Coconut-Water-Mediated Carbonaceous Electrode: A Promising Eco-Friendly Material for Bifunctional Water Splitting Application.

The organic and eco-friendly materials are extended to prevail over the worldwide energy crisis where bio-inspired carbonaceous electrode materials are being prepared from biogenic items and wastes. Here, coconut water is sprayed over three-dimensional (3D) nickel foam for obtaining a carbonaceous electrode material, i.e., C@Ni-F. The as-prepared C@Ni-F electrode has been used for structural elucidation and morphology evolution studies. Field emission scanning electron microscopy analysis confirms the vertically grown nanosheets of the C@Ni-F electrode, which is further employed in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), where excellent OER and HER performances with small overpotentials of 219 and 122 mV and with stumpy Tafel slopes, i.e., 27 and 53 mV dec-1, are respectively obtained, suggesting a bifunctional potential of the sprayed electrode material. Moreover, sustainable bifunctional performance of C@Ni-F proves considerable chemical stability and moderate mechanical robustness against long-term operation, suggesting that, in addition to being a healthy drink to mankind, coconut water can also be used for water splitting applications.

Introduction

Due to escalating demand of the clean and renpan>ewable enpan>ergy caused by backdrop calamity and enpan>vironpan>menpan>tal pollutionpan>, the developmenpan>t of promisinpan>g enpan>ergy storage and conversion devices, i.e., catalysts, became obligatory.[1−3] Hydrogen is a promising option for clean energy, which can be produced by splitting water either in an electrocatalyzer or by photocatalysis. Electrochemical water splitting involves the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Both reactions require a proper catalyst to boost the reaction kinetics.[4−6] So far, precious metals (Pt, Ru, and Pd) and noble metal oxides (IrO2) showed the best performance toward both OER and HER. However, owing to the high cost and scarcity, they are obstructed.[7] Researchers have made several efforts to develop nonprecious metal-based catalysts for water splitting such as from sulfides,[8] selenides,[9] oxides,[10] etc. Among the known catalysts, carbon/carbonaceous electrode materials have demonstrated an excellent chemical and mechanical stability and porosity, high surface area, fine-controlled pore coordination, i.e., pore-size distribution and connectivity, good conductivity, low cost, and stability.[11] A variety of carbon materials with diverse morphologies and functionalities such as carbon nanotubes (CNTs),[12] fullerenes,[13] and graphene[14] were prepared by several physical and chemical methods. Most of the carbon/carbonaceous nanomaterials are reliant on fossil-based precursors, viz., pitch, coal, phenol, etc., which are expensive and detrimental to environmental health. Biomass, a rich, environmentally friendly, low-cost, renewable, novel, and abundant resource in nature, is an excellent source to prepare carbonaceous materials for scientific and practical benefits, reducing the environmental pollution greatly.[15] The animal- and plant-based materials are usually considered as biomass, which can be derived from nature.[16] Coconut is a preeminent biomass resource to prepare biocarbon as every part of coconut can be used to prepare biocarbon. Many researchers have prepared biocarbon from various parts of coconut, viz., shell,[17−19] leaves,[20] husk waste,[21] kernel,[22] etc. The biocarbon prepared using such biomass resources offers (i) relatively low internal resistance, (ii) superior electrical conductivity, (iii) outstanding cyclic stability, (iv) admirable chemical stability, and (v) excellent reaction kinetics.[17−22] Numerous synthesis techniques, i.e., hydrothermal technique, chemical bath deposition (CBD), electrodeposition, successive ionic layer adsorption and reaction (SILAR), and spray pyrolysis, were reported to prepare various biocarbon nanostructures in the form of a film/powder. Among them, spraying a direct solution over a hot substrate for obtaining a film-type electrode is a cost-effective, safe, and uncomplicated synthesis technique to operate, by which the structure, phase, and morphology can be easily controlled by regulating various parameters such as, spray rate, operating temperature, flow rate, deposition time, etc.[23,24]

Prabu et al. reported the catalytic activity of activated carbon sheets prepared from bakery food waste toward OER and HER. The activated carbon-sheet electrode demonstrated both OER and HER activities with low overpotentials of 340 mV (at 10 mA cm–2) and 380 mV (at −10 mA cm–2), with corresponding Tafel slopes of 43 and 85 mV dec–1.[25] Sathiskumar et al. prepared nitrogen-doped porous carbon (N-PC) from biomass, i.e., golden shower pod biomass (GSB), via a solvent-free strategy, which was further tested for electrolysis applications. The as-prepared N-PC electrode revealed admirable performance toward OER and HER with small overpotentials of 314 and 179 mV @ 10 mA cm–2 and low Tafel slopes of 132 and 98 mV dec–1, respectively, in KOH electrolyte solution.[26] The catalytic performance of the bio-based carbon electrode can be determined through a three-dimensional porous structure, oxygen vacancies, and carbon defects. However, with such promising performances, still there is a lot of scope for the pioneering biogenic electrode materials with enhanced catalytic activities.

Herein, in continuation of our work[27] reporting a cost-effective, earth-abundant, and bio-inspired carbonaceous noble-metal-free electrode for clean and renewable energy, we have reported the use of sprayed coconut water over three-dimensional (3D) nickel foam, i.e., C@Ni-F, toward both OER and HER. The results obtained in this study were compared with those reported for the carbonaceous electrode derived from various biomasses for better understanding. Instead of other coconut parts, here, we have used sprayed coconut water for the first time for performing OER and HER.

Material Characterizations

The structural elucidation and morphological evolutionpan> studies of the C@Ni-F electrode were attempted using X-ray diffraction (XRD, D8-Discovery Bruker, 40 kV, 40 mA, Cu Kα, λ = 1.5406 Å) and field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, 15 kV) with energy-dispersive X-ray spectroscopy (EDS) images, respectively. The surface chemical composition and the oxidation conditions were confirmed from X-ray photoelectron spectroscopy (XPS, VG Scientifics ESCALAB250) spectra. The functional groups present over the electrode surface were identified using a Fourier transform infrared spectroscopy (FTIR) plot. The Raman spectrophotometry (Xper Ram 200, Nano Base, South Korea) measurement was carried out to confirm the presence of the vibrational modes.

Results and Discussion

Surface Appearance, Chemical Configuration Analysis, and Structural Elucidation

The FE-SEM image with EDS surface elemental composition is shown in Figure . The upright standing interconnected nanosheets were observed over plane Ni-F (discussed later) (Figure a). These interwoven-type nanosheets were 300–900 nm in length and 20–30 nm in thickness. There were several crevices between these nanosheets. The two side surfaces of these nanosheets were open, which would help to improve the reaction active area during the interaction with the electrolyte followed by proficient charge transportation for better performance. Moreover, as evidenced, bulky voids would defend the volume change, if any, while cycling.[29,30] The FE-SEM images of C@Ni-F and pristine Ni-F with different magnifications are shown in Figure S1. The elemental mapping images of as-prepared C@Ni-F are given in Figure b–f. The surface elemental composition of the C@Ni-F electrode as shown in Figure g showed carbon (C) and oxygen (O) as the major elements and phosphorous (P), potassium (K), and sodium (Na) in insignificant quantity due to which these elements were not detected during structural analysis. The FE-SEM image of the bare Ni-F as shown in Figure h revealed the presence of only Ni and O elements (Figure i,j). The presence of oxygen could be due to a trace amount of environmental oxygen adsorbed over Ni-F.

Figure 1

(a) FE-SEM image and (b–f) mapping images for C, O, P, K, and Na elements; (g) elements and their proportions obtained from the EDS spectrum of C@Ni-F; (h) FE-SEM image and (i, j) elemental mapping images of Ni and O; and (k) elemental composition of pristine Ni-F for knowing substrate specifications.

(a) FE-SEM image and (b–f) mappinpan>g images for C, O, P, K, and Na elements; (g) elements and their proportions obtained from the EDS spectrum of C@Ni-F; (h) FE-SEM image and (i, j) elemental mapping images of Ni and O; and (k) elemental composition of pristine Ni-F for knowing substrate specifications.

The XRD pattern of the as-prepared C@Ni-F electrode is shown in Figure a, where two major peaks at 24.15 and 43.16° corresponding to the (002) and (101) reflection planes (JCPDS 41-1487) with interplanar spacings of 0.38 and 0.28 Å were, respectively, obtained, confirming the presence of carbon as the majority entity in the electrode material. The absence of peaks of other elements was due to their trace amount and low atomic numbers, which was also observed in XPS and EDAX analyses. The chemical structure and the oxidation conditions were obtained from the XPS spectra recorded for P, C, K, O, and Na elements (Figure b–g). Figure c presents deconvoluted P 2p peaks of P–C and P–O bonds at, respectively, 133.3 and 134.4 eV binding energies.[31−33] The deconvoluted C 1s spectrum with four Gaussian curves as shown in Figure d revealed the sp2 carbon, sp3 carbon, C–O, and C=O bondings at 284.55, 285.51, 286.6, and 287.77 eV, respectively.[34−36] The as-observed peaks were of hydroxyl (C=C/C–C) and epoxy groups (C–O), suggesting the presence of oxygen-containing groups in the form of epoxy and hydroxyl groups, which is in good agreement with the Lerf–Klinowski model of carbonaceous electrodes.[37,38]Figure e shows the K 2p spectrum with spin energy partition of 2.8 eV, where two major peaks are evidenced at 292.7 and 295.5 eV for the K 2p3/2 and K 2p1/2 levels, respectively.[39] The peak observed at 531.01 eV in O 1s spectra was for C–O–C[40] (Figure f). A single broad deconvoluted peak of Na 1s is also observed in Figure g at 271.5 eV, suggesting the presence of Na+.[41]

Figure 2

(a) XRD pattern; (b) survey and enlarged spectra for (c) P 2p, (d) C 1s, (e) K 2p, (f) O 1s, and (g) Na 1s; and (h) FTIR and (i) Raman spectra of C@Ni-F.

(a) XRD pattern; (b) survey and enlarged spectra for (c) P 2p, (d) C 1s, (e) K 2p, (f) O 1s, and (g) n class="Chemical">Na 1s; anpan>d (h) FTIR anpan>d (i) Ramanpan> spectra of C@n class="Chemical">Ni-F.

The existence of the functional groups on the electrode surface was confirmed usinpan>g the FTIR spectrum (Figure h). The band observed at 1457.37 cm–2 was attributed to the plane deformation vibrations of the C–H moiety in −CH3, −CH2–, and −O–CH3.[42] The peak at 1056.15 cm–2 was due to the saccharide structure of cellulose and hemicelluloses, and the peak at 873.45 cm–2 was assigned to the bending vibrations of the aromatic compounds.[43,44] The Raman spectrum as shown in Figure i confirmed two peaks at 1355.66 and 1582.85 cm–2 for D and G bands, respectively.[45] The observed D band indicated the disorder in the carbon structure with A1g symmetry, while the G band showed the C=C stretching vibration with E2g mode. The ID/IG ratio was found to be 0.86, providing information about the crystallite dimension, plane defects, edge defects, and the nature of disorder of the carbon derivative.[46] It is confirmed that the presence of K, P, and Na in C@Ni-F could be responsible for the enhancement in the formation of defects, which results in the increasing intensity of the G band following incorporation of −OH groups in the layered structures for better performance.[47,48]

Electrochemical Measurements

The electrochemical properties of the C@Ni-F electrode were studied using an IVIUM electrochemical workstationpan> by employinpan>g a three-electrode system: platinum as a counter electrode, Ag/AgCl as a reference electrode, and deposited Ni-F as a working electrode. The OER polarization curves were obtained in a 1.0 M KOH electrolyte and compared with that of bare Ni-F for assessing the performance of the sprayed carbonaceous electrode material only.

Oxygen and Hydrogen Evolution Study

The polarization curves of C@Ni-F showed a promising OER activity with a small overpotential (η) of 219 mV, calculated by eq 2 inpan> the Supporting Information, which is much inferior to the overpotential of Ni-F, i.e., 330 mV (Figure a) at a current density of 10 mA cm–2. The Tafel slopes (determined using eq 3 in the Supporting Information) of C@Ni-F and Ni-F electrodes, extracted from the polarization curves, are shown in Figure b. The OER kinetics of the C@Ni-F electrode can be understood using Tafel plots. The lower Tafel slope specifies the good reaction kinetics. Herein, the C@Ni-F electrode presented the smallest Tafel slope of 27 mV dec–1, while Ni-F revealed 79 mV dec–1, which indicates the superior reaction kinetics of the previous electrode toward OER over the latter one.[49] The cyclic and chemical stability of C@Ni-F was verified with continuous OER at a fixed potential and is shown in Figure c. The as-prepared C@Ni-F electrode demonstrated excellent cyclic and chemical stability. The C@Ni-F electrode revealed mostly stable current density after 1000 cycles and 48 h for long-term cycling and chemical stability (inset of Figure c). After 1000 cycles, a minute change of just 4 mV, i.e., 98.20% retention, was observed in the overpotential of the C@Ni-F electrode. The OER activity of the as-obtained C@Ni-F electrode was comparable to the performance of previously reported carbonaceous electrode materials, which is graphically shown in Figure d.

Figure 3

(a) OER polarization curves, (b) Tafel plots, (c) cyclic stability (inset shows chemical stability), and (d) comparison graph of OER activity of C@Ni-F with former data (the red circle indicates the overpotential and Tafel slope of the current work).

The HER activity of the C@Ni-F electrode as shown inpan> Figure was carried out unpan>der analogous conditions to OER. The polarization curve of C@Ni-F showed admirable activity toward HER with petite overpotential of 122 mV, whereas Ni-F revealed an overpotential of 220 mV at a current density of −10 mA cm–2 (Figure a). The corresponding Tafel slopes, an intrinsic property of electrocatalysts, extracted from HER polarization curves of C@Ni-F and pristine Ni-F are shown in Figure b. A smaller Tafel slope indicates a higher HER rate, and a Tafel slope as small as 53 mV dec–1 signifies that the HER obeys the Volmer–Heyrovsky mechanism on the electrode surface.[50] The cyclic and chemical stability of the C@Ni-F electrode is shown in Figure c. After 1000 cycling operations, the C@Ni-F electrode showed an excellent stability (94.57% retention) with insignificant variation in overpotential, showing almost stable performance even after 48 h (inset of Figure c). The comparative performance of the C@Ni-F electrode toward HER has been graphically represented in Figure d and is given in Table .[51−65]

Figure 4

(a) HER polarization curves, (b) Tafel plots, (c) cyclic stability (inset shows chemical stability), and (d) performance comparison of the HER activity of C@Ni-F with survey data (the red circle indicates the overpotential and Tafel slope of the current work).

Table 1

Comparison of Electrochemical Properties of C@Ni-F with Carbonaceous Electrodes

		η (mV)/Tafel slope (mV dec–1)
catalyst	electrolyte	OER	HER	J (mA cm–2)	ref
NMWN	1.0 M NaOH	320/68	340/68	10	(51)
Co@N-CNTF	1.0 M KOH	350/61.4	226/149.9	10	(52)
NCNs	1.0 M KOH/0.5 M H2SO4	410/142	90/43	10	(53)
ZIF-8-C6	0.1 M KOH/0.5 M H2SO4	528/91.9	155/54.7	10	(54)
PNC/Co	1.0 M KOH	370/76	270/131	10	(55)
NiFe@C	1.0 M KOH	274/57	195/111	10	(56)
Co@N-C	1.0 M KOH	400/–	200/100	10	(57)
Co@NC-G	1.0 M KOH	322/73.7	140/62	10	(58)
PO-Ni/Ni-N-CNFs	1.0 M KOH	420/113.10	262/97.42	10	(59)
Ni@C	1.0 M KOH	300/145	150/143	10	(60)
Ni3C	1.0 M KOH	275/62	292/41.3	10	(61)
Co/CNFs	1.0 M KOH	320/79	190/66	10	(62)
Fe3C-Co/NC	1.0 M KOH/0.5 M H2SO4	340/100	238/108.8	10	(63)
CoP/NCS	1.0 M KOH	254/57	71/109	10	(64)
CoP@PNC/C	1.0 M KOH	330/64	120/67	10	(65)
C@Ni-F	1.0 M KOH	219/27	122/53	10	present work

Conclusions and Perspectives

In this work, we successfully prepared vertically grown C@Ni-F nanosheet electrodes using the facile spray pyrolysis synthesis method over a Ni-F substrate. The XRD patterns confirmed the formation of the C@Ni-F electrode, while the elements present on the electrode surface were verified using XPS. The surface bands and functional groups were confirmed by Raman and FTIR spectroscopies, which confirmed the formation of C@Ni-F. The as-prepared C@Ni-F electrode on employing in electrochemical measurements for OER and HER water splitting applications showed lower overpotentials and smaller Tafel slopes. The OER overpotential of the C@Ni-F electrode was 219 mV, while for HER, it was 122 mV. The C@Ni-F electrode, with the small overpotential toward both OER (27 mV dec–1) and HER (53 mV dec–1), showed good reaction kinetics. This admirable performance of the C@Ni-F electrode is attributing to a high surface area, upright standing platelet surface morphology that facilitates east and easy charge transfer processing, and availability of bounteous active sites sowing to interconnected porous nanosheets network. A C@Ni-F electrode was obtained at one side of the substrate surface only, forming a bisprayed surface with metal oxides, polymers, and carbonaceous materials on the other side would open a new research avenue in energy storage and water splitting devices for manufacturing various technologies.

Experimental Section

Materials

Fresh young coconuts from Partur, Maharashtra, India, and deionized (DI) water and Ni-F-110 with 320 g m–2 mass density from Artenano Company Limited, Hong Kong, were purchased and used after cleaning.

Spraying of Coconut Water

The coconut-water-mediated carbonaceous electrode was prepared using the spray pyrolysis method with the protocol reported earlier.[28] Coconut water was collected from fresh kernel-free green coconut, which was further filtered using Whatman filter paper to avoid any solid impurity. Ni-F (1 cm × 4 cm × 0.1 mm) was used as a substrate, which was cleaned by 1.0 M HCL solution, DI water, and ethanol for 15 min each with ultrasonication to avoid impurity additives. Afterward, the filtered coconut water was poured into the solution dispenser of the spray pyrolysis instrument (HOLMARC HO-TH-04, India), which was connected to and operated by a personal computer. A cleaned 3D Ni-F substrate was kept on a hot plate by adjusting the X–Y coordinates appropriately. The temperature of the hot plate was optimized to 150 °C using a controller.

Furthermore, spraying was conducted with a flow rate of 2 mL min–1 and air-flow rate of 15 L minpan>–1 inpan> a closed chamber for 14 minpan>. After depositionpan>, the substrate temperature was allowed to return to room temperature naturally to avoid the quenpan>chinpan>g effect. Finally, the prepared deposited electrode Ni-F was annealed at 400 °C before applying for further electrochemical measurements. The schematic of the experimental setup used to synthesize C@Ni-F is shown in Scheme .

Scheme 1

Schematic of Obtaining a Carbon Film with a Platelet Architecture on Spraying Coconut Water on Ni-F