Fig. S7a, at overpotential of

0.294 and 0.353 V the geometrical current density for LiFexNi1-xPO4@C/CC was

found to be 10 and 20 mA/cm2, respectively. In contrast, pristine CC shows a

negligible current density at the same overpotential, which further confirms the high

OER activity of our LiFexNi1-xPO4@C. Continuous OER at static overpotential was

performed. As shown in Fig. S7b, the time dependence of the current density for

LiFexNi1-xPO4@C/CC at an overpotential of 300 mV shows only a little degradation

even after a long period of 20000 s, suggesting the LiFexNi1-xPO4@C are of excellent

stability for OER.

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