Facile design of ultrafine CuFe2O4 nanocrystallines coupled porous carbon nanowires: Highly effective electrocatalysts for hydrogen peroxide reduction and the oxygen evolution reaction

Journal Pre-proof Facile design of ultrafine CuFe2O4 nanocrystallines coupled porous carbon nanowires: Highly effective electrocatalysts for hydrogen peroxide reduction and the oxygen evolution reaction Mian Li, Mingjiao Lu, Jirong Yang, Jie Xiao, Lina Han, Yingjie Zhang, Xiangjie Bo PII:

S0925-8388(19)32999-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.151766

Reference:

JALCOM 151766

To appear in:

Journal of Alloys and Compounds

Received Date: 23 March 2019 Revised Date:

30 June 2019

Accepted Date: 7 August 2019

Please cite this article as: M. Li, M. Lu, J. Yang, J. Xiao, L. Han, Y. Zhang, X. Bo, Facile design of ultrafine CuFe2O4 nanocrystallines coupled porous carbon nanowires: Highly effective electrocatalysts for hydrogen peroxide reduction and the oxygen evolution reaction, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151766. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

Facile design of ultrafine CuFe2O4 nanocrystallines coupled porous carbon

2

nanowires: highly effective electrocatalysts for hydrogen peroxide reduction and

3

the oxygen evolution reaction

4

Mian Li,a,* Mingjiao Lu,a Jirong Yang,a Jie Xiao,a Lina Han,c Yingjie Zhang*,a and

5

Xiangjie Bo b,*

6

a

7

Materials Preparation Technology, Key Laboratory of Advanced Battery Materials of

8

Yunnan Province, Faculty of Metallurgical and Energy Engineering, Kunming

9

University of Science and Technology, Kunming 650093, China.

National and Local Joint Engineering Laboratory for Lithium-ion Batteries and

10

b

11

Province, Key Laboratory of Polyoxometalate Science of Ministry of Education,

12

National & Local United Engineering Laboratory for Power Batteries, Department of

13

Chemistry, Northeast Normal University, Changchun, Jilin Province 130024, PR

14

China.

15

c

16

Technology, Kunming 650093, China.

17 18 19

* Corresponding authors

20 21

Key Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin

Faculty of Material Science and Engineering, Kunming University of Science and

E-mail

address:

[email protected],

[email protected]

[email protected] (Y.J. Zhang); [email protected](X. J. Bo).

22 1 / 32

(M.

Li);

1

Abstract

2

Designing low-cost, high-efficiency and non-noble metal-based electrocatalysts is

3

fairly essential for the commercial utilization of electrochemical sensors and energy

4

conversion devices. Low-cost CuFe2O4 spinel has been widely researched as an

5

electrocatalyst in electrochemical sensors and catalysis. Nevertheless, the low

6

utilization of active sites in bulk CuFe2O4 and the poor electro conductivity of

7

CuFe2O4 invariably restrict its upgrade in catalytic efficiency. Herein, by utilizing the

8

facile electrospinning technique and without involving any template or surfactant, we

9

successfully design three-dimensional (3D) hierarchically porous architecture woven

10

by abundant ultrafine CuFe2O4 crystal-coupled porous carbon nanowires (denoted as

11

CuFe2O4/PCFs). Characterization results verify the 3D net-like textural structures of

12

CuFe2O4/PCFs. Especially, the hierarchically porous structure, high surface area, and

13

abundant carbon edges boost the uniform dispersion of tiny CuFe2O4 crystals; these

14

obviously promote the amounts of electrochemically available CuFe2O4 active sites

15

while decreasing the mass transport resistance of CuFe2O4/PCFs in electrocatalytic

16

processes. Meanwhile, introducing carbon matrices can drastically enhance the

17

electrical conductivity of CuFe2O4/PCF nanowires. All these advances in structural

18

and physical performances truly make tremendous progress for CuFe2O4/PCFs for

19

H2O2 reduction and oxygen evolution reaction (OER) catalysis compared with bulk

20

CuFe2O4. For instance, the CuFe2O4/PCF catalyst exhibits a high sensitivity of 69.18

21

µA mM−1 cm−2, low detection limit of 1.20 µM and wide linear range of 0.11 to 22.0

22

mM for H2O2 sensing. Meanwhile, the CuFe2O4/PCF catalyst just needs a potential 2 / 32

1

value of 1.589 V (vs. reversible hydrogen electrode) to achieve the OER catalysis

2

current density of 10 mA cm−2 in 1.0 M KOH, it only shows a small Tafel slope of

3

89.34 mV dec−1 for the OER as well. Our catalyst design strategy of CuFe2O4/PCF

4

nanowires not only demonstrates the successful design of a novel high-efficiency

5

non-noble-metal catalyst for both OER and H2O2 reduction catalysis but also affords a

6

new methodology for boosting the electrocatalytic abilities of spinel-type hybrid

7

materials by designing a 3D structure and improving conductivity.

8

Keywords: CuFe2O4; electrospinning; H2O2 reduction; oxygen evolution reaction;

9

porous carbon nanowires

10

1. Introduction

11

It is well known that octahedral MO6 structures sharing abundant edges can act as

12

active centers toward various electrochemical catalysis processes.[1, 2] In fact,

13

AB2X4-type spinel is a closely packed matrix of X2− ions with A2+ cations and B3+

14

cations replacing parts of octahedral sites, respectively. In addition to the presence of

15

abundant octahedral edges, solid-state redox A3+/A2+ and B3+/B2+ couples in AB2X4

16

spinel can provide specific electro-catalytic activity toward various electrochemical

17

catalysis reactions compared with other metal oxides. [3, 4] To date, AB2X4-based

18

catalysts have been widely studied in energy storage applications,[5] energy

19

conversion devices,[6] electrochemical sensors,[7] and clean energy applications.[8]

20

Nevertheless, based on previous studies, two factors have consistently impeded the

21

further promotion of the electrochemical activities of AB2X4. (1) AB2X4 has relatively

22

low conductivity compared with the electronic conductor at room temperature.[9] (2) 3 / 32

1

The AB2X4 powders are prone to aggregate into compact AB2X4 films, which

2

markedly decreases the number of exposed active sites and then leads to a relatively

3

low utilization of AB2X4 active sites.[10, 11] For AB2X4 spinel-based catalysts,

4

overcoming the two abovementioned bottlenecks is the precondition for acquiring

5

excellent catalytic activity close to that of noble metal-based catalysts (such as Pt, Ru,

6

and Ir) in various electrochemical applications (such as electrosensors, energy storage

7

devices, fuel cells and water splitting devices).

8

Constructing the typical three-dimensional (3D) architectures has been proven as

9

an efficient approach for boosting AB2X4’ catalytic efficiency. This occurs because the

10

enriched macroporosity of the 3D architectures can expose more AB2X4 lattice planes

11

to electrolyte solutions, which extremely promotes the utilization of AB2X4 active

12

sites in catalytic processes.[11] In addition, those hierarchically porous channels can

13

decrease the mass transport resistances in catalytic processes as well, which will boost

14

the transmission efficiency of reactants from electrolytes to active sites and products

15

from active centers into electrolytes.[12, 13] This advance will lead to a tremendous

16

advance in electrochemical activity. Finally, the 3D architectures with excellent

17

mechanical strength can keep the long lifetime of superior catalytic activity as

18

well.[14] Further enhancing the electronic transmission capacity of spinel-type AB2X4

19

materials is another powerful approach for boosting AB2X4’s catalytic ability because

20

of their relatively poor electric conductivity compared with an electronic conductor.

21

Coupling AB2X4 particles on various carbonaceous supports has been proven as the

22

efficient operation of obviously enhancing AB2X4’s electrocatalysis abilities.[15] 4 / 32

1

Summarizing from the above facts, designing the 3D hierarchically porous

2

carbonaceous matrices with vast AB2X4 nanoparticles coupling along surfaces could

3

be very promising for developing high-efficiency substitution catalysts toward noble

4

metals.

5

Among various AB2X4 spinel materials, CuFe2O4 is low cost because Fe and Cu

6

elements are all highly abundant and inexpensive. In addition, CuFe2O4 exhibits

7

multiple valence and low toxicity, which will lead to the abundant surface defects

8

being beneficial for electrochemical catalysis applications.[16] Nevertheless, similar

9

to other AB2X4 spinels, the inherently insufficient electron conductivity has also

10

limited the further enhancement in the electrochemical activity of CuFe2O4.[17]

11

Herein, we report the design and preparation of novel ultrafine CuFe2O4

12

crystal-coupled

13

CuFe2O4/PCFs) by utilizing the simple, controllable and rapid electrospinning method

14

without involving any template or surfactant. The 3D hierarchically porous net-like

15

structures woven by porous carbon nanowires can effectively boost the exposure

16

degree of active sites and mass transmission in electrocatalysis. Reasonably

17

controlling the metal contents in precursor solution can obviously restrict the particle

18

sizes of CuFe2O4 crystal nanoparticles benefitting from the regulation of abundant

19

porous structures and defects dispersed along surfaces, which can eminently promote

20

the utilization ratio of CuFe2O4 active centers. The excellent electric conductivity of

21

porous carbon nanowires will further boost the electron transmission efficiency of

22

CuFe2O4. The high surface area, excellent porosity, ultrafine CuFe2O4 crystals and

3D

hierarchically

porous

5 / 32

carbon

nanowires

(denoted

as

1

advanced electron conductivity make the 3D hierarchically porous CuFe2O4/PCFs

2

nanowires very attractive for various electrochemical applications. By taking

3

advantage of advanced structure properties for CuFe2O4/PCFs, the 3D hierarchically

4

porous CuFe2O4/PCF catalyst shows superior catalytic activity toward both

5

nonenzymatic H2O2 reduction/detection and the oxygen evolution reaction (OER)

6

without using any precious metals (such as Ir, Pt and Ru). Our morphological and

7

structural design strategy is fairly effective for boosting the utilization and electron

8

conductivity of CuFe2O4, which provides new methodology for designing

9

high-efficiency CuFe2O4/C hybrid catalysts with catalytic efficiency comparable to

10

noble metals.

11

2. Experimental

12

2.1. The synthesis of 3D hierarchically porous CuFe2O4/PCFs nanowires

13

Scheme 1 shows the detailed procedures of synthesizing 3D hierarchically porous

14

CuFe2O4/PCF nanowires. An appropriate dosage of polyvinylpyrrolidone (PVP, K90)

15

was first dissolved into 100 mL of N, N-dimethylformamide (DMF) at room

16

temperature under vigorous stirring until the system became a clear and ropy solution

17

(denoted as PVP-DMF). Then, Cu(CH3COO)2·H2O and Fe(CH3COO)2·4H2O metallic

18

salts with a stoichiometric Cu: Fe ratio of 1: 2 were added to the PVP-DMF solution.

19

The resultant precursor slurry with 10 wt.% PVP and 8 wt.% metallic salts was

20

denoted

21

as-prepared PVP-DMF-Cu(CH3COO)2/Fe(CH3COO)2 precursor slurry was woven

22

into the 3D porous membrane consisting of PVP/DMF/Cu(CH3COO)2/Fe(CH3COO)2

23

nanowires by utilizing an electrospinning technique. The stainless steel needles

as

PVP-DMF-Cu(CH3COO)2/Fe(CH3COO)2.

6 / 32

Following

closely,

the

1

equipped on the plastic syringes are 0.9 mm in diameter. The nickel mesh collector

2

was used as the support for collecting precursor membranes; the distance between the

3

nickel mesh collector and the orifice of the plastic syringe was controlled as 15 cm.

4

The voltage of electrospinning was set at 20 kV. The 3D hierarchically porous

5

CuFe2O4/PCF nanowires were successfully synthesized through pyrolyzing the

6

as-dried PVP/DMF/Cu(CH3COO)2/Fe(CH3COO)2 precursor nanowires in air at

7

550 °C for 3 h under a heating rate of 5 °C min-1 in a tube furnace. On the other hand,

8

for comparison, the pure bulk CuFe2O4, Fe2O3 particles and PCFs materials were also

9

synthesized. For the preparation of pure bulk CuFe2O4, the Cu(CH3COO)2 and

10

Fe(CH3COO)2 metal salts with a stoichiometric Cu: Fe ratio of 1: 2 were dissolved

11

into water, dried and calcined in air at 550 °C for 3 h. The Fe2O3 particles were

12

directly synthesized through heating Fe(CH3COO)2 metal salt at 550 °C for 3 h in air.

13

The pure PCFs were synthesized through carbonizing the PVP/DMF precursor

14

nanowires in air at 300 °C for 3 h at a heating rate of 5 °C min-1 in a tube furnace.

15 16

2.2. The Characterizations of 3D hierarchically porous CuFe2O4/PCFs, bulk CuFe2O4 and Fe2O3

17

For the characterization methods and technologies of 3D hierarchically porous

18

CuFe2O4/PCFs, bulk CuFe2O4 and Fe2O3, one can find details in the Experimental

19

Section in the Supporting information (SI).

20

2.3. Electrochemical characterization methods

21

All relevant electrochemical measurements were performed at room temperature

22

using the typical three-electrode system. In detail, for H2O2 reduction and detection,

23

the catalyst particle-modified glassy carbon electrodes (GCE, d = 3 mm) acted as the 7 / 32

1

working electrodes, Ag/AgCl acted as the reference electrode and Pt wire acted as the

2

counter electrode. For the OER measurement, self-supporting CuFe2O4/PCFs (1 cm ×

3

1 cm), bulk CuFe2O4 and Fe2O3 particles-modified rotating disk electrodes (RDE; 5

4

mm in diameter, 0.19625 cm2) acted as the working electrodes, respectively. Ag/AgCl

5

and Pt wire acted as the reference and counter electrodes, respectively. All potentials

6

for the OER test appearing in this paper refer to the reversible hydrogen electrode (i.e.,

7

ERHE = EAg/AgCl +0.059 pH+0.197 V, where ERHE is the potential vs. reversible

8

hydrogen electrode (RHE), EAg/AgCl is the potential vs. the Ag/AgCl reference

9

electrode, and pH is the pH value of the 1.0 M KOH electrolyte. All current densities

10

are the ratios of as-recorded currents to the geometric area of electrodes.

11

To prepare working electrodes, 3 mg of catalyst powders were first dispersed in

12

1.0 mL of Nafion solution (0.5 wt%), generating homogeneous inks. Afterwards, 10

13

µL of the catalyst dispersion inks were transferred onto the GCE to build the working

14

electrodes for H2O2 detection, and 30 µL of the dispersion inks were transferred onto

15

the RDE to construct the working electrodes for OER catalysis. The as-modified GCE

16

and RDE electrodes were then dried at room temperature for standby application.

17

For the OER electrochemical test, the Tafel plots were transformed from

18

corresponding polarization curves and plotted as potentials (E vs. RHE) vs. log |j (mA

19

cm−2)|, which can be used to assess the OER kinetics of different catalysts. Through

20

fitting the Tafel plots in the linear portion by utilizing the Tafel equation (η= b log (|j|)

21

+ a), the Tafel slopes (b values) can be successfully calculated. Finally, all linear

22

sweep voltammetry (LSV) curves in this paper were reported without iR 8 / 32

1

compensation.

2

3. Results and discussions

3

X-ray diffraction (XRD) characterization was first implemented to confirm the

4

typical crystalline structures of as-synthesized Fe2O3 particles, bulk CuFe2O4 and

5

CuFe2O4/PCFs. As shown in Figure 1 (black curve), the XRD pattern of pure Fe2O3

6

only reveals the characteristic diffraction peaks of the hematite-type Fe2O3 crystal; the

7

peaks located at approximately 24.14°, 33.15°, 35.61°, 40.85°, 49.48°,

8

54.09°, 57.59°, 62.45°, 63.99°, 71.94°, 80.71°, 82.94°, 84.91°, and 88.54°

9

correspond to (012), (104), (110), (113), (024), (116), (018), (214), (300), (1010),

10

(128), (0210), (134), and (226) planes (JCPDS:33-0664), respectively. The CuFe2O4

11

bulk displays the typical features of spinel-type CuFe2O4 crystal. As shown in Figure

12

1 (blue curve), its XRD diffraction peaks located at ~18.32°, 30.56°, 34.72°,

13

35.86°, 37.12°, 41.83°, 43.77°, 53.92°, 55.48°, 57.03°, 57.83°, 62.16°,

14

63.63°, 74.58° and 79.09° correspond to the (101), (200), (103), (211), (202),

15

(004), (220), (312), (105), (303), (321), (224), (400), (413), and (404) planes

16

(JCPDS:34-0425), respectively.[18] XRD characterization proves that the spinel-type

17

CuFe2O4 crystal was successfully synthesized for bulk CuFe2O4. The XRD pattern of

18

CuFe2O4/PCFs exhibits the diffraction peaks of the spinel-type CuFe2O4 crystal as

19

well (as shown in red curve of Figure 1), which demonstrates that CuFe2O4 crystals

20

were successfully grown on the CuFe2O4/PCFs hybrid sample. In addition to the

21

diffraction peaks of the CuFe2O4 crystal, another wide XRD diffraction peak located

22

at ~25° appears for the CuFe2O4/PCFs as well, which corresponds to the diffraction 9 / 32

1

peak of the carbon matrices. Even so, the diffraction peak of carbon is seemingly

2

weak, illustrating the low crystallinity of as-formed carbon matrices at low pyrolysis

3

temperature. From the aforementioned XRD characterization results, we preliminarily

4

speculate that the CuFe2O4/PCFs composite has been synthesized.

5

Scanning electron microscopy (SEM) images and energy dispersive X-ray

6

spectroscopy (EDS) spectra were measured to observe the topographic characteristics

7

and elemental constituents of pure bulk Fe2O3, bulk CuFe2O4 and CuFe2O4/PCFs. As

8

shown in Figure 2a, the pure Fe2O3 is stacked by vast secondary bulk Fe2O3. The

9

pure CuFe2O4 is also stacked by abundant bulk CuFe2O4 (Figure 2b). Nevertheless,

10

the CuFe2O4/PCF compound is a 3D network woven by large numbers of

11

CuFe2O4/PCFs nanowires (Figure 2c), in which the diameters of CuFe2O4/PCFs

12

nanowires are less than 1 µm. This microscopic morphology of CuFe2O4/PCFs is

13

similar to the precursor membrane (Figure S1), but the CuFe2O4/PCF nanowires

14

display rougher surfaces than the precursor. Even so, the pyrolysis process cannot

15

destroy the 3D net-like topological structure. To determine the surface elemental

16

compositions of the three control samples, their EDS spectra were further measured.

17

Only the Fe and O elements can be tested in the EDS spectrum of pure Fe2O3 (Figure

18

2d). The Fe and O contents are 39.65 at.% and 60.35 at.%, respectively, with a Fe: O

19

stoichiometric ratio of 2: 3.04; this molar ratio corresponds to that in hematite-type

20

Fe2O3. The EDS spectrum of pure CuFe2O4 exhibits the coexistence of Cu, Fe and O

21

(Figure 2e). The Cu, Fe and O contents are 14.15 at.%, 28.65 at.% and 57.21 at.%,

22

respectively, with a Cu: Fe: O stoichiometric ratio of 1: 2.02: 4.04; this molar ratio 10 / 32

1

corresponds to that in the chemical formula of the spinel-type CuFe2O4 crystal (1: 2:

2

4). Finally, the EDS spectrum of the CuFe2O4/PCFs displays the coexistence of C, Cu,

3

Fe and O (Figure 2f). The C, Cu, Fe and O contents are 41.62 at.%, 6.15 at.%, 13.17

4

at.% and 39.06 at.%, respectively. It is clear that the Cu: Fe: O stoichiometric ratio in

5

the CuFe2O4/PCF compound is 1: 2.14: 6.35; the molar number of O in

6

CuFe2O4/PCFs is ~1.59 times of that in pure CuFe2O4. As with the presence of high

7

amounts of C in CuFe2O4/PCFs, the excess O in CuFe2O4/PCFs may be attributed to

8

the symbiotic oxygen-containing functional groups dispersed along the surfaces of

9

CuFe2O4/PCFs.

10

Transmission electron microscopy (TEM) images were further measured to

11

observe the microstructures of CuFe2O4/PCF nanowires. As shown in Figure 3a, the

12

3D networks are woven by many CuFe2O4/PCF nanowires with diameters less than

13

600 nm. Meanwhile, the surfaces of each CuFe2O4/PCF nanowire are coupled to

14

copious tiny CuFe2O4 nanoparticles. The high-magnification TEM (HRTEM) image

15

of an individual CuFe2O4/PCF nanowire reveals the abundant porous structures on the

16

surfaces of the CuFe2O4/PCF nanowire (Figure 3b). In addition, large numbers of

17

tiny CuFe2O4 crystal nanoparticles are uniformly dispersed along the surfaces of the

18

CuFe2O4/PCF nanowires (as shown in Figure S2). The size distribution histogram of

19

CuFe2O4 crystal nanoparticles appearing in Figure S2 is recorded in Figure 3c.

20

Except for a few CuFe2O4 nanoparticles with diameters larger than 50 nm, the

21

diameters of most CuFe2O4 nanoparticles are mainly distributed between 5 and 40 nm,

22

providing an average particle size of 15.62 nm. Sequentially magnifying the HRTEM 11 / 32

1

image of an individual CuFe2O4/PCF nanowire, one can see that a portion of the

2

CuFe2O4 nanoparticles were tightly coated by several carbon layers (Figure 3d), but

3

another portion of CuFe2O4 nanoparticles were directly dispersed along the surfaces

4

of the CuFe2O4/PCFs nanowires (Figure 3e). Finally, as revealed in Figure 3d,

5

sequentially magnifying the HRTEM image, one can see the lattice fringes with the

6

spacing values of 0.25 and 0.207 nm, which correspond to the (211) and (220) planes

7

of spinel CuFe2O4 crystals, respectively. Figure S3a exhibits a typical SEM image of

8

pure PCFs, which reveals the existence of 3D networks stacked by vast PCF

9

nanowires with diameters less than 500 nm. The HRTEM image (Figure S3b) proves

10

that only small mesopores (with diameters less than 5 nm) can be observed along the

11

surfaces of pure PCF nanowires.

12

N2 adsorption−desorption isotherms of pure CuFe2O4 and CuFe2O4/PCFs were

13

measured to further verify their porous features (Figure 4a). The N2

14

adsorption−desorption isotherm of CuFe2O4/PCFs reveals the typical IV sorption

15

behavior, and a distinct hysteresis loop can be clearly observed, which demonstrates

16

the existence of numerous mesoporous microstructures. Nevertheless, the N2

17

adsorption−desorption isotherm of bulk CuFe2O4 shows the typical I sorption

18

behavior, proving that the mesopores are insufficient. By utilizing the BJH method,

19

we successfully calculated the pore size distribution status of CuFe2O4/PCF nanowires

20

and pure CuFe2O4 from their desorption branches. As revealed in Figure 4b, except

21

for a small number of micropores, no obvious large pores can be found in the pore

22

size distribution curve of pure CuFe2O4. The pore-size distribution curve of the 12 / 32

1

CuFe2O4/PCF catalyst exhibits three peaks between 2-4 nm, 4-10 nm and 20-100 nm,

2

respectively; demonstrating the coexistence of micropores, mesopores and

3

macropores. The pores with diameters less than 60 nm mainly result from the

4

pyrolyzation of the PVP precursor with escaping CO2 and H2O gas.[19] Nevertheless,

5

those pores with diameters larger than 60 nm are attributed to the stacked architecture

6

of CuFe2O4/PCF nanowires. Benefitting from the abundant porous structures of 3D

7

hierarchically porous CuFe2O4/PCF nanowires, it exhibits a relatively high BET

8

specific surface area of 97.85 m2 g−1 compared to pure CuFe2O4 (3.64 m2 g−1), which

9

is extremely desirable for active site dispersion and then boosting electrochemical

10

reaction efficiency.

11

XRD, EDS and TEM characterization results all indicate the presence of carbon

12

matrices in CuFe2O4/PCFs; we therefore examined the graphitization degree and

13

defect state of hierarchically porous CuFe2O4/PCF nanowires by utilizing a Raman

14

technique. As shown in Figure 5, the Raman spectrum of CuFe2O4/PCFs shows a D

15

band at ~1354.6 cm−1 and a G band at ~1590.8 cm−1. It is well known that the D band

16

relates to the disorder of carbon atoms with abundant defects, and the G band

17

corresponds to the ordered sp2 carbon atoms in-plane. Herein, the Raman spectrum of

18

CuFe2O4/PCF nanowires exhibits a larger D band and a smaller G band with a large

19

ID/IG ratio of 1.61. Raman characterization proves the weak crystallinity of the carbon

20

matrices in CuFe2O4/PCF nanowires, which is in agreement with the XRD

21

characterization. Nevertheless, the abundant carbon edges/defects can boost the

22

dispersion of active sites along the surfaces of CuFe2O4/PCF nanowires. 13 / 32

1

X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate

2

the chemical binding states of each element in CuFe2O4/PCFs. As shown in Figure 6a,

3

XPS survey spectrum exhibits the characteristic peaks located at binding energy

4

ranges of 282-290 eV, 526-536 eV, 703-737 eV and 930-967 eV ascribed to the C1s,

5

O 1s, Fe 2p, and Cu 2p peaks, respectively. This demonstrates the coexistence of C, O,

6

Fe and Cu in CuFe2O4/PCFs. XPS analysis results display the elemental contents of

7

59.00 at.%, 33.96 at.%, 4.69 at.% and 2.35 at.% for C, O, Fe and Cu, respectively.

8

The Cu: Fe molar ratio is ~1: 1.996, indicating that all Cu and Fe atoms existed in the

9

form of CuFe2O4 crystals. The Cu: O molar ratio is 1: 14.45, which is larger than the 1:

10

4 of the CuFe2O4 crystal, indicating the existence of other types of O (i.e., the

11

oxygen-containing functional groups dispersed on surfaces of carbon matrices). The

12

high resolution C 1s XPS spectrum can be fitted into three parts: two main peaks

13

located at ~283.85 and ~284.83 eV are assigned to the sp2 carbon and sp3 carbon

14

(Figure 6b) and another weak peak at ~288.14 eV is ascribed to the C-O species. The

15

sp3 carbon amount is much larger than that of sp2 carbon, indicating the existence of

16

abundant carbon edges/defects and a weak graphitization degree of CuFe2O4/PCFs,

17

which corresponds to the XRD and Raman results. The high resolution O 1s XPS

18

spectrum can be fitted into three components located at ~530.27, ~532.39 and

19

~533.47 eV, corresponding to the M-O, C=O and C-OH species, respectively (Figure

20

6c). M-O corresponds to the CuFe2O4 crystal, C=O and C-OH are attributed to the

21

oxygen-containing functional groups dispersed along carbon matrices. The Fe 2p core

22

level peak of CuFe2O4/PCFs is divided into two areas for Fe 2p3/2 and Fe 2p1/2 14 / 32

1

(Figure 6d). It is clear that both Fe 2p3/2 and Fe 2p1/2 peaks are successfully

2

designated into the Fe(III) oxidation states (at ~709.83 and ~722.77 eV) and satellites,

3

which indicates that Fe mainly exists as Fe3+ ions in the CuFe2O4 crystal.[20] Finally,

4

the Cu 2p XPS peak of CuFe2O4/PCFs can be divided into two areas of Cu 2p3/2 and

5

Cu 2p1/2 as well (Figure 6e), and Cu mainly exists in the form of Cu2+ (at ~933.78 and

6

~954.16 eV).[21] In conclusion, the SEM, TEM, EDX, N2 adsorption–desorption

7

isotherms, Raman and XPS test results demonstrate that the 3D hierarchically porous

8

networks woven by mesoporous carbon nanowires coupled with copious tiny

9

CuFe2O4 crystal particles have been successfully synthesized.

10

We first explored the application possibility of CuFe2O4/PCFs for H2O2 reduction

11

and detection in 0.1 M NaOH solution. Meantime, as the control samples, the H2O2

12

sensing efficiencies of pure Fe2O3 and CuFe2O4 bulk materials were also studied by

13

utilizing the cyclic voltammetry (CV) curves recorded at a potential scan rate of 50

14

mV s-1. As shown in Figure 7a, in the negative potential range between -0.6 and -0.2

15

V vs. Ag/AgCl, the background currents of all control samples are featureless. Once

16

adding 10 mM H2O2 into 0.1 M NaOH solution, three control catalysts reveal entirely

17

different catalysis efficiency toward H2O2 reduction. Figure 7b shows the

18

electrocatalytic responses (∆j = jH2O2 – jbackground) of H2O2 reduction between -0.6 and

19

-0.2 V vs. Ag/AgCl for pure Fe2O3, pure CuFe2O4 and CuFe2O4/PCFs increasing by

20

the order of Fe2O3 < CuFe2O4 < CuFe2O4/PCFs. H2O2 reduction efficiencies of pure

21

PCFs, CuFe2O4 bulk and CuFe2O4/PCFs were also studied by utilizing CV curves

22

recorded with or without adding 10 mM H2O2 into 0.1 M NaOH solutions (Figure 15 / 32

1

S4a). As revealed in Figure S4b, the ∆j values of H2O2 reduction between -0.6 and

2

-0.2 V vs. Ag/AgCl for pure PCFs, CuFe2O4 bulk and CuFe2O4/PCFs increase by the

3

order of pure PCFs < CuFe2O4 bulk < CuFe2O4/PCFs. The pure PCFs exhibit poor

4

H2O2 reduction efficiency, but the CuFe2O4 bulk displays obvious current responses

5

toward H2O2 reduction; which indicates that pure PCFs cannot act as high-efficiency

6

active sites for H2O2 reduction, but CuFe2O4 crystals can serve as efficient active sites

7

toward H2O2 reduction. CuFe2O4/PCFs porous nanowires reveal excellent H2O2

8

reduction catalytic ability surpassing both pure PCFs and bulk CuFe2O4, which may

9

be attributed to the synergistic effect of the porous PCF matrices and ultrafine

10

CuFe2O4 nanoparticles coupled to them. In detail, the 3D hierarchically porous

11

networks and abundant porous structures dispersed along the surfaces of

12

CuFe2O4/PCF nanowires can boost the uniform dispersion of ultrafine CuFe2O4

13

nanoparticles, which will visibly enhance the number of electrochemically available

14

CuFe2O4 active sites.

15

To verify the possibility of CuFe2O4/PCFs for H2O2 sensing, we further

16

implemented the amperometric j-t test to acquire the H2O2 detection performances.

17

From the CV test results, when potential values are more negative than -0.55 V vs.

18

Ag/AgCl, the ∆j values of H2O2 reduction remain unchanged. We therefore chose

19

-0.55 V vs. Ag/AgCl as the working potential of the j-t measurement. As shown in

20

Figure 7c, in continuously stirred 0.1 M NaOH solution with N2-saturation, when

21

repeatedly adding H2O2 into the electrolyte system, the current responses changed

22

invariably. As revealed in the inset of Figure 7c, for an individual H2O2 injection, the 16 / 32

1

current response would immediately increase and rapidly reach 90% of the

2

steady-state j value within 1.8 s, indicating the excellent response time of

3

CuFe2O4/PCFs toward H2O2 detection. The corresponding calibration plot of the j-t

4

curve in Figure 7c was calculated and recorded in Figure 7d. The limit of detection

5

(LOD) was first calculated by using the formula of LOD=3 Sd/s (s is the slope of

6

calibration plot in the linear range and Sd is the standard deviation of current heights

7

for ten zigzag waves in the background range without the presence of H2O2). By

8

utilizing this method, the LOD of CuFe2O4/PCFs was calculated to be ~1.2 µM

9

(S/N >3). One can see from the corresponding calibration curve, the CuFe2O4/PCFs

10

catalyst has a linear range from 0.1 to 22.0 mM with a large sensitivity of 69.18 uA

11

mM-1 cm-2. Compared with previously reported catalysts (as shown in Table 1)，

12

CuFe2O4/PCFs exhibit a faster response time toward H2O2 detection than those of

13

pure CuFe2O4 crystals,[22] RGO/CuFe2O4/CPE,[12] CuFe2O4 nanotubes,[23]

14

meso-ZnCo2O4, [24] and Ni3N-PCF.[25] CuFe2O4/PCFs also reveal a wider linear

15

range than those of RGO/CuFe2O4/CPE,[12] meso-ZnCo2O4,[24] rGO-CuFe2O4,[26]

16

O-MoS2/graphene,[27] GDC/NiO, [28] CuxMnO2·nH2O,[29] Fe3O4/RGO [30] and

17

CoFe2O4 NFs.[31] Meanwhile, CuFe2O4/PCFs exhibit a larger H2O2 detection

18

sensitivity value than those of Ni3N-PCF,[25] CuxMnO2·nH2O,[29] Fe3O4/RGO,[30]

19

and CoFe2O4 NFs.[31] Especially, CuFe2O4/PCFs show a lower detection limit value

20

than those of CuFe2O4 nanotubes,[23] Fe3O4/RGO, [30] and CoFe2O4 fibers.[31] The

21

abovementioned electrochemical results reveal the excellent H2O2 detection efficiency

22

of the CuFe2O4/PCF nanowires. 17 / 32

1

Another major challenge for H2O2 detection is the interferences caused by other

2

electroactive substances (i.e., glucose, ascorbic acid (AA), dopamine (DA), urea acid

3

(UA) and NaCl). We therefore accomplished selectivity studies of CuFe2O4/PCFs

4

against glucose, AA, DA, UA and NaCl interferences. As shown in Figure 7e, the

5

amperometric responses of CuFe2O4/PCFs with successive additions of 3 mM H2O2, 1

6

mM glucose, 1 mM AA, 1 mM DA, 1 mM UA, 1 mM NaCl and 3 mM H2O2 in 0.1 M

7

NaOH solution at an applied potential of −0.55 V vs. Ag/AgCl are recorded. It is clear

8

that CuFe2O4/PCFs only reveal negligible responses toward the interferences,

9

demonstrating the outstanding selectivity of CuFe2O4/PCFs toward H2O2 sensing.

10

Finally, CuFe2O4/PCF nanowires also possess excellent long-term stability for

11

H2O2 sensing. In detail, we recorded the current responses of CuFe2O4/PCFs to 3 mM

12

H2O2 every 10 days within a 2-month period through the typical amperometric

13

method (after each electrochemical test, CuFe2O4/PCFs sensor was stored in a

14

refrigerator at -4 °C). Figure 7f shows that CuFe2O4/PCFs retain 96.5% of the initial

15

current value after 2 months, indicating the excellent stability of CuFe2O4/PCFs; this

16

also proves the possibility of using CuFe2O4/PCFs in practical H2O2 sensor

17

applications.

18

Designing low-cost catalysts for high-efficiently OER catalysis is important for

19

realizing the cheap production of clean H2 energy through electrochemical water

20

splitting. We therefore explored the possibility of using CuFe2O4/PCF nanowires for

21

high-efficiency OER catalysis. LSV curves were first tested for comparing the OER

22

catalytic performances of all control samples (as shown in Figure 8a); the 18 / 32

1

corresponding OER catalytic onset potential (Eonset) and potential at j = 10 mA cm-2

2

(E10) values were recorded in Figure 8b. It is clear that, in 1.0 M KOH electrolyte

3

solution, the CuFe2O4/PCF nanowires show high OER catalytic activity with much

4

more negative Eonset (1.553 V vs. RHE) and E10 (1.589 V vs. RHE) values than those

5

of pure Fe2O3 (Eonset =1.781 V vs. RHE and E10 =1.877 V vs. RHE) and bulk CuFe2O4

6

(Eonset =1.607 V vs. RHE and E10 =1.704 V vs. RHE), demonstrating the highly

7

superior OER catalytic ability of CuFe2O4/PCF nanowires compared to pure Fe2O3

8

and bulk CuFe2O4. What needs illustration is that, although the Eonset (1.553 V vs.

9

RHE) and E10 (1.589 V vs. RHE) values of CuFe2O4/PCFs nanowires are ~92 mV and

10

~87 mV worse than those of commercial RuO2 (Eonset =1.462 V vs. RHE and E10

11

=1.502 V vs. RHE), respectively, the j values in the positive potential range (> +1.7 V

12

vs. RHE) are much larger than those of commercial RuO2 (Figure 8a). This also

13

reveals the excellent OER catalytic ability of self-supported CuFe2O4/PCF nanowires.

14

OER catalysis efficiencies of pure PCFs, bulk CuFe2O4 and CuFe2O4/PCF porous

15

nanowires were also studied by utilizing LSV curves recorded in 1.0 M KOH solution.

16

As shown in Figure S5, the pure PCFs exhibit negligible OER catalysis efficiency,

17

demonstrating the deficiency in catalytic active sites toward the OER of pure PCFs.

18

The bulk CuFe2O4 displays obvious OER catalysis current responses, which indicates

19

that CuFe2O4 crystals can serve as efficient active sites for OER catalysis. Especially,

20

CuFe2O4/PCF porous nanowires exhibit the maximum OER catalytic ability,

21

surpassing both pure PCFs and bulk CuFe2O4, which may be attributed to the

22

synergistic effect of the porous PCF matrices and ultrafine CuFe2O4 nanoparticles 19 / 32

1

coupled to them. In detail, the 3D hierarchically porous networks woven by

2

CuFe2O4/PCF nanowires with abundant porous structures dispersed on them can boost

3

the uniform dispersion of ultrafine CuFe2O4 nanoparticles along surfaces of the

4

porous

5

electrochemically available CuFe2O4 active sites toward OER catalysis.

carbon

nanowires,

which

will

hugely

enhance

the

amounts

of

6

Moreover, we further examined the OER kinetics of catalysts by using Tafel

7

plots. The linear regions can be fitted by utilizing the Tafel equation (η = b log j + a,

8

where η is the overpotential value, j is current density and b is the Tafel slope). As

9

shown in Figure 8c, the lower Tafel slope of 89.43 mV dec-1 for self-supported

10

CuFe2O4/PCF nanowires provides solid evidence for its enhanced OER kinetics

11

compared to the two control samples: Fe2O3 (Tafel slope = 201.22 mV dec-1) and bulk

12

CuFe2O4 (Tafel slope = 164.58 mV dec-1). Meanwhile, the Tafel slope value of

13

CuFe2O4/PCF nanowires (Tafel slope = 89.34 mV dec-1) is just slightly larger than

14

that of commercial RuO2 (Tafel slope = 78.36 mV dec-1). The negative OER catalytic

15

potential, large OER catalytic current and small Tafel slope indicate that the

16

CuFe2O4/PCF catalyst is a highly active OER catalyst. Compared with inexpensive

17

element-based OER catalysts reported in previous works, the self-supported

18

CuFe2O4/PCF nanowires still show greater advantages. As shown in Table 2, except

19

the NiMn-LDHs nanosheets, [2] Cu3Mo2O9/NF, [32] CoFe(3:1)-N, [33] and

20

Co2N0.67-CMFs, [34] the hierarchically porous CuFe2O4/PCF nanowires reveal a more

21

negative OER E10 value than other control catalysts, such as: CoMn-LDHs-W,[1] pure

22

CuFe2O4 crystals,[22] CuFe2O4 fibers,[35] CoFe2O4 NFs,[31] CuFe2O4 NFs,[31] 1D 20 / 32

1

hollow CuFe2O4 fibers,[35] Thiospinel CuCo2S4,[36] Co/CoFe2O4,[37] ZnxCo3−xO4

2

polyhedrons,[38] porous MoO3 [39] and Ni3Fe thin films.[40] Meantime, the

3

hierarchically porous CuFe2O4/PCF nanowires also exhibit a smaller Tafel slope value

4

than those of CuFe2O4 NFs,[31] Cu3Mo2O9/NF,[32] Thiospinel CuCo2S4 [36] and

5

porous MoO3.[39] The above-mentioned electrochemical results all reveal the

6

excellent OER catalytic efficiency of the self-supported CuFe2O4/PCF nanowires.

7

We further investigated the OER catalysis kinetics of CuFe2O4/PCF nanowires

8

by utilizing the electrochemical impedance spectroscopy (EIS) method. The EIS plots

9

recorded at +1.65 V vs. RHE for pure Fe2O3, bulk CuFe2O4 and CuFe2O4/PCF

10

nanowires are recorded in Figure 8d. The corresponding Nyquist plots and their data

11

fittings implemented using the Randles circuit demonstrate that bulk CuFe2O4 has a

12

smaller charge transfer resistance (Rct) value than pure Fe2O3, proving the better

13

electron conduction ability and OER catalysis ability of spinel-type CuFe2O4 crystal

14

versus the hematite-type Fe2O3. Especially, the 3D hierarchically porous networks

15

woven by CuFe2O4/PCFs nanowires exhibit a much smaller Rct value than the bulk

16

CuFe2O4, which increases the advantages of porous carbon nanowires in boosting

17

electron conduction and the OER catalytic abilities of tiny CuFe2O4 crystal particles.

18

Furthermore, for an excellent OER catalyst, the OER catalytic stability is as

19

important as OER catalytic efficiency for its practical energy conversion utilization.

20

As displayed in Figure 8e, after 3000 CV scans between 1.5 and 1.7 V vs. RHE in 1.0

21

M KOH, the LSV curve of the CuFe2O4/PCF nanowires is almost the same as the

22

initial one. Only a minor positive potential shift (~20 mV) appears for the LSV curve 21 / 32

1

of CuFe2O4/PCF nanowires after the OER stability test (denoted as post-OER

2

CuFe2O4/PCFs) after OER polarization. We also observe the chronoamperometric

3

response changes at 1.65 V vs. RHE for CuFe2O4/PCF nanowires (Figure 8f). The j-t

4

plot of CuFe2O4/PCF nanowires shows excellent durability in alkaline electrolyte.

5

Even after 30 h of a continuous polarization process, the j-t plot just exhibits 11.03%

6

anodic current loss. To understand why the CuFe2O4/PCF nanowires exhibit superior

7

stability and activity toward the OER, the SEM and HRTEM images of post-OER

8

CuFe2O4/PCF nanowires were studied. As shown in Figure 9a, after 30 h of a

9

long-term stability test, the 3D hierarchically porous net-like architecture woven by

10

the CuFe2O4/PCF nanowires is still unbroken. The steady 3D hierarchically porous

11

net-like architecture will ensure the fast transfer of OH- from the depths of KOH

12

solution to active sites and boost the rapid release of O2 products from active sites’

13

surfaces. Furthermore, the HRTEM image of post-OER CuFe2O4/PCFs (Figure 9b)

14

indicates that those tiny CuFe2O4 nanocrystals coupled on porous carbon nanowires

15

separate from each other even after 30 h of the long-term stability test, which can

16

ensure that sufficient active sites are present

17

experimental results show that the 3D hierarchically porous CuFe2O4/PCF nanowires

18

exhibit excellent long-term stability toward the OER because of the superior structural

19

stability in the 3D hierarchically porous net-like architecture and the coupled tiny

20

CuFe2O4 nanoparticles. The remarkable features of the CuFe2O4/PCF catalyst

21

(including high activity, favorable kinetics, and strong durability) suggest that it is an

22

excellent promising candidate for catalyzing OER in water splitting with low 22 / 32

for OER catalysis. The above

1

manufacturing cost.

2

On the basis of the abovementioned experimental results, combining with the

3

SEM, TEM, N2 adsorption-desorption and Raman characterization parameters, we can

4

confirm the major factors of an excellent H2O2 sensor and OER catalytic efficiencies

5

of 3D hierarchically porous CuFe2O4/PCFs nanowires: (1) the 3D hierarchically

6

porous net-like structures woven by porous CuFe2O4/PCF nanowires can effectively

7

decrease the mass transport resistances compared with bulk CuFe2O4. (2) The 3D

8

net-like textural structure and abundant porous structures dispersed along the surfaces

9

of

porous

CuFe2O4/PCF

nanowires

collectively

form

abundant

10

micro/meso/macropores; this type of structure obviously increases the surface area

11

and then exposes more available active sites for H2O2 reduction and OER catalysis. (3)

12

The abundant carbon defects and mesopores on porous carbon nanowires restrict the

13

unlimited growth of CuFe2O4 crystals and meanwhile output copious tiny

14

monodisperse CuFe2O4 crystals. This progress can hugely improve the utilization

15

efficiency of CuFe2O4 active sites and then boost their H2O2 reduction and OER

16

catalytic abilities. (4) The carbon nanowires with excellent conductivity can boost the

17

electron transmission capacity of CuFe2O4 crystals coupled along surfaces of

18

CuFe2O4/PCFs nanowires. Benefitting from the abovementioned advantages, the 3D

19

hierarchically porous CuFe2O4/PCFs nanowires exhibit superior catalytic efficiencies

20

for both H2O2 reduction/detection and the OER.

21

Conclusion

22

In summary, we have successfully designed and synthesized novel CuFe2O4 23 / 32

1

crystal-coupled CuFe2O4/PCF nanowires by using the electrospinning technique. The

2

3D precursor net-like membrane was successfully transformed into the 3D

3

hierarchically porous architecture woven by porous CuFe2O4/PCF nanowires after the

4

pyrolysis process. SEM, TEM, XRD, EDS, XPS, Raman and N2 adsorption–

5

desorption characterization results indicate that the surfaces of the resultant

6

CuFe2O4/PCF nanowires disperse abundant meso/macroporous channels, and these

7

porous CuFe2O4/PCF nanowires can weave into the special 3D networks. The

8

surfaces of CuFe2O4/PCF nanowires dispersed numerous defects and porous

9

structures; large amounts of ultrafine CuFe2O4 crystal nanoparticles are uniformly

10

embedded/dispersed on the porous PCFs, which will increase the densities of exposed

11

electrocatalysis active sites. The hierarchically porous structures of CuFe2O4/PCF

12

nanowires can decrease the mass transport resistances as well. Meanwhile, the

13

conductive carbon matrix further boosts the electron conductibility of CuFe2O4/PCFs.

14

Benefitting from the synergistic effect of excellent electron conductibility ability,

15

abundant meso/macroporous channels and the inherently superior electrocatalytic

16

ability of CuFe2O4 actives, the 3D hierarchically porous CuFe2O4/PCF nanowires

17

reveal excellent efficiencies for H2O2 reduction and OER catalysis. The

18

high-efficiency, low-cost and mild strategy for synthesizing 3D hierarchically porous

19

CuFe2O4/PCF nanowires in the present study is suitable for extending to synthesize a

20

wide range of other catalysts with 3D architectures and high catalytic activities

21

comparable to non-noble metals.

22 24 / 32

1

Acknowledgement

2

The authors gratefully acknowledge the financial support provided by the

3

scientific research start-up fund from Kunming University of Science and Technology

4

of introducing talents (130214119417) and the analysis and test fund from Kunming

5

University of Science and Technology (2018T20170019).

6 7

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Figure captions

8 9 10 11

Scheme 1 Schematic illustration for the preparation procedures of 3D hierarchically porous CuFe2O4/PCFs nanowires.

12 13 14 15 16

Figure 1. The typical powder XRD patterns of pure Fe2O3 (black curve), bulk CuFe2O4 (blue curve) and CuFe2O4 /PCFs (red curve). Figure 2. SEM images (a, b, c) and EDX spectra (d, e, f) of pure Fe2O3 (a, d), bulk CuFe2O4 (b, e) and CuFe2O4 /PCFs (c, f).

17 18 19 20 21

Figure 3. (a) The TEM image and (b) locally amplified TEM image of the CuFe2O4/PCFs nanowires. (c) The particle size distribution curve of CuFe2O4 crystal nanoparticles dispersed on CuFe2O4/PCFs nanowire (as shown in Figure S2). (d, e and f) HRTEM images of the as-synthesized CuFe2O4/PCFs.

22 23 24 25

Figure 4. Nitrogen adsorption-desorption isotherms (a) and the corresponding pore size distribution curves (b) of the bulk CuFe2O4 (red curves) and CuFe2O4/PCFs (black curves).

26 27

Figure 5. Raman spectrum of the CuFe2O4/PCFs nanowires.

28 29 30

Figure 6. The XPS survey spectrum (a) and high resolution C 1s (b), O 1s (c), Fe 2p (d) and Cu 2p (e) XPS spectra of the CuFe2O4/PCFs sample.

31 32

Figure 7. (a) CV curves of bulk Fe2O3 (black curves), bulk CuFe2O4 (blue curves) and 31 / 32

1 2 3 4 5 6 7 8 9 10 11 12 13

CuFe2O4/PCFs nanowires (red curves) with the absence (dot lines) and presence (solid lines) of 10 mM H2O2 in 0.1 M NaOH solution under a potential scan rate of 50 mV s−1. (b) The corresponding current responses of bulk Fe2O3 (black curves), bulk CuFe2O4 (blue curves) and CuFe2O4/PCFs porous nanowires toward H2O2 in 0.1 M NaOH solution between -0.6 and -0.2 V vs. Ag/AgCl. (c) The amperometric j-t curve of CuFe2O4/PCFs nanowires with the successive addition of H2O2 at an operating potential of -0.55 V vs. Ag/AgCl in 0.1 M NaOH solution, inset is the response time of CuFe2O4/PCFs nanowires toward H2O2 reduction. (d) The calibration curve of CuFe2O4/PCFs nanowires toward H2O2 detection. (e) The j–t curve of CuFe2O4/PCFs nanowires with the successive additions of 3.0 mM H2O2, 1.0 mM interference substances (glucose, AA, DA, UA, and NaCl) and 3.0 mM H2O2 at -0.55 V vs. Ag/AgCl. (f) Stability measurement of the CuFe2O4/PCFs nanowires to H2O2 with testing a data every ten days in two months.

14 15 16 17 18 19 20 21

Figure 8. The OER LSV polarization curves (a), Eonset and E10 values (b), and Tafel plots (c) for bulk Fe2O3, bulk CuFe2O4, CuFe2O4/PCFs nanowires and RuO2. (d) EIS spectra of bulk Fe2O3, bulk CuFe2O4 and CuFe2O4/PCFs nanowires recorded at 1.65 V vs. RHE. (e) The LSV curves of the self-supporting CuFe2O4/PCFs nanowires before and after 3000th potential cycles between 1.5 and 1.7 V vs. RHE. (f) The typical chronoamperometric j-t responses of CuFe2O4/PCFs nanowires recorded at 1.65 V vs. RHE.

22 23 24 25

Figure 9. The SEM image (a) and HRTEM image (b) of post-OER CuFe2O4/PCFs nanowires after the OER polarization measurement.

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Table 1. Comparison of the H2O2 detection performances between our catalyst and other reported H2O2 sensors. Samples

Linear range (mM)

sensitivity (µA cm−2 mM−1)

Detection limit (µM)

CuFe2O4/PCFs nannowires

0.1 to 22.0

69.18

1.2

CuFe2O4 nanotubes

0.5 to 25.0

219

22.0

meso-ZnCo2O4

1×10-6 to 0.9 1×10-3 to 2.08 2.08 to 50.5 1×10-3 to 11.0 0.25 to 16.0 0.01 to 3.9 2.0 to 5.0 (nM) 0.1 to 6.0 5×10-3 to 3.0 3.0 to 17.0

658.92 51.81 38.32 265.57 269.7 107.22 96.9 688.0 45.25 25.94

1×10-4

Ni3N-PCF rGO/CuFe2O4 O-MoS2/graphene GDC/NiO CuxMnO2·nH2O Fe3O4/RGO CoFe2O4 NFs

0.1 0.35 0.12 0.5 nM 3.2 1.4

Applied potential (V vs.Ag/AgCl) -0.55 +5×10-4 V vs. Hg/HgO +0.50 -0.37 -0.40 -0.40 +0.13 -0.15 -0.30 -0.48

Response time (s)

References

1.8

This work

3.0

[23]

5.2

[24]

3.0

[25]

5.0 5.0 5.0

[26] [27] [28] [29]

[30]

3.0

[31]

Table 2. Comparison of OER catalysis performances of CuFe2O4/PCFs nanowires toward other previously reported catalysts. Samples

E10 (V vs. RHE)

Tafel slope (mV dec-1)

CuFe2O4/PCFs CoMn-LDHs-W NiMn-LDHs nanosheets CuFe2O4 crystals CoFe2O4 NFs CuFe2O4 NFs Cu3Mo2O9/NF CoFe(3:1)-N Co2N0.67-CMFs CuFe2O4 fibers Thiospinel CuCo2S4 Co/CoFe2O4 ZnxCo3−xO4 polyhedron Porous MoO3 Ni3Fe thin films

1.589 1.625 1.586 ~1.810 1.670 1.718 1.555 1.430 1.490 1.597 1.625 1.620 1.660 ~1.738 ~1.615

89.34 45.00 47.00 ~52.00 82.75 93.97 146.00 42.00 53.90 82.00 115.00 66.00 66.30 125.00 -

References This work [1] [2] [22] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

1. Ultrafine CuFe2O4 nanocrystallines coupled porous carbon nanowires. 2. The 3D hierarchical porous net-like architecture consist of CuFe2O4/PCFs nanowires. 3. Riched catalytic active sites and excellent electroconductibility for electrocatalysis. 4. Wide linear range (0.11 to 22.0 mM) and high sensitivity (69.18 µA mM−1 cm−2) toward H2O2 sensing. 5. A negative OER E10 value of 1.589 V vs. RHE and a low Tafel slope of 89.34 mV dec-1.