Engineered porous Co–Ni alloy on carbon cloth as an efficient bifunctional electrocatalyst for glucose electrolysis in alkaline environment

Journal Pre-proof Engineered porous Co–Ni alloy on carbon cloth as an efficient bifunctional electrocatalyst for glucose electrolysis in alkaline environment Chong Lin, Panjing Zhang, Shengying Wang, Qiaoli Zhou, Bing Na, Huiqin Li, Jingyang Tian, Yu Zhang, Cui Deng, Liqing Meng, Jiaxin Wu, Chengzhi Liu, Junyuan Hu, Limin Zhang PII:

S0925-8388(20)30147-X

DOI:

https://doi.org/10.1016/j.jallcom.2020.153784

Reference:

JALCOM 153784

To appear in:

Journal of Alloys and Compounds

Received Date: 11 September 2019 Revised Date:

8 January 2020

Accepted Date: 9 January 2020

Please cite this article as: C. Lin, P. Zhang, S. Wang, Q. Zhou, B. Na, H. Li, J. Tian, Y. Zhang, C. Deng, L. Meng, J. Wu, C. Liu, J. Hu, L. Zhang, Engineered porous Co–Ni alloy on carbon cloth as an efficient bifunctional electrocatalyst for glucose electrolysis in alkaline environment, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153784. 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. © 2020 Published by Elsevier B.V.

The engineered porous CoNi alloy on carbon cloth can drive glucose electrolysis for hydrogen production with much lower cell voltage than that of water electrolysis.

1

Engineered porous Co–Ni alloy on carbon cloth as an efficient bifunctional

2

electrocatalyst for glucose electrolysis in alkaline environment

3

Chong Lin1†, Panjing Zhang1†, Shengying Wang1†, Qiaoli Zhou1†, Bing Na1*, Huiqin

4

Li2*, Jingyang Tian1, Yu Zhang1, Cui Deng2, Liqing Meng2, Jiaxin Wu1, Chengzhi

5

Liu1, Junyuan Hu1, Limin Zhang1

6

1

7

Devices, School of Chemistry, Biology and Materials Science, East China University

8

of Technology, Nanchang, 330013, P. R. China.

9

2

Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and

Department of Chemistry and Chemical Engineering, Baoji University of Arts and

10

Sciences, Baoji, 721013, P. R. China.

11

Corresponding author: [email protected], [email protected].

12

†

13

Abstract

implies equal contribution

14

Toward water splitting, the cell voltage is limited by the sluggish anodic oxygen

15

evolution reaction (OER), requiring a much higher overpotential than cathodic

16

hydrogen evolution reaction (HER). Herein, we replace the OER with more favorable

17

glucose oxidation reaction (GOR), that is glucose electrolysis in alkaline environment,

18

which requires a much lower cell voltage than that of water electrolysis. The surface

19

of porous Co–Ni alloy on carbon cloth is partial oxidized, serving as an efficient

20

bifunctional electrocatalyst for both GOR and HER. The active sites, conductivity,

21

intrinsic activity and stability can be synergistically improved with the

22

engineered porous Co–Ni alloy on carbon cloth. In result, the cell voltage is as low

23

as 1.39 V when the current density reaches to 10 mA cm-2 in glucose

24

electrolysis. While without the glucose in the electrolyte, the voltage reaches as

25

high as 1.65 V. This work can make a contribution to develop the other highly

26

active and robust bifunctional electrocatalysts for glucose or similar organic

27

molecular electrolysis.

28

1. Introduction

29

Hydrogen energy gradually replace fossil-fuel energy often requires advanced and

30

cost-effective hydrogen production technologies.[1–6] Steam reforming and coal 1

1

gasification are traditional method for hydrogen production in large quantities.[7]

2

However, massive greenhouse gases will be given off, threating the environment and

3

human health.[7] Water electrolysis provides a promising way to produce high-purity

4

hydrogen, but the costly price limits its large-scale application in industry. The

5

primary reason for its high cost is the sluggish oxygen evolution reaction (OER),

6

which is responsible for the major energy loss of water electrolysis. More importantly,

7

the O2 produced at the anode side is low-value, which is a byproduct of the whole

8

process. The essential purpose of water oxidation is to extract electrons and transport

9

to the cathode side for hydrogen production. Recently, replacing anodic OER with

10

more favorable small organic molecules oxidation reaction in thermodynamic [8–11]

11

can decrease voltage input of the electrolytic. The widely reported organic substrates

12

such as urea[9,10], hydrazine[8,11] etc. electrolysis require much lower cell voltage

13

than water electrolysis, in result saving the energy of water electrolysis. However, the

14

so-called “sacrificial agent” like urea and hydrazine will yield low-value CO2 or N2.

15

In addition, these organic substrates are costly. Therefore, several organic molecules

16

containing hydroxyl or amino groups, such as methanol[12], ethanol[13,14], primary

17

amine compounds[15] and 5-hydroxymethylfural[16,17] display promising potential

18

in promoting water electrolysis. With these organic molecules, hydrogen and

19

chemicals will be simultaneously generated at the cathode and the anode side,

20

respectively.

21

Here, we intent to adopt more favorable glucose oxidation reaction (GOR) to

22

replace sluggish OER, that is glucose electrolysis in alkaline environment. Glucose

23

can be regarded as a promising organic substrate for boosting water electrolysis,

24

which has several advantages. Glucose is the most abundant monosaccharide with

25

polyhydroxyl aldehyde in nature[18]. So, the cost of glucose production is extreme

26

cheap. Glucose is highly soluble in water even at room temperature. It could not be

27

simply considered as just a “sacrificial agent”, which can be upgraded to value-added

28

gluconolactone or gluconic acid in a relative low oxidation potential.[19] Furthermore,

29

it may yet not happen the situation of gas products emissions except glucose molecule

30

is completely oxidized. In fact, GOR has been widely applied in glucose fuel cells[19] 2

1

and glucose sensors[20] etc. However, highly active, low-cost and robust GOR

2

electrocatalysts are urgent in the consideration of poisoning and scarcity of the

3

precious metals.[21,22] The Co or Ni based electrocatalysts have robust, low-cost and

4

superior electrochemical activity in alkaline environment, serving as promising

5

candidates for electrochemical applications[23–31]. Currently, in-situ growth of

6

electrocatalysts on the conductive substrates (carbon cloth, Ni foam etc.) can deliver

7

more catalytic sites and better conductivity than the corresponding powders[31–37].

8

Based on this structure engineering, the extra conductive additives and binders are no

9

more needed, which sometimes will sacrifice massive active sites of powder

10

materials.

11

Herein, on the basis of micro-nano structure engineering, the doping engineering is

12

further applied. Namely, the porous Co–Ni alloy has been in-situ coupled with carbon

13

cloth by a thermal reduction of metal hydroxides precursors on carbon cloth in this

14

work. The obtained porous Co–Ni alloy is not fully reduced or is partial oxidized,

15

displaying abundant meso-macro pores and three dimensional (3D) superstructures,

16

which can serve as an efficient bifunctional electrocatalyst in both anodic GOR and

17

cathodic HER. In result, the glucose electrolysis over engineered porous Co–Ni

18

alloy on carbon cloth requires a much lower cell voltage than that of water

19

electrolysis, which will open a new avenue to develop more superior

20

bifunctional electrocatalyst or more favorable organic molecules.

21

2. Experimental

22

2.1 Materials

23

Cobalt

nitrate

hexahydrate

(Co(NO3)2·6H2O),

nickel

nitrate

hexahydrate

24

(Ni(NO3)2·6H2O), hexamethylenetetramine (HMT), absolute methanol, ethanol (AR)

25

are purchased from Shanghai Titan technology co. Ltd. Potassium hydroxide (KOH,

26

45 wt% aqueous solution) is acquired from Sigma-Aldrich. Carbon cloth (CC) is

27

obtained from Shanghai Hesen electric co., Ltd. All the chemicals are used as received

28

without purification. Water is purified with the Millipore Direct-Q system (18.2

29

MΩ·cm).

30

2.2 Materials Preparation 3

1

To prepare Co–Ni alloy on carbon cloth (Co/CC, Ni/CC and Co0.5Ni0.5/CC), we

2

firstly need to prepare corresponding metal hydroxides on carbon cloth (Co(OH)2/CC,

3

Ni(OH)2/CC and Co0.5Ni0.5(OH)2/CC). In a typical synthesis of Co(OH)2/CC or

4

Ni(OH)2/CC, 1 mmol Co(NO3)2·6H2O or 1 mmol Ni(NO3)2·6H2O and 2 mmol HMT

5

are dissolved in 15 mL methanol, respectively. Then, the above solutions are mixed

6

evenly under sonication for 10 min. In the meantime, the cleaned and dry CC is

7

placed into a Teflon lining (100 mL) against the inner wall. After that, the mixed

8

solution is carefully poured into the Teflon lining. Then, the Teflon-lined stainless

9

autoclave is sealed and heated to 120 oC for 24 h. The obtained CC is subjected to

10

ultrasonic treatment in methanol for certain time for the exfoliation of the excessive

11

samples on the CC. Subsequently, the CC is washed with methanol and dried at 60 oC

12

for future use. The Co0.5Ni0.5(OH)2/CC is obtained using 0.5 mmol Co(NO3)2·6H2O

13

and 0.5 mmol Ni(NO3)2·6H2O under the same condition. Finally, the as-prepared

14

Co(OH)2/CC, Ni(OH)2/CC and Co0.5Ni0.5(OH)2/CC are treated with a low temperature

15

(300 oC) thermal reduction under the atmosphere of Ar/H2 for 2h. Accordingly, the

16

Co/CC, Ni/CC and Co0.5Ni0.5/CC is obtained, respectively. The mass loading of Co,

17

Ni or Co–Ni alloy on carbon cloth is ~ 2 mg cm-2.

18

2.3 Materials Characterization

19

Field emission scanning electron microscopy (FE-SEM) is carried out on a Nova

20

NanoSEM 450. X-ray diffraction (XRD) is conducted on a Bruker AXS D8 Advance

21

instrument with Cu Kα radiation. Nitrogen adsorption isotherm test is performed on a

22

micromeritics surface area measurement analyzer (Quantachrome Instruments,

23

Autosorb-iQ). X-ray photoelectron spectroscopy (XPS) is performed on a Kα X-ray

24

photoelectron spectroscope (Thermo Fisher, E. Grinstead, UK) with an Al Kα X-ray

25

radiation (1486.6 eV photons) for excitation.

26

2.4 Electrochemical measurements

27

We have performed all the electrochemical measurements on a CHI 760E

28

workstation. Thereinto, a standard three-electrode system is applied to analyze the

29

activity of HER, OER and GOR, in which the working electrode, the counter

30

electrode and the reference electrode are porous metal on carbon cloth (Co/CC, Ni/CC 4

1

and Co0.5Ni0.5/CC, 0.5 cm×1.0 cm), platinum plate and saturated Ag/AgCl,

2

respectively. The potential is calibrated by iR compensation and is also referenced to

3

reversible hydrogen electrode (RHE) via Evs.RHE = Evs. Ag/AgCl + EθAg/AgCl + 0.059 × pH

4

– i × R. For overall water electrolysis or glucose electrolysis, the porous metal on

5

carbon cloth is used as both cathode and anode. The liner sweep voltammetry (LSV)

6

is carried out at a scan rate of 5 mV s-1. The stability of each sample is examined by a

7

chronopotentiometry (CP) test at 10 mA cm-2 without iR compensation. To be noted,

8

all the electrochemical data is collected until a stable cyclic voltammetry is obtained.

9

3. Results and discussion

10

3.1 Morphology and structure characterization

11 12

Figure 1. Morphology and structure characterization. LR-SEM images of (a, b)

13

Co/CC, (d, e) Ni/CC and (g, h) Co0.5Ni0.5/CC, respectively. The insets in (b), (e) and

14

(h) are corresponding HR-SEM images, respectively. (c), (f) and (i) are corresponding

15

elements mapping of (b), (e) and (h), respectively.

16

The porous Co/CC, Ni/CC and Co0.5Ni0.5/CC are obtained according to a low

17

temperature thermal reduction of corresponding precursors of Co(OH)2/CC,

18

Ni(OH)2/CC and Co0.5Ni0.5(OH)2/CC. The preparation procedures of these metal 5

1

hydroxide precursors are inherited from our previously reported methanol

2

solvothermal method with a little modification.[37] In detail, the HMT can serve as a

3

kind of organic base, homogeneously alkalizing the solution by its hydrolysis in a

4

high temperature. The methanol can dissolve many organic base and metal precursors.

5

The water-deficiency environment caused by the methanol solution can further

6

control the hydrolysis of HMT. In result, the ultrathin metal hydroxide on carbon

7

cloth can be obtained. After a thermal reduction of under an atmosphere of Ar/H2, the

8

metal hydroxide can be converted into metal.

9

The morphology and structure characterization of the engineered porous Co–Ni

10

alloy is conducted by FE-SEM. Figure 1a, Figure 1e and Figure 1g display the

11

FE-SEM images of Co/CC, Ni/CC and Co0.5Ni0.5/CC, respectively. All these metals

12

are evenly distributed on the carbon fibers. The self-aggregation caused by the

13

excessive metals is hardly observed. With an enlarged view of these metals (Figure 1b,

14

Figure 1e and Figure 1h), we can find out that the wavy metal nanosheets spread out

15

the carbon fiber in a vertical growth mode. These metals have constructed a

16

wide-open 3D and hierarchical superstructures, possessing abundant macropores by

17

the interconnection of the metal nanosheets. From the high-resolution SEM (HR-SEM)

18

images of the metals (the insets of Figure 1b, Figure 1e and Figure 1h), we can also

19

observe abundant mesopores, which may be formed by the thermal decomposition

20

and reduction of metal hydroxides precursors under the atmosphere of Ar/H2.

21

Therefore, the obtained metals can maintain the structure and morphology of

22

flake-like metal hydroxides with low temperature thermal reduction. To further

23

characterize the structure of these engineered metals,

24

XRD test is performed (Figure S1). All the characteristic diffraction peaks are in

25

accordance with cubic metallic phase and no signals of metal oxides can be observed.

26

These metal elements are uniformly on carbon fibers in the corresponding element

27

mappings of Co/CC, Ni/CC and Co0.5Ni0.5/CC (Figure 1c, Figure 1f and Figure 1i).

28

However, the elements of O also can be clearly observed. In fact, is difficult to

29

convert Co–Ni hydroxide to pure Co–Ni alloy even under a high temperature, just like

30

the Mo–Ni alloy nanoparticles prepared in a higher temperature of 500 oC.[38] In 6

1

addition, the nanomaterials also can be inevitably oxidized when they are exposed to

2

air, even the precious metal like Rh nanoparticles.[39] Therefore, the engineered three

3

materials on carbon cloth are still called Co/CC, Ni/CC and Co0.5Ni0.5/CC if there is

4

no special statement. It is worth mentioning that the calculated compositions of

5

Co/CC,

6

Co0.42Ni0.46O0.12/CC. The calculation is based on the EDS analysis of these metals

7

listed in Table S1. In addition, to better prove the crystal structure of the Co–Ni alloy.

8

The TEM images and SAED pattern of the Co–Ni alloy are displayed in Figure S2

9

and Figure S3, respectively. In accordance with the SEM image, the Co–Ni alloy

10

display flake-like morphology and the lattice fringe in its HR-TEM can be indexed to

11

d (111) plane (0.204 nm). From its SAED pattern which taken in a large area, we can

12

discover some metal oxides like Co2O3/Ni2O3 and Co3O4. The result has further

13

confirmed the existence of the element of O in the Co–Ni alloy prepared by a low

14

temperature thermal reduction.

15

3.2 Nitrogen adsorption-desorption isotherm characterization

Ni/CC

and

Co0.5Ni0.5/CC

are

Co0.84O0.16/CC,

Ni0.84O0.16/CC

and

16 17

Figure 2. (a) Nitrogen adsorption isotherms. (c) pore size distribution.

18

To further analyze the specific surface area (SSA) and pore width of these metals,

19

nitrogen adsorption-desorption isotherm measurement is also performed. Figure 2a

20

displays the isotherms of the three metals. All the isotherms of the three metals have

21

hysteresis loops, which can be considered to be characteristic Type IV isotherm[40].

22

In accordance with the SEM images, all these metals have abundant mesopores. A 7

1

larger hysteresis loop of the metal, more mesopores it possesses. The SSAs of the

2

three metals are calculated by the Brunauer-Emmett-Teller (BET) method. The SSAs

3

of porous Co, Ni and Co0.5Ni0.5 are 22.9 m2 g-1, 62.8 m2 g-1 and 83.5 m2 g-1,

4

respectively. Figure 2b presents the corresponding pore size distributions, which are

5

analyzed by the Barrett-Joyner-Halenda (BJH) method. Two main peaks appear in the

6

location of 15~19 nm and 38 nm, which are assigned to the mesopores. In addition,

7

massive pores in the range of larger than 50 nm can be observed, suggesting these

8

metals indeed have many macropores. Table S2 has listed the Langmuir types, SSAs,

9

pore volumes and average sizes of these metals. For porous Co, Ni and Co0.5Ni0.5, the

10

pore volumes are 0.203 cm3 g-1, 0.471 cm3 g-1 and 0.630 cm3 g-1, and the average pore

11

sizes are 19.1 nm, 15.3 m and 38.8 nm, respectively. Therefore, the Co–Ni alloy has

12

much larger SSA, pore volume and average pore sizes, which can bring about faster

13

transportation of glucose or electrolyte and more active sites than the bare Co or Ni.

14

3.3 XPS analysis

15 8

1

Figure 3. (a) XPS of Co 2p3/2 in Co/CC, (b) Co 2p3/2 in Co0.5Ni0.5/CC, (c) Ni 2p3/2 in

2

Ni/CC and (d) Ni 2p3/2 in Co0.5Ni0.5/CC, respectively.

3

In order to get more information about the chemical state of porous Co or Ni in

4

these metals, XPS analysis is performed. As a truism, Co 2p or Ni 2p spectrum will

5

spin-orbit splitting into 2p1/2 and 2p3/2. The 2p1/2 and 2p3/2 contain the same chemical

6

information of Co or Ni.[41] Therefore, in an effort to conveniently describe and

7

analyze the chemical state, we just display the XPS of Co 2p3/2 or Ni 2p3/2 of these

8

metals in Figure 3. Figure 3a gives the Co 2p3/2 spectrum in porous Co, in which the

9

each binding energy located at 778.8 eV, 780.6 eV, 782.7 eV and 786.5 eV is assigned

10

to Co(0), Co(III) and Co(II) and satellite structure, respectively.[42,43] Furthermore,

11

no metal oxides can be inferred from the XRD patterns in Figure S1. In combination

12

with the surface analysis technology of XPS, we can learn that the metallic Co surface

13

is oxidized to a large degree. In the case of the Co 2p3/2 spectrum in porous Co–Ni

14

alloy (Figure 3b), we can also observe that similar locations with Co 2p3/2 spectrum in

15

porous Co (778.4 eV, 780.4 eV, 782.7 eV and 786 eV, respectively) appear four peaks.

16

It indicates that the Co species in porous Co–Ni alloy have similar chemical state with

17

Co species in porous Co. However, the porous Co–Ni alloy contains more metallic Co

18

than that in porous Co. Figure 3c displays the Ni 2p3/2 spectrum in porous Ni, the

19

appeared three peaks are located at 852.8 eV, 853.9 eV, 856.1 eV and 860.7 eV,

20

respectively. They are indexed to Ni(0), Ni(II), Ni(III) and satellite structure,

21

respectively. We can learn that the porous Ni surface is also inevitably oxidized. In

22

the case of the Ni 2p3/2 spectrum in porous Co–Ni alloy (Figure 3d), the similar peak

23

locations at 852.9 eV, 853.9 eV, 856.0 eV and 860.8 eV can be observed, respectively.

24

In addition, the Ni species in porous Co–Ni alloy also have similar chemical state

25

with Ni species in porous Ni. However, the metallic Ni content in porous Co–Ni alloy

26

has no obvious difference with that in porous Ni. The porous Co–Ni alloy with more

27

metallic Co has more superior conductivity, further has more superior intrinsic

28

activity than porous Co and Ni.

29

3.4 Anodic electrochemical characterization 9

1 2

Scheme 1. Illustration of the mechanism of anodic OER (a) and GOR (b),

3

respectively.

4

5 6

Figure 4. (a) LSV plots of Co/CC, Ni/CC and Co0.5Ni0.5/CC in the electrolyte of 1 M

7

KOH, respectively. (b) corresponding Tafel plots, respectively. (c) LSV plots of

8

Co/CC, Ni/CC and Co0.5Ni0.5/CC in 1 M KOH & 0.1 M glucose, respectively. (d)

9

corresponding Tafel plots, respectively.

10

In general, metals especially precious metals have superior HER activity due to the 10

1

appropriate hydrogen adsorption free energy. Metal oxides have superior OER activity

2

due to their superior adsorption capacity with the intermediates of O species. In this

3

work, the Co–Ni alloy is obtained with a low-temperature thermal reduction. The XPS

4

(Figure 3) or SAED (Figure S3) analysis demonstrates it is partial oxidized, making it

5

can synergistically serve as both cathodic HER and anodic OER or GOR. So, the NiO,

6

or CoO or Co-NiO compound at the surface of the catalysts can contribute to the

7

anodic OER or GOR, and the metals can contribute the cathodic HER during the

8

water electrolysis or glucose electrolysis, respectively. Scheme 1 illustrates the brief

9

mechanism of anodic OER and GOR in alkaline environment. The Co–Ni alloy is

10

abbreviated as CoNiOx as it is partial oxidized. The CoNiOx will be further oxidized

11

to CoNiOxOH in a relative high potential. In the case of OER (Scheme 1a), the OH-

12

will be oxidation to O2. As for GOR (Scheme 1b), glucose will be oxidation to

13

gluconic acid or gluconolactone[30] by the CoNiOxOH. In result, the CoNiOxOH will

14

be reduced back to CoNiOx in both OER and GOR. The anodic OER or GOR can be

15

briefly described with the following equation: CoNiOx + OH- + H2O → CoNiOxOH

16

+ e- (1); CoNiOxOH + OH- → CoNiOx + O2 + H2O or CoNiOxOH + C6H12O6 + OH-

17

→ CoNiOx + C6H12O7/C6H10O6 + e- (2).

18

To evaluate the anodic GOR or OER activity of these engineered porous metals, a

19

series of electrochemical tests are carried out. Figure 4a displays the LSV curves of

20

porous Co/CC, Ni/CC and Co0.5Ni0.5/CC in the electrolyte of 1 M KOH. All the LSVs

21

are performed after several cycles to get stable performance. In the case of porous

22

Co/CC and Ni/CC, the potential at the current density of 100 mA cm-2 (P100) is 1.650

23

V vs. RHE and 1.608 V vs. RHE, respectively. While a P100 of 1.585 V vs. RHE is

24

obtained toward Co0.5Ni0.5/CC, which is 65 mV and 23 mV lower than that of Co/CC

25

and Ni/CC, respectively. From the potential difference between porous Co or Ni and

26

Co–Ni alloy in Figure 4a, we can know that Co doping can improve the OER activity

27

by the alloy effect. In addition, a strong redox peak located at 1.29 V vs. RHE and a

28

weak redox peak located at 1.44 V vs. RHE are observed in porous Ni. They can be

29

indexed the conversion of Ni(0) to Ni(II) and Ni(II) to Ni(III), respectively. However,

30

the peak for Co/CC is disappeared, which can be ascribed to the irreversible reaction 11

1

between Co in low-valence state and high-valence state.[44] In the case of porous

2

Co-Ni alloy, the redox peak shifts to 1.25 V vs. RHE, indicating a smaller

3

thermodynamic barrier in the conversion of Ni species in different chemical state.

4

However, the kinetics of the porous Co–Ni alloy to drive OER has an insignificant

5

increase than the porous Co, which can be analyzed by the corresponding Tafel slopes

6

in Figure 4b. This is because the porous Co-Ni alloy even has higher Tafel slope (87

7

mV dec-1) than porous Co (82 mV dec-1). But it is still much lower than the porous Ni

8

(117 mV dec-1). When the electrolyte is added into 0.1 M glucose (1 M KOH & 0.1 M

9

glucose), a much higher current density can be delivered by the three porous metals in

10

a much lower potential than the region of OER (Figure 4c). The onset potentials of

11

GOR for the porous Co, Ni and Co–Ni alloy are 1.143 V vs. RHE, 1.138 V vs. RHE

12

and 1.096 V vs. RHE, respectively. These onset potentials are even lower than the

13

thermodynamic potential of water oxidation (1.23 V vs. RHE). Therefore, to drive the

14

constant current density, the anodic GOR indeed requires a much lower potential than

15

that of OER, especially in the low-potential region before OER has a response.

16

Furthermore, the Tafel slope of the porous Co and Ni have been increased to 175 mV

17

dec-1 and 125 mV dec-1, respectively. It can be ascribed to the high viscosity of the

18

glucose solution. However, the Tafel slope of the porous Co–Ni alloy still remains at a

19

very low level (88 mV dcc-1), suggesting an enhanced GOR kinetics even in a

20

high-viscosity glucose solution (Figure 4d).

21

In addition, Nyquist plots (Figure 5a) for all the three metals have a small

22

semicircle over the high-frequency region, following by a straight line over

23

low-frequency region. The radius of the semicircles over the high-frequency region

24

can be assigned to the charge transfer resistance (Rct). The slope pf the straight lines

25

over the low-frequency zone is corresponding to diffusive resistance (Warburg

26

impendence, W). All the materials exhibit small circle diameters and large slopes,

27

indicating low Rct and W, respectively. The relationships between Z and Frequency is

28

displayed in Figure 5b. The conductivity of metals can be recorded the DC resistances

29

at about a frequency of 10 kHz, which are 3.13 Ω, 2.65 Ω and 2.29 Ω, respectively. It

30

shows the porous Co–Ni alloy has the optimist conductivity among all the three 12

1

metals.

2 3

Figure 5. EIS. (a) Nyquist plots. (b) The relationship between Z and Frequency.

4 5

Figure 6. Column diagram of P20 and P100 in 1 M KOH and 1 M KOH & 0.1 M

6

glucose with Co/CC, Ni/CC and Co0.5Ni0.5/CC, respectively.

13

1 2

Figure 7. (a) CP tests at the current density of 10 mA cm-2 in 1 M KOH & 0.1 M

3

glucose for 32 h. (b) Corresponding potential at the current density of 10 mA cm-2

4

before and after CP tests. (c) CP tests at the current density of 10 mA cm-2 in 1 M

5

KOH & 0.1 M glucose for 32 h. (d) Corresponding potential at the current density of

6

10 mA cm-2 before and after CP tests.

7

To visually compare the potential difference in different electrolyte, corresponding

8

column diagrams of P20 and P100 are illustrated in Figure 6. In the case of OER, the

9

P20 for the porous Co, Ni and Co–Ni alloy is 1.591 V vs. RHE, 1.521 V vs. RHE and

10

1.519 V vs. RHE, respectively. The P100 increases to 1.650 V vs. RHE, 1.608 V vs.

11

RHE and 1.586 V, respectively. While in the case of GOR, he P20 for the porous Co,

12

Ni and Co–Ni alloy decreases to 1.290 V vs. RHE, 1.237 V vs. RHE and 1.172 V vs.

13

RHE, respectively. In the region of high current density, the each P100 is 1.563 V vs.

14

RHE, 1.355 V vs. RHE and 1.244 V vs. RHE, respectively. We can find out that the

15

porous Ni and Co–Ni alloy still have a leading GOR activity even in high-current 14

1

region due to the much lower potential than that in OER. However, the potential

2

difference in the two electrolytes is quite low in the case of the porous Co, indicating

3

a leading OER activity in high-current region. Furthermore, we can learn from the

4

diagram that the GOR activity of the porous Co–Ni alloy is enormously enhanced

5

even though the enhancement of OER is indistinctive.

6

The long-term stability is another an important parameter except the activity to

7

evaluate the electrocatalysts. The activity and stability are associated with the intrinsic

8

activity and the active sites of the electrocatalysts. To evaluate the stability of the

9

three engineered porous metals, CP test at a constant current electrolysis of 10 mA

10

cm-2 throughout the entire OER or GOR process is measured. Figure 7a and Figure 7c

11

display the CP tests at the current density of 10 mA cm-2 for 32 h using the porous Co,

12

Ni and Co–Ni alloy in 1 M KOH and 1 M KOH & 0.1 M glucose, respectively.

13

Obviously, the porous Co–Ni alloy delivers the smallest and steady potential to drive

14

the anodic OER and GOR. However, the porous Co displays a steep growth of the

15

potential in the latter period of OER and a rapid increasement in the earlier stage of

16

GOR. More narrowly, in the case of 1 M KOH, the initial potentials before CP test of

17

the porous Co, Ni and Co–Ni alloy are 1.594 V vs. RHE, 1.528 V vs. RHE and

18

1.526V vs. RHE, respectively (Figure 7b). After CP test for 32 h, the potentials

19

change to 1.679 V vs. RHE, 1.523 V vs. RHE and 1.491 V vs. RHE, respectively

20

(Figure 7b). In the case of 1 M KOH & 0.1 M glucose, the initial potentials before CP

21

test of the porous Co, Ni and Co–Ni alloy are 1.291 V vs. RHE, 1.248 V vs. RHE and

22

1.125 V vs. RHE, respectively (Figure 7d). After CP test for 32 h, the potentials

23

change to 1.590 V vs. RHE, 1.405 V vs. RHE and 1.328 V vs. RHE, respectively

24

(Figure 7d). We can know that even though the materials display superior stability in

25

OER except Co/CC, the potential differences between OER and GOR are very high

26

during the entire period of electrolysis. However, for the porous Co, the OER activity

27

get closer to the GOR activity with the electrolysis continuing. It can be ascribed to its

28

split away off the carbon fibers, while the Ni and the Co–Ni alloy still evenly cover

29

on the carbon fibers (Figure S4). In addition, the XRD after CP tests also shows no

30

signals of metallic Co, while the metallic Ni and Co–Ni alloy still can be observed 15

1

(Figure S5). However, the signals of metallic Co or Ni could not be detected in the

2

XPS (Figure S6) after CP test. Therefore, the surface of the Co–Ni alloy is completely

3

oxidized.

4

3.5 Cathodic electrochemical characterization

5 6

Figure 8. LSV plots of Co/CC (a), Ni/CC (b) and Co0.5Ni0.5/CC (c) in 1 M KOH and

7

1 M KOH & 0.1 M glucose, respectively. (d) LSV plots of Co/CC, Ni/CC and

8

Co0.5Ni0.5/CC in 1 M KOH & 0.1 M glucose, respectively.

9

In fact, we need to evaluate the influence of glucose on the cathodic HER activity

10

over the engineered porous metals. The corresponding results are displayed in Figure

11

7. For the HER process of the porous Co (Figure 8a) and Ni (Figure 8 b), negative

12

shifts can be observed in the region of low current density. In the electrolyte of 1 M

13

KOH, the overpotential at the current density of 10 mA cm-2 (η10) of the porous Co

14

and Ni is 206 mV and 110 mV, respectively. When the glucose is added into the

15

electrolyte, the corresponding η10 increases to 259 mV and 152 mV, respectively. In

16

the case of the porous Co–Ni alloy, the glucose nearly has no effect on the HER. The 16

1

corresponding η10 maintains at 104 mV in alkaline environment (Figure 7c). The

2

negative shift in the HER process is negligible in comparison with the great

3

promotion in the GOR process. In addition, the porous Co–Ni alloy displays the

4

optimal HER activity in 1 M KOH & 0.1 M glucose among the three metals.

5

3.6 Overall glucose electrolysis

6 7

Figure 9. (a, b) LSV plots of Co/CC, Ni/CC and Co0.5Ni0.5/CC in 1 M KOH and 1 M

8

KOH & 0.1 M glucose, respectively. (c, d) Corresponding column diagram of V10 and

9

V50. respectively.

10

From the anodic and cathodic electrochemical characterization, the engineered

11

porous Co–Ni alloy on carbon cloth has been confirmed to be an efficient and robust

12

bifunctional electrocatalyst toward HER and GOR in alkaline solution. Therefore, a

13

H-type electrolytic tank is used for electrolysis, in which the three materials are used

14

as both anode and cathode in alkaline media. In the case of 1 M KOH solution, all the

15

three metals need very large voltage to drive the water electrolysis (Figure 9a). As

16

expected, the porous Co–Ni alloy shows superior water electrolysis activity than that

17

of the porous Co and Ni. Figure 9b shows the corresponding column diagrams of the 17

1

cell voltage at the current density of 10 mA cm-2 (V10) and 50 mA cm-2 (V50) using

2

these engineered porous metals (Figure 9b). For the porous Co, Ni and Co–Ni alloy,

3

V10 is 1.74 V, 1.71 V and 1.65 V, respectively. The corresponding V50 reaches as high

4

as 1.92 V, 1.89 V and 1.79 V, respectively. The Co–Ni alloy has more active sites than

5

Co or Ni (Figure 2), the efficiency of glucose electrolysis is also higher than that of

6

Co or Ni. In detail, as the glucose nearly has no impact on the cathodic HER activity,

7

the efficiency of glucose electrolysis is mainly determined by the anodic current

8

density difference between GOR and OER. It also can be calculated based on the

9

equation as follow: η =

× 100%, in which, the η is the electrolytic

10

efficiency of glucose at one potential,

is the corresponding current density of

11

glucose electrolysis,

12

potential. Taking the potential of 1.5 V as example (Figure 9a and Figure 9b), the

13

corresponding the η of Co/CC, Ni/CC and Co0.5Ni0.5/CC are calculated as 65.8%, 79.9%

14

and 94.6%, respectively. Furthermore, toward glucose electrolysis in alkaline media,

15

the cell voltage to drive the water splitting is much lower than that in water

16

electrolysis (Figure 9c). The V10 and V50 of the porous Co–Ni alloy in glucose

17

electrolysis reduce to 1.39 V and 1.52 V, respectively (Figure 9d). It is much lower

18

than that with the current electrocatalysts in water electrolysis or organic molecular

19

electrolysis (Table S1). As for the porous Co and Ni, the V10 and V50 are 1.49 V, 1.67

20

V and 1.40 V and 1.60 V, respectively. Therefore, the porous Co–Ni alloy delivers the

21

smallest cell voltage, indicating the optimal activity among the three porous metals.

is the current density of water electrolysis at the same

18

1 2

Figure 10. CP tests without iR corrected at the current density of 10 mA cm-2 in 1 M

3

KOH & 0.1 M glucose for 12 h with Co/CC, Ni/CC and Co0.5Ni0.5/CC, respectively.

4

In addition, a slight increase in the cell voltage after CP test for 12 h is observed in

5

the case of Co–Ni alloy, suggesting an excellent activity and durability (Figure 10).

6

The cell voltage at the end of CP test is still as low as 1.61 V. It is worth to be noted

7

that the CP test is not iR corrected. The porous Ni, especially the porous Co delivers a

8

poor stability in glucose electrolysis. The corresponding cell voltages of the porous

9

Co and the porous Ni at the end of CP tests are 1.82 V and 1.70 V, respectively. The

10

cell voltage in glucose electrolysis is still much lower than that in water electrolysis

11

(Figure S4). The decay trend in glucose electrolysis is in accordance with the

12

durability in GOR. Therefore, the Co–Ni alloy can simultaneously improve the

13

activity and stability, even in the high-viscosity glucose solution.

14

Conclusions

15

In summary, a pioneer work has been done with glucose electrolysis. In which, we

16

have used engineered porous Co–Ni alloy on carbon cloth as a model bifunctional

17

electrocatalyst toward GOR and HER, displaying a robust and efficient overall water

18

splitting. A low cell voltage of 1.39 V to drive the current density of 10 mA cm-2 is

19

acquired in a two-electrode electrolytic tank in the electrolyte of 1 M KOH & 0.1 M

20

glucose. The potential is lower than that with the current electrocatalysts in water

21

electrolysis or organic molecular electrolysis. The numbers of active sites,

22

conductivity, intrinsic activity and stability are synergistically boosted by the 19

1

engineered porous Co–Ni alloy on carbon cloth. This work can open a new avenue

2

to boost water splitting with glucose electrolysis over the other excellent Co or

3

Nibased electrocatalysts.

4

Acknowledgement

5

This work is financially supported by the National Natural Science Foundation of

6

China (51702006), the Doctoral research project of Baoji University of Arts and

7

Sciences (ZK2017027), the Doctoral Scientific Research Starting Foundation of East

8

China University of Technology (DHBK2018039) and the Project of Educational

9

Commission of Jiangxi Province of China (GJJ180408).

10

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1. The glucose electrolysis requires a much lower cell voltage than that of water electrolysis. 2. The surface of the porous Co–Ni alloy on carbon cloth is partial oxidized, which can serve as bifunctional electrocatalysts in glucose electrolysis. 3. The numbers of active sites, conductivity, intrinsic activity and stability are synergistically boosted by the engineered porous Co–Ni alloy on carbon cloth.

Prof. Bing Na and Prof. Huiqin Li conceived and designed the study. Dr. Chong Lin analyzed the data. Dr Chong Lin, Pan Zhang, Shengying Wang, Qiaoli Zhou wrote the paper. Yu Zhang, Cui Deng, Liqing Meng, Jiaxin Wu, Chengzhi Liu, Junyuan Hu, Limin Zhang performed the experiments. Dr. Jingyang Tian reviewed and edited the manuscript. All authors read and approved the manuscript.

The authors declare no conflict of interest.