Decoration of green synthesized S, N-GQDs and CoFe2O4 on halloysite nanoclay as natural substrate for electrochemical hydrogen storage application

Halloysite nanotubes (HNTs) with high active sites are used as natural layered mineral supports. Sulfur- and nitrogen-co doped graphene quantum dots (S, N-GQDs) as conductive additive and CoFe2O4 as the electrocatalyst was decorated on a HNT support to design an effective and environmentally friendly active material. Herein, an eco-friendly CoFe2O4/S, N-GQDs/HNTs nanocomposite is fabricated via a green hydrothermal method to equip developed hydrogen storage sites and to allow for quick charge transportation for hydrogen storage utilization. The hydrogen storage capacity of pure HNTs was 300 mAhg−1 at a current density of 1 mA after 20 cycles, while that of S, N-GQD-coated HNTs (S, N-GQDs/HNTs) was 466 mAhg−1 under identical conditions. It was also conceivable to increase the hydrogen sorption ability through the spillover procedure by interlinking CoFe2O4 in the halloysite nanoclay. The hydrogen storage capacity of the CoFe2O4/HNTs was 450 mAhg−1, while that of the representative designed nanocomposites of CoFe2O4/S, N-GQDs/HNTs was 600 mAhg−1. The halloysite nano clay and treated halloysite show potential as electrode materials for electrochemical energy storage in alkaline media; in particular, ternary CoFe2O4/S, N-GQD/HNT nanocomposites prove developed hydrogen sorption performance in terms of presence of conductive additive, physisorption, and spillover mechanisms.

www.nature.com/scientificreports/ most challenging source for energy devices in the future 18 . Hydrogen is identified as a nontoxic and low-priced power origin for static and transferable avails. Hydrogen supplies reproducible energy after ignition 19 . Scientists have drawn consideration to hydrogen as an encouraging future fuel because of its substantial features. These features are, containing plenty in the world, an undefiled energy source, and the lightest fuel 20,21 . Electrode material properties are crucial for the energy storage performance of storage devices. It has been well appointed that nanostructured electrode materials can enhance the capacity and cycle stability of storage devices. Mass transport and electrode kinetics can be elevated by decreasing the charge transportation path and ion diffusion distance. In addition, the cooperation of several mechanisms and storage processes can promote the potential of electrode materials for ion adsorption. In layered materials, especially clay minerals with unique structures, a spillover mechanism can occur by decorating electrocatalyst materials on their surfaces. Adsorbed H 2 molecules on the decorated electrocatalyst migrate to the surface of clay minerals, and hydrogen can be transported on a surface to another surface. Additionally, designing carbon bridges between the dissociation source (electrocatalyst) and receptor (clay) leads to diffusion. Graphene quantum dots (GQDs), as a functional carbon source with distinctive physicochemical properties, play ideal roles in the energy storage process 22,23 . The presence of GQDs in the texture of electrodes has been reported in previous studies for studying electrochemical energy storage systems such as Li-ion batteries, Li-S batteries, and supercapacitors [24][25][26][27][28] . Yushan Liu groups reposted the preparation of HNTs/RGO nanocomposites which provided superior performances as electrode material in supercapacitors 29 . Nano assembly of N-doped graphene quantum dots anchored Fe 3 O 4 /halloysite nanotubes designed by Akhilesh BabuGanganboina et.al. for high performance supercapacitor 30 .
According to the mentioned literature, the HNT nanoclay, CoFe 2 O 4 and S, N-GQDs were selected as support, electrocatalyst and conductive receptor components, respectively. Hence, ternary nanocomposites were designed by decorating CoFe 2 O 4 and S, N-GQDs on halloysite nanotubes. CoFe 2 O 4 as a transition metal oxide in the category of spinel materials has been widely utilized in electrochemical energy storage systems 31,32 . Electrochemical energy storage devices undergo redox reactions with high power, which store electrical charges at the surface and in the structure pores 33 . The achieved ternary CoFe 2 O 4 /S, N-GQD/HNT nanocomposites were prepared through a green hydrothermal method with synergistic spillover mechanisms, redox processes and physical adsorption as potential hydrogen storage materials. Decoration of the S, N-GQD layer as a conductive receptor and CoFe 2 O 4 nanoparticles as an electrocatalyst on halloysite nanotubes was investigated using structural analysis of XRD, FE-SEM, TEM, FT-IR and EDS. The electrochemical abilities of the resulting composites were compared to reach ideal electrode materials in terms of function, mechanism and performance.

Results
Structure characterization. The halloysite nanotubes were investigated in terms of morphology and purity using FE-SEM micrographs and XRD diffractograms. The schematic diagram for decoration process of S, N-GQDs and CoFe 2 O 4 nanoparticles on halloysite nanotubes is presented in Fig. 1a-c. Also, the mechanism for hydrogen storage process through spillover represented in Fig. 1d and e. As shown in Fig. S1a and b, the structure of halloysite is nanotubes with different lengths 34 . Additionally, the XRD diffractogram for pristine HNTs is  www.nature.com/scientificreports/ shown in Fig. S1c, which confirms the structure of Al 2 Si 2 O 5 (OH) 4 with a reference code of 29-1487. The peak at 2Ɵ = 25 degrees is present in the texture of halloysite nanotube minerals related to SiO 2 (JCPDS = 82-1554), which is available in clay materials 35,36 . The purity of the as-fabricated pure S, N-GQDs and S, N-GQDs/HNTs nanocomposite samples was studied and is shown in Fig. 2a and b using XRD technique. Figure 2a displays a broad peak at approximately 15-25 degrees of carbon texture, which supports the formation of graphene quantum dots from red onion juice in the hydrothermal process. In addition, the formation of S, N-GQDs on the halloysite nanotube substrate was confirmed by investigation of the XRD diffractogram in Fig. 2b. Figure 2b simultaneous displays the sharp peaks of aluminum silicate hydroxide (29-1487) and silica (82-1554) and a broad peak of carbon (08-0415). In addition, Fig. 3a    www.nature.com/scientificreports/ Chemical composition of onion includes several carboxylic acids, sugars, etc. Seven organic acids were identified and quantified in onion samples such as Malic acid, Citric acid, Tartaric acid, Oxalic acid, Ascorbic acid, Succinic acid and Pyruvic acid. Also, sulfur and nitrogen containing compounds detected in the onion extract as organosulfur and amino acids. These compounds consist of 1-Propanethiol, Propylene sulfide, Dimethyl sulfide, Aspartic acid, Serin and, etc. 40,41 .
The possible mechanism of formation of GQDs from the organic acids was explained as follows; the organic acids such as tartaric acid or ascorbic acids when heated at their melting temperature decomposes and the hydronium ion formed from the acid, acted as a catalyst in subsequent decomposition reaction stages. The significant path in mechanism was that the condensation and cyclo-addition followed by the formation of aromatization and aromatic clusters. During the pyrolysis, adjacent dehydrated acid molecules reacted with each other to form GQDs and the functional groups located at the edge of each GQDs acts as passivation layer at the surface 42 . Additionally, presence of sulfur or nitrogen source along with carbon sources leads to formation of S, N doped GQDs. The compounds of thiamine or pantothenic acid in the onion extract appear to be responsible for doping the S or N in the GQDs structures 43 .
The surface specifications were determined by the BET method to compare the surface data of pristine HNTs and representative nanocomposites of CoFe 2 O 4 /S, N-GQDs/HNTs. Surface characterization in terms of specific surface area and porosity is an essential factor for ion adsorption and insertion on the surface or in pores. Therefore, the resulting data can act as important properties for energy storage materials. As seen in Fig. S3a and c, a typical type-IV isotherm can be determined for two BET plots. The obtained isotherms have H3 hysteresis loop. The type-IV isotherm is defined for the materials by mesoporous structures 44 . The specific surface area, total pore volume and average pore diameter for pristine HNTs and nanocomposites of CoFe 2 O 4 /S, N-GQDs/HNTs are summarized in Table S1. The pore size distribution for the samples is shown in Fig. S3b and d. The pore size distribution for pristine HNTs showing broad pore size distribution in the range of 1-55 nm with maximum around pores of 2 nm diameter (Fig. S3b). As shown in Fig. S3d  www.nature.com/scientificreports/ The role of hydrogen spillover in hydrogen storage capacity enhancement is still disputable due to lack of understanding of the exact mechanism, and the contrary results on the hydrogen uptake characteristics. Spillover is the dissociation of the hydrogen molecules into atoms by transition metals and subsequent diffusion of these  www.nature.com/scientificreports/ atoms to the host material, in which hydrogen atoms can hydrogenate the unsaturated C-C bonds (i.e., activated carbon, graphite, and single walled carbon nanotubes) or the benzene ring (i.e., MOFs, COFs, and polymers). Among the various steps of spillover, dissociation of hydrogen molecules and diffusion of hydrogen atoms are considered as instantaneous and barrierless, respectively; however, hydrogen atoms need to overcome high energy barriers for migration to the host material which makes migration the rate limiting step. Therefore, minimum clustering and fine molecular level dispersion of transition metals within the host material is desirable to make the migration path as short as possible 45 . Spillover is a conceivable mechanism in hydrogen sorption systems that comprise electrocatalysts to dissociate H 2 to H atoms. In the spillover concept, adsorbed hydrogen on the electrocatalyst migrates to the surface of support materials such as carbon or layered structures such as graphite www.nature.com/scientificreports/ or clay, and hydrogen can transport on a surface to another surface. Spillover processes occur in the CoFe 2 O 4 / HNT and CoFe 2 O 4 /S, N-GQD/HNT composites when hydrogen is adsorbed on the CoFe 2 O 4 nanoelectrocatalysts and two-step spillover occurs. Primary spillover occurs through transportation of atomic hydrogen from the electrocatalyst nanoparticle to the support, and secondary spillover occurs through transportation of hydrogen to the receptor (HNT). The diffusion procedure is obligatory for secondary spillover 46 . Therefore, the formation of carbon bridges (S, N-GQDs) between the dissociation source (CoFe 2 O 4 ) and receptor (HNTs) leads to diffusion. Secondary spillover was substantiated to develop the hydrogen storage capacity by mixing the adsorbent with a catalyst that is capable of dissociating hydrogen 47,48 . The explained spillover mechanism in the CoFe 2 O 4 /S, N-GQDs/HNTs nanocomposites is shown in detail in Fig. 1d and e. In addition to the role of the carbon bridge of S, N-GQDs in the spillover mechanism, the hydrogen sorption ability in the presence of codoped doped GQDs (CoFe 2 O 4 /S, N-GQDs/HNTs) is better than that of CoFe 2 O 4 / HNTs owing to the electron transfer in the S, N-GQD-based electrodes being properly quick. The formation of S, N-GQDs on electrodes can alter the discharge capacity because of the numerous edge areas produced by small sized S, N-GQDs and the adequate conductivity correlated to CoFe 2 O 4 bond structures. The high efficiency of CoFe 2 O 4 /S, N-GQDs/HNTs can be elucidated owing to the trap states formed by both dopants and edge states, which can adsorb charge carriers to increase the storage capability. The electron density of graphene layers can be modified through doping by electron acceptor or electron donor species. Sulfur or nitrogen as heteroatoms in the graphene structure can be replaced with carbon atoms. Therefore, electron migration occurs, which leads to the creation of electrons and holes. This structure alters with cooperating S, N-GQDs and plays a great role in electrochemical and electrocatalyst systems for energy storage applications 22,49 . Additionally, nitrogen as a donor atom with the ability to electronically modify graphene can improve the conductivity and catalysis behaviors of graphene structures 50,51 .
Operation of hydrogen sorption in metal oxides situates through physisorption. The produced H + from aqueous electrolyte adsorbs on the surface of the metal oxide (charge process). During the discharge path, H 2 migrates from the working electrode under alkaline circumstances and becomes water again while freeing an electron. In the next step, adsorbed hydrogen on the electrocatalyst of metal oxides migrates from nanoparticles to the surface of the carbon support, and hydrogen can be transported on a surface to another surface 52 .
Actually, the Heyrovsky process occurred due to the suitable electrocatalytic activity of synthesized CoFe 2 O 4 nanoparticles 53  According to Eq. (4), the electrolyte of KOH dissociated and the ions of OH − and H + were produced during the charge direction. Next, the produced H + migrates to the working electrode. In the cathodic reaction, the H + adsorbs on the surface of active materials. During an oxidization process and anodic reaction, the charging mechanism was performed in the counter electrodes (Pt). During the discharge direction, the stored H 2 in the working electrode migrates through freeing an electron under alkaline circumstances and water is again produced. The Fe 3+ as a central metal ion is reduced to Fe 2+ [Eq. (5)]. Finally, the adsorbing H + can equilibrate the total charge of CoFe 2 O 4 . The Fe-H bonds created in terms of construction favored the formation of OH − ions with a reduction in the redox couple of Fe 3+ /Fe 2+ .
Cyclic voltammetry. The electroanalytical technique of cyclic voltammetry (CV) was customarily employed for the comparative study of electrode material attributes. Fascinating and effective scientific insights can be provided by associating the attributes of the material, such as structure and morphology, with their electrochemical response. The electrochemical response of HNTs decorated with CoFe 2 O 4 and GQDs was investigated by CV for comparison with pristine HNTs. Figure 7e exhibits the CV curves for the pristine HNTs and representative nanocomposites of CoFe 2 O 4 /S, N-GQDs/HNTs over a potential region of − 1.1 to − 0.2 V. The electrode response (anodic and cathodic peaks) for all samples is listed in Table S2. The CoFe 2 O 4 /S, N-GQDs/HNT nanocomposites express a better electrode response than pristine HNTs. It is fascinating to note that covering S, N-GQDs on HNT structures modifies the electrochemical attributes of the as-fabricated electrode materials for energy storage applications. Additionally, CoFe 2 O 4 as an electrocatalyst can improve the energy storage mechanism, which is compatible with chronopotentiometry charge-discharge results.

Conclusion
Halloysite nanoclay as a unique substrate shows favorable properties in terms of hydrogen adsorption using incorporating physisorption and spillover mechanisms. The green synthesized Sulfur and Nitrogen co-doped graphene quantum dots as conductive components and cobalt ferrite nanoparticles as electrocatalyst was decorated on the halloysite substrate. The morphology and purity of designed ternary nanocomposites confirm the

Methods
Precursors and materials. The chemical precursors and starting materials utilized in the synthesis process of samples, including halloysite nanotubes (HNTs), FeCl 3 ·6H 2 O, Co(CH 3 CO 2 ) 2 ·4H 2 O, CTAB and NaOH, were purchased from a Merck company and applied without further purification. XRD diffractograms were recorded by an X-ray diffractometer device using Ni-filtered Cu Ka radiation (Philips-X'pertpro). FT-IR spectra were obtained on a Nicolet Magna-550 spectrometer in KBr pellets. SEM micrographs were obtained on a LEO-1455VP equipped with energy dispersive X-ray spectroscopy. EDS analysis with a 20 kV hasten voltage was performed. TEM images were captured on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. The BET analysis was conducted at − 196 °C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). The distribution of pore size was measured by applying the desorption branch of the isotherm in the BJH method.
Green synthesis of S, N-GQDs. The red onion was selected as the green source for the synthesis of graphene quantum dots using a hydrothermal procedure. To use onion extract, the fresh red onion was squeezed, and the onion juice was filtered from the pulps. The red onion juice was filtered again and transferred to a 50 ml sealed autoclave and heated for 8 h at 180 °C. The obtained product was filtered, and the GQD solution was separated from the resulting powders. The obtained solution contains an S, N-codoped GQD solution. Electrochemical experiments. The collected cells possessed the resulting electrode material for the working electrode, platinum plate as a counter electrode, Ag/AgCl as a reference electrode and KOH in DI water as the electrolyte. The working electrode was designed by coating the synthesized materials on the copper substrate.

Preparation of S, N-GQDs
In three-electrode cells, it is necessary to calculate the potential changes of the working electrode, so this change is studied with respect to a reference electrode (a potentially defined electrode). These electrodes provide the facility of specifying the working electrode potential by providing constant and repeatable potential. Ag/AgCl electrodes are the most widely used reference electrodes in this system 55 . These resulting copper substrates were applied as working electrodes in three-electrode hydrogen storage cells. Galvanostatic charge-discharge investigations were performed on the SAMA 500 testing device at desired current densities. Cyclic voltammogram experiments for the obtained electrodes were performed in similarly collected cells at a scan rate of 0.10 Vs −1 .