Synthesis and characterization cobalt phosphate embedded with N doped carbon for water splitting ORR and OER

1 Introduction

With the increasing of the global energy problems and the increasingly serious environmental pollution, the utilization of photo and electro catalytic technology to get hydrogen has attracted extensive attention; due to hydrogen energy is considered to be one of the most favorable options to fossil fuels (Ashok et al., 2019; Béjar et al., 2019; Chakrabarty et al., 2019). Water splitting is deeming as one of the most favorable strategy for hydrogen production (Chen et al., 2020; Cheng et al., 2020; Deng et al., 2019). Water splitting include, oxygen reduction (ORR, O₂ + H₂O + 4e⁻ → 4OH⁻) and oxygen evolution (OER, 4OH⁻ → O₂ + 2H₂O + 4e⁻) reactions, which plays a paramount role in energy conversion and storage devices, including metal air-batteries and fuel cells (Fang et al., 2018; Giacco et al., 2018). The greatest challenge for water splitting is to fabricate an efficient catalyst for both the ORR and OER (Li et al., 2020b; Zhang et al., 2019; Zheng et al., 2019; Zhu et al., 2018). Currently, several precious metals including Pt, Ir and Ru based catalyst have been used for water splitting (Ding et al., 2019; Fang et al., 2019; Filimonenkov et al., 2019). The use of these metal based catalysts still limited due their high cost and rear source. Moreover, using of the different catalysts for each side reaction also increase the processing costs and time. Therefore, it is the great demand to developed low coast highly stable bi-functional catalyst (Ishizaki et al., 2020; Jung et al., 2015; Li et al., 2020a). Recently several novel materials, such as metal oxides, carbides, nitrides, sulfides and carbonaceous materials, have been used a catalyst for water splitting (Li et al., 2018; Liu et al., 2017; Lv et al., 2019; Mamtani et al., 2018). Among these materials, metal phosphate especially cobalt phosphate has been used for electrocatalyst for water splitting (Yuan et al., 2016). Moreover, the nanocomposites of hetroatoms doped carbon with these metal phosphates and oxide increased the electrical conductivity as well as the stability of the catalyst in both the acidic and basic condition. Based on these study, herein we have in-situ fabricate Co₃(PO₄)₂ nanoparticles in to N doped carbon matrix. The fabricated Co₃(PO₄)₂-NC nanocomposite was analyzed using several techniques such as FTIR, TGA, XRD, Raman, SEM, TEM, BET and XPS. As-fabricated Co₃(PO₄)₂-NSC nanocomposite has been used for water splitting ORR and OER in 1.0 M KOH solution. The fabricated catalyst shows excellent catalytic efficiency for both OER and ORR as well as excellent reusability, long term stability with fast electron transfer during the catalytic reactions. These outcomes revealed that the fabricated nanocomposites can be used as low-cost, efficient catalyst for water splitting ORR and OER in metal-air batteries and fuel cells.

2 Experimental

3 Results and discussion

Co₃(PO₄)₂ nanoparticles embedded into N doped carbon [Co₃(PO₄)₂-NC] was fabricated using hydrothermal and post-pyrolysis method as illustrated in Fig. 1. The FTIR spectra of the nanocomposite as shown in Fig. 2, show several adsorption peaks at 3416–3234, 2945–2862, 1624, 1534, 1456 and 1392 cm⁻¹ assigned to the O–H/N–H, C–H, C⚌N, C⚌C, C–O and C–N respectively (Ahamad et al., 2020b; Ahamad et al., 2020c; Ahamad et al., 2019; Al-Kahtani et al., 2018; Naushad et al., 2019). Another peaks at 1053 and 568 cm^-1corresponds to P-O and Co-O stretching respectively. The X-ray diffraction (XRD) as illustrated in Fig. 1(a), show diffraction peaks at 2θ = 20.8, 25.84, 31.8, 38.02 43.8, 51.01, and 55.47° and assigned to (0 1 1), (2 1 0), (2 2 0), (0 3 1), ( −1 2 2), ( −1 3 2), and (3 1 2) planes respectively for Co₃(PO₄)₂ (JCPDS No.01–0373) (Liu et al., 2018; Shi et al., 2020). Another broad diffraction peak at 2θ = 26.0°, suggest the formation of graphitic structure in the nanocomposite. The presence of the graphite carbon in the nanocomposite and doping of the N was further supported using Raman spectra. The nanocomposite shows two peaks Raman peaks at 1346 and 1583 cm^-1assinged to the G and D band respectively. Another peaks at low frequency at 971 cm⁻¹ support the formation of (PO₄)^3- into the carbon matrix. The porosity of the nanocomposite was determine using the N₂ adsorption and desorption isotherm and the BET surface area was found to be 372.32 m²/g, Moreover the pore size distribution was calculated from desorption data using the BJH model and was found in the range of 4–5 nm as shown in Fig. 1(c).

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Fig. 1

(a) XRD (b) Raman (c) N₂ adsorption and desorption isotherm (d) SEM (e) TEM and (f) HRTEM and SAED of Co₃(PO₄)₂-NC.

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Fig. 2

(a) XPS of Co₃(PO₄)₂-NC (b) C1s deconvoluted peaks (c) Co 2p deconvoluted peaks (c) CV (e) LSV and (f) tafel plot for Co₃(PO₄)₂-NC in 1.0 M KOH solution.

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The morphology and structure of the as-fabricated Co₃(PO₄)₂-NC was determine by scanning electron microscopy (SEM). As shown in Fig. 1(d), The SEM images show the spherical Co₃(PO₄)₂nanoparticles were embedded uniformly into the carbon matrix. Fig. 1(e) display the details of the nanocomposites and show that Co₃(PO₄)₂-NC is composed of a large amount of Co₃(PO₄)₂ nanoparticles with the range of 24–26 nm and dispersed on the surface or into the carbon matrix. The strong interaction between the Co₃(PO₄)₂ nanoparticles and the Co₃(PO₄)₂-NC, enhancing the overall electron transfer capability. The high resolution TEM images as shown in Fig. 3(f), show the crystalline nature of the Co₃(PO₄)₂nanoparticles and the lattice fringes was found to be0.244 nm for (1 2 1) plans. The selected area electron diffraction pattern as illustrated in inserted figure display the electron diffraction plane (1 0 0), (0 1 1) and (01–1) of monoclinic cobalt phosphate(Zhou et al., 2016).The elemental composition and the metal oxidation state of the nanocomposite was determine using XPS, as shown in Fig. 2(a), the nanocomposite shows the presence of C, N, O, Co and P elements. The spectra for C1s is shown in Fig. 2(b) and split four peaks at 283.6, 284.4, 285.3 and 287.42 eV corresponding to the C–C/C = C, C-N/C-O and C = O/C = N and –O–C⚌O respectively (Ahamad et al., 2020a).

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Fig. 3

(a) LSV at different rotation (b) K-L plot (c) chronopotentiometric curves for ORR (d) LSV for OER (e) Tafel plot (d) chronopotentiometric curves for OER.

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Fig. 2(c) show the Co 2p deconvoluted into four peaks at 782.21 (2P_3/2), 786.23, 796.3 (2P_1/2) and 804.21 eV support the presence of Co²⁺. The N1s spectra was split into three peaks belongs to the pyridinic N, pyrrolic N, graphitic N and display the peaks at 398.17, 499.23, and 401.05 eV respectively as shown in figure SF-3 (Khalaf et al., 2019). The P 2p region of the sample displayed two peaks at 133.2 eV and 134.4 eV binding energies assigned to the to the 2p3/2 and 2p1/2 core levels of the central phosphorous atoms in the phosphate groups. The O1s shown two peaks at 532.12 and 533.54 eV due to the carbon oxygen and phosphate oxygen bond respectively as shown in supporting figure SF-4.

4 Electro-catalytic activities

The electrons transfer and the electrical conductivity of catalysts were determine using electrochemical impedance spectroscopy (EIS). As shown in supporting figure (SF-5), It was notices that Co₃(Po)₄-NC shows the smaller semicircle radius of the Nyquist plots indicate the lower charge transfer resistance and a higher electron and charge transfer efficiency compared to Co₃(Po)₄. The electro-catalytic activities of the catalyst for ORR was carried out in 0.1 M KOH, as the CV was presented in the Fig. 2(d), these results revealed that under the O₂ saturated solution exhibit the ORR peak at 0.834 V, while in the presence of Ar saturated solution this peak was diaper. These results support the fabricated nanocomposite show the ORR behavior in the presence of oxygen. Further the ORR was carried out using linear sweep voltammetry (LSV) with a rotating rate between 400 and 2500 rpm. The LSV curve with a rotating rate of 1600 rpm and is presented in Fig. 2(e), the onset and half wave-potential for Co₃(PO₄)₂-NC was found to be 0.964 and 0.831 V. The CV and the LSV results of Co₃(PO₄)₂-NC was compared with commercial Pt/C catalyst. To determine the kinetics of the ORR the tafel slope was plotted. As shown in Fig. 2(f) the tafel slop was determine via based on the Fig. 2(e) and the tafel slope for Co₃(PO₄)₂-NC was found to be 56 mV/decade, that is smaller than that of Pt/C (64 mV/ decade) and show excellent ORR activities in 0.1 M KOH. As shown in Fig. 3(b), the transfer electron number was calculated using Koutecky–Levich (K–L) plots which were derived from the RED results as shown in Fig. 3(a). the results revealed that the ORR react was carried out about to 4 electron system as shown in supporting information(Yuan et al., 2016). It was reported that an Co atom in Co₃(PO₄)₂-NC catalyst structure helps to develop strong back bonding with the adsorbed molecular oxygen, which can cause an increase in O–O bond distance and thus facilitates ORR, based on the molecular orbital theory.(Zhao and Yuan, 2017), In alkaline medium the reaction was proceed via 4 e- way (O₂ + 2H₂O + 4e⁻ → 4OH⁻) .

To determine the stability of the electrocatalyst the chronoamperometry was carried out and only 3.6% of the current loss was observed after 48 h, while in the case of Pt/C about to 20.2% current loss was noticed in the similar condition. The OER performance of Co₃(PO₄)₂-NC nanocomposite was investigated in the O₂ saturated 1.0 M KOH solution and corresponding polarization curve were obtained from LSV. As shown in Fig. 3(d), the LSV plot of the RuO₂ and Co₃(PO₄)₂-NC nanocomposite show the over potential 293 mV and 296 mV to get a current density of 10 mA cm⁻². The OER kinetics was determine via the tafel plots (53 mV dec⁻¹) derived from polarization curves and the results show that the fabricated nanocomposite show smaller slope then that of RuO₂ (74 mV dec⁻¹)(Huang et al., 2019). These outcomes revealed that the Co₃(PO₄)₂-NC catalyst show faster OER than commercial catalyst RuO₂. The results showed that the OER reaction is proposed to be assisted by phosphate groups which are directly connected to cobalt and act as proton acceptors. Phosphates can move and shuttle protons freely as the Co₃(PO₄)₂-NC catalyst experiences iterative oxidation/reduction cycles which influence the charge transfer. In alkaline medium the reaction proceeds as, 4OH⁻_(aq) → 2H₂O_(aq) + 4e⁻+ O_2(g) as shown below in equation (1)

The chronopotentiometric curves at current density of 20 mA cm⁻², was carried out for 24 h and the results revealed that the real-time potential presents slight increase, whereas the RuO₂ catalyst exhibits a continuous increasing the potential during the test.

5 Conclusion

Herein, we have successfully fabricated the Co₃(PO₄)₂-NC nanocomposite and used as bifunctional electrocatalyst for ORR and OERin basic medium. As-fabricated Co₃(PO₄)₂-NC show excellent catalytic performance for ORR and OER, equivalent to or better than commercial catalyst Pt/C and RuO₂. Due to the easy synthetic process, extensive material availability, and high catalytic activity and stability, the fabricated nanocomposite will also be helpful for other electro catalytic applications.