Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High

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A Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High-Efficiency Water Splitting Yanqun Tang, Xiaoyu Fang, Xin Zhang, Gina Fernandes, Yong Yan, Dongpeng Yan, Xu Xiang, and Jing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10338 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

A Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High-Efficiency Water Splitting Yanqun Tang, † Xiaoyu Fang, ‡ Xin Zhang, † Gina Fernandes, § Yong Yan, § Dongpeng Yan,‡* Xu Xiang †*, Jing He † †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beijing 100029, People’s Republic of China ‡

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education,

College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China §

Department of Chemistry and Environmental Science, New Jersey Institute of Technology,

University Heights, Newark, New Jersey 07102, United States of America

KEYWORDS: water splitting, bifunctional electrocatalyst, space-confined growth, layered double hydroxide, monolithic catalyst

ABSTRACT Hydrogen generation from water splitting could be an alternative way to meet increasing energy demands while also balancing the impacts of energy being supplied by fossilbased fuels. The efficacy of water splitting strongly depends on the performance of

ACS Paragon Plus Environment

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electrocatalysts. Herein, we report a unique space-confined earth abundant electrocatalyst having bi-functionality of simultaneous hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), leading to high-efficiency water splitting. Outperforming Pt/C or RuO2 catalysts, this mesoscopic, space-confined, bifunctional configuration is constructed from a monolithic zeolitic imidazolate framework@layered double hydroxides (ZIF@LDH) precursor on Ni foam. Such a confinement leads to a high dispersion of ultrafine Co3O4 nanoparticles within the Ndoped carbon matrix by temperature-dependent calcination of the ZIF@LDH. We demonstrate that the OER has an overpotential of 318 mV at a current density of 10 mA cm-2, while that of HER is -106 mV at -10 mA cm-2. The voltage applied to a two-electrode cell for overall water splitting is 1.59 V to achieve a stable current density of 10 mA cm-2 while using the monolithic catalyst as both the anode and the cathode. It is anticipated that our space-confined method which focuses on earth-abundant elements with the structural integrity may provide a novel and economically sound strategy for practical energy conversion applications.

INTRODUCTION The over-dependence of fossil fuels and their potential depletion possibility have continually aroused worldwide concerns about the environmental problems and the energy issues. These concerns have stimulated extensive research to pursue renewable and clean energy alternatives to compensate for the utilization of fossil-based fuels throughout recent decades. Because H2 fuel is consumed with zero emission and minimal environmental impact, electrochemical or photoelectrochemical (PEC) splitting of water offers a promising strategy to producing clean fuel. The efficiency of water splitting strongly depends on catalysts, which promote two


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associated half-reactions, i.e. HER and OER. The OER is significantly hurdled by its unfavorable thermodynamic barriers and sluggish kinetics, which involve a multi-protoncoupled, four-electron process.1-3 Consequently, the OER is typically identified as a bottleneck of the overall water splitting. And in most cases, OER catalysts are designed to be effective in basic condition.4-6 For example, transition-metal oxides and layered double hydroxides (LDHs), such as NiFeOx,4 Co3O4,5 Mn3O4,6 ACo2O4 (A: Ni, Cu, Zn),7 NiO/CoO/Fe2O3,8 NiFe-LDHs,9 and Zn/Co hydroxy sulfate

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were reported to be OER active in alkaline electrolytes. Although less

challenging, HER catalysts are still nontrivial but are normally active in acid solution. Transition-metal sulfides, carbides, and phosphides have shown promising HER catalytic performance in acidic electrolytes.11-14 Some noticeable exceptions are noted. For example, a HER catalyst on GaInP2 photoelectrode was reported to be highly active in a basic aqueous solution.15 In addition, a cobalt-based bimetallic sulfide polyhedron was found to possess superior HER activity in an alkaline electrolyte.16 It is of great desire to construct a bifunctional catalyst having simultaneous HER and OER activities operating under the consistent electrolyte in a one-compartment electrolysis cell.17-19 So far, limited bifunctional catalysts have been reported to be effective in overall splitting of water in this regards, e.g. Co-, Ni-, Fe- and Mn-based materials.20-25 Surface engineering the catalysts is a vital way to enhance overall catalytic activity via dispersing and exposing active sites. In this context, metal–organic frameworks (MOFs), could be promising catalysts. MOFs are best described as a complex in which metal ions are coordinated to ligands forming 1D, 2D, or 3D structures, which possess a high dispersion of active sites.26 MOFs as templates have been demonstrated to derive a variety of nano-structured carbon/metal oxides for high-efficiency water splitting catalysts.26-29 Lately, Jiang and co-workers reported template-directed MOF


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synthesis and the derivatives showed superior electrochemical water splitting capability, highlighting further the advantages of MOFs as catalyst precursors.30 The ordered arrangement of metal centers and C, N-containing ligands in a MOF precursor leads to high dispersion and efficient anchoring of metal/metal oxides into carbon or N-doped carbon matrix subject to calcination. However, MOFs themselves are mostly synthesized in the form of powder. Consequently, the direct transformation of MOFs into their derivatives generally results in the catalysts being in the form of a powder or a particle. Thus, it requires a coating or binding process to fabricate an electrode applicable in electrochemical water splitting. As a result, a structured catalyst derived from precursors to MOFs is highly desired. Layered double hydroxides i.e., LDHs consist of alternatively distributed divalent and trivalent cations in the layers and charge-compensating anions in the interlayer.31 LDHs exhibit excellent OER catalytic ability.32-34 The array of LDH structures have shown enhanced catalytic activity and stability.35,36 Nevertheless, to design functional materials by using the mesoscopic space of LDH arrays has not yet been reported. Herein, we design a 3D mesoscopic spaceconfined ZIF@LDH structure that enables superior bifunctional catalytic ability for simultaneous activation of OER and HER in a basic media. The stability of such designed materials also guarantees the durability of this catalytic system for the complete water splitting process. Our strategy to successfully synthesize this superior bifunctional catalyst with ability for overall water splitting is realized via the following. First, matching the size of ZIFs to the channels of the LDH nanowalls allows the ZIF particles to strongly anchor into the 3D space of the channels. Second, after simultaneous topological transformation of ZIFs and LDHs to metal oxides@Ndoped carbon, a structured catalyst was derived on the framework of nickel foam showing superior OER and HER activity in basic media. The space-confined growth strategy is


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responsible for both the high dispersion of metal oxides into the carbon matrix as well as the structural integration of multi-active components.

RESULTS AND DISCUSSION The ZIF@LDH@Ni foam-x electrodes (x refers to the calcination temperature) were constructed following four consecutive steps (Scheme 1). The detailed procedure was stated in the Experimental Section. Briefly, the ZnCo-LDHs nanowalls were first prepared onto the surface of the Ni foam by an electrodeposition method, similar to our previous work.37 Subsequently, ZIF67 was nucleated within the channels of the LDH nanowalls by a space-confined solution method. The ZIF-67 particles were further aged and anchored into the channels of the LDHs, where the size of ZIF-67 matched the channels well. Finally, the ZIF@LDH@Ni foam was carbonized at different temperatures (550, 600 or 650°C) in an inert atmosphere to delicately tune the properties of the derived catalysts. The space-confined growth of ZIF-67 and the subsequent carbonization favor the stabilization of catalysts onto the Ni foam substrate, which is promising from an engineering prospective.


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Scheme 1. Synthesis illustration of the ZIF@LDH@Ni foam-x electrode (x refers to the calcination temperature) SEM and TEM observations revealed the morphologies and structure of products. The asprepared LDHs showed an array structure consisting of vertically-aligned nanowalls with an average gap of 200-300 nm surrounded by the nanosheets (Figure 1A, B). After the growth of ZIF-67 from the solution phase, the regular dodecahedrons appeared, which were uniformly and densely anchored onto the underlying LDH array (Figure 1C, D). The size of aged ZIF-67 particles were measured ca. 200-300 nm, which makes a perfect match with the gaps of the LDHs. It was observed that the newly-born ZIF nanoparticles dropped into the channels of the LDH nanowalls at the initial growth stage (1 min, Figure S1). As we expected, the subsequent carbonization of the ZIF@LDH@Ni foam changed the overall structure of ZIF-67 dodecahedrons. As shown in (Figure 1E, F), the dodecahedrons wrinkled and further fused with the calcined LDHs. This is due to the shrinking and collapse of the framework of ZIF-67 upon the high-temperature calcination.


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Figure 1. SEM images of (A), (B) LDH@Ni foam, (C), (D) ZIF@LDH@Ni foam, (E), (F) ZIF@LDH@Ni foam-600 High-resolution TEM (HRTEM) observations showed that the ultrafine Co3O4 nanoparticles (3-5 nm) were dispersed in the carbon matrix. The lattice images presented the d-spacings of 0.24 nm and 0.34 nm, which correspond to plane (311) of Co3O4 and plane (002) of graphic carbon, respectively (Figure 2A).38,39 This provides evidence that the calcination made the ZIF@LDH transform to metal oxides (Co3O4) and the ZIF-67 carbonize. The confined growth favored the dispersion and embedment of Co3O4 into the surrounding carbon layer, enhancing


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the activity towards HER or OER.40-42 The energy dispersive X-ray spectroscopy (EDS) mappings of ZIF@LDH@Ni foam-600 were conducted to verify the elemental distributions (Figure 2B), which showed C, N, O, Co and Zn elements were highly dispersed throughout the structure. The C and N come from the ZIF-67, while the Co could come from the ZIF-67 and/or LDHs. It was found that the morphologies of ZIF@LDH@Ni foam-x samples were affected by the calcination temperature (Figure S2). At a lower temperature, the ZIF@LDH@Ni foam-550 nearly maintained the similar morphology (Figure S2B) to that of ZIF-67 (Figure S2A). With the calcination temperature increased to 600 oC, the ultrafine, highly-dispersed nanoparticles appeared, and the framework appearance of ZIF-67 was partially destroyed (Figure S2C). When the temperature rose to 650 oC, the framework of ZIF-67 collapsed completely and the ultrafine nanoparticles were hardly observed (Figure S2D).

Figure 2. (A) HRTEM lattice image and (B) EDS mappings of ZIF@LDH@Ni foam-600 The chemical environment and valence states were detected by X-ray photoelectron spectroscopy (XPS) technique. The O1s spectra of LDH@Ni foam and ZIF@LDH@Ni foam600 can be fitted into three deconvoluted peaks (Figure 3A). These peaks originate from the O2− at 530.8 eV, OH− at 531.6 eV and adsorbed water at 532.9 eV.43 The binding energy (B.E.) hardly shifted in the two samples. However, the peak area ratio of OH−/O2− decreased from 1.4


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in LDH@Ni foam to 0.9 in ZIF@LDH@Ni foam-600, suggesting a decrease of OH− species in the latter. This is due to dehydroxyl of the LDHs containing abundant surface hydroxyl (OH-) and transformation to metal oxides during calcination. The Co2p spectrum of ZIF@LDH@Ni foam-600 is deconvoluted into three peaks in the Co2p3/2 range (Figure 3B). The ones at 781.8 and 780.0 eV were assigned to the Co3+ and Co2+ species, respectively, which implies the presence of Co3O4.43 The Zn2p spectrum displayed two peaks at 1044.9 and 1021.8 eV (Figure S3) with a line separation value of 23.1 eV, as the characteristics of wurtzite ZnO.44 Furthermore, the C1s spectrum showed three contributions (Figure 3C). The peaks at 288.3, 285.9 and 284.6 eV can be assigned to the contributions of C-N bond, the C atoms directly bonded to oxygen in the OH configuration (i.e. C-OH), and the graphitic carbon, respectively.45 The EDS analyses confirmed the presence of nitrogen. The binding states of N were further studied from N1s spectrum analyses (Figure 3D). The contribution at 400.6 eV was ascribed to graphitic nitrogen species, that is to say, the nitrogen atom is incorporated into the graphene layer and is bonded with three sp2 carbon atoms. The other at 398.5 eV was assigned to the pyridinic nitrogen species, i.e., one nitrogen atom bonded to two carbon atoms in a hexagonal ring.45


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Figure 3. (A) XPS O1s spectra of LDH@Ni foam and ZIF@LDH@Ni foam-600, (B) Co2p spectum, (C) C1s spectrum, and (D) N1s spectrum of ZIF@LDH@Ni foam-600 The quantitative analyses indicate that the calcination temperature has a critical impact on the evolution of elements in the ZIF@LDH@Ni foam-x. The total nitrogen content gradually decreased with the increasing calcination temperature, possibly due to the high-temperature leaching of elements (Table 1). The graphitic N/pyridinic N ratio was found to increase with the elevated temperature (Figure S4). The ZIF@LDH@Ni foam-600 contained a larger percentage of graphitic nitrogen (3.45%) than ZIF@LDH@Ni foam-550 (1.82%) and ZIF@LDH@Ni foam650 (2.77%). The percentages were calculated according to both the total nitrogen content and the graphitic N/pyridinic N ratio in the samples (Table 1). The theoretical studies have indicated that the graphitic nitrogen tends to preserve the high mobility of charge carriers as a result of


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direct substitution of carbon in the graphene, whereas the pyridinic nitrogen has a lower charge transfer rate due to the formation of vacancy defects.45,46 Therefore, the ZIF@LDH@Ni foam600 with the highest graphitic nitrogen ratio could be anticipated to show better catalytic activity in the electrolysis of water, where charge transfer is a key factor to impact the activity. Table 1. Elemental analyses of ZIF@LDH@Ni foam-x and ZIF-67 N wt%

Graphitic N wt%

C wt%

O wt%

Co wt%

ZIF@LDH@Ni foam-550

18.16

1.82

36.55

11.17

29.54

ZIF@LDH@Ni foam-600

13.26

3.45

34.37

15.48

33.80

ZIF@LDH@Ni foam-650

9.54

2.77

33.80

18.32

35.12

ZIF-67

20.28

-

44.64

22.65

12.42

The HER activity of ZIF@LDH@Ni foam-600 was assessed in an aqueous solution of KOH (1 M) in a three-electrode cell. ZIF@LDH@Ni foam, LDH@Ni foam and Pt/C cast onto Ni foam were prepared for comparison. The data measured were subject to iR correction to minimize the effect of ohmic resistance.47 From the polarization curves (Figure 4A), the ZIF@LDH@Ni foam-600 exhibited the smallest onset potential for HER in contrast to LDH@Ni foam and ZIF@LDH@Ni foam. The overpotential@-10 mA cm-2 is -91, -106, -176 and -214 mV for Pt/C, ZIF@LDH@Ni foam-600, LDH@Ni foam and ZIF@LDH@Ni foam, respectively. The overpotential@-10 mA cm-2 of ZIF@LDH@Ni foam-600 is even superior to excellent Cobased and MOF-derived bifunctional catalysts, e.g. the porous N-rich carbon/Co (-298 mV),39 CoOx@CN (-230mV),40 CoFe LDH/Ni foam (-255 mV),48 Ni@CoO@CoNC (-190 mV),30 Ni0.5Co0.5/nitrogen-doped carbon (-176 mV) 49 and CoP/reduced graphene oxide (-150 mV).50 Generally, the high HER activity could be attributed to metallic Co and cobalt oxide39,40 or carbon-based materials incorporated with cobalt or nickel species.49 For instance, the CoOx@CN


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catalyst reported by Jin et al was composed of Co0, CoO, Co3O4 and N-doped carbon.40 It is considered that the high activity of CoOx@CN for HER originated from the conductive feature of carbon, the synergy of cobalt and cobalt oxide, and electron-rich nitrogen. In our case, the synergy of Co3O4 and graphitic nitrogen could play a major role in high activity for HER. The HER activity is enhanced in the presence of ultrafine Co3O4 particles and with increasing content of graphitic nitrogen. Tafel plots of ZIF@LDH@Ni foam-600 and the controls were shown in Figure 4B. The linear region of the plots were fitted using the Tafel formula Z = blogj + a, where Z refers to overpotential, j refers to current density and b refers to Tafel slope. The Tafel slope of ZIF@LDH@Ni foam-600 (-109 mV dec-1) is smaller than that of ZIF@LDH@Ni foam (-146 mV dec-1) and LDH@Ni foam (-136 mV dec-1). The Pt/C sample showed the smallest Tafel slope (-102 mV dec-1) as expected. It is believed that the HER process might follow either the Volmer-Tafel pathway (1) or the Volmer−Heyrovsky one (2) under basic conditions.51 Both pathways include the adsorption of a water molecule, the dissociation of adsorbed water to be adsorbed H atom and OH−, desorption of OH−, and the transformation of adsorbed H into H2. H2O + e-→Hads + OH- (Volmer) and Hads + Hads→ H2 (Tafel)

(1)

H2O + e-→Hads + OH- (Volmer) and Hads + H2O + e- → H2 + OH- (Heyrovsky)

(2)


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-10

A -0.4

Potential (V vs. RHE) -0.3

-0.2

-0.1

0.0

B

0.1 0.05

ZIF@LDH@Ni foam-600 ZIF@LDH@Ni foam LDH@Ni foam Pt/C

Potential (V vs. RHE)

-2

-20 -30 -40 -50

0.00

ZIF@LDH@Ni foam-600 ZIF@LDH@Ni foam LDH@Ni foam Pt/C

-1

-102 mV dec

-0.05 -1

-0.10

-109 mV dec

-1

-136 mV dec

-0.15 -1

-146 mV dec

-0.20

-60 -1.5

0

C -0.4

-1.4

-1.3

-1.2

-1.1

-0.3

-0.2

-0.1

0.0

-2

-30 -40 -50 -60 -1.3

-1.2

-2.4

-2.2

-2.0

-1.8

-1.6

D -2

-20

-1.4

-2.6

-2

0.1

st

-1.5

-2.8

Log (j / mA cm )

1 cycle th 500 cycle

-10

-3.0

-1.0

Potential (V vs. SCE) Potential (V vs. RHE) Current density (mA cm )

Current density (mA cm )

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.1

-1.0

0 -5 -10 -15 -20 -25 -30 0

1

Potential (V vs. SCE)

2

3

4

5

Time (h)

Figure 4. (A) HER polarization curves and (B) Tafel plots of ZIF@LDH@Ni foam-600 and the controls. (C) Polarization curves of the ZIF@LDH@Ni foam-600 in the 1st and the 500th cycles of CV. Scan rate: 2 mV s-1, 1 M KOH, pH=14. (D) Time-dependent current density curve of the ZIF@LDH@Ni foam-600 under a static potential of -120 mV vs. RHE for 5 hours. The initial data measured were iR corrected in the experiments to minimize the effect of ohmic resistance. As previously reported, Co3O4 could be hydroxylated into dissociate water.52 Herein, the OH− ions generated by decomposition of H2O could preferentially attach to the surface of Co3O4 due to their intensive electrostatic affinity to the positively charged Co2+ and Co3+ species. The HER mechanism in a basic media was not clearly understood based on the literature’s


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reports.40,53 Nevertheless, the Tafel slopes reveal that HER might be dominated by a Volmer−Heyrovsky pathway (2), where the adsorbed H atom (Hads) reacts with water molecule to generate H2 and release OH-. It was recognized that the HER could follow a Volmer−Heyrovsky mechanism when the Tafel slopes do not match the value of 39 mV dec-1.5355

In addition, the LDH@Ni foam-600 without the involvement of ZIF was compared, which

showed far lower activity (an overpotential of -164 mV@-10 mA cm-2) than ZIF@LDH@Ni foam-600. Also, the HER of the ZIF-600 and ZIF+LDH@Ni foam-600 without the confinement of 3D LDH framework were inferior to that of the ZIF@LDH@Ni foam-600 (Figure S5, S6A). This indicates that neither LDH- nor ZIF-derived products are capable of contributing high HER activity and the confined growth strategy is critical to the high-efficiency. Compared to ZIF@LDH@Ni foam-550 and ZIF@LDH@Ni foam-650, ZIF@LDH@Ni foam-600 exhibited the highest HER activity (Figure S7A) due to having the largest ratio of graphitic nitrogen in the catalyst. The durability of the ZIF@LDH@Ni foam-600 for HER was assessed by continuous cyclic voltammetric (CV) at a scan rate of 2 mV s-1 for 500 cycles (Figure 4C). The CV curves showed little change between the 1st and the 500th scan. In addition, the HER stability of the ZIF@LDH@Ni foam-600 was further evaluated under an overpotential of -120 mV. The initial current density was at ~12 mA cm-2 and had a ~15% decrease after continuous electrolysis of 5 hours (Figure 4D). The fluctuation of the curve originated from the evolution of H2 bubbles from the electrode surface. The HRTEM and SEM observation showed that the ZIF@LDH@Ni foam-600 remained the original microstructure even after long-term HER testing (Figure S8). These findings suggested that the ZIF@LDH@Ni foam-600 possesses excellent HER activity and durability under alkaline conditions. The amount of H2 evolution was also detected by gas


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chromatography (Figure S9). The theoretical H2 yield was obtained according to the quantity of the electrons transferred. And the Faradaic yield for hydrogen production is >90%, indicating that the current generated is mostly ascribed to HER. It is believed that the oxygen-evolving half-reaction is more challenging because it is a kinetically-sluggish, multi-proton coupled four-electron process accompanied by the breaking of O–H bond and formation of O=O bond. The as-synthesized catalysts were also evaluated in the OER under the same alkaline conditions (1M KOH). The ZIF@LDH@Ni foam-600 exhibited high OER activity having an overpotential of 318 mV at 10 mA cm-2 (Figure 5A). The superior OER activity is due to the highly dispersed ultrafine Co3O4 particles in the N-doped carbon matrix (Figure 2). It is extensively accepted that the spinel-type Co3O4 is an efficient catalyst towards OER.5,56,57 In addition, the OER activity could be improved by embedding the cobalt species into carbon or N-doped carbon matrix.39,40 In contrast, the LDH@Ni foam showed much lower activity. This suggests that the Co-hydroxide grown on the Ni foam is less active although several types of LDHs have been reported to be effective OER catalysts e.g. NiFe-LDHs in the form of powder or array. Another control ZIF@LDH@Ni foam hardly showed OER activity even at a potential of 1.6 V or above. This is an indication that the ZIF-67 itself is less catalytically active towards OER. The ZIF@LDH@Ni foam-600 is even more active than the RuO2 catalyst, which is used as a benchmark in the OER. In addition, the control LDH@Ni foam-600 was measured, which showed much lower activity (an overpotential of 415 mV@10 mA cm-2) than ZIF@LDH@Ni foam-600. Also, the OER performance of the ZIF-600 and ZIF+LDH@Ni foam-600 were inferior to that of the ZIF@LDH@Ni foam-600 (Figure S6B, S10). This indicates that the confined growth is the key to the high-performance catalyst. Neither LDH- nor ZIF-derived products are high-efficiency OER catalysts. That is to say, the synergy of


15


Co3O4 and N-doped carbon could be responsible for high OER activity, which corresponds to several recent findings in the literatures.5,40,57 In addition, ZIF@LDH@Ni foam-600 has the highest OER activity in contrast to ZIF@LDH@Ni foam-550 and ZIF@LDH@Ni foam-650 (Figure S7B). It is noted that the ZIF-67 cannot directly be deposited on Ni foam without the support and confinement of LDHs. Therefore, the 3D LDH array is required for constructing the high performance structured bifunctional catalyst.

A

1.3

1.4

1.5

1.6

2.0

ZIF@LDH@Ni foam-600 ZIF@LDH@Ni foam LDH@Ni foam RuO2

80 70

Potential (V vs. RHE )

-2


90

B

Potential (V vs. RHE)

1.2 100

60 50 40 30 20 10

ZIF@LDH@Ni foam-600 ZIF@LDH@Ni foam LDH@Ni foam RuO2

1.9

-1

185 mV dec

1.8

-1

170 mV dec

1.7

-1

128 mV dec

1.6

-1

97 mV dec

1.5

0 0.2

0.4

0.5

-2.0

0.6

1.3

1.5

70 60 50 40 30 20 10 0 0.3

0.4

0.5

-1.2

20

st

0.2

-1.4

D

1.6

1 cycle th 500 cycle

80

-1.6

-2

-2

90

1.4

-1.8

Log (j / mA cm )


C

-2

0.3

Potential (V vs. SCE) Potential (V vs. RHE)

1.2 100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

18 16 14 12 10 8 6 4 0

1

Potential (V vs. SCE)

2

3

4

5

Time (h)

Figure 5. (A) OER polarization curves, (B) Tafel plots of ZIF@LDH@Ni foam-600 and the controls. (C) Polarization curves of ZIF@LDH@Ni foam-600 in the 1st and the 500th cycles of CV. Scan rate: 2 mV s-1, 1 M KOH, pH 14. (D) Current density-time curve of the


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ZIF@LDH@Ni foam-600 at a static potential of 1.56 V vs. RHE for 5 hours. The initial data measured were iR corrected in the experiments to minimize the effect of ohmic resistance. In comparison, the OER performance of the ZIF@LDH@Ni foam-600 is superior to Cobased or MOF-derived bifunctional catalyst ever reported, e.g. porous N-rich carbon/Co (370 mV@10 mA cm-2),39 cobalt-phosphorous-derived films (345 mV@10 mA cm-2),21 CoOx@CN (340 mV@10 mA cm-2),40 cobalt poly-phosphide (overpotential of 343 mV@10 mA cm-2)58 and carbonized ZIF-8@ZIF-67 (340 mV@10 mA cm-2),59 and also comparable to the excellent MOF-derived Ni@CoO@CoNC catalyst (309 mV@10 mA cm-2)30 (Table 2). Typically, Ma and co-workers reported the hybrid Co3O4-carbon porous nanowire arrays as the OER catalyst, which was derived from Co-based MOFs.5 It is proven that the homogeneous distribution of metal oxide nanoparticles and in-situ formed carbon species contributed to the high OER activity. As a consequence, the highly dispersed Co3O4 nanoparticles and the carbon matrix from the ZIF@LDH@Ni foam precursor can be anticipated to contribute to the high OER activity. In addition, the bifunctional catalyst was grown on Ni foam with an integrated structure. The Ni foam could provide high surface area for catalyst growth and allow exposing active sites more efficiently, thus contributing high current densities in HER/OER. The porous characteristic of Ni foam favors the evolution and release of gases (H2 or O2) produced from the electrode surface. Table 2. Comparison of HER and OER activity of various bifunctional catalysts OER Catalysts

HER

Two-electrode system

η (mV)@10 mA cm-2 η (mV)@-10 mA cm-2 E (V)@10 mA cm-2

Refs

NiCo2S4 nanowire

260

-210

1.63

60

Porous N-rich carbon/Co

370

-298

1.64

39

CoFe LDH@Ni Foam

260

-166

1.63

48


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Ni@CoO@CoNC

309

-190

-

30

CoOx@CN

260

-230

~1.4

40

Ni0.5Co0.5/nitrogen-doped carbon 300

-176

1.75

49

CoP/reduced graphene oxide

-150

-

50

Cobalt-phosphorous-derived films 345

-94

-

21

Cobalt poly-phosphide

343

-78

-

58

ZIF@LDH@Ni foam-600

318

-106

1.59

This work

340

The Tafel slope of ZIF@LDH@Ni foam-600 was 97 mV dec-1, smaller than the value of ZIF@LDH@Ni foam (185 mV dec-1), LDH@Ni foam (170 mV dec-1), and even RuO2 (128 mV dec-1) (Figure 5B). This strongly suggests that the ZIF@LDH@Ni foam-600 has favorable kinetics towards OER. The durability of ZIF@LDH@Ni foam-600 was evaluated by continuous sweeps of CV (scan rate 2 mV s-1, 1 M KOH solution) (Figure 5C). The polarization curve showed very limited decay after 500 cycles. Furthermore, the stability was tested at 330 mV overpotential and the current density remained at ~11.5 mA cm-2 for a period of 5 hours (Figure 5D). The HRTEM and SEM images indicated that the microstructure of ZIF@LDH@Ni foam600 showed little change after long-term OER testing (Figure S11). The amount of O2 evolution was measured using an O2 fluorescent probe at an overpotential of 330 mV (Figure S12). The theoretical O2 amount was calculated according to the number of the transferred electrons. And the Faradaic yield for O2 production is nearly 100%, confirming that the current generated is due to OER. As a consequence, the ZIF@LDH@Ni foam-600 is a highly active electrocatalyst in both HER and OER. The overall splitting of water was tested in a two-electrode configuration cell using ZIF@LDH@Ni foam-600 as both an anode and a cathode (1 M KOH). It only requires a cell voltage of 1.59 V to reach a current density of 10 mA cm-2 (Figure 6), lower than the values of


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Pt/C (anode) // RuO2 (cathode), RuO2 (anode) // RuO2 (cathode) and Pt/C (anode) // Pt/C (cathode). Also, the voltage is lower than those of Co-based and MOF-derived bifunctional catalysts, e.g. porous N-rich carbon/Co (1.64 V),39 NiCo2S4/carbon cloth (1.68 V),60 CoSe/Ti mesh (1.65 V)61 and CoFe LDH (1.63 V).48 The structured electrode also exhibited excellent stability for water splitting in an electrolyzer of two-electrode configuration (Figure S13, S14).

Figure 6. LSV plots of overall water splitting at ZIF@LDH@Ni foam-600 // ZIF@LDH@Ni foam-600, Pt/C // RuO2, RuO2 // RuO2 and Pt/C // Pt/C as cathode and anode, respectively. Scan rate: 2 mV s-1, 1 M KOH, pH 14. To better understand the behavior of charge transfer, the electrochemical impedance spectra (EIS)

tests

were

carried

out.35

The

charge-transfer

resistances

(Rct)

across

the

electrode/electrolyte interface were obtained from the Nyquist plots (Figure S15).35, 62 The Rct value of ZIF@LDH@Ni foam-600 (375 Ω) is much smaller than that of LDH@Ni foam (1137 Ω) and ZIF@LDH@Ni foam (1764 Ω). The ZIF@LDH@Ni foam has the largest Rct value,


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indicative of the poor electrical conductivity of ZIF itself. The decreased Rct value is favorable to rapid charge transport, which is significant for high activity in both HER and OER.

Figure 7. The capacitive currents at 0.66 V for ZIF@LDH@Ni foam-600, ZIF@LDH@Ni foam and LDH@Ni foam. Conditions: 1 M KOH, pH 14. To obtain insight into the enhanced electrochemical performance of ZIF@LDH@Ni foam600 in comparison with the controls, the electrochemically active surface areas (ECSA) were measured and estimated based on the electrochemical double-layer capacitance (Cdl). This was conducted by collecting CV in a non-Faradaic potential range of 0.608 ~0.708 V vs. RHE (Figure S16). It is extensively recognized that the ECSA of the electrode materials is proportional to their Cdl values,63 which is derived from the linear slope of plots of the current density vs. scan rate (Figure 7). It is unambiguous that the Cdl value of the ZIF@LDH@Ni foam-600 reached 17 mF cm−2, 3 times and 12.5 times, respectively, higher than that of the LDH@Ni foam-600 (5.43 mF cm−2) and that of the ZIF@LDH@Ni foam (1.36 mF cm−2). The higher ECSA of the ZIF@LDH@Ni foam-600 revealed that the 3D hierarchically porous


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configuration could facilitate gas penetration and release during water splitting. Consequently, the enhanced exposure and the better utilization of active sites (i.e. Co3O4 and graphitic nitrogen species) on the large electrochemical active surface greatly contributed to the high catalytic activity.

CONCLUSION Overall, we have designed an earth-abundant element bifunctional electrocatalyst for highly efficient overall splitting of water. Our bifunctional configuration is realized via a 3D confined growth method leading to hierarchical ZIF@LDH matrix. This matrix provides a perfect structure for the catalytic architecture of monolithic Co3O4@N-doped carbon. HRTEM and EDS mapping analyses verify the formation of ultrafine Co3O4 particles (3~5 nm) and the homogeneous distributions of the elements. XPS characterizations confirm the content of graphitic N, which affects the catalytic activity. The OER and HER performances of the electrocatalysts are greatly optimized by delicately adjusting the dispersion of Co3O4 nanoparticles and the graphitic nitrogen species at varied calcination temperatures. The OER and HER overpotentials achieved 318 mV and -106 mV, respectively, at current density of 10 and 10 mA cm-2. Furthermore, the cell voltage was 1.59 V@10 mA cm-2 during the overall splitting of water in a two-electrode electrolysis cell, which is even superior to the commercial RuO2 and Pt/C counterparts. This work inspires a newly monolithic bifunctional catalyst for overall splitting of water, which could be an elegant example for designing high-efficiency electrocatalysts through a microstructure-tunable and space-confined MOF@LDH functional precursor.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details, Additional SEM and TEM images, XPS spectra, HER and OER polarization curves, HRTEM lattice and SEM images before and after HER/OER reactions, Time-dependent current density curves, Polarization curves of overall water splitting, Nyquist plots, Cyclic voltammograms AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. * E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the 973 Program (Grant 2014CB932104), the National Natural Science Foundation of China (Grant 21576016, U1507202), the Innovative Research Groups of National Natural Science Foundation of China (Grant 21521005), the Beijing Natural Science Foundation (Grant 2152022).

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(60) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661-4672. (61) Liu, T.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. An Amorphous CoSe Film Behaves as an Active and Stable Full Water-Splitting Electrocatalyst under Strongly Alkaline Conditions. Chem. Commun. 2015, 51, 16683-16686. (62) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes. Energy Environ. Sci. 2012, 5, 7626-7636. (63) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477-4485.


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Table of Contents Graphic and Synopsis

A monolithic Co3O4@N-doped carbon bifunctional catalyst was constructed by a unique 3D space-confined growth strategy, and exhibited impressive catalytic activity and structural integrity towards electrochemical overall water splitting.


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Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High

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