Nitrogen-Doped Carbon Composites for

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ZIF-67 Derived Cobalt/Nitrogen-Doped Carbon Composites for Efficient Electrocatalytic N2 Reduction Yunnan Gao, Zishan Han, Song Hong, Tianbin Wu, Xin Li, Jieshan Qiu, and Zhenyu Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01135 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Energy Materials

ZIF-67

Derived

Cobalt/Nitrogen-Doped

Carbon

Composites for Efficient Electrocatalytic N2 Reduction Yunnan Gao,a Zishan Han,a Song Hong,*,b Tianbin Wu,c Xin Li,a Jieshan Qiua and Zhenyu Sun*,a,d a

State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing

University of Chemical Technology, Beijing 100029, P. R. China. E-mail: [email protected] b

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing

100029, P. R. China. E-mail: [email protected] c

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and

Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China d

Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical

Technology, Beijing 100029, P. R. China

ABSTRACT: Electrochemical N2 reduction (ENR) provides a potential approach for NH3 synthesis. To facilitate ENR, the development of naturally abundant, cheap, and effective electrocatalysts is critically important. Herein we report Co/nitrogen-doped carbon composites comprising a large number of single Co sites for efficient N2 electrofixation at ambient conditions. The N configurations and Co species in the catalysts are readily tunable by controlling calcination temperatures. Such low-cost catalysts enhanced electrochemical reduction of dinitrogen to ammonia, yielding a maximum ammonia production rate of about 5.1 μgNH3 h-1

mgcat.-1 at -0.4 V (vs. the reversible hydrogen electrode, RHE) and a Faradaic efficiency of up to 10.1% at 0.1 V (vs. RHE) in 0.1 M KOH electrolyte. We inferred that single Co atoms together with pyrrolic N may be major active sites for N2 activation. This work would provide an easy and alternative method for the synthesis of transition metal catalysts for ENR.

KEYWORDS:

N2 reduction, Electrocatalysis, Cobalt, Nitrogen-doped carbon, Metal-organic

frameworks

INTRODUCTION N2 is abundant in atmosphere, whereas the molecule is difficult to activate and react because N2 has a high bonding energy, no dipole moment, and low polarizability.1 Until now, the Haber-Bosch process is still widely used in industry, which relies on H2 to convert N2 to NH3.2 However, this process has intrinsic disadvantages of low energy inefficiencies and dramatic CO2 emissions. Thus environmentally friendly nonthermal catalytic routes have been considered for N2 activation and reduction. Electroreduction of N2 to produce NH3 is an expanding field in recent years. Despite significant progresses that have been achieved in this direction, electrochemical N2 reduction (ENR) is still plagued by two major issues. One is low NH3 Faradaic efficiencies (FEs) (low selectivity), which are typically only a few percent or even below 1% because of severe competitive proton reduction catalyzed at similar potentials.3 The other is unsatisfactory ENR activities (low NH3 production rate). High FE is often accompanied by low ammonia yield rate. As such, it is highly desirable to develop selective and energy-efficient electrocatalysts, which can afford significant

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ammonia FE and reasonable ammonia production rate at the same time. Besides, natural abundance and low price are another two important aspects for consideration. So far, a variety of metals,4-11 metal oxides (hydroxides),12-18 metal nitride,19 doped carbons and other nonmetallic electrocatalysts20-26 have been reported for N2 reduction reaction. Of these materials, low-cost transition metal electrocatalysts were calculated to be able to catalyze ENR according to theoretical prediction.27 However, there have been few experimental reports on Co catalysts for ENR, which thus deserves exploration. Metal-organic frameworks (MOFs) are composed of organic ligands and tunable metal ion centers featuring with large surface areas and diverse structural topologies, which offer inherent advantages and accessible active sites when using as catalysts.28-29 MOFs can also serve as a sacrificial template in material synthesis. Direct carbonization of MOFs could lead to nanoporous carbon materials that possess adjustable N contents and species, which provide an effective way to facilitate N2 adsorption and dissociation, thereby promoting ENR.21-22, 30,31-34 In this work, we demonstrate efficient ENR to produce NH3 under ambient conditions by using a new catalyst of cobalt- and nitrogen-doped porous carbon which was obtained by carbonization of cobalt-based zeolitic imidazolate frameworks (ZIF-67). Of interest is that the structural topologies of ZIF-67 preserves after carbonization, offering high surface areas and excellent stability. Meanwhile, the contents of cobalt and nitrogen could be readily tuned by varying heating temperature to facilitate ENR. The resulting catalyst could yield a maximum NH3 production rate of about 5.1 μgNH3 h-1 mgcat.-1 at -0.4 V (vs. the reversible hydrogen electrode, RHE) and a Faradaic efficiency of up to 10.1% at -0.1 V (vs. RHE) at ambient conditions in an alkaline electrolyte. Such performance even outperforms noble metal catalysts (Pt/C, Ru/C, and Pd/C) under similar experimental conditions.

RESULTS AND DISCUSSION The synthesis of ZIF-67 was conducted according to literature. Different heating temperatures were used to pyrolyze ZIF-67 to yield samples labelled as Co/NC_T (T refers to treating temperature). Fig. 1a displays the XRD patterns of the samples obtained at varying carbonization temperatures from 400 to 700 oC. The samples calcined at 400 and 500 oC possess similar diffraction peaks with the original ZIF-67, indicating maintenance of the MOF structure.35 Nevertheless, the reflection intensity became weakened as the temperature increased. When the temperature was further raised to 600 and 700 oC, the structure of ZIF-67 collapsed and crystallized Co phase prevailed. The pronounced diﬀraction peak located at about 44.3° was assigned to the (111) reflection of facecentered-cubic cobalt.32 The particle size was estimated to be about 2.1 nm based on the (111) reflection utilizing Scherrer’s equation.


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Intensity/a.u.

Intensity/a.u.

(a) simulated ZIF-67 ZIF-67

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Pyridinic N

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0

Co (sat) Co 2p1/2

Co 2p3/2

Co/NC_600

Co/NC_700 810

805

Binding Energy/eV

800

795

790

785

780

775

Binding Energy/eV

Figure 1. (a) XRD patterns, (b) N 1s and (c) Co 2p XPS spectra of the samples obtained at varying carbonization temperatures.

X-ray photoelectron spectroscopy (XPS) was carried out to probe surface composition of the obtained materials. Intense Co 2p, C 1s, O 1s, and N 1s XPS peaks could be clearly seen in all the cases (Fig. S2). As observed in Fig. 1b, the deconvoluted N 1s spectra of both Co/NC_600 and Co/NC_700 can be divided into four peaks at about 398.2, 398.8, 400.15, and 401.4 eV, in accordance with pyridinic N, Co-N, pyrrolic N, and graphitic N, respectively.

36-37

The Co-N is

formed by bonding of N atoms with Co atoms. While graphitic N configuration is not identified for the samples treated at temperatures less than 600 °C. Fig. 1c shows deconvoluted Co 2p XPS spectra. Relative to Co/NC_400 and Co/NC_500 that only display three peaks of Co2+ (779.6 eV)38, Co-N (~781 eV), and Co-C (782.4 eV), an apparent additional peak of Co0 (778.2 eV)39 can be discernible for Co/NC_600 and Co/NC_700.37, 40 The presence Co0 in the samples suggests that Co2+ has been partly reduced to metallic Co, agreeing well with the afore-mentioned XRD results. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) imaging (Fig. 2a) of Co/NC_500 shows preservation of a polyhedral shape typical of the ZIF-67



precursor albeit with a gradual shrinkage shape. Shown in Fig. 2b-d are corresponding energydispersive X-ray spectroscopy (EDS) maps and spectrum, displaying uniform distributions of C, N, and Co elements in the Co/NC_500 sample. Close inspection by TEM (Fig. 2f and g) reveals a number of evenly dispersed granules attributed to Co nanoparticles with sizes of about 4 nm, resulting from thermal reduction of Co2+ during heating of ZIF-67. The particle size is larger than the value estimated by XRD is probably due to the presence of carbon layers around the metal nanoparticles. The fast Fourier transform (FFT) image (inset of Fig. 2g) taken from the region encased with a white dashed square illustrates typical (222) and (311) planes of metallic Co. Further high-resolution STEM examination (Fig. 2h) illustrates a lattice spacing of about 0.21 nm (derived from a distance of 1.464 nm for seven planes) corresponding to the (111) plane of crystalline Co. Co nanoparticles are likely coated with amorphous carbon layers. Interestingly, there are many bright spots dispersed on the carbon support, as clearly shown in Fig. 2h and i. These spots are most likely single Co atoms, which are circled in yellow to guide the eyes because Co has a higher atomic number than carbon and is thus brighter during STEM observation.41 The atomically dispersed Co is probably formed due to coordination interaction of Co and N species resulting from decomposition of ZIF-67 at elevated temperatures.36

Figure 2. (a) STEM image of Co/NC_500. Corresponding EDS maps of (b) C, (c) N, and (d) Co elements, and (e) EDS spectrum. (f) TEM image and (g)-(i) High-resolution STEM images of Co/NC_500. The inset in g is the FFT of the region encased with the white dashed square shown in g. The inset in h illustrates the distance of seven planes denoted by the while arrow shown in h. In image I, some Co atoms are annotated with yellow dashed circles to guide the eyes.

XRD, XPS, and TEM analyses confirmed the formation of a high density of small Co nanoparticles along with single Co sites supported on N-doped carbon matrix. Such materials with high exposure of Co sites warrant further exploration in electrocatalysis. Motivated by this


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advantage, we investigated the ENR activities of the resultant Co/NC samples. The electrocatalytic activities of Co/NC catalysts were examined in an H-type cell separated by a Nafion 117 membrane under ambient temperature and pressure.42 Fig. 3a shows the linear sweep voltammetries (LSVs) of Co/NC_500 in 0.1 M aqueous KOH electrolyte saturated with argon and dinitrogen. The current density in N2 was observed to be larger than that in Ar with an onset potential of ~-0.15 V. We use the indophenol blue method43 and the Watt and Chrisp method44 to probe NH3 and N2H4, respectively. During the reaction, we detected only NH3 without N2H4, indicating good selectivity for NH3. To probe the source of NH3, we performed blank and control experiments. Very little or nearly no NH3 was observed when using carbon paper electrode in the absence of catalyst, or with solely the Nafion solution binder, or at an open circuit (Fig. 3b). A small amount of NH3 was detected over Co/NC_500 catalyst in Ar-saturated KOH, which may arise from reduction of adsorbed environmental N2 and/or NH3. Significant amounts of NH3 were produced in N2-saturated KOH, confirming that NH3 was predominantly generated from electroreduction of dissolved N2 over the catalyst surface.

(b) 0.30

0

N2

-20 -25 -0.8

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(d)

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3 2

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1

Co/NC_400 ZIF-67

0 -0.4

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Potential/V (vs. RHE)

650

700

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Co/NC_400 Co/NC_600 0.3

NH3 Faradaic Efficiency/%

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0.20


(c) -1

Ar gas Open circuit Carbon paper Binder

Ar

-10 -15

N2 gas

0.25

Absorbance

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NH3 Faradaic Efficiency/%

Current Density/mA cm

-2

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NH3 Yield Rate/g h mg

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


Co/NC_500 Co/NC_700 @-0.4 V

0.2

0.1

4

ZIF-67

0.0

2 0

-0.4

-0.3

-0.2

-0.1


Figure 3. (a) The LSVs of Co/NC_500 electrode in Ar- (black line) or N2- (red line) saturated 0.1 M KOH solution with a scan rate of 5 mV s-1. (b) UV-vis absorption spectra of the electrolytes after electrolysis at −0.2 V under different conditions. (c) The yield rate and (d) FE of NH3 at various potentials over Co/NC_400, Co/NC_500, Co/NC_600, and Co/NC_700. The inset in d shows comparison of NH3 FE among the four Co/NC catalysts and ZIF-67 at −0.4 V.



We studied the ENR catalytic properties of the resultant Co/NC catalysts fabricated under different calcination temperatures. The NH3 yield rates at applied potentials ranging from -0.1 to 0.4 V are plotted in Fig. 3c. The NH3 production rate was found to increase with the increase of overpotential, reaching up to approximately 5.1 μgNH3 h-1 mgcat.-1 (~0.4 μmolNH3 h-1 cm-2) at -0.4 V over Co/NC_500. This value is higher than ~1.1 μg·h-1 mg-1 for tetrahexahedral Au nanorods45 and Fe/CNT (0.012-0.097 μmol h-1 cm-2)40 at similar potentials. The electrocatalytic performance follows the trend Co/NC_500 > Co/NC_600 > Co/NC_700 > Co/NC_400. All Co/NC samples are more active than the ZIF-67 precursor which shows very weak activity in ENR. The NH3 Faradaic efficiency decreased with shift of potential toward more negative values in all the cases. Notably, the Co/NC_500 provided an NH3 FE as high as about 10.1% at -0.1 V. Similarly, the ammonia partial current densities of Co/NC_500, are the highest among the catalysts tested at different potentials (Fig. 4a). We note that both the NH3 yield rate and FE of Co/NC_500 catalyst are higher than those of many previous reported electrocatalysts (Table S1) and commercial noble metal-based catalysts including 5 wt% Pt/C, 5 wt% Ru/C, and 10 wt% Pd/C (Fig. S3). The stability of the Co/NC_500 in ENR was examined at a fixed potential of -0.2 V for 6 hour (Fig. 4b). Although there occurred decrease of NH3 yield rates and FEs in the first three hours over Co/NC_500, both parameters tended to be stable from the fourth hour. Possible reasons for this initial reduction in performance may result from gradual consumption of adsorbed environmental N2 on the surface of catalyst, which seems to be complete after continuous reaction for three hours. STEM measurements (Fig. S4) showed that the catalyst structure comprising a large number of single Co sites preserved after electrolysis, indicating its good stability.


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2.0 1.5

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Figure 4. (a) Partial current densities of NH3 at various potentials over the samples obtained at varying carbonization temperatures. (b) The long-term durability test at -0.2 V on Co/NC_500 electrode. (c) The contents of pyridinic N, pyrrolic N, Co-N, and graphitic N of the samples obtained at varying temperatures. (d) The yield rates of NH3 at various potentials over Co/NC_600 before and after subjected to acid treatment (Co/NC_600-soak).

We attempted to understand the impact of pyrolysis temperature on catalytic performance. We found that the proportion of Co-N species dropped when the pyrolysis temperature was improved from 400 to 700 oC, derived from XPS measurements (Fig. 4c). This is reasonable due to easier reduction and aggregation of Co sites at higher temperatures. The content of pyrrolic N is in the trend Co/NC_700 < Co/NC_600 < Co/NC_400 < Co/NC_500, consistent with the trend of Faradaic efficiencies. This phenomenon suggested that the pyrrolic N is probably responsible for promoting the selectivity of ammonia synthesis, which was also corroborated by DFT calculations demonstrated in previous literature.21 It was also predicted that N2 molecules were unlikely adsorbed on either graphitic N or neighbouring C atoms, which may not significantly accelerate the N2 conversion.22 For the Co/NC_400 catalyst, the skeleton structure of ZIF-67 seems to be still the main component, which is very poor in catalysing ENR (Fig. 3c). The less efﬁcient activities of Co/NC_600 and Co/NC_700 may be closely associated with the smaller numbers of single Co sites than Co/NC_500. To obtain insight into the influence of Co and N, Co/NC catalysts were soaked in 2 M H2SO4 for 2 h. The treated samples were then washed with a solution of water and ethanol. We found that the


Faradaic Efficiency/%

-1 -1

Co/NC_500


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(b)


NH3 Current Density/mA cm

(a)

Content/%

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-2

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NH3 yield rates of soaked Co/NC_600 remained nearly constant (Fig. 4d). It should be aware that most Co crystallites can be removed by acid leaching, while single Co atoms and pyrrolic N can sustain H2SO4 treatment.41,42 We thus suppose that the catalytic activity in Co/NC originates predominantly from stable single Co sites as well as pyrrolic N species. The NH3 production rates of soaked Co/NC_500 decreased (Fig. S5), probably due to the decrease of unstable Co-N moieties in Co/NC_500.

CONCLUSIONS In summary, we have demonstrated encouraging activity and selectivity for ENR in alkaline media by using Co/N-doped C composite catalysts derived from ZIF-67. Such low-cost catalysts afforded an NH3 production rate of up to 5.1 μgNH3 h-1 mgcat.-1 at -0.4 V (vs. RHE) and a Faradaic efficiency of about 10.1% at -0.1 V (vs. RHE), even surpassing noble metals. The nitrogen configurations and cobalt species can be readily tailored by manipulating pyrolysis temperature. Single Co sites in combination with pyrrolic N groups were found to be critically important in electrocatalytic N2 conversion. This work offers a potential route for efficient electrochemical N2 fixation. Further research in controlling Co-N coordination structures for ENR is underway.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, UV-vis curves of indophenol assays with NH4+ ions; wide-survey XPS spectra; the yield rates of NH3 at various potentials over unsoaked and soaked samples of Co/NC_500

AUTHOR INFORMATION Corresponding Authors *(Z.S.) E-mail: [email protected]. *(S.H.) E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the State Key Laboratory of Organic-Inorganic Composites (No. oic201503005, oic-201901001); Beijing Natural Science Foundation (No. 2192039); State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No. M2-201704).

REFERENCES


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Table of Contents

We report Co/nitrogen-doped carbon composites comprising a large number of single Co sites for efficient electrochemical N2 fixation at ambient conditions.


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Nitrogen-Doped Carbon Composites for

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