Molybdenum Carbide-Anchored N

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Energy, Environmental, and Catalysis Applications

In-situ fabrication of Nickel/Molybdenum Carbide anchored N-doped graphene/CNT hybrid: An efficient (pre)catalyst for OER and HER Debanjan Das, Saswati Santra, and Karuna Kar Nanda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09941 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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ACS Applied Materials & Interfaces

In-situ fabrication of Nickel/Molybdenum Carbide anchored N-doped graphene/CNT hybrid: An efficient (pre)catalyst for OER and HER Debanjan Das, Saswati Santra and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore - 560012, India. KEYWORDS: Molybdenum carbide, CNT-graphene hybrid, electrocatalysis, hydrogen evolution reaction (HER), oxygen evolution reaction (OER)

ABSTRACT: Despite the recent promise of transition metal carbides as non-precious catalysts for hydrogen evolution reaction (HER), their extension to oxygen evolution reaction (OER) in order to achieve the goal of overall water splitting remains a significant challenge. Herein, a new Ni/MoxC (MoC, Mo2C) nanoparticles supported N-doped graphene/CNT hybrid (NC) catalyst is developed via a facile, one-step integrated strategy which can catalyze both the HER and OER in an efficient and robust manner. The catalyst affords low overpotentials of 162 and 328 mV to achieve a current density of 10 mA/cm2 for HER and OER, respectively, in alkaline medium which either compares favorably or exceeds most of the Mo-based catalysts documented in literature. It is believed that an electronic synergistic effect between MoxC, Ni and NC wherein a tandem electron transfer process (Ni MoxC NC) may be responsible for promoting the HER as well as OER activity. This work opens a new avenue towards the development of multi-component, highly efficient but inexpensive electrocatalysts for overall water splitting.

Ever-increasing global energy demands coupled with growing concerns regarding the use of fossil fuels have resulted in tremendous efforts towards development of alternative energy sources.1-2 Hydrogen, often deemed as the future fuel is an ideal energy carrier. Compared to the current fossil-fuel based energy technologies, hydrogen burns without emitting any greenhouse gases and can be generated on a large scale by water electrolysis using renewable energy sources such as sun and wind.3 Electrochemical water splitting consists of two half-cell reactions, cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) both of which requires a considerable overpotential (η) to proceed in a practical manner.4-5 Currently, Pt-based materials are considered as the state-of-art HER catalysts while Ir/Rubased materials are the OER benchmark.6-7 Unfortunately, the low earth-abundance and exorbitant price of these metals significantly hinders their widespread usage. Therefore, the development of low-cost but efficient alternative bi-functional HER and OER catalytic materials is highly desired.8-11 12 13

density unoccupied d-orbitals in Mo.17 While this favours the H+ reduction (i.e., Volmer step), it restricts desorption of the adsorbed hydrogen, Hads (i.e., the Tafel/Heyrovsky step) proving to be a bottleneck for the overall HER kinetics. Further improvement of HER activity would require optimizing the electron density on Mo2C for a more favourable kinetics. One way to achieve this goal would be the modification of Mo2C with Group VIII metals since the electron rich dopants can effectively reduce the unoccupied d-orbitals of Mo. Recently, it was established that among the 3d transition metals (e.g. Fe, Co, Ni, Mn), Nibased catalysts exhibit the best HER performance in alkaline medium owing to the optimum OH-Ni2+δ (0< δ Ni-MoxC/NC-50 (11.7 mF/cm2) (inset of Figure 5c). Interestingly, when the current density of the samples obtained from the polarization curves were normalized with their respective ECSA’s [J ECSA (mA/cm2] (Figure 5c) they were found to be very close to each other. This points to the fact that the intrinsic activities of the Ni-MoxC/NC samples are similar and depends upon the availability of active sites, while a small disparity in the performance may be attributed to the

Figure 7. (a) HER and (b) OER polarization curves of NiMoxC/NC-100 in commercial and Fe-free 1M KOH solution

tructural features of the samples that might affect the electrolyte permeation. The role of the individual components in Ni-MoxC/NC100 is explained as follows: i) Molybdenum carbide: Molybdenum carbides have been found to be good catalyst for HER due to a similar d-band electronic configuration as Pt. With a strong hydrogen adsorption energy (Hads), MoxC facilitates the intermediate step (in both Volmer-Heyrovsky and Volmer-Tafel pathway) of HER, i.e. adsorbing H atom on the catalyst surface. ii) Nickel: The role of nickel lies in bringing about a synergistic effect between molybdenum carbide and itself. As discussed above, molybdenum carbide possesses high hydrogen adsorption energy making subsequent steps of bond breaking and generation of gaseous H2 difficult. However, after incorporating nickel the strength of Hads bound to Mo atoms could be reduced, thus facilitating the desorption process. This was achieved by electron

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transfer from Ni to molybdenum carbide (Fig. 10 a) which resulted in lowering the valence state of Mo. More importantly, it was found that Ni 2+ species was the active site for the water dissociation (Volmer step) in alkaline medium. These facts are consistent with our findings where indeed the chemical state of Ni in Ni-MoxC/NC100 was found to be +2 (Fig.10 a) and Volmer step was found to be the rate determining step from the Tafel slope (Fig. 5 b). Thus, incorporation of Ni was a key factor in improving the catalytic activity of molybdenum carbide. iii) N-doped carbon: The role of the N-doped carbon support was manifold. Its presence prevented the aggregation and detachment of the particles during the pyrolysis process thus helping to retain the structural integrity of the composite. Similarly, under the electrocatalytic condition, it limited the corrosion and overall morphology collapse in alkaline media. The porous carbon support ensures a good contact between the catalyst and the electrolyte by facilitating charge and mass transfer. Particularly, for Ndoped carbon, the dopants have been shown to increase the electron density on the graphene surface resulting in improved HER. Durability against strongly alkaline medium over an extended period is of utmost importance for a catalyst used in water electrolyzer. Figure 5d shows the HER chronoamperometric response of Ni-MoxC/NC-100recorded over a period of 50,000s (>14h) with continuous operation at an overpotential of 165 mV that shows a retention of 90.7% of the initial current density, a testimony to the stability of the catalyst. Additionally, accelerated degradation test (ADT) carried out at a scan rate of 100 mV/s shows only a slight positive shift of 9 mV to achieve a current density of 10 mA/cm2 after 2000 cycles (inset of Figure 5d). The electrocatalytic OER performance of the catalyst was next assessed by LSV in 1.0 M KOH solution at a scan rate of 1 mV/s (Figure 6a). As expected, bare GCE does not exhibit any catalytic activity towards OER. Ni-MoxC/NC100 shows the highest activity as compared to others, requiring an overpotential of only 328 mV which is 32, 43 and 61 mV less than that required by Ni-MoxC/NC-200, Ni-MoxC/NC-50 and Mo2C/NC-100, respectively, to reach a current density of 10 mA/cm2, a metric relevant to solar fuel synthesis. Interestingly, the activity of Ni-MoxC/NC100 even surpasses RuO2 (benchmark OER catalyst) that requires an overpotential of 339 mV to achieve a current density of 10 mA/cm2, which is significant considering the fact that molybdenum carbides have rarely been reported as OER catalysts and that too with quite poor activity,23 making it an attractive alternative for OER in alkaline medium. Moreover, the OER activity of the catalyst developed here was either comparable or superior to most of the Ni or transition-metal carbide OER catalyst reported till date (Table S2).

By plotting overpotential (η) against log (J), the Tafel slope of OER was evaluated (Figure 6b). The Tafel slope was found in the order of: RuO2 (72 mV/dec) < NiMoxC/NC-100 (74 mV/dec) < Ni-MoxC/NC-200 (91 mV/dec) < Ni-MoxC/NC-50 (102 mV/dec). A smaller Tafel slope is indicative of faster kinetics which is duly reflected on the catalytic performance of the samples in the same order. Additionally, the Nyquist plots also indicates the charge transfer resistance (Rct) of the catalysts in the order of: Ni-MoxC/NC-100 (17.9 Ω) < Ni-MoxC/NC-200 (31.4 Ω) < Ni-MoxC/NC-50 (38.1 Ω) indicating a favorable electron transfer at the electrode interface in the same order. The double layer capacitance was then measured in a faradaically silent region to estimate the ECSA of the catalyst and found that Ni-MoxC/NC-100 possessed the highest Cdl value (27.2 mF/cm2) followed by Ni-MoxC/NC-200 and Ni-MoxC/NC-50 (14.1 and 9.5 mF/cm2, inset Figure 6c). When normalized to the ECSA, the polarization curves of the three samples shows only a small difference (Figure 6c) suggesting the same intrinsic activity of all the three samples. The OER mechanism of Ni-based catalyst in alkaline medium typically consists of four elementary steps:47 Ni2+ + 3OH- ↔ NiOOH + H2O + e-

NiOOH + OH ↔ NiO(OH)2 + e

Step (1)

-

Step (2)

NiO(OH)2 + 2OH- ↔ NiOO2 + 2H2O + 2e-

Step (3)

NiOO2 + OH- → NiOOH + O2 + e-

Step (4)

-

Summary: 4 OH

↔

O2 + 2H2O + 4e

-

Step (1), (2) and (3) are reversible and determine the overall rate of the process while step (4) is fast and irreversible. From the results obtained for the Ni-MoxC/NC catalysts towards OER, it is expected that a similar mechanism is followed. Surface Ni atoms are first partially oxidized into NiOOH to form NiOOH/Ni as the actual active surface as evidenced by the oxidation peak of Ni (II) to Ni (III) at ~1.36 V, which then facilitates the oxidation of OHinto molecular O2. It is worth noting that the presence of metallic Ni nanoparticles in Ni-MoxC/NC catalysts is advantageous to nickel oxide/hydroxide in view of their higher electrical conductivity. Interestingly, the intensity of the oxidation peak varies in the order of Ni-MoxC/NC100 > Ni-MoxC/NC-200 > Ni-MoxC/NC-50 signifying a synergistic effect in the same order between the Mo2C/MoC and Ni nanoparticles that contributes to the generation of NiOOH species, subsequently promoting the OER performance. Furthermore, the N-doped carbon framework (CNT/graphene hybrid) which supports the Ni-MoxC catalyst plays a crucial role in improving the OER activity, wherein the N-dopant renders the adjacent carbon atoms positively charged which can then adsorb OH- via electrostatic adsorption leading to its subsequent oxidation.48

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Moreover, the direct role of MoxC nanoparticles towards OER cannot be completely ignored. Recent findings have shown MoO2 to be an independently good catalyst for OER49 whose performance is further enhanced in conjugation with other transition metal oxides (e.g. CoO).50 MoxC particles can undergo surface oxidation during OER and enhance the activity further in association with insitu formed NiOOH/Ni(OH)2 species. However, the exact nature of their interaction and its role for improved OER performance remains unclear at this point.

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HER and post-OER are shown in Figure 8 a. For the sample collected post-OER ADT, barring the Ni (111) peak at 44.2o, the peaks corresponding to Mo2C and MoC all but disappears pointing to the severe oxidation of molybdenum carbide during the OER process as discussed earlier which may be a factor for the relatively poor longterm stability of the catalyst. Additionally, a small peak corresponding to β-Ni(OH)2 (100) is observed at 17.7o which may have aroused via oxidation of surface Ni atoms.

Recent findings of Boettcher and co-workers revealed that even trace amount of Fe-impurities (often present in commercial KOH) can significantly enhance the activity (especially OER) of Ni-based catalysts. To verify if that is the case, polarization curves in Fe-free KOH was acquired to compare with the results obtained in commercial KOH solution. Figure 7 (a) shows no apparent difference in the HER polarization curves of Ni-MoxC/NC-100 in commercial and Fe-free 1M KOH suggesting that incidental Fe impurities are not responsible for the HER activity of the catalysts. Though an observable increase in the OER activity was noted (Figure 7b), it was not as pronounced as reported by Boettcher and co-workers.31, 51 This may be explained by the fact that the catalysts used in those studies Ni/Co- Fe (oxy) hydroxides were electrodeposited on the working electrode and were directly used for OER thus providing a large “bare” surface to interact with the Fe-impurities present in the KOH solution. Our catalysts were different for a couple of reasons: i) The MOOH species (M= 3d transition metal, Ni in our case) which is especially susceptible to iron contamination was generated in-situ during the oxidation cycle and not the sole catalyst component as in the previously discussed ones, ii) additionally, since the Ni nanoparticles were supported by a protective graphitic carbon layer the contamination with Fe-impurities was probably limited. Therefore, the Feimpurities did not play a significant role towards the activity of our catalyst. The long-term stability of Ni-MoxC/NC-100 was evaluated by chronoamperometry in 1M KOH solution at an overpotential of 330 mV (Figure 6d) which shows a 75.9 % retention of the original current density over a period of 50,000s (> 14h). Moreover, ADT of the catalyst was carried out at 100 mV/s for 2000 continuous cycles. The LSV curves before and after the ADT is (inset: Figure 6d) reveal that the catalyst (Ni-MoxC/NC-100) after ADT, requires an additional overpotential of 37 mV to reach a current density of 10 mA/cm2 as compared to the fresh catalyst. It is evident that despite the promising OER activity of the catalyst, its stability is a matter of concern. This is due to the easy oxidation of Mo which still remains an exceeding challenge to overcome.52 53 To gain further insight regarding the stability of the catalyst, they were characterized after ADT for HER and OER. The XRD patterns of the three samples i.e. fresh, post-

Figure 8. (a) XRD pattern and high-resolution (b) Ni 2p and (c) Mo 3d XPS spectra of fresh (no carbon paper support), post-OER and post-HER samples of Ni-MoxC/NC-100.

The post-HER sample however retains all the characteristic peaks of the fresh sample which suggests that the catalyst is structurally more stable in HER environment. Note that the sharp graphitic carbon peak appearing at ~ 26o and 54oin the post HER & OER samples are because of the carbon paper on which the catalyst was supported during ADT (the XRD of the fresh sample was acquired in pow-

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der form, i.e. unsupported). The high-resolution XPS spectra of Ni 2p (Figure 8b) reveal that the post-HER sample shares similar features with the fresh sample, whereas the post-OER sample exhibits a peak at 855.9 eV corresponding to Ni3+ species which is known to be highly active for OER.54 In the case of Mo 3d spectra Table 1: Total water splitting activity of Ni-MoxC/NC100 compared to some of the recently developed electrocatalysts in alkaline medium

(Figure 8c), the post HER samples exhibited a pattern with peak position similar to the fresh sample, however,

Catalyst

Loading density (mg cm-2)

Current density (mA cm-2)

Voltage

VOOH hollow nanospheres

0.8

10

1.62

55

NiFeOx/CFP

1.6

10

1.61

56

NiSe/NF

2.8

10

1.63

57

Ni5P4/NF

3.5

10

1.69

58

CoPx/NC

1

10

~1.71

59

SrNb0.1Co0.7Fe0. 2O3–δ nanorods

0.464

10

1.68

60

NiFe LDH/NF

N/A

10

1.7

26

3.5 nm Pt /NF

N/A

10

1.71

26

Ni(OH)2/NF

N/A

10

1.82

26

Ni3S2/NF

1.6

13

1.76

61

L-0.5/rGO nanohybrids

N/A

10

~1.65

62

Ni/Mo2CPC/NF

2

10

1.66

63

Ni-MoxC/NC100

1

10

1.57

(V)

Ref.

This work

the post OER samples showed an obvious shift in peak position to a higher binding energy. Interestingly, Cui and co-workers made a similar observation in case of MoO2 nanosheets which was proposed to improve the OER activity of the catalysts.64 Therefore, a similar effect may be presumed in this case, though the exact nature of the contribution remains uncertain at this point. This loss of feature is more pronounced than the XRD spectra, which may be due to the fact that XPS is sensitive to the surface where the effect of ADT would be more severe, compared to the core. In view of its excellent catalytic properties, we examined the possibility of using Ni-MoxC/NC-100 for practical wa-

ter splitting. We therefore, loaded the catalyst on commercial carbon paper and used it as a cathode as well as an anode for homemade electrolyser. Polarization curves of Ni-MoxC/NC-100// Ni-MoxC/NC-100 couple in comparison to RuO2//Pt-C, RuO2//RuO2 and Pt-C//Pt-C is shown in Figure 9a, wherein a water splitting current density of 10 mA/cm2 was achieved at a cell voltage of 1.72 V which is comparable to most of the recently developed bifunctional total water splitting electrocatalyst (Table S3). The cell voltage required to drive a current density of 10 mA/cm2 was deduced from the cathodic scan of the CV curve of the Ni-MoxC/NC-100 in order to eliminate the contribution from the redox couple. The volume of H2 and O2 evolved during the potentiostatic electrolysis experiment was monitored by water displacement method similar to one used by Zhang and colleagues.65 The amount of gas collected were in close agreement to the calculated values based on Faraday’s law suggesting nearly 100% faradaic efficiency in both cases (Figure 9b). Moreover, the experimentally recorded gas volumes as a function of water-splitting time exhibited a linear slope of 0.034 mL/min and 0.0165 mL/min for H2 and O2, respectively. The slope ratio of 2.06 is very close to the theoretical ratio of 2, as anticipated for hydrogen and oxygen production by water splitting. The most plausible explanation for the enhanced overall water splitting activity of Ni-MoxC/NC composite catalysts is the electronic synergistic effect between MoxC, Ni and N-doped C that can be rationalized as follows: Ni 2p XPS spectra suggest metallic Ni (i.e. Ni0) in Ni/NC-100 and a +2 oxidation state66 in Ni-MoxC/NC-100 (Figure 10 a), while Mo 3d of Ni-MoxC/NC-100 shifts to a lower binding energy than Mo2C/NC-100 (Figure 10 b). The change of oxidation state of Ni along with the shift in Mo 3d peak manifests the transfer of electron density from Ni to the closely coupled MoxC. It is worth mentioning that Ni 2+ species are the active site for water dissociation (Volmer step)67-68 and is consistent with the Tafel slope obtained for HER. In addition, pyridinic-N in Ni-MoxC/NC-100 activates their adjacent C atoms via modifying its band structure, raising the density of the π states near the Fermi level and lowering the work function, thus, manifesting additional active sites for HER.14 69 70 These results point to a tandem electron transfer processes ( Ni Mo C N) as shown in Figure 10 (c) that is proposed to be responsible for HER activity. The synergistic effect is also responsible for the enhanced OER activity of NiMoxC/NC-100 as compared to Ni/NC-100. The OER mechanim of Ni-based catalyst in alkaline medium relies on the surface oxidation of Ni to form NiOOH/Ni that promotes the oxidation of OH- to O2. The transformation from Ni (II) to Ni (III) is evidenced by an oxidation peak near 1.36 V. Interstingly, a shift in the oxidation peak for Ni-MoxC/NC-100 towards a negative potential and higher

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current density as compared to Ni/NC-100 (Figure 6a) was observed.

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vented a strong electronic coupling between the two species resulting in an impeded synergistic effect towards the formation of NiOOH species.

(c) Figure 9. (a) Linear polarization curves of Ni-MoxC/NC100// Ni-MoxC/NC-100, RuO2//Pt-C, RuO2//RuO2 and PtC//Pt-C couples for overall water splitting in 1M KOH (catalyst loading: 1mg cm-2). (b) O2 and H2 production volumes (theoretically calculated and experimentally measured) as a function of water-splitting time.

This demonstrates that the synergestic effect between MoxC and Ni generated in Ni-MoxC facilitates the formation of NiOOH species which subsequently leads to an improved OER activity. This observation can also shed some light on the apparent deviation of the OER activity trend away from the Ni composition for Ni-MoxC/NC200. Elemental analysis of the samples revealed the Ni composition (atomic %) to be 2.26 %, 4.39% and 8.57 % while Mo was found to constitute 1.94%, 4.01% and 8.29% of Ni-MoxC/NC-50, 100 and 200 respectively. It is therefore expected that Ni-MoxC/NC-200 having the largest amount of Ni should naturally result in the formation of OER active NiOOH species. However, the large size of the Ni and MoxC particles in case of Ni-MoxC/NC-200 pre-

Figure 10. (a) High-resolution Ni 2p XPS spectra of NiMoxC/NC-100 and Ni/NC-100, (b) high-resolution Mo 3d XPS spectra of Ni-MoxC/NC-100 and Mo2C/NC-100, and (c) schematic illustration of the electron transfer process facilitating total water splitting.

CONCLUSIONS In summary, we report in-situ synthesis of novel Ni/MoxC supported N-doped graphene/CNT hybrid architecture by thermal treatment of nickel molybdate nanorods with melamine along with a comprehensive mechanistic un-

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derstanding of the process. This facile, cost-effective and scalable method is a significant improvement over recently developed procedure to obtain Ni/MoxC hybrids which involves multiple step synthesis and considerable technical expertise. This low-cost composite catalyst can efficiently catalyze both HER and OER in alkaline medium with striking kinetic parameters, including low overpotential (η) values of 162 mV and 328 mV to reach a current density of 10 mA/cm2 for HER and OER respectively. The synergistic effect between biphasic molybdenum carbide and Ni nanoparticles along with their strong chemical coupling with N-doped carbon (CNT/graphene) is believed to drive the excellent catalytic activity which is better or comparable to most of the transition metal carbide based bi-functional electrocatalyst for HER and OER reported in recent literature, which is among the first instances of a transition metal carbide based overall water splitting catalyst. Finally, the facile synthesis procedure described here can be extended to similar transition metal (Fe, Co, Mn)-MoxC/carbon hybrid architecture which may be employed for a myriad of applications such as lithium-ion batteries, supercapacitors, electromagnetic shielding, oxygen reduction reaction etc. ASSOCIATED CONTENT Supporting Information Complete experimental details and procedures including metal complex syntheses, hydrogenation procedures, and characterization data for all compounds. The materials may be downloaded free of charge at pubs.acs.org AUTHOR INFORMATION Corresponding Author * Email for Prof. K. K. Nanda: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Part of the reported research (characterization) was carried out using the facilities at CeNSE, IISc. DD gratefully acknowledges Bristol-Myers Squibb for a graduate fellowship and Prof. Dr. Heinz Grafsmaa (Deutsches Elektronen-Synchrotron,DESY) for helpful discussion. SS acknowledges financial assistance from Dr. DS Kothari post-doctoral fellowship, UGC, Govt. of India. The authors acknowledge Dr. Vanaraj Solanki for helping in the analysis of XPS results. References 1. Roger, I.; Shipman, M. A.; Symes, M. D., EarthAbundant Catalysts for Electrochemical and

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Molybdenum Carbide-Anchored N

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