An Ni–P/C electro-catalyst with improved activity for the carbohydrazide oxidation reaction

Abstract

Ni–P/C nanoparticles (NPs) were prepared through a two-step liquid phase reduction method, which exhibited a significant enhancement for electro-catalytic activity towards the carbohydrazide oxidation reaction compared with Ni/C NPs. The proposed mechanism of the enhanced activity can be attributed to the improved anti-oxidation ability of Ni atoms and the charge transfer from Ni to P atoms in the catalysts.

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Direct hydrazine fuel cells (DHFCs) have drawn considerable attention because of their high energy density and green emission since the development in the 1970s for vehicle propelling units.^1–4 However, the high toxicity and carcinogenicity were the crucial obstacles regarding the use of hydrazine.^5,6 To pursue more desirable fuels for substituting hydrazine, the derivatives of hydrazine hydrate have received much consideration.^1,7–11 Among them, carbohydrazide (CHZ) was believed to be an ideal underlying substitute due to its advantages, that is, (a) solid state which could cut down the risk of hydrazine leaking, (b) high theoretical electric motive force of direct carbohydrazide fuel cell (+1.65 V),^12,13 which is higher than direct hydrazine fuel cell (+1.61 V)^14,15 and hydrogen oxygen fuel cell (+1.23 V), (c) higher energy density (4.2 kW h L⁻¹) than that of direct hydrazine fuel cell (3.5 kW h L⁻¹), (d) releasing 8 electrons by fully oxidizing of each carbohydrazide molecule, as shown in eqn (1)–(3):

CH₆N₄O + 8OH⁻ → CO₂ + 7H₂O + 2N₂ + 8e⁻ (1)

2O₂ + 4H₂O + 8e⁻ → 8OH⁻ (2)

CH₆N₄O + 2O₂ → CO₂ + 3H₂O + 2N₂ (3)

However, there were only few reports for carbohydrazide oxidation reaction (CHZOR) in recent years due to the sluggish kinetic rate in comparison to that of hydrazide oxidation reaction. Asazawa et al.¹ investigated various metal-disk electrodes as electro-catalysts for hydrazine and hydrazine derivatives in alkaline media and demonstrated the electrocatalytic activities toward carbohydrazide oxidation reaction of various metal electrodes (Au, Ag, Pt, Ni, Cu) were much weaker than those of hydrazine. Recently, Li group reported that CNT¹² and 3D graphene¹⁶ as metal-free catalysts with improved performance for carbohydrazide oxidation reactions compared to that of carbon black or multi-layer graphene in single electrode and single fuel cell.

The non-metal elements, such as B, N, P or S, have been successfully used to improve the activity of metal electro-catalysts in recent works.^17–24 For example, Pd–B was prepared and applied as oxygen reduction reaction catalyst with improved activity compared with commercial Pd or Pt catalysts.²⁵ In addition, Pd–P nanoparticles obtained a superior electro-catalytic activity towards oxygen reduction reaction than Pt/C or Pd/C.²⁶ Recently, Ni–B/C was prepared with improved electro-catalytic performance towards hydrazide oxidation reaction in our previous work.²⁷

Herein, the Ni–P/C nanoparticles (NPs) were prepared through a two-step liquid phase reduction method and characterized using XRD, TEM and XPS methods. The obtained NPs exhibited a significant enhancement for the electro-catalytic activity towards CHZOR compared with Ni/C NPs at 0.4 V (vs. RHE). The proposed mechanism of improved activity can be attributed to the change of electronic structure of Ni.

A two-step route was adopted to obtain Ni–P/C NPs through the preparation of Ni/C by ethylene glycol reduction and followed by hydrothermal method using NaH₂PO₂ as P source. The detailed process can be found in the ESI.† The X-ray diffraction (XRD) patterns of Ni/C NPs in Fig. 1a showed three peaks at 2θ = 44.39°, 51.93° and 76.48° which clearly indicated the face centered cubic (fcc) structure. These diffraction peaks were assigned to the (111), (200), and (220) planes of cubic Ni respectively. Ni–P particles showed similar XRD patterns which indicated Ni–P/C also showed fcc structure. Furthermore, the (200) peak of Ni–P/C showed slight shift to smaller angle compared to that of Ni/C as shown in Fig. 1b. The average particle size of Ni/C and Ni–P/C NPs was 12.3 nm and 13.7 nm respectively using Scherrer equation from the Rietveld refinement of (200) peaks.

Fig. 1 (a) XRD patterns and (b) partial enlarged detail of (200) crystalline plane of typical Ni–P/C and Ni/C NPs. (c) TEM image of Ni–P/C NPs; inset of (c): SAED pattern of Ni–P/C NPs. (d)–(f) EDX elemental mapping images of Ni–P/C NPs (red box).

The micro-morphologies of Ni/C and Ni–P/C NPs were characterized by transmission electron microscopy (TEM) as shown in Fig. S1† and 1c. The Ni/C and Ni–P/C NPs both showed quasi spherical shapes with particle size of about 15 nm, which was consistent with the XRD results. The observed selected area electron diffraction (SAED) patterns (insert of Fig. S1† and 1c) of both NPs presented a disordered diffraction spotted pattern,²⁸ verifying the polycrystalline structure of the products. Additionally, the energy dispersive X-ray spectroscopy (EDX) elemental mapping images of the Ni–P/C NPs, as shown in Fig. 1d–f, confirmed the existence of P atoms on the surface of Ni nanoparticles to form a core–shell structure.

The surface chemical state and chemical component of Ni–P/C and Ni/C NPs were studied using the X-ray photoelectron spectroscopy (XPS) as shown in Fig. 2 and Fig. S2.† The P 2p orbital binding energies of Ni–P/C and Ni/C NPs were displayed in Fig. 2a. The P 2p orbitals of Ni₁₀₀P_9.6/C and Ni₁₀₀P_14.4/C showed two peaks with binding energies ∼133.9 and 130.4 eV, which corresponded to P(V) and P(0) respectively.²⁹ This result further corroborated the existence of P atoms. In addition, the peaks at binding energy of 855.9 and 861.4 eV in Ni/C were assigned to Ni(II) and its satellite peak. Furthermore, for Ni 2p3/2 orbitals in Ni₁₀₀P_9.6/C, four peaks at 853.2, 856.4, 858.1 and 862.0 eV were assigned to Ni(0), Ni(II), satellite peak of Ni(0) and satellite peak of Ni(II) respectively³⁰ (as shown in Fig. 2b). For Ni 2p3/2 energy level in Ni₁₀₀P_14.4/C, the positions of all peaks (853.3, 856.5, 858.4, 862.2 eV) were corresponding to those of Ni₁₀₀P_9.6/C, and there is clear peak shift of Ni 2p3/2 orbitals, which was possibly caused by the formation of nickel phosphides and could improve the anti-oxidation ability of Ni/C NPs. The possible reason could be attributed to the decreased outer-layer electrons density of Ni and the weakened interaction between Ni and O atoms.³¹ This was also confirmed by the Density Functional Theory (DFT) calculation, which showed the adsorption energy of O atom on Ni/C was −0.48 eV and changed to be −0.34 eV for Ni–P particles. Furthermore, the content of P atoms on Ni NPs surface could be obtained from the integration of XPS peaks. In this work three Ni–P/C NPs with different P atom ratio were obtained and denoted as Ni₁₀₀P_x/C, where x = 7.0, 9.6 or 14.4.

Fig. 2 XPS spectra of Ni–P/C and Ni/C NPs. (a) P 2p orbital binding energy regions of Ni–P/C NPs and Ni/C NPs. (b) Ni 2p orbital binding energy of Ni–P/C and Ni/C NPs.

The electrochemical activities of Ni–P/C NPs and Ni/C NPs were measured in 1.0 M KOH solution with 0.1 M carbohydrazide. As displayed in Fig. 3a and b, the CV curves with or without carbohydrazide of Ni/C and Ni–P/C NPs were obtained at a scan rate of 50 mV s⁻¹ from 0 V to 0.4 V (vs. RHE). Fig. 3a clearly showed that Ni/C NPs had quite poor electro-catalytic activity towards CHZOR. As a comparison, Ni–P/C NPs could afford much enhanced catalytic activity towards CHZOR above the potential of 0.17 V (vs. RHE). Because there was no Ni(0) in Ni/C and it only appeared in Ni–P/C as show in Fig. 2, it's believed that Ni(0) should be the active site for CHZOR.

Fig. 3 CV curves of (a) Ni/C and (b) Ni–P/C NPs in 1 M KOH compared with that in 1.0 M KOH + 0.1 M carbohydrazide solution purged with N₂ at a scan rate of 50 mV s⁻¹ respectively, which clearly shows Ni/C NPs has poor catalytic activity towards CHZOR while Ni–P/C NPs exhibits much enhanced activity. (c) Linear sweep voltammetry curves of different catalysts at a scan rate of 10 mV s⁻¹. (d) Current density of CHZOR with varied P ratio in Ni–P/C NPs at potential of 0.4 V (vs. RHE), which shows the catalyst has best activity with P ratio of 9.6. (e) Nyquist plots of Ni–P/C and Ni/C NPs at 0.4 V (vs. RHE) with frequency range of 10⁰–10⁵ Hz and equivalent circuit (inset). (f) Current–time curves of carbohydrazide oxidation at 0.4 V (vs. RHE).

The comparison of catalytic performances between Ni/C and Ni–P/C NPs was further evaluated by linear sweep voltammetry (LSV) at a scan rate of 10 mV s⁻¹ as displayed in Fig. 3c. The result indicated that Ni/C had quite poor electro-catalytic activity towards CHZOR, which was consistent with Asazawa's work.¹ However, Ni–P/C NPs exhibited obvious electro-catalytic activity towards CHZOR. The onset potential for CHZOR of Ni–P/C NPs was ∼0.05 V and the mass current density of Ni–P/C NPs was up to 26.62 mA mg_Ni⁻¹ at 0.4 V (vs. RHE). In order to better analyze the current density, the electrochemically active surface areas of Ni/C and Ni–P/C NPs were measured as shown in Fig. S3† and it was 7.616 and 6.884 m² g⁻¹ for Ni/C and Ni–P/C NPs respectively. However, the current density increased from 0.013 mA cm⁻² to 0.211 mA cm⁻² for Ni/C and Ni–P/C NPs (Table S1†). The improved electro-catalytic activity could be ascribed to the electron transfer from Ni atoms to P atoms as indicates by the shift to high energy level of Ni 2p orbitals in Fig. 2b.

In addition, the optimized catalytic activities for CHZOR by changing the ratio of P and Ni sources were presented in Fig. 3c and d, which showed the optimized catalytic activity could be obtained by Ni₁₀₀P_9.6/C NPs. The possible mechanism could be addressed as following: (1) P atoms would affect the catalytic activity of Ni atoms through electronic and ligand effect. As P ratio increased, more P atoms would occupy Ni active sites which resulted in negative influence to the catalytic activity. The DFT calculation revealed that there was electron transfer from Ni to P atoms in Ni–P/C particles, and more electrons were transferred from Ni to P as P ratio increased (Fig. S4 and Table S2†), which was in consistent with the XPS results in Fig. 2 and (2) the conductivity of Ni–P/C could be decreased as P ratio became larger similar to phosphide materials.^32,33 (3) The influence of P ratio on the interaction energy between Ni NPs and CHZ molecules could be another possible reason for different catalytic activity of Ni catalysts. Ni atoms will have different electronic structure with varied P ratio which would have influence on the interaction energy between catalyst and CHZ molecules. It will be considered in our future work.

The electrochemical impedance spectroscopy (EIS) analysis was carried out at 0.4 V (vs. RHE) to investigate the electro-chemical resistant for CHZOR of Ni/C and Ni₁₀₀P_9.6/C NPs. As shown in Fig. 3e, the Nyquist plots for two electrodes showed similar semicircles at high frequency zone. The diameters of the semicircles represented charge transfer resistance (R_ct), corresponding to the electro-catalytic activity of catalysts. The equivalent circuit consisting of the intrinsic resistance (R_s), the charge transfer resistance (R_ct) arising from interface of electrodes and electrolyte, and electric double layer (C_dl) was fitted with the Nyquist plots, as shown in Fig. 3e inset. The fitting results showed that R_ct of Ni–P/C NPs is 123.7 Ω cm², which was lower than that of Ni/C NPs (317.9 Ω cm²). These data suggested that Ni–P/C NPs exhibited a higher conductivity than that of Ni/C NPs, as well as a higher electro-catalytic performance for CHZOR.

The long-term electrochemical stability of the Ni/C and Ni–P/C NPs was compared by chronoamperometry at the peak potential of 0.4 V (vs. RHE) for 1800 s in 1.0 M KOH solution with 0.1 M carbohydrazide electrolyte, as displayed in Fig. 3f. Remarkably, the mass current density of Ni–P/C NPs towards CHZOR was 0.90 mA mg_Ni⁻¹ at t = 1800 s, which was ∼18 times higher than that of Ni/C NPs. Furthermore, Ni–P/C NPs could maintain excellent durability for CHZOR under 2 h test at 0.4 V (vs. RHE) as shown in Fig. S5.† Therefore, there was no obvious P loss during the performance test. To further confirm the durability of Ni–P/C NPs, the TEM image after catalysis was shown in Fig. S6,† which showed there was no obvious morphology change of the Ni–P NPs after catalysis.

Conclusions

In summary, Ni–P/C NPs were prepared through a two-step liquid phase reduction method. Though there was no obvious electro-catalytic activity of Ni/C, Ni–P/C exhibited obvious activity towards carbohydrazide oxidation reaction. The proposed mechanism of the enhanced activity contributed to the improved anti-oxidation ability of Ni atoms and the charge transfer from Ni to P atoms in Ni–P/C NPs. This work provides a strategy to design efficient electro-catalysts for carbohydrazide oxidation reaction.

Acknowledgements

The authors thank the financial support by grants from the National Natural Science Foundation of China (No. 21576007, 51422301, 21503010), and International Science & Technology Cooperation Program of China (2015DFG52700).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: TEM image and CV curve of Ni/C. See DOI: 10.1039/c6ra22925e

‡ Equal contribution.

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