The Loading of Cluster Based Coordination Compound on Biomass

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The Loading of Cluster Based Coordination Compound on Biomass Derived N-doped Mesoporous Carbon Matrix: a Bi-functional Electrocatalyst for Overall Water Splitting Xiaoxuan Wang, Xinxin Xu, Zhongmin Feng, Huo Y., and Bian Lijun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01636 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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ACS Sustainable Chemistry & Engineering

The Loading of Cluster Based Coordination Compound on Biomass Derived N-doped Mesoporous Carbon Matrix: a Bi-functional Electrocatalyst for Overall Water Splitting Xiao-Xuan Wang, Xin-Xin Xu*, Zhong-Min Feng, Yu-Qiu Huo* and Li-Jun Bian* Department of Chemistry, College of Science, Northeastern University, NO.3-11, Wenhua Road, Heping District, Shenyang, Liaoning Province, 110819, People’s Republic of China

*Author to whom correspondence should be addressed. Tel: +86-024-83684533, Fax: +86-024-83684533. * E-mail: [email protected] (Prof. X. X. Xu), [email protected] (Prof. Y. Q. Huo) and [email protected] (Prof. L. J. Bian) ACS Paragon Plus Environment

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ABSTRACT To improve the electrocatalytic activity of a cluster based coordination compound constructed by Cu(I) and 4, 6-Dimethyl-2-mercaptopyrimidine ligand, its particles were loaded on N-doped mesoporous carbon matrix derived from peach juice. In this electrocatalyst, nanoparticles of the cluster based coordination compound disperse homogenously on mesoporous carbon matrix doped by nitrogen atoms. The size of nanopraticle ranges from 6 to 8 nm. Surface area of the electrocatalyst is 296.4 m2·g-1. In 1.0 M KOH, this electrocatalyst exhibits outstanding performance for hydrogen evolution reaction (HER). To achieve current of 10 mA·cm-2, the overpotential is only 60 mV and Tafel slope 63 mV·dec-1. This electrocatalyst also shows outstanding oxygen evolution reaction (OER) performance. Under the same condition, to obtain current of 10 mA·cm-2, its overpotential is only 298 mV. The electrocatalyst possesses excellent durability, after 2000 cycles as well as 10 h long-term HER and OER tests, the current keeps stable. To achieve overall water splitting, an electrolyzer is constructed with this bi-functional electrocatalyst as both cathode and anode at the same time. With 1.55 V voltage, this electrocatalyst can obtain a current of 10 mA·cm-2. The electrolyzer possesses excellent stability, which keeps constant for 48 h in overall water splitting. We expect this bi-functional electrocatalyst can act as a new material for H2 production. KEYWORDS:

Cluster

based

coordination

compound;

Biomass;

Mesoporous carbon matrix; Electrocatalyst; Overall water splitting

ACS Paragon Plus Environment

N-doped

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INTRODUCTION With the accelerated developing of human society, serious energy shortage and environmental pollution have emerged due to continuous consumption of coal, oil and natural gas.1-4 Now, H2 is regarded as a new replacer for above traditional fossil fuels for its renewable, zero carbon dioxide discharge and high energy density.6-9 Compared with other H2 production methods, electrocatalytic overall water splitting seems more attractive, which can obtain H2 and O2 simultaneously through hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).10-16 Noble metal based materials, such as Pt and RuO2, IrO2, have been proved as efficient electrocatalysts, which are widely used in HER and OER.17-19 The advantages of these noble metal based materials are remarkably, but the low reserves and high price limit their large scale

development.

The

application

of

earth-abundant

elements

derived

electrocatalysts is proved as an ideal method to resolve the contradiction between cost and efficiency. 20-25 Recently, metal-sulfide, such as Cu2S, has been explored as a new bi-functional electrocatalyst, which possesses low overpotential and stability in both HER and OER.26,27 In this aspect, cluster based coordination compounds constructed by Cu(I) and -SH containing N-heterocycle ligand inherit the excellent electrocatalytic activity of Cu2S.28-32 In addition, they also possess controllable and various structures.33,34 The application of cluster based coordination compounds in overall water splitting looks solid, but the further enhancement of electrocatalytic property is restricted by poor conductivity. To resolve this problem, a feasible method is to load the cluster based coordination compounds on carbon based material, which possesses excellent conductivity. In the family of carbon material, porous carbon exhibits more merits in electrochemistry, because of large surface area and abundant electrocatalytic active sites. With these



advantages, porous carbon becomes an ideal candidate carrier for cluster based coordination compounds in HER and OER.35,36 More importantly, for mesoporous carbon materials, after the doping of hetero atoms, such as N, P, S and B, their electrocatalytic performance in HER and OER can be improved in great extent, which originates from the polarization effect between hetero atoms and carbon atoms caused by the differences in their electro negativities.37,38 To obtain hetero atoms doped mesoporous carbon, fruit juice is an ideal precursor. At first, the sugar in fruit juice can convert to mesoporous carbon after hydrothermal carbonization.39 In general; fruit juice possesses a high sugar content, which achieves as high as 80 to 100 mg/mL. Secondly, in fruit juice, the inorganic salts, such as NaCl, KCl and MgCl2 can act as directing agents in pore generation.40 More importantly, the amino acids and proteins in fruit juice can also induce hetero atoms, such as N and P in mesoporous carbon. Based on these points, a new electrocatalyst (abbreviated as Cu6L6@NMC), was synthesized through the loading of [Cu6(DMMPY)6]·H2O (Cu6L6, DMMPY = 4,6-Dimethyl-2-mercaptopyrimidine), a cluster based coordination compound on peach juice derived mesoporous carbon matrix (NMC) doped by nitrogen atoms. In the structure of this electrocatalyst, small Cu6L6 nanoparticles distribute homogenously in NMC. Cu6L6@NMC exhibits excellent HER activity. In 1.0 M KOH, It merely requires 60 mV overpotential to obtain 10 mA·cm-2 current. During HER process, the Tafel slope is 63 mV·dec-1. In acidic condition, the overpotential increases to 134 mV and the Tafel slope becomes 76 mV·dec-1. Cu6L6@NMC also possesses excellent durability. Its current maintains stable after 2000 cycles and 10 h long-term HER tests. The OER performance of Cu6L6@NMC is also perfect. The overpotential is only 298 mV. Cu6L6@NMC//Cu6L6@NMC, an overall water splitting electrolyzer, is set up with bi-functional Cu6L6@NMC as cathode and anode


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at the same time. To get the current of 10 mA·cm-2, the electrolyzer only needs 1.55 V overpotential. With Cu6L6@NMC//Cu6L6@NMC, water splitting can be achieved with a 1.5 V battery. It is the first time that cluster based coordination compound was employed for overall water splitting.

Figure 1 (a) The fundamental unit of Cu6L6; (b) Two-dimensional supramolecular network of Cu6L6 RESULTS AND DISCUSSION Structure and morphology and characterization In the fundamental unit of Cu6L6, there exist three independent Cu(I) ions. Cu(1) links with two S atoms in two DMMPY molecules (Scheme S1). The Cu-S bond distances ranges are 2.242 and 2.246 Å respectively. In Cu(1), the other coordination site is occupied by N atom and the Cu-N bond distance is 2.017 Å. This leads to a distorted trigonal CuS2N geometry of Cu(1). Cu(2) and Cu(3) adopt the same coordination modes as Cu(1), but with different bond lengths and angles. Three adjacent Cu atoms are connected by DMMPY with µ3-bridging mode. Detailed speaking, in DMMPY, the thiolate-S atom links with two Cu atoms in µ2-bridging mode and the pyrimidine-N atom connects to one Cu atom. This results in a chair-shaped [Cu3S3] ring. Two such [Cu3S3] rings are bound together by DMMPY ligands, which generate a pseudohexagonal prismatic [Cu6S6] cluster (Figure 1a). In this [Cu6S6] cluster, the Cu-Cu distances in this cluster range from 2.75 to 2.85 Å,



which is obviously smaller than the sum of van der Waals radii in two Cu atoms.41,42 These distances are close to the reported [Cu6S6] based compounds and confirm the existence of Cu-Cu interactions in Cu6L6.43,44 With π-π interactions, these Cu6L6 units are connected and form 2D supramolecular network (Figure 1b).

Figure 2 (a) PXRD of Cu6L6@NMC; (b) Raman spectra of Cu6L6@NMC

Figure 3 (a) SEM image of NMC; (b) SEM image of Cu6L6@NMC(B); (c) TEM image of Cu6L6@NMC(B), inset, elemental maps of S and Cu; (c) HRTEM image of Cu6L6@NMC(B) The structure of Cu6L6@NMC was studied with PXRD at first. Cu6L6@NMC exhibits similar diffraction pattern with Cu6L6, which illustrates in the electrocatalyst, the structure of Cu6L6 is well still retained (Figure 2a). The diffractions from carbon


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materials disappear in PXRD analysis and this maybe originates from the overlap of the peaks from carbon materials with Cu6L6. To confirm the existence of NMC in Cu6L6@NMC, Raman spectrum was applied (Figure 2b). There exist two peaks locating at 1334 and 1563 cm-1 in Raman spectrum. These two peaks originate from D and G bands, which represent disorder and graphite carbon.45 They are prominent features in carbon based materials. ID/IG, the intensity ratio between of D band to G band, is a significant indicator to study the structure of carbon materials. In general, the large ID/IG value reflects the high concentration of defect and the low number of graphite carbon atoms. From Cu6L6@NMC(A) to Cu6L6@NMC(C), their ID/IG values are 0.98, 0.92 and 1.04 respectively.46 The increasing of Cu6L6 can weaken the graphite structure of NMC, which leads to the generation of more disordered carbon. So, Cu6L6@NMC(C) possesses higher ID/IG value than Cu6L6@NMC(A) and Cu6L6@NMC(B).

Figure 4 (a) XPS survey of NMC; (b) XPS survey of Cu6L6@NMC(B); (c) Cu 2p of



Cu6L6@NMC(B); (c) N 1s of Cu6L6@NMC(B) The morphologies of N-doped mesoporous carbon matrix and Cu6L6@NMC were studied with SEM and TEM. Here, Cu6L6@NMC(B) was chosen as an example. SEM demonstrates N-doped mesoporous carbon matrix and Cu6L6@NMC(B) both exhibit spherical appearance with the diameter about 400 to 500 nm. The surface of Cu6L6@NMC(B) and N-doped mesoporous carbon matrix looks relatively smooth (Figure 3a and 3b). In SEM image, Cu6L6 particles disappear completely and this illustrates Cu6L6 particles with small size loaded on NMC. Further structural details of Cu6L6@NMC(B) were obtained by TEM. As illustrated by its image, there are a large number of small nanoparticles distribute homogenously in N-doped mesoporous carbon matrix, which exhibits spherical looks with the size in the range of 6 to 8 nm (Figure 3c). Furthermore, it can be observed clearly, Cu and S elements distributes evenly in Cu6L6@NMC(B) (Figure 3c, inset). In high resolution TEM (HRTEM) image, there exists a lattice plane with the distance 0.228 nm. This is in agreement with (001) plane of Cu6L6 (Figure 3d). The composition as well as the electronic state of NMC and Cu6L6@NMC(B) was studied with X-ray photoelectron spectroscopy (XPS). The peaks of N-doped mesoporous carbon matrix appear at 284.2 eV, 400.5 eV and 532.2 eV. These peaks originate from C 1s, N 1s and O 1s respectively (Figure 4a). On the contrary, in Cu6L6@NMC(B), besides above mentioned elements, the signs belonging to S 2p and Cu 2p are also observed at 132.1 eV and 932.1 eV (Figure 4b). In high resolution Cu 2p spectrum, the peaks at 932.1 eV and 952.2 eV can be ascribed to Cu 2p3/2 and Cu 2p1/2 (Figure 4c).47 This illustrates Cu element exists in Cu(I) form. After deconvolution, there are three independent peaks appear at 284.8 eV, 285.8 eV and 287.4 eV, which corresponds to C1s spectra of Cu6L6@NMC(B) (Figure S1).48


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Besides Cu 2p and C 1s, high-resolution N 1s are also analyzed. The broad N 1s peak of Cu6L6@NMC(B) can be separated into four peaks (Figure 4d). The peaks appearing at 397.9 eV, 399.4 eV and 401.6 eV are ascribed to pyridinic-N, pyrrolic-N and graphitic-N.49 In an electrocatalyst, the existence of graphitic-N is benefit for the movment of electron. This is significant for the enhancement of electrochemical activity as well as the performance in HER and OER. In addition to three peaks mentioned above, the fourth peak appears at 396.4 eV, it is the character of N 1s in Cu-N bond from Cu6L6.

Figure

5

Nitrogen

adsorption/desorption

of

(a)

Cu6L6@NMC(A);

(b)

Cu6L6@NMC(B); (c) Cu6L6@NMC(C); Pore distribution of Cu6L6@NMC electrocatalysts To study the specific surface area as well as porous character of Cu6L6@NMC electrocatalyst, the specific surface area was calculated with BET method. There exists an obviously slope in middle relative pressure region from 0.4 to 0.8 in N2 adsorption/desorption isotherms of Cu6L6@NMC electrocatalysts. This is the



character of type-IV isotherm. From Cu6L6@NMC(A) to Cu6L6@NMC(C), their BET specific surface area is 288.2, 296.4 and 301.7 m2·g-1 (Figure 5a to 5c). The large surface area facilitates adequate contact between electrolyte and Cu6L6@NMC electrocatalyst, which can enhance the process of HER and OER. The pore-size distribution of above Cu6L6@NMC electrocatalysts was calculated from BJH model and the results imply their average pore diameter is 4.82, 4.05 and 3.56 nm (Figure 5d). The surface area and average pore diameter of Cu6L6@NMC electrocatalysts are almost similar with N-doped mesoporous carbon matrix (308.2 m2·g-1 and 4.27 nm) (Figure S 2a and 2b). This suggests the loading of Cu6L6 particles does not block the pore of NMC.

Figure 6 LSV of Cu6L6@NMC and NMC (a) in 1.0 M KOH; (b) in 0.5 M H2SO4; (c) in 0.1 M PBS; (d) Tafel plots of Cu6L6@NMC in 1.0 M KOH HER Performance To evaluate the HER activity of Cu6L6@NMC, the sample was loaded on glassy carbon electrode and measured with three electrode setup in 1.0 M KOH at first. In


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these Cu6L6@NMC electrocatalysts, Cu6L6@NMC(B) performs the most excellent HER activity. It exhibits a smaller onset potential (j = 1.0 mA·cm-2) of 5.2 mV (vs. RHE) than other two electrocatalysts. Furthermore, to obtain 10 mA·cm-2 current, Cu6L6@NMC(B) only needs a low overpotential of 60 mV. This value is much smaller than Cu6L6@NMC(A) (111 mV) and Cu6L6@NMC(C) (160 mV) (Figure 6a). This overpotential is much lower than Cu2S based electrocatalysts and even comparable with Pt (Figure S3a).50,51 For comparison, HER activity of merely NMC was studied. The results illustrate NMC is nearly inert under this condition, which confirms Cu6L6 is account for the enhanced HER performance. Furthermore, the activity of Cu6L6 is also studied. Cu6L6 exhibits higher overpotential than Cu6L6@NMC electrocatalysts and this suggests the loading of Cu6L6 on N-doped mesoporous carbon matrix is benefit for the improvement of HER activity (Figure S3b). In 0.5 M H2SO4, the advantage of Cu6L6@NMC(B) is also retained (Figure 6b). To obtain 10 mA·cm-2 current, Cu6L6@NMC(B) needs an overpotential of 132 mV. For Cu6L6@NMC(A) and Cu6L6@NMC(C), this value becomes 155 and 191 mV respectively. For Cu6L6@NMC, the HER performance was also investigated in PBS solution with pH value 7.0. The results illustrates in neutral condition, the overpotential is much higher (Figure 6c). Based on these results, it can be concluded the basic electrolyte is more suitable for Cu6L6@NMC in HER. For Cu6L6@NMC, to explore the mechanism and the reaction type in HER, Tafel slope is an important parameter, which reflects the inherent properties of an electrocatalyst. For Tafel plot, in general, its linear section fits perfectly with the Tafel equation. Tafel plots of Cu6L6@NMC electrocatalysts were obtained from the polarization curves in basic condition. For Cu6L6@NMC(B), its Tafel slope is 63 mV·dec-1, which is much smaller than Cu6L6@NMC(A) (83 mV·dec-1) to



Cu6L6@NMC(C) (99 mV·dec-1) (Figure 6d). Compared with the other two electrocatalysts, Cu6L6@NMC(B) possesses the smallest Tafel slope value and this implies a faster enhancement of the HER rate upon increasing the overpotential. All these Tafel slopes fall in the range of 40 to 120 mV·dec-1, which suggests the HER occurring on Cu6L6@NMC surface follows a Volmer-Heyrovsky mechanism. At first, H+ is reduced by electron and generates H atom adhered on Cu6L6@NMC (Volmer process). Then the ion-atom reaction (Heyrovsky reaction, Hads + H+ + e− → H2) happens and produces H2.

Figure 7 (a) Linear fitting of ∆j for Cu6L6@NMC vs. scan rates; (b) Electrochemical impedance spectra of Cu6L6@NMC in 1.0 M KOH; (c) LSV of Cu6L6@NMC(B) before and after 2000 cycles in 1.0 M KOH; (d) Time dependence current density of Cu6L6@NMC(B) The advantage of Cu6L6@NMC(B) in HER can also be suggested by electrochemical active surface area (EASA). To calculate the value of EASA, cyclic voltammetry (CV) is an effective method, because EASA is in proportion with double


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layer capacitance (Cdl) of an electrocatalyst. CV was conducted with scan rates in the range of 20 to 200 mV·s-1 (Figure S3). The Cdl values are 9.5, 12.2 and 3.3 mF·cm-2 for Cu6L6@NMC(A), Cu6L6@NMC(B) and Cu6L6@NMC(C) (Figure 7a). From Cu6L6@NMC(A) to Cu6L6@NMC(C), with the increasing of Cu6L6, the EASA increases at first. If the content of Cu6L6 increases continuously, the poor electric conductivity of Cu6L6 hinders further enhancement of EASA. Compared with the other two electrocatalysts, the large EASA of Cu6L6@NMC(B) provides more active sites, which is benefit for HER. For Cu6L6@NMC electrocatalysts, their difference in HER performance can also be illustrated from electrochemical impedance spectra (EIS) clearly. The Nyquist plot of Cu6L6@NMC(B) shows a smaller arc than Cu6L6@NMC(A) and Cu6L6@NMC(C) (Figure 7b). The charge-transfer resistance (Rct) of Cu6L6@NMC(B) is 6.2 Ω, which exhibits obviously decrease relative to Cu6L6@NMC(A) (13.8 Ω) and Cu6L6@NMC(C) (21.5 Ω). This implies Cu6L6@NMC(B) possesses higher conductivity and faster electron transfer than other composite materials. In HER, stability is very significant. For Cu6L6@NMC(B), the continuous HER test was conducted in 1.0 M KOH. After recycled for 2000 times, the polarization curve keeps in consistence with initial one (Figure 7c). The long term durability of Cu6L6@NMC(B) was also studied and the current density still keeps stable for 10 h (Figure 7d). Furthermore, Cu6L6@NMC(B) is also stable in 0.5 M H2SO4 (Figure S5). All these facts imply the stability of Cu6L6@NMC(B) is excellent in HER. OER activity of Cu6L6@NMC(B) was also studied in 1.0 M KOH. To obtain a current with density of 10 mA·cm-2, it only requires overpotential of 298 mV (Figure 8a). In this process, the corresponding Tafel slope of Cu6L6@NMC(B) is 89 mV·dec-1 (Figure 8b). Compared with Cu6L6 and NMC, OER performance of



Cu6L6@NMC(B) is improved greatly, which can be ascribed to synergistic effect between Cu6L6 and NMC (Figure S6a and 6b). The activity of Cu6L6@NMC(B) is also much better than Cu2S based electrocatalysts and even comparable with the performance of IrO2/C (Figure S6c).52 Cu6L6@NMC(B) possesses excellent durability, after 10 h as well as 2000 cycles long-term OER tests, the current keeps stable (Figure 8c and 8d). All these results suggest Cu6L6@NMC(B) is an active and stable electrocatalyst for OER.

Figure 8 (a) LSV of Cu6L6@NMC(B); (b) Tafel plot of Cu6L6@NMC(B) in 1.0 M KOH; (c) Time dependence current density for Cu6L6@NMC(B); (b) Polarization curves of Cu6L6@NMC(B) before and after 2000 cycles OER Performance and electrocatalytic water splitting Because of outstanding HER and OER activities of Cu6L6@NMC(B), electrocatalytic water splitting was studied with 1.0 M KOH serving as electrolyte. The reaction was conducted in Cu6L6@NMC//Cu6L6@NMC electrolyzer, in which bi-functional Cu6L6@NMC(B) acts as cathode and anode simultaneously. This


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electrolyzer only require 1.55 V to obtain a current with the density of 10 mA·cm-2 (Figure 9a). Long term stability of this electrolyzer was also studied and the current density retain as high as 97.6 % of the original value after 48 h reaction (Figure 9b). The molar ratio between obtained O2 and H2 is very close to 0.5 and this value is in consistence with the theoretical data. This illustrates in water splitting the Faradic efficiency is almost 100 % (Figure 9c). For Cu6L6@NMC//Cu6L6@NMC, the low overpotential and excellent stability is benefit for its practical usage in H2O splitting. With a 1.5 V battery (AA size) as energy source, Cu6L6@NMC//Cu6L6@NMC can achieve H2O splitting. It can be observed clearly a large number of H2 and O2 bubbles emerge at the same time (Figure 9d). The results illustrate the outstanding activity of Cu6L6@NMC//Cu6L6@NMC in overall H2O splitting.

Figure 9 (a) LSV for overall water splitting of Cu6L6@NMC//Cu6L6@NMC; (b) Time dependence current density of Cu6L6@NMC//Cu6L6@NMC; (c) The amount of H2 and O2 at 20 mA·cm-2; (d) Picture of water splitting device powered by 1.5 V battery (AA size).



CONCLUSIONS In conclusion, a noble metal free bi-functional electrocatalyst, Cu6L6@NMC, has been fabricated successfully. In this electrocatalyst, small Cu6L6 nanoparticles distribute homogenously in peach juice derived N-doped mesoporous carbon matrix. As revealed by the low overpotentials, Cu6L6@NMC possesses excellent HER and OER activity. Either in HER or OER process, Cu6L6@NMC both possesses good stability, which keeps stable after 2000 cycle tests. Furthermore, at a given overpotential, the current density of Cu6L6@NMC retains for 10 h. An electrolyzer, Cu6L6@NMC//Cu6L6@NMC is constructed to realize electrocatalytic H2O splitting, which only needs 1.55 V to obtain the current with density of 10 mA·cm-2. During this process, the electrolyzer also exhibits remarkable stability in 48 h. These advantages make Cu6L6@NMC an ideal option to explore H2 energy as a promising replacer to classical fossil resource. ASSOCIATED CONTENT Supporting Information Experimental details; the structure of DMMPY ligand; XPS spectra of C 1s of Cu6L6@NMC(B); N2 adsorption/desorption and pore distribution of NMC; LSV of Pt and Cu6L6; CV of Cu6L6@NMC with scan rates in the range of 20 to 200 mV·s-1; Time dependence current density of Cu6L6@NMC(B) in 0.5 M H2SO4; LSV of Cu6L6; NMC and IrO2/C in 1.0 M KOH. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Fundamental Research Funds for the Central University (N170504025).


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Table of Contents Graphic

A bi-functional electrocatalyst built by cluster based coordination compound and biomass derived N-doped carbon has been synthesized for water splitting.


The Loading of Cluster Based Coordination Compound on Biomass

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