Synthesis and characterizations of Ni-NiO nanoparticles on PDDA-modified graphene for oxygen reduction reaction

We are presenting our recent research results about the Ni-NiO nanoparticles on poly-(diallyldimethylammonium chloride)-modified graphene sheet (Ni-NiO/PDDA-G) nanocomposites prepared by the hydrothermal method at 90°C for 24 h. The Ni-NiO nanoparticles on PDDA-modified graphene sheets are measured by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) pattern for exploring the structural evidence to apply in the electrochemical catalysts. The size of Ni-NiO nanoparticles is around 5 nm based on TEM observations. The X-ray diffraction (XRD) results show the Ni in the (012), (110), (110), (200), and (220) crystalline orientations, respectively. Moreover, the crystalline peaks of NiO are found in (111) and (220). The thermal gravimetric analysis (TGA) result represents the loading content of the Ni metal which is about 34.82 wt%. The electron spectroscopy for chemical analysis/X-ray photoelectron spectroscopy (ESCA/XPS) reveals the Ni0 to NiII ratio in metal phase. The electrochemical studies with Ni-NiO/PDDA-G in 0.5 M aqueous H2SO4 were studied for oxygen reduction reaction (ORR).


Background
Although platinum-based nanoparticles are thought as the best catalysts for oxygen reduction reaction (ORR), the increasing cost of Pt and the low abundance triggered scientists to develop non-noble metal catalysts for fuel cell applications [1][2][3][4]. The transition metal-based catalysts (based on Co, Ni, and Fe) are considered as a promising alternative due to their cheap cost and availability and have thus been studied for decades [5,6].
Catalysts for ORR of fuel cells (PEMFC and DMFC) have been the focus in recent years from the combination of Pt with varying metals to non-Pt-based metals [7][8][9]. Furthermore, carbon-supported nanocatalysts are also of great interest for scientists and engineers [7,[10][11][12][13][14]. The ORR cathode is 6 or more orders of magnitude slower than the anode hydrogen oxidation reaction and thus limits performance, so almost all research and development focus on improving the cathode catalysts and electrodes [5]. The ORR catalysts are considered for mass production with the following factors: lower production of H 2 O 2 during the ORR and higher tolerance of the impurities (Cl − for instance). They must have the satisfied durability, and must be cost-effective. The three phenomena which lower the performance of fuel cells are kinetic losses, mass transport losses, and iR losses [5,7,15,16]. The ORR dominates the kinetic loss of fuel cells because the enhancement of the ORR activity would gain only 60 to 70 mV and kinetic losses are challenging. Moreover, the progress in catalyst development so far has achieved only modest cell voltage gains of tens of millivolts [5,[17][18][19]. How to improve and enhance the catalyst electrochemical performances is the focus of scientists and engineers. Carbon-supported materials were introduced for fuel cell application. The supported materials would provide the surfaces for anchoring the catalysts and increasing the surface areas of the catalysts. Also, the supported material provides higher volume-to-mass ratio to make a good dispersive paste for electrode assembly. The size of Pt nanoparticles for the commercial Pt on carbon (Pt/C) is about 2 to 5 nm [5,20]. In addition to that, the Pt-based bimetallic system is interesting for ORR application, and the Pt 3 Ni bimetallic electrocatalyst on carbon support has also been known to serve as a catalyst for ORR [21]. Herein, we introduced additionally poly-(diallyldimethylammonium chloride) (PDDA) which further assists in the formation of a layer-to-layer structure for graphene surface modification (PDDA-G) on carbonsupported materials [22][23][24][25]. The synthesis of Ni-NiO nanoparticles on PDDA-G is done using the hydrothermal method. The results on hydrothermal synthesis of the Ni-NiO nanoparticles on PDDA-modified graphene for ORR application would be presented in this study.

Methods
Graphene was prepared from graphite using the microwave synthesis method. Graphite (0.1 g; Sigma-Aldrich Co., St. Louis, MO, USA) was put into a 25-mL roundbottomed flask, and the flask was treated using a CEM Discover Du7046 microwave synthesis system (CEM, Matthews, NC, USA) with a power output of 20 W at 80°C for 10 s. The graphene was produced in 2 to 5 s with a sound of a bomb. Fifty milliliters of 3.5 wt% aqueous PDDA (Sigma-Aldrich) and 100 mg of graphene prepared by the method as mentioned were put into a 100-mL flask and then heated at 90°C for 4 h with a flux apparatus.
About 0.45 mmol Ni(NO 3 ) 2 · 2.5H 2 O was added into the above mentioned PDDA-G solution, followed by the addition of hydrazine hydrate of about 20 mmol. Then, the mixed solution was transferred into a Teflon-lined autoclave and heated at 90°C for 24 h. The mixture was centrifuged and washed for three times prior to drying at 90°C to produce the Ni-NiO nanoparticles on the PDDA-modified graphene (Ni-NiO/PDDA-G).
The crystalline structure of Ni-NiO/PDDA-G was examined by X-ray diffraction (XRD) using a Bruker D8 diffractometer (Bruker AXS, Karlsruhe, Germany) equipped with CuKα X-ray source. The chemical environments of Ni-NiO/PDDA-G were analyzed by electron spectroscopy for chemical analysis/X-ray photoelectron spectroscopy (ESCA/XPS) using a Thermo VG ESCAlab 250 (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a dual-anode (MgKα/AlKα) X-ray source.
The microstructures of Ni-NiO/PDDA-G were investigated with the high-resolution microstructural images produced using the JOEL FEM 2100F (JEOL Ltd., Akishima, Tokyo, Japan) equipped with an Oxford energy-dispersive X-ray spectroscope (EDS) for element analysis.
Thermal gravimetric analysis (TGA) for nanoparticle loading was carried out using a PerkinElmer Pyris 1 instrument (PerkinElmer, Waltham, MA, USA) and by applying a heating rate of 10°C/min from room temperature to 800°C in an oxygen-purged environment.
The ORR study was examined using an Autolab potentiostat/galvanostat PGSTAT30 (Eco Chemie BV, Utrecht, The Netherlands). The reference electrode is Ag/AgCl (ALS Co. Ltd., Tokyo, Japan), and the counter electrode is a 0.5 mm × 10 cm platinum wire. The working electrode is the glassy carbon whose surface is

Results and discussion
The crystallization of Ni-NiO/PDDA-G was examined by XRD as shown in Figure 1. The peaks of the (002) plane in the PDDA-modified graphene was shifted from 20.5°to 22°, which revealed the change in the layer-tolayer distance of graphene due to incorporation of PDDA [21]. The hydrothermal method for synthesis of the Ni-NiO alloy nanoparticles was one-pot synthesis with a mixture of PDDA-G, Ni precursors, and hydrazine hydrates at 90°C for 24 h. The XRD result of Ni-NiO/ PDDA-G indicated peaks assigned as Ni (111), Ni (200), Ni (012), Ni (222), NiO (111), NiO (012) and NiO (220), respectively [26,27].
The microstructures of the Ni-NiO/PDDA-G nanohybrids are shown in Figure 2. The PDDA-modified graphene is a layer-by-layer structure, shown in Figure 2a Figure 5c). Figure 5c shows no significant difference, as evidenced by the blue line denoting the O 2 -saturated ORR first scan and the green line denoting the tenth scan. The inset in Figure 5c is the ORR test in the N 2 -saturated 0. 5 Figure 5d. The semicircle curve of Ni-NiO/ PDDA-G in the 0.5 M aqueous H 2 SO 4 is higher than that in the 0.5 M aqueous H 2 SO 4 + 0.5 M CH 3 OH, showing the higher chemical reaction ability. Thus, the Ni-NiO/PDDA-G is more suitable for ORR than for the methanol oxygen reaction.

Conclusions
We have successfully synthesized the Ni-NiO/PDDA-G nanohybrids, and the size of Ni-NiO nanoparticles was about 2 to 5 nm. The morphologies and chemical composition of Ni-NiO/PDDA-G were evaluated by TGA, XRD, TEM, and ESCA/XPS. The results show the phase of the Ni-NiO/PDDA-G, and the loading content of Ni-NiO is about 35 wt%. The CV and EIS results of Ni-NiO/PDDA-G in 0.5 M aqueous H 2 SO 4 are better than those in 0.5 M aqueous H 2 SO 4 + 0.5 M CH 3 OH. Therefore, Ni-NiO/PDDA-G in 0.5 M aqueous H 2 SO 4 is more suitable as ORR electrocatalyst and could be a candidate of for cathode electrocatalyst of fuel cells.