A Simple, Ultrafast Combustion Synthesis of Co3O4 Dispersed

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Pick a Wick: A Simple, Ultrafast Combustion Synthesis of Co3O4 Dispersed Carbon for Enhanced Oxygen Evolution Kinetics Dheeraj Kumar Singh, Soumita Chakraborty, Arunava Saha, Srinivasan Sampath, and Muthusamy Eswaramoorthy ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00906 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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ACS Applied Energy Materials

Pick a Wick: A Simple, Ultrafast Combustion Synthesis of Co3O4 Dispersed Carbon for Enhanced Oxygen Evolution Kinetics Dheeraj Kumar Singh,[a] Soumita Chakraborty,[a] Arunava Saha,[a] Srinivasan Sampath,[b] and Muthusamy Eswaramoorthy*[a] a b

Nanomaterials and Catalysis Lab; JNCASR, Bengaluru 560064

Inorganic and Physical Chemistry Lab; Indian Institute of Science, Bengaluru 560010

*[email protected]

Abstract: Mass transport and charge transfer at an interface plays a crucial role in governing the electrochemical performance of a material. Wider meso-macropores are expected to enhance the reaction kinetics by facilitating the ion-transport to and fro from an active interface thereby continuously regenerating it at accelerated rates. Herein, we report a generic, simple, and ultrafast synthetic method to obtain highly graphitized porous carbon containing well-dispersed Co3O4 nanoparticles (~ 1 wt.% Co) using cobalt acetate and piperidine precursors. The obtained catalyst (Co3O4@CS) exhibits onset potential and oxygen evolution kinetics similar to that of the state-of-the-art catalyst, RuO2. For oxygen evolution reaction (OER), the synthesized material exhibits excellent cycling performance over 2000 cycles. Such performance metric can be attributed to the uniform dispersion of active sites (Co3O4) over low density, highly interconnected conducting carbon matrix leading to facile mass transport and charge transfer respectively.

Keyword: Carbon composite, Electrocatalyst, Oxygen evolution reaction, Electron-transfer, Combustion. Water electrolyzers1-2 and fuel cells (FCs)3-4 are two key players to address future problems pertinent to energy and environmental issues posed by fossil fuels. Generation of H2/O2 (water splitting) via renewable resources solves the problem of intermittency in tapping the solar as well as wind energies even in their absence.5-8 However, the hindrance to the development of practical

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prototypes (at large scale) stems from sluggish oxygen evolution kinetics at the anode in water electrolyzers (and conversely, oxygen reduction reaction, ORR in FCs).9-14 In other words, the H2 generation at the cathode in water electrolyzers is also affected by impeded multi-electron (4e-) kinetics involved in oxygen evolution reaction (OER) at the anode.15-18 The O2 generation in general has two-fold issues, firstly, the higher overpotential and secondly, its interference with hydrogen generation. Consequently, efficient catalysts are required to drive such reactions near their thermodynamic potentials at current densities close to practical applications.19 Traditionally, Ru based catalysts are used for OER15,

20-22

but the development of practical

energy devices is deeply plagued by their prohibitively high cost along with their toxic nature. Alternative earth-abundant elements such as Ni, Fe, Co etc. which are being used to carry out above reactions are limited by their stabilities under extreme pH conditions.6, 17, 23-25 Metal in the form of oxides are typically non-conducting in nature.26-29 Usually, higher loadings of such oxides are required to obtain significant activity due to the low density of active sites which is further aggravated by poor textural parameters. Even when used in phosphide forms (to enhance conductivity),27 oxide formation at the interface is inevitable (due to the participation of lattice oxygen)30 leading to the loss of electrode integrity thereby affecting long-term performance.27, 3132

Therefore, it is imperative to select low-density carbon supports (to stabilize catalysts) which

will not only be conducting but also porous in nature for easy access of the electrolytes.33-37 Furthermore, the catalytic oxides should be in intimate contact with the conducting carbon to ensure higher activity even at much lower loadings due to large number of exposed active sites (high dispersion).38-39 Also, the rate of accessibility of active sites will be synergistically boosted by the rapid flux of electrolytes aided by the favorable textural parameters of the composites.40-43 The incorporation of oxides on the preformed carbon supports (carbon nanotubes, graphene etc.)44-46 often lead to poor interfacial contact leading to high internal polarization and leaching of oxides

thus compromising its stability. Simulatneous formation of conducting, three-

dimensional porous carbon and metal-oxide nanostructure by an insitu-synthetic method is expected to overcome the issues like stability, poor interfacial contact and dispersion of active sites.40, 47 Herein we report an innovative, ultrafast (less than a minute), one-step strategy to synthesize Co3O4 embedded carbon composites whose OER activity is better than the state-of-the-art RuO2


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catalyst. The strategy adopted here is a combustion technique involving simple carbon precursor, piperidine, mixed with Co(II) acetate as a fuel. Addition of cobalt(II) acetate to excess piperidine results in the formation of Co(II)-piperidine complex indicated by an instantaneous color change from pink (Co(II) acetate) to a violet solution which was further confirmed by UV-Vis spectroscopy (Figure S1). The cobalt acetate containing piperidine solution thus obtained was allowed to combust through capillarity action (Figure 1) using a cotton wick to produce Co3O4 containing carbon soot (Co3O4@CS) over a glass slide as shown in Figure 1b. For control experiments, blank piperidine without Co(II) precursor was used as a fuel to obtain CS. Field emission scanning electron microscopic (FESEM) images of Co3O4@CS (Figure S2a) and CS (Figure S3a) show interconnected foamy network of carbon with ample porosity. Transmission electron microscopic (TEM) investigations (Figure 2a and Figure S3b) further support FESEM data indicating prominent interconnected porous structure with hierarchical distribution of pores at meso-macro length scales (>30 nm). High-resolution transmission electron microscopic (HRTEM) investigations of Co3O4@CS reveal intimate and uniform dispersion of Co3O4 particles in highly graphitic carbon which is conducive for good electronic conductivity (Figure 2b and Figure S4). Most of the particles were found to be smaller than 2 nm while at very few places bigger particles (5-10 nm) were also observed. Energy dispersive X-ray spectroscopic (Figure S5) elemental mapping indicate the presence of Co in the predominat carbon matrix. Lattice spacings analyzed for a number of regions suggests that the dispersed nanoparticles predominantly exists as Co3O4 species (Figure 2c-d and Figure S2b). The (220) planes (0.286 nm) and (311) planes (0.244 nm) of Co3O4 were observed in most of the regions (Figure 2c and Figure S2b).48 In addition at fewer places, (400) planes (0.202 nm) of Co3O4 were also present.48 Selected area electron diffraction (SAED) pattern of Co3O4@CS shows (Figure 2d) the presence of (220), (311), (400), (511) and (440) planes associated with polycrystalline Co3O4.49 The Co3O4 phase was further confirmed by the presence of very weak peaks (a corollary to low loading of Co ~1.wt.%) at 36.9o and 31.3

o

(2θ) in powder X-ray

diffraction (PXRD) pattern (Figure S6a-b) corresponding to (311) and (220) planes respectively which corroborates well with SAED pattern (Figure 2d) (JCPDS card 43-1003). Figure S7 shows the PXRD pattern of Co3O4. TEM analyses reveals high degree of graphitization in Co3O4@CS (Figure 2b) as compared to CS (Figure S3c) probably due to metal oxide nanoparticle induced catalytic graphitization.50

Furthermore, N2 sorption measurements at 77 K indicate a typical



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type II behavior51 Co3O4@CS and CS (Figure 3) indicating interconnected meso-macroporous structure. Furthermore, the interconnected hierarchical structure is well supported by the pore size distribution calculated by quenched solid density functional theory (QSDFT).52 Both Co3O4@CS and CSshows broad pore size distribution (Figure 3-Figure S8) in the entire mesopore regime associated with the inter-particle induced porosity with aggregation density dictating the non-vanishing distribution. BET analyses shows higher specific surface area (SSA) for CS (165 m2 g-1) compared to that of Co3O4@CS (124 m2 g-1). Since, both the materials are synthesized in an analogous way the observed difference in SSA can be attributed to the higher degree of graphitization (as evident from TEM images; Figure 2b and Figure S3c and is further confirmed by impedance measurements; vide infra) in Co3O4@CS induced by the presence of Co.50, 53-54 The observation of higher intensity of G band (compared to D band) in the Raman spectra for Co3O4@CS (ID/IG = 0.89) as compared to CS (ID/IG = 0.96) (Figure S9) further supports the positive role of metal/metal-oxide nanoparticles in inducing higher degree of graphitization in the former.50,

55

Inductively coupled optical emission

spectroscopic (ICP-OES) analysis of Co3O4@CS indicates that the Co content of the sample is around 1.0 wt.%. However, X-ray photoelectron spectroscopic investigations did not show any Co signal (Figure S10) which can be attributed to the combined effect of low cobalt loading along with the fact that the analyses is surface sensitive with a small penetration depth of the photons (1-10 nm).56 The absence of N 1s in the survey scan is attributed to the favorable thermodynamic equilibrium for the formation of N2 or nitrogen-oxides in the combustion process. OER activity of the Co3O4@CS was evaluated in 1.0 M KOH and was compared with CS, Co3O4 and state-of-the-art catalyst RuO2 (Sigma-Aldrich). Polarized linear sweep voltammograms (LSVs) at rotating disk electrode (RDE) indicates (Figure 4a) that the activity of Co3O4@CS is similar to that of RuO2 in terms of the onset potential and much better compared to bulk Co3O4. However, the activity of Co3O4@CS showed significant improvement at higher overpotentials (η). For instance, RuO2 (1.54 V) requires 20

mV higher

overpotential

compared

to

Co3O4@CS (1.52 V) to achieve an anodic current density of 10 mA cm-2. While for the current densities of 20 mA cm-2 and 30 mA cm-2 the differences being 40 mV and 70 mV respectively. Since an electrochemical current is a measure of the kinetics of a reaction, the observed improved current density (j) for Co3O4@CS results from better oxygen evolution kinetics. Such


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kinetics stems from the accelerated gas detachment at the active interfaces or electrodics (highly dispersed Co3O4 in low density conducting carbon matrix) synergistically aided by the effective mass transport through the highly interconnected meso-macroporous networks leading to rapid generation of electrocatalytically active sites per unit time. Whereas, the activity of CS was found to be inferior compared to both RuO2 and Co3O4@CS (Figure 4a). Tafel analyses in the kinetically dominant region reveal smaller value for Co3O4@CS (61 mV dec-1) compared to RuO2 (71 mV dec-1), Co3O4 (68 mV dec-1) and CS (178 mV dec-1) indicating improved OER kinetics of the former (Figure 4b). To gain insights into the observed OER performance electrochemical impedance spectroscopic (EIS) measurements were performed at their respective onset potentials. Clearly, Co3O4@CS shows least charge transfer resistance (Rct) (Figure 4c) exhibiting a value of 142 Ω which is much smaller compared to RuO2 (497 Ω) and CS (1770 Ω) indicating excellent electronic conductivity of the former. The higher conductivity of Co3O4@CS can be attributed to the higher degree of graphitization (as evident from HRTEM images (Figure 2b-c and Figure S2b) and Raman spectra; Figure S6) a direct corollary to the beneficial presence of metal nanoparticles. Since oxygen evolution is a multi-electron (4e-) process the efficiency of such faradaic process is governed by the effectiveness with which the electrons are transferred to the adsorbed intermediates which in turn is governed by the conductivity of the adsorbent. An electrical equivalent circuit fit for the observed impedance response (Figure 4c) for Co3O4@CS, pertaining to different interfacial phenomena57-61 is presented in Figure S11 and the involved electrode processes is discussed in detail therein.

Clearly, the observed activity of the

Co3O4@CS can be attributed to the excellent conducting nature of the carbon matrix. Practical implications of such catalysts in industrial water electrolyzers are governed by their cyclic stability performance. Accelerated degradation test reveal superior stability of Co3O4@CS even after 2000 cycles (Figure 4d). The observed performance of Co3O4@CS is much better compared to other heteroatom-doped carbon systems with higher Co loading reported previously in terms of overpotential at a current density of 10 mA cm-2 as shown in Table S1. Furthermore, chronoamperometric performance of Co3O4@CS at high (10 mA cm-2) and low (5 mA cm-2) current densities indicate a stable performance over 12 h (Figure 5 and Figure S12). The increase in current after certain period can be attributed to the improved wettability of the electrode interface during the run leading to better exposure of active sites and thereafter it assumes a steady value. Also, it is to be noted that interface does not remain constant in gas



evolution reactions as it is continuously getting regenerated (dictated by the surface tension of the bubbles).

In conclusion, we have shown that metal salt complexed organic solvents (Co(II) and piperidine in our case) can be directly used as a fuel to synthesize catalytically active metal/metal-oxide containing carbon composites. The synthesized material on account of its unique pore architecture and highly accessible active sites shows excellent OER activity in terms of kinetics compared to the state-of-the-art catalyst at higher overpotentials.

Associated Content: Supporting Information Available Notes: The authors declare no competing financial interest.

Acknowledgements We thank Prof. C.N.R. Rao for his constant encouragement and support. D.K.S. thanks CSIR for fellowship. S.C. and A.S. gratefully acknowledges financial support from U.G.C. E.M. acknowledges Sheikh Saqr Fellowship. References:

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Figure 1. (a) Digital photographs indicating the distinct color change (violet) upon complexation of Co(II) acetate (pink; not shown here) with piperidine. (b) Images showing the utilization of piperidine and cobalt acetate complex as a fuel for the synthesis of carbon soot. The inset shows the collected soot over a glass slide.



Figure 2. (a) TEM image of Co3O4@CS indicating mesh like structure indicating highly interconnected meso-macroporous structure. (b) HRTEM image of Co3O4@CS indicating graphitic nature of the carbon walls along with highly dispersed Co3O4 nanoparticles. (c) Shows HRTEM image of Co3O4 indicating lattice fringes corresponding to (220) planes. (d) Selected area electron diffraction (SAED) pattern of Co3O4@CS with indexed bright spots corresponding to the (220), (311), (400), (511) and (440) planes of Co3O4 species superimposed over polycrystalline graphitic carbon rings.


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450

300

CoCo-Pip O @CS 3 4

0.015

0.2 0.010 0.1 0.005

0.0

0.000

150

0 0.0

0

10

20

30

40

Pore diameter (nm)

Cumulative pore volume (cc g -1)

0.3

0.020

dV/dD (cc nm -1g -1)

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


Volume (cc g-1 @ STP)

Page 13 of 16

50

CS Co3O4@CS

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Figure 3. N2 adsorption (closed symbols)-desorption (open symbols) isotherms of Co3O4@CS and CS. The inset shows pore size distribution of Co3O4@CS along with corresponding cumulative uptake calculated via QSDFT method.


(b)

60

40

CS RuO2

30 20 10

Page 14 of 16 Co3O4 Co3O4@CS CS RuO2

0.6

Overpotential (V)

50

Co3O4 Co3O4@CS

0.5 0.4

68 mV dec-1

178 mV dec-1 61 mV dec-1

0.3 0.2

71 mV dec-1

0.1 0 1.3

1.4

1.5

1.6

1.7

1.8

Potential (V vs. RHE)

2000

Co3O4@CS 600 CS RuO2

1600

(d)

Co3O4@CS RuO2

400

200

1200

0

800

0

200

Z'

400

600

400 0 0

400

800

1200

1600

2000

Current density (mA cm-2)

1.2

(c)

-Z"

(a)

-Z"

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



-1.0

100

-0.5

0.0 0.5 1.0 Log j (mA cm-2)

1.5

Co3O4@CS Co3O4@CS after 2000 cycles

80 60 40 20 0

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Potential (V vs. RHE)

Z'

Figure 4. (a) Polarized LSVs at RDE for Co3O4,Co3O4@CS, CS, and RuO2 with corresponding Tafel analyses (b) in their respective kinetic regions. (c) EIS data indicating least charge transfer resistance for Co3O4@CS; inset shows the expanded view of the higher frequency region. (d) Stability performance of Co3O4@CS before and after 2000 cycles.


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Co3O4@CS

80 40 0 -40 -80

10 mA cm-2 0

10000

20000

30000

Time (s) Figure 5. i-t curve of Co3O4@CS at 10 mA cm-2.


40000


Table of contents:

Trick in a wick: Ultrafast synthesis of highly dispersed Co3O4 carbon composites is prepared through combustion using cobalt acetate and piperidine precursors as fuel for enhanced OER kinetics.


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A Simple, Ultrafast Combustion Synthesis of Co3O4 Dispersed

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