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Enhancing Electron Transfer and Electrocatalytic Activity on Crystalline Carbon-Conjugated g-C3N4 Wenhan Niu, Kyle Marcus, Le Zhou, Zhao Li, Li Shi, Kun Liang, and Yang Yang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Catalysis

Enhancing Electron Transfer and Electrocatalytic Activity on Crystalline Carbon-Conjugated g-C3N4 Wenhan Niu†, Kyle Marcus‡, Le Zhou‡, Zhao Li‡, Li Shi†, Kun Liang† & Yang Yang†,‡,* †

NanoScience Technology Center, University of Central Florida, Orlando, FL 32826, United

States ‡

Department of Materials Science and Engineering, University of Central Florida, Orlando, FL

32826, United States *

E-mail: [email protected]

ABSTRACT: Carbon nitride (g-C3N4) materials are electro-activated for oxygen reduction (ORR) and oxygen evolution (OER) reactions when they are supported by conductive carbons. However, the electrocatalytic process on semiconductor-based heterostructures such as carbon supported g-C3N4 still suffers from a huge energy loss because of poor electron mobility. Here, we demonstrated a concept that the conjugation of g-C3N4 with crystalline carbon can improve the in-plane electron mobility and make interior triazine units more electro-active for ORR and OER. As a result, the Co metal coordinated g-C3N4 with crystalline carbons (Co-C3N4/C) showed a remarkable electrocatalytic performance toward both ORR and OER. For example, it displayed an onset potential of 0.95 V for ORR and an overpotential of 1.65 V for OER at 10 mA cm-2, which are comparable and even better than those of benchmark Pt, RuO2, and other carbon nitride-based electrocatalysts. As a proof-of-concept application, we employed this catalyst as an

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air electrode in the rechargeable aluminum-air battery, which showed more rechargeable and practicable than those of Pt/C and RuO2 catalysts in two-electrode coin cell battery. The characterization results identified that the good performance of Co-C3N4/C was primarily attributed to the enhanced in-plane electron mobility by crystalline carbon conjugation and the Co-coordinated g-C3N4 along with nitrogen-doped carbons.

KEYWORDS: g-C3N4; crystalline carbon; oxygen reduction; oxygen evolution; aluminum-air battery

With an ever-growing global energy economy, the development of environmentally friendly and sustainable energy alternatives to traditional fossil fuel products are becoming more significant to mankind. Metal-air batteries (MABs) have attracted extensive attention owing to their high theoretical energy density, low fabrication cost, and facile fuel recycling capabilities.1-3 Although there are many advantages in MABs, the sluggish kinetics process for oxygen reduction (ORR) and oxygen evolution (OER) reactions remain the main challenge for the large-scale commercialization. Platinum group metals (PGMs such as Pt, Ru and Ir etc.) possess high electrocatalytic activity toward ORR and OER, but high cost and poor stability greatly hamper their practical application in MABs.4-6 Therefore, considerable effort has been devoted to developing PMGs-free electrocatalyst materials such as carbon, polymer and transition metal compounds.7-10 Among these,


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nitrogen-contained electrocatalysts like carbon nitride (g-C3N4) are of great interest because of their high content of pyridinic-like nitrogen active sites.11 However, unlike carbon materials, the electrocatalytic performance of g-C3N4 is greatly limited by its conductivity.12-16 At this point, it is necessary to design a g-C3N4-based electrocatalyst with high electron mobility for efficiently catalyzing ORR or OER. There are many developed ideas concerning the optimization of electron conductivity of g-C3N4. For example, the construction of one- or two-dimensional g-C3N4 nanostructures can increase the interfacial area between the carbon support and g-C3N4, simultaneously shortening the electron transfer pathway from conductive support to nitrogen active sites. Unfortunately, by only relying on the van der Waals contacts in the semiconductor-based heterostructures, the electron transfer process still suffers a tremendous energy loss because of the n-type semiconductor characteristic of g-C3N4 (Figure S1a).17 It is acceptable to assume that the pyridine-like nitrogen in a triazine configuration could provide a lone-electron pair to adjacent carbon atoms.14 In this case, it is essential to introduce a π-electron system into the electro-insulating pyridinic nitrogen atoms in triazine, which will undoubtedly result in delocalization of lone-pair-electrons on pyridinic nitrogen and thus make it actively electron-transportable for electrocatalysis. As typical representatives of π-electron system species, graphene and carbon tubes could serve as the conjugation systems with abundantly movable electrons.18 However, the conjugation between g-C3N4 and these carbons is extremely difficult to be achieved so far based on current synthesis strategies, which requires the simultaneous growth of carbon skeleton and g-C3N4 framework. Therefore, the conjugation of highly conductive


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carbon materials with g-C3N4 for obtaining a high-performance g-C3N4 based electrocatalyst toward ORR and OER is still a colossal challenge. Herein, we are the first to report a strategy for in-plane conjugation of g-C3N4 sheets with crystalline carbons by introducing lactose as a carbon source and Co metal as a catalyst promoter. The resultant crystalline, carbon-conjugated g-C3N4 with Co species (referred to as Co-C3N4/C; the configuration can be described by Figure S1b) showed an improved electron mobility and electrocatalytic performance toward ORR and OER when compared with those of benchmarking PGMs and other reference samples. Typically, the sheet-like Co-C3N4/C was synthesized by a space-confined growth method using NaCl salt as a template (see supporting information). More specifically, the precursor for Co-C3N4/C was prepared by mixing melamine, lactose, Co salt and the NaCl template together. Then, the Co-C3N4/C precursor underwent polymerization and the conjugation process of g-C3N4 and amorphous carbon continued under low temperature (the first step in Figure 1a). With an increase in pyrolysis temperature, the Co metal acted as a catalyst to facilitate the formation of crystalline carbon domains within the g-C3N4 framework. After a post-etching process, most Co compounds were removed, which subsequently left crystalline carbon domains and Co coordinated g-C3N4 moieties within the g-C3N4 framework (the second step in Figure 1a and Figure S2). Scanning electron microscopy (SEM) images revealed a sheet-like morphology of Co-C3N4/C (Figure 1b and c). Transmission electron microscopy (TEM) image in Figure 1d confirms that the Co-C3N4/C has a wrinkled surface with a few scattered metal nanoparticles. A closer observation by HRTEM in Figure 1e shows that the entire sample


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consists of crystalline and amorphous domains. For example, the HRTEM image of Figure 1f shows that the lattice space of 0.35 nm in the crystalline domain (yellow line in Figure 1f), consistent with (002) plane of crystalline carbon. The area (blue line) adjacent to crystalline domain exhibits an amorphous structure, which could be comprised of disordered triazine units from the g-C3N4 framework. The similar morphology can also be found in Figure 1g and h. Fourier Transform Infrared Spectroscopy (FTIR) further confirms the triazine units in Co-C3N4/C (Figure S3). The formation of crystalline carbon domains within the Co-C3N4/C framework could be attributed to Co compounds acting as catalysts in promoting the graphitization degree of adjacent carbons during pyrolysis.19 This is evidenced by the similar graphite lattices observed on metal nanoparticle surface (Figure 1d). Furthermore, the lattice space of the metal nanoparticle in Figure 1d coincides with that of the (200) plane of Co metal, which verifies encapsulation of trace amounts of Co nanoparticles within the carbon. The energy dispersive X-ray spectroscopy (EDS) linear scans show that N, C, and Co elements are highly distributed in the selected area without visible Co nanoparticles (Figure S4), confirming the formation of Co coordinated g-C3N4 configurations within Co-C3N4/C. X-ray diffraction (XRD) patterns in Figure 2a indicate that a graphite phase exists in both Co-C3N4/C and C3N4/C, while the full width at half maximum (FWHM) of (002) peaks of graphite in Co-C3N4/C is much smaller than that of the reference sample C3N4/C, which confirms again the effect of Co metal in facilitating the local growth of crystalline carbon in g-C3N4. The crystalline carbon formation in Co-C3N4/C could also be identified by Raman analysis. As can be seen from Figure 2b, the


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typical D, G and 2D band are found at 1343, 1573 and 2677 cm-1 for Co-C3N4/C, which are consistent with characteristic features of crystalline graphite. Generally speaking, the intensity ratio of ID/IG is inverse proportional to graphitization degree of carbon materials.20-21 The lower value of ID/IG indicates the more content of highly ordered carbon structure in the nano-carbon system. These values are calculated to be 0.93 and 1.02 for Co-C3N4/C and C3N4/C, respectively, indicative of the high content of crystalline carbon domain existing in Co-C3N4/C. Additionally, the in-plane crystallite sizes of sp2 hybridization in the g-C3N4 framework can be quantified from the formula: La (nm) = (2.4 ×10−10)λ4(ID/IG)−1 (λ denotes the Raman excitation wavelength).22 At last, the value of La is calculated to be 20.6 for Co-C3N4/C and 18.8 for C3N4/C, respectively, suggesting that the mean size of crystalline carbon domain in Co-C3N4/C is apparently larger than that in C3N4/C, which is consistent with previous XRD observation. X-ray photoelectron spectroscopy (XPS) analysis was carried out to identify the composition of Co-C3N4/C. As shown in Figure S5, N, C and O elements are all detected in Co-C3N4/C and C3N4/C, but no Co element appearing in C3N4/C. The Co 2p orbital of Co-C3N4/C can be deconvoluted into three pairs of peaks, where the two pairs of peaks at 797.9/782.2 eV and 795.9/779.4 eV corresponding to Co2+ 2p 1/2 and Co2+ 2p 3/2, respectively. There are two peaks at 783.0 and 803.1 eV belonging to satellite peaks of Co 2p.23-24 Meanwhile, the high-resolution N 1s spectra present with three fitted peaks at 398.4±0.2, 400.1±0.1, and 401.0±0.1eV, which are corresponded to pyridinic N from the triazine configuration, and pyrrolic N and graphitic N from nitrogen-doped carbon species, respectively. This is indicative of the existence of both triazine


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and nitrogen doped crystalline carbon configurations within Co-C3N4/C.25 Recent theory and experimental observations demonstrated that both graphitic N and pyridinic N-doping could lead to redistribution of electron density of adjacent C and make it electro-active for O2 adsorption/desorption, and thus facilitate the ORR and OER.22,

26

Therefore, based on the

integrated peak areas, the elemental ratio of nitrogen species can be quantified as presented in Table S1, where the pyridinic N content of Co-C3N4/C (4.0 at.%) is higher than that of C3N4/C (3.4 at.%) (Figure S6), which implies the Co metal assists in the formation of more pyridinic N during pyrolysis. To gain a better insight into the electrocatalytic activity of all samples, linear sweep voltammogram (LSV) measurements were conducted to determine the ORR performance. Evidently, the onset potential of the Co-C3N4/C electrode stands out as the highest with 0.95 V (vs. RHE) among the serial electrodes. It is worth noting that the Co-C3N4/C electrode exhibits a more positive onset potential when compared with those of C3N4/C and C3N4-rGO, which sufficiently prove that the crystalline carbon-conjugated g-C3N4 is more favorable for electron transfer in electrocatalytic ORR. The electron transfer number and HO2- yield also identify the high selectivity of the Co-C3N4/C electrode toward ORR with 4 e- transfer process (Figure S7). This result coincides with the highest pyridinic N of the Co-C3N4/C sample in series (Table S1), indicating that the Co coordinated pyridinic N likely plays a major role in the determination of ORR performance. This phenomenon could be also explained by electrochemical impedance spectroscopy (EIS) analysis (Figure 3b). The diameter sizes of semicircles of EIS are increased


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with the following order: R(Co-C3N4/C)< R(C3N4/C)< R(C3N4-rGO), which elucidates the implantation of crystalline carbon into g-C3N4 framework resulting in the high electron mobility for

ORR.

The

double-layer

capacitance

in

Figure

S8

reveals

the

largest

electrochemically active surface area (ECSA) for the Co-C3N4/C electrode among the series, which indicates the sufficiently exposed active sites for ORR. The Co-C3N4/C electrode also showed a superior ORR performance to that of Pt/C electrode and those of other carbon-based electrodes reported recently (Table S2). Moreover, the ORR stability testing for the Co-C3N4/C electrode showed the almost no LSV attenuation after 3000 cycles testing. In contrast, the half-wave potential of LSV on benchmark Pt/C electrode was sharply increased by 20 mV when compared to the initial one. This phenomenon demonstrates the active sites including the Co coordinated g-C3N4 and graphitic N are more stable than Pt nanoparticles for long-term ORR operation. With respect to OER performance, the overpotential (Ej=10) at 10 mA cm-2 was measured to be 1.65 V for Co-C3N4/C electrode, which was higher than 1.58 V for the RuO2/C electrode. The Tafel Plots in Figure S9 show a similar slope (53 mV dec-1) of Co-C3N4/C to that of RuO2/C (51 mV dec-1), implying a favorable 4e- oxidation kinetics for water to O2 conversion on the Co-C3N4/C sample. However, the Ej=10 of RuO2/C electrode dramatically fell to 1.77 V after the cycling test, whereas the Co-C3N4/C electrode still maintained a high OER activity with Ej=10 at 1.66 V, confirming the robustly active sites for OER on Co-C3N4/C electrode. In order to identify the effect of Co species on the activity of ORR and OER, the SCN- poisoning test was conducted to probe the Co-relevant active sites. As shown in Figure S10, the LSVs of ORR and


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OER on the Co-C3N4/C electrode are declined by 30 mV and 38 mV, respectively, after adding KSCN into electrolyte, confirming that the Co species such as nitrogen-coordinated cobalt along with pyridinic and graphitic nitrogen likely serves as the active sites toward ORR and OER. Given a dramatically enhanced catalytic performance on Co-C3N4/C, it is reasonable to consider the use of Co-C3N4/C as a bifunctional air electrode for a rechargeable aluminum-air battery system. There are very few reports for fabrication of rechargeable aluminum-air battery electrodes by using bifunctional g-C3N4-based electrocatalysts, which leads to the meaningful development of our catalysts for this practical application. Figure S11 depicts a schematic illustration of the aluminum-air coin battery assembly. We initially examined the applicability of the aluminum-air coin battery with the Co-C3N4/C electrode by connecting it to a timer with 1.5 V open potential. Obviously, the aluminum-air coin battery can successfully supply the timer with sufficient power, as evidenced by the insets of Figure 3e. The excellent rechargeability of Co-C3N4/C electrode was also identified by the discharge/charge cycling test over 22 cycles, which outperforms 4 cycles of commercial Pt/C+RuO2/C hybrid electrode (Figure 3e). In a careful observation of cycling test, the discharge and charge currents were maintained at ~1.1 V and ~2.0 V, respectively, for Co-C3N4/C electrode under 10 mA cm-2. These values are slightly better than the ~1.0 V and ~2.2 V of Pt/C+RuO2/C hybrid electrode, implying the highly efficient catalytic activity and robust chemical stability enable the application of Co-C3N4/C electrode in the rechargeable aluminum-air battery. Furthermore, as depicted in Fig. 3f, a peak power density of 89 mW cm-2 at 120 mA cm-2 is achieved on the aluminum-air battery using Co-C3N4/C


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electrode, which is higher than 79 mW cm-2 at 103 mA cm-2 for the benchmarking Pt/C+RuO2 hybrid electrode and those of other reported aluminum-air batteries.27-28 Note that the charge polarization curve of the Co-C3N4/C electrode is lower than that of the Pt+RuO2/C hybrid electrode, which is indicative of the good OER performance of the Co-C3N4/C electrode in an aluminum-air battery system. These parameters highlight the outstanding catalytic activity of Co-C3N4/C in an aluminum-air battery system, ascribing to the high electron mobility within Co-C3N4/C offering sufficient active sites to electrocatalytic reactions. In summary, we have successfully demonstrated a strategy to enhance the catalytic performance of g-C3N4 through conjugation with crystalline carbon and coordination with a Co metal species. The resultant Co-C3N4/C catalyst exhibited impressive activity and stability toward OER and ORR, owing to an optimal balance of Co-relevant active sites， nitrogen-doped carbon and electron conductivity. As a proof-of-concept, the Co-C3N4/C was further examined by rechargeable aluminum-air coin battery, which exhibited sufficient rechargeability and practicality. In conclusion, this work will provide an avenue for the development of advanced bifunctional g-C3N4-based catalysts for reversible energy conversion and storage systems.

ASSOCIATED CONTENT Supporting Information

Detailed method and characterization.

AUTHOR INFORMATION

Corresponding Author *E-mail [email protected] Notes 10 ACS Paragon Plus Environment

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All authors declare no competing financial interest.

ACKNOWLEDGMENT：：

This work was financially supported by the University of Central Florida through a startup grant (No. 20080741).

REFERENCE

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Figure 1. (a) Schematic illustration of the preparation process of Co-C3N4/C. (b) and (c) SEM image, (d) TEM and (e) HRTEM images of Co-C3N4/C. (The inset of Figure 1e shows a lattice space of 0.17 in the selected area of the nanoparticle). (f-h) represent the HRTEM images of the selected area with label 1, 2 and 3 (yellow colored square area) in the Figure 1e, respectively. (Note that the right images of Figure 1f, g and h indicate the lattice spaces in the corresponding areas, respectively) 14 ACS Paragon Plus Environment

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Figure 2. (a) XRD patterns and (b) Raman spectrums of C3N4/C and Co-C3N4/C. High-resolution XPS spectra of (c) Co 2p and (d) N 1s in Co-C3N4/C.


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Figure 3. (a) LSVs and (b) EIS of all samples for ORR (electrode rotating speed: 1600 rpm). LSVs before and after cycling test for (c) ORR and (d) OER on Co-C3N4/C electrode, Pt/C and RuO2/C hybrid electrodes. (e) galvanostatic charge-discharge cycling (10 mA cm-2, 1 hour for each cycle) of the aluminum–air coin batteries using different air electrodes. (f) Polarization curves and power density curves of the aluminum-air coin batteries using Co-C3N4/C electrode and Pt/C+RuO2/C hybrid electrode. (the insets of Figure 3e show photographs of aluminum-air coin battery to start

a timer with open potential of 1.5 V) 16 ACS Paragon Plus Environment

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