Carbide decorated carbon nanotube electrocatalyst for high-efficiency hydrogen evolution from seawater

Abstract

Pursuit of robust electrocatalysts has been a persistent objective for hydrogen evolution from seawater. We present here an experimental realization of an efficient Co₃Mo₃C decorated carbon nanotube (Co₃Mo₃C/CNT) electrocatalyst with exceptionally high electrocatalytic activity and good durability through a simple thermal decomposition method for hydrogen evolution reaction (HER), yielding an onset overpotential as low as 42 mV and an exchange current density up to 0.415 mA cm⁻² in seawater. Notably, the resultant Co₃Mo₃C/CNT electrocatalyst displays a promising stability when suffering persistent operation over 26 h in seawater. In comparison with pristine Ni foam electrode, the Co₃Mo₃C/CNT can markedly reduce the overpotential for hydrogen evolution and therefore enhance HER kinetics.

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1. Introduction

Hydrogen energy is a renewable and abundant resource to reduce our dependence on fossil fuels and reduce greenhouse gas emission,^1,2 and water splitting is believed to be a simple method for hydrogen evolution.^3–5 Since the creative detection on water splitting by TiO₂ electrode,⁶ it has been more than forty years, however, there are still some technical challenges in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).^7–13 Pt-group metals are currently the preferred electrocatalysts for HER, but the high cost of these noble metal are gigantic burden for their commercial application.^14–17 By addressing this issue, many focuses have been placed on creating Pt-like candidates such as transition metal carbides (TMCs) without sacrificing catalytic activity.^18–21 TMCs are an important class of compounds with remarkable catalytic activities,^22–26 and notably Co₃Mo₃C is active for catalyzing the hydrodesulfurization (HDS) reaction.^27–29 Since HDS and HER both rely on the reversible binding of the catalyst and hydrogen,^30,31 it is expected that Co₃Mo₃C may also be active for the HER in water splitting. Apart from electrocatalytic activity, the long-term stability is still crucial for the practical utilization. Unfortunately, most of the electrocatalysts are commonly active in acidic or alkaline solutions,^32–34 which not only needs to add sodium hydroxide or sulfuric acid into water system for increasing electrical conductivity, but also leads to great challenges in developing anticorrosion-resistant electrodes. One of the solutions to this impasse is to develop seawater-stable and non-noble-metal HER catalysts and to directly realize hydrogen evolution from seawater.

Since the pioneering creation of Co₃Mo₃C by Newsam,³⁵ Bussell has followed the concept for bulk and alumina supported Co₃Mo₃C by a temperature-programmed nitridation and subsequent carburization route.³⁶ Afterwards, Co–Mo carbide was prepared by Liang through a carbothermal hydrogen reduction method with activated carbon material as carbon source and support.³⁷ Recently, Co₃Mo₃C and MCM41-supported Co₃Mo₃C catalyst have also been successfully prepared by Wang through a simple thermal decomposition method, showing an efficient activity in both reactions of HDS and hydrodenitrogenation (HDN).²⁹ Due to the similar reaction mechanisms for HDS and HER, we present here the preparation of the Co₃Mo₃C/CNT electrocatalyst by a simple hexamethylenetetramine-based one-step thermal decomposition method and study on salient HER performance. Hexamethylenetetramine (HMT) has been used as a reduction agent and molybdate ion ligand, which is an organic compound containing both carbon and nitrogen elements.^38,39 Compared to the synthesis of bimetallic nitride, a higher HMT dosage in the precursor is required synthesize bimetallic carbide. In other words, there is a phase transformation from nitride to carbide in the carbon-enriched pyrolysate produced from decomposition of excess HMT in the precursor.²⁹ Meanwhile, the existence of the Co elements facilitate the formation of the carbon nanotubes. We finally get an efficient Co₃Mo₃C decorated CNT electrocatalyst with exceptionally high electrocatalytic activity and good durability through a simple one-step thermal decomposition method for hydrogen evolution reaction. The wormlike and porous microstructure of the Co₃Mo₃C/CNT electrocatalyst increases the specific surface area for HER. The closely interconnected structure is beneficial of shortening the electron transfer pathway and enabling rapid diffusion of ions and electrons, allowing for an enhanced electrocatalytic activity and good durability. When suffered HER in pristine seawater, the Co₃Mo₃C/CNT electrocatalyst is highly active with an onset overpotential of 42 mV, a Tafel slope of 0.249 V dec⁻¹, and an exchange current density of 415 mA cm⁻². The Co₃Mo₃C/CNT electrocatalyst maintains its activity for at least 26 h and needs overpotentials (η) of 124 and 782 mV to attain current densities of 10 and 200 mA cm⁻², respectively.

2. Experimental

2.1. Materials

All chemicals were used as received without any further purification unless noted. Nickel foam (>99.5%, 1.5 mm) was purchased from Taiyuan Yingze Lizhiyuan Battery. Nafion (5 wt%) was purchased from Sigma-Aldrich. Pt/C (20 wt% Pt) was purchased from Johnson Matthey. The CNT we used was multi-walled carbon nanotube purchased from Shenzhen Nano-port Co., Ltd. with more than 5 μm in length and 7–15 nm in diameter. The target seawater was taken from Yellow Sea (China) and filtrated to remove organic and suspended species.

2.2. Preparation of the working electrode

A precursor was prepared by dissolving (NH₄)₆Mo₇O₂₄·4H₂O, Co(CH₃COO)₂·4H₂O, and HMT with a mole ratio of 1 : 7 : 37 in 15% NH₃·H₂O solution under agitation. The mixture was evaporated at room temperature under stirring for approximate 7 days to form a homogeneous slurry. Subsequently, the abovementioned slurry was dried under vacuum at 333 K over night and ground to obtain the powder precursor, which was then placed in a porcelain boat under a flow of argon (99.99%) and heated to 1023 K at a heating rate of 5 K min⁻¹. After reaction at 1023 K for 2 h, the as-synthesized Co₃Mo₃C/CNT powders were cooled naturally under argon.

Ni foam was thoroughly rinsed by ultrasonic dispersing in acetone, deionized water, and hydrochloric acid for 30 min, respectively, followed by being rinsed with deionized water and dried in the air. Catalyst ink was prepared by dispersing 5 mg of the Co₃Mo₃C/CNT electrocatalyst into a solvent containing 990 μL of isopropanol and 10 μL of 5 wt% Nafion solution. The mixture was then ultrasonicated for 30 min to generate a homogeneous ink. The working electrodes was prepared by soaking a piece of Ni foam in the catalyst ink for about ten seconds and then dried under vacuum at 333 K for 12 h. The electrocatalyst loading amount was controlled at 0.89 mg cm⁻². The preparation procedures of CNT or Pt/C modified Ni foam as well as loading amount were identical to that for Co₃Mo₃C/CNT modified Ni foam.

2.3. Electrochemical characterizations

All the electrochemical measurements were conducted on a CHI660E electrochemical workstation in a typical three-electrode setup, comprising an Ag/AgCl reference electrode, a counter electrode (CE) composed of a Pt sheet, and a working electrode, with an electrolyte solution of seawater. The schematic diagram of the device and the working electrode is shown in Scheme 1. Linear sweep voltammetry (LSV) was carried out in seawater with a scan rate of 1 mV s⁻¹. If not specially instructed, the first cycle LSV curves were utilized to compare the catalytic performances. Onset overpotentials were determined according to the beginning of the linear regime in the Tafel plot. Electrochemical impedance spectroscopy (EIS) measurements were also conducted in the frequency range of 10⁶ Hz to 10⁻² Hz. The time dependency of catalytic currents during electrolysis for the catalyst was tested in seawater at η = 170 mV. In all measurements, the Ag/AgCl reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). RHE calibration was performed experimentally according to reported method.⁴⁰ In seawater, E_RHE = E_Ag/AgCl + 0.197 V + 0.059 pH, and the pH of the seawater was 6.44. In the current work, Pt sheet could be stable under persistent operation as a counter electrode because the dissolution of Pt sheet initiated at 0.95 V (vs. RHE) in 0.1 M HClO₄ at 23 °C.^41,42 It was well known that Pt dissolution processes may be accelerated in an increasing pH aqueous solution during persistent operation. However, neutral seawater instead of acidic solution was employed as a target electrolyte for HER, therefore the dissolution reactions of Pt can be markedly restricted.

Scheme 1 Schematic diagram of the water-splitting device as well as microstructure of working electrode.

2.4. Characterizations

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on SU8020 and Tecnai G2 F20 FEI instruments, respectively. X-ray diffraction (XRD) data were obtained on Bruker D8 Advance with CuKα radiation (λ = 1.5418 Å). Energy dispersive spectroscopy (EDS) measurements were performed on QUANTA250FEG. The BET surface area, pore volume, and pore size were measured on a micromeritics instrument 3Flex at liquid N₂ temperature.

3. Results and discussion

SEM in Fig. 1a and b images shows a homogeneously interpenetrated morphology by wormlike carbon nanotubes with 1–2 μm in length and around 100 nm in diameter, leaving loose feature for diffusion and increase active surface area, and Co₃Mo₃C grafted to these carbon nanotubes. The TEM characterize is utilized to cross-check the distinctive microstructure, as shown in Fig. 1c and d. The d-spacing values of 0.213 nm corresponds to the (511) crystallographic planes for Co₃Mo₃C, which proves the small particles are Co₃Mo₃C. The determination of d-spacing value at 0.339 nm suggests the presence of CNT (Fig. 1e). Meanwhile, the element mappings for C, Co and Mo give a credible evidence for the presence of Co₃Mo₃C on CNTs (Fig. 1f). The characteristic peaks centered at 2θ = 26.7°, 35.3°, 42.4°, 46.4°, 48.6°, 52.2°, 69.4°, 72.4°, and 74.7° correspond to the (311), (331), (511), (440), (531), (620), (733), (822), and (662) facets of Co₃Mo₃C (JCPDS no. 80-0339), respectively. The other characteristic peaks correspond to the facets of C (JCPDS no. 05-0625) (Fig. 1g). The Co₃Mo₃C/CNT electrocatalyst was submitted to thermogravimetric analysis (TGA) (Fig. 1h). The stable line below 369 °C suggested that there are no thermal decomposition reactions for Co₃Mo₃C/CNT electrocatalyst, while an increased weight peak at 369–508 °C was attributed to oxidation reactions of Co and Mo species. The increased weight arises from oxygen species in corresponding metal oxides. The abrupt weight reduction in temperature range of 508–647 °C allows for combustion of CNTs. After calculation, the Co₃Mo₃C loading on corresponding Co₃Mo₃C/CNT electrocatalyst is around 73.4 wt%. The nitrogen adsorption/desorption isotherm plot shows a BET specific surface area (SSA) of 30.8 m² g⁻¹ for the Co₃Mo₃C/CNT electrocatalyst (Fig. 1i), while the average pore diameter is about 3.8 nm which has been determined by BJH pore-size distribution curve (Fig. 1j). The large SSA can expose more catalytic sites for hydrogen adsorption. However, the porous microstructure provides channels for charge transportation and therefore increased HER kinetics.

Fig. 1 (a and b) SEM and (c and d) TEM images, (e) HRTEM image, and (f) element mapping of corresponding Co₃Mo₃C/CNT catalysts. (g) X-ray diffraction (XRD) profile, (h) thermogravimetric analysis (TGA) weight lost curve, (i) nitrogen adsorption/desorption isotherm plot, and (j) pore radius distribution curve of the Co₃Mo₃C/CNT electrocatalyst.

The XPS spectra for the Co₃Mo₃C/CNT electrocatalyst are shown in Fig. 2. Carbon is the main ingredient (Fig. 2a). As shown in Fig. 2b, the dominant peak (64.73%) in the Co 2p_3/2 XPS spectrum at 781.43 eV indicates the presence of Co²⁺,^43,44 and the peak at 793.5 eV is assigned to Co 2p_1/2.⁴⁵ The weak peak located at 786.23 eV and 802.14 eV are attributed to satellite peaks.⁴⁶ The main peaks of Mo 3d appear at 228.81 eV, 232.42 eV, and 235.66 eV, coming from the Mo⁴⁺ 3d_5/2, Mo⁴⁺ 3d_3/2, and Mo⁶⁺ species. The peaks at 228.81 (Mo⁴⁺ 3d_5/2) and 232.42 eV (Mo⁴⁺ 3d_3/2) confirm that the molybdenum is basically in its Mo(IV) state (Fig. 2c).^47–50 In order to study the formation mechanism of Co₃Mo₃C/CNT catalysts, Wang has carefully tuned HMT dosages during synthesis processes.²⁹ Their results demonstrate that low HMT dosage in the precursor results in bimetallic nitride, while high HMT dosage is beneficial to bimetallic carbide such as Co₃Mo₃C. Therefore, excessive HMT is crucial in realizing phase transformation from bimetallic nitride to bimetallic carbide.

Fig. 2 XPS spectra of the Co₃Mo₃C/CNT electrocatalyst: (a) survey spectrum, (b) Co 2p, (c) Mo 3d.

The polarization curve of Co₃Mo₃C/CNT modified Ni foam electrode in seawater with a scan rate of 1 mV s⁻¹ is shown in Fig. 3a. Pristine Ni foam, CNT modified Ni foam and Pt/C modified Ni foam were also studied for comparison. Due to the effects from ohmic resistance, the as-measured currents cannot demonstrate the intrinsic charge-transfer behaviors of electrocatalysts. To address this issue, an iR compensation has been applied to all electrochemical characterizations. Pristine Ni foam and CNT modified Ni foam show very poor HER activity. In sharp contrast, the Co₃Mo₃C/CNT modified Ni foam electrode is highly active toward the HER with an onset overpotential of 42 mV (vs. RHE) and it can get a current density as high as 200 mA cm⁻² at an overpotential of 782 mV. The cathodic current density has an increasing evolution at more negative potentials, requiring overpotentials as low as 124, 577, and 782 mV to afford current densities of 10, 100, and 200 mA cm⁻², respectively. There are not apparent deviations for other electrochemical parameters. By comparing the overpotential of 695 mV for pristine Ni foam to yield a current density of 10 mA cm⁻², the markedly reduced overpotential for Co₃Mo₃C/CNT electrocatalyst demonstrates it to be promising for HER from seawater splitting.

Fig. 3 (a) Polarization curves of Ni foam, CNT modified Ni foam, Pt/C modified Ni foam and Co₃Mo₃C/CNT modified Ni foam electrode. (b) The corresponding Tafel plots recorded at a scan rate of 1 mV s⁻¹.

The Tafel plots of Ni foam and Co₃Mo₃C/CNT modified Ni foam electrode in seawater determined at a scan rate of 1 mV s⁻¹ are shown in Fig. 3b. The linear portions of the Tafel plots are fitted by Tafel equation (η = b log j + a, where η is the overpotential, b refers to Tafel slope, j is the current density, and a is the intercept interrelate with the exchange current density j₀). The Tafel slopes of Ni foam and Co₃Mo₃C/CNT modified Ni foam electrode are calculated to be 0.262 and 0.249 V dec⁻¹, respectively. The exchange current density (j₀) of the Co₃Mo₃C/CNT modified Ni foam electrode is approximately 0.415 mA cm⁻², which is 13 times larger than that of Ni foam electrode and much higher than those from other HER catalysts.^43,51–55 The higher j₀ and the more positive onset potential for the Co₃Mo₃C/CNT modified Ni foam electrode are possibly a result of the HER occurring on a porous matrix with higher SSA and the high catalytic activity of Co₃Mo₃C/CNT.^56,57

Electrochemical impedance spectroscopy (EIS) is investigated at various HER overpotentials to demonstrate the relative enhancement of the catalytic activity. As shown in Fig. 4 the Nyquist and Bode EIS plots (Fig. 4b and c), the plot for the Co₃Mo₃C/CNT modified Ni foam electrode has two semicircles at all overpotentials, indicating two electrochemical reaction processes for realizing HER in seawater. The first one at high frequency correlates with the large surface of the electrode, whereas the other at low frequency region is related to the kinetics of the HER process. From the low-frequency charge-transfer resistance (R_ct) as a function of overpotentials presented in Fig. 4d, one can find that the R_ct decreases from 15.8 ohm cm² at 42 mV to 0.1 ohm cm² at 700 mV, allowing for accelerated HER process at increasing overpotentials. This conclusion can be supported by the electron lifetime (τ = 1/2πf, f is the frequency at maximum peak), and the electron lifetime of the carbon nanotube supported Co₃Mo₃C electrocatalyst and at various HER overpotential was shown in Table 1.

Fig. 4 (a) Nyquist and (b and c) Bode EIS plots of the Co₃Mo₃C/CNT modified Ni foam electrode at various HER overpotentials in seawater. (d) The low-frequency charge transfer resistance R_ct and equivalent electrical circuit used to model the HER process on the Co₃Mo₃C/CNT modified Ni foam electrode at various overpotentials.

Table 1 The electron lifetime of the Co₃Mo₃C/CNT modified Ni foam electrode at various HER overpotentials

Overpotential (mV)	42	100	300	400	500	600	700
Rct (ohm cm^2)	15.8	11.1	5.2	2.5	0.8	0.3	0.1
τ (ms)	74.1	50.4	28.3	23.4	8.9	3.4	1.6

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Stability of the catalytic response is further tested by cycling the Co₃Mo₃C/CNT modified Ni foam electrode continuously for 5000 cycles at a scan rate of 100 mV s⁻¹, and the potential ranges from +0.20 to −0.20 V vs. RHE. At the end of the cycling procedure, the Co₃Mo₃C/CNT modified Ni foam electrode afforded similar LSV polarization curves to the initial cycle with negligible loss in cathodic current density. The long-term stability of the Co₃Mo₃C/CNT modified Ni foam electrode is also evaluated by electrolysis at a fixed potential of 170 mV. Fig. 5a shows that the current density remains at around 14 mA cm⁻² over 26 h, and it tends to increase with time elapsing. These results demonstrate that the Co₃Mo₃C/CNT modified Ni foam electrode has superior stability under the experimental conditions in a long-term electrochemical process. As a reference, the stability of the Ni foam electrode is also performed under the same conditions, yielding an obvious loss in cathodic current density. Its long-term stability is evaluated at a potential of 623 mV. Fig. 5b shows that the catalytic current density is only around 4 mA cm⁻² for 16 hours, and there is a 19% reduction after 20 hours.

Fig. 5 (a) Stability of the Co₃Mo₃C/CNT modified Ni foam electrode with an initial LSV polarization curve and after 5000 cycles in seawater at a scan rate of 100 mV s⁻¹, and the time dependence of catalytic currents during electrolysis at an overpotential of 170 mV (inset). (b) Stability of the Ni foam electrode with an initial LSV polarization curve and after 500 cycles in seawater at a scan rate of 100 mV s⁻¹, and the time dependence of catalytic currents at an overpotential of 623 mV (inset).

4. Conclusions

In summary, the Co₃Mo₃C/CNT decorated Ni foam electrode with exceptionally high catalytic activity and good durability has been successfully prepared by a simple one-step thermal decomposition method for realizing hydrogen evolution from seawater. The preliminary results demonstrate that the Co₃Mo₃C/CNT modified Ni foam electrode is highly active for HER in seawater with an onset overpotential as low as 42 mV, a Tafel slope of 0.249 V dec⁻¹, and an exchange current density of 0.415 mA cm⁻². The overpotentials of 125 and 795 mV are required to attain current densities of 10 and 200 mA cm⁻² for the Co₃Mo₃C/CNT modified Ni foam electrode, respectively, which are much lower than those from pristine Ni foam electrode. Additionally, the resultant Co₃Mo₃C/CNT modified Ni foam electrode is relatively stable under persistent operation in seawater over 26 h. Our present study is far from optimization, but the preliminary results demonstrate that the cost-effective Co₃Mo₃C/CNT electrocatalyst is promising for hydrogen evolution from seawater.

Acknowledgements

The authors would like to acknowledge financial supports from National Natural Science Foundation of China (21503202, U1037604), and Collaborative Innovation Center of Research and Development of Renewable Energy in the Southwest Area (05300205020516009).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17839a

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