Preparation Of Nickel And Cobalt Based 3D Graphene Electrocatalysts And Their Hydrogen And Oxygen Evolution Reactions

Environmental pollution and the energy crisis have become two pivotal issues for human society that seriously threaten the existence of terrestrial lives.Hydrogen received special attention as best possible solution of the environmental and energy crises due to their safe and high energy density.Recently,the splitting of water by electrocatalytic process becomes the center of interest to obtain hydrogen energy with free carbon–base emission from a cheap and abundant source.Unfortunately,the highly expensive and rare nature of the element Pt and compounds IrO₂ or RuO₂ makes them difficult be industrialized.In this regard,the scientists investigate other transition metals and their derivative in order to replace the precious metals and realize the practical application of electrocatalysis of water for the sake of environmental and energy crises,and most of such material show excellent performances.Among transition metals the Ni and Co are deeply studied for HER and OER and show better results.In addition to good performance these metals are abundant,cheap and have low or zero impact on environment.The large size of Ni and Co cause high intrinsic resistance with low active surface area which may affect the performances.In this research work,we developed cheap and simple methods to obtain micro and nano–sized Ni and Co particles on 3D graphene?3DG?as a source of support.The 3DG not only provide wide surface area for the deposition of Ni and Co and their derivatives but also ease the transport of electrons due to their conductive lamella,which may enhance the electrocatalytic performances results.The dispersion of micro and nano–sized Ni and Co increase the active spots for reaction and reduce the intrinsic resistance.The obtained Ni and Co based electrocatalyst are active toward HER and OER with good stability.The obtained electrocatalyst deliver high current density at low overpotential as well as displayed low resistance during reaction.Ni based electrocatalysts are comparatively better in performance than Co based electrocatalyst.The work is classified in five chapters which is summarized as follow.1.Nickel particles loaded 3DG by simple ion exchange/activation method was synthesized.In this method the resin was treated with Ni salt followed by KOH and ethanol and then calcinated to obtain 3DG.In the next step 3DG was decorated with Ni particles by simple chemical reduction and precipitation method.Ni loaded 3DG with different concentration of Ni were synthesized and investigated 1 M KOH solution so as to determine the best electrocatalyst with optimum concentration of metal.Ni–3DG–2 shows overpotential of 0.205 V?vs RHE?at a current density of 10 mA cm^–2 with better stability for 4 h continuously and lower R_ct.Ni–3DG–2 shows only 15%loss for HER after 4 h.The effect of Ni on electrocatalytic performance was investigated by using different concentration of Ni with constant amount of 3DG.The Ni Nano particles are the active sites for the reaction and 3DG facilitate electron transport due to their conductive nature.This study showed that Ni–3DG–2is better in performance as compared to other as–prepared electrocatalysts and previously studied Ni–based electrocatalysts.2.The Ni is very similar to Co in many properties,and mostly studied together.The 3DG were decorated with a series of hexagonal micro–coin Co?OH?₂ particles by a simple double displacement reaction.Different analytical techniques confirmed that the 3DG exhibit rose petal–like structure,which is decorated with Co?OH?₂ hexagonal micro coin structure which are the actual active sites for electrochemical reactions.The as–synthesized electrocatalysts are used to study different electrochemical measurement in an alkaline?1 M KOH?solution.Different concentrations of Co were also used to study the effect of Co on electrochemical measurements.The data shows that Co?OH?₂–3DG–0.5 exhibited better performance than the other as–prepared Co?OH?₂ based electrocatalysts.The as–prepared Co?OH?₂–3DG–0.5delivered the overpotential value of–0.367 and 1.599 V?vs RHE?(10 mA cm^–2),the calculated Tafel slope values were 96 and 110 mV dec^–11 for hydrogen and oxygen evolution reactions correspondingly.The as–prepared electrocatalyst also shows low R_ct and reasonable stability for hydrogen and oxygen evolution reaction.3.The interest in morphology and composite composition attract the attention for further studies of Co–based materials and 3DG were ornamented with different concentration of CoO nanoparticles by chemical reduction and thermal treatment.The graphene shows three–dimensional lamellae which are composed of a few layers of graphene.The ultrathin 3DG lamella offers the large surface area for cobalt oxide particles to deposit over its surface.Cobalt oxide particles of different size and shape which accumulates on the surface of 3DG layers provide large active sites for OER and HER.The as–prepared electrocatalyst acts an efficient bifunctional electrocatalyst for OER and HER in 1 M KOH solution.The as–prepared 3DG–3electrocatalyst required an OER overpotential of 1.6 V?vs RHE?and HER overpotential of0.36 V?vs RHE?for the current density of 10 mA cm^–2.The 3DG–3 electrocatalyst exhibits Tafel slope value of 62 and 104 mV dec^–1 for OER and HER respectively.The 3DG–3 also shows good stability for 4 h with lower R_ct for OER and HER.4.The better performance increase the interest to obtained the metal and metal oxide simultaneously of both metals and simplify the method.Ni–NiO@3DHPG and Co–CoO@3DHPG were obtained by an efficient,cheap and simple synthetic method for oxygen evolution reaction?OER?hydrogen evolution reaction?HER?.Nickel/nickel oxide@3DG?Ni-NiO@3DG?and cobalt/cobalt oxide@3DG?Co–CoO@3DG?electrocatalysts were obtained from cation exchange resin,nickel acetate and cobalt nitrate as a source of carbon,Ni and Co respectively.In Ni–NiO@3DG and Co–CoO@3DG,metal and metal oxide nanoparticles embedded in the 3DG layers,were confirmed by different techniques.The 3DG in Ni–NiO@3DHPG and Co–CoO@3DHPG were thinned wall,highly porous structure with large specific surface area.The Ni–NiO@3DHPG composite displays onset potential,overpotential(10 mA cm^–2)and Tafel values of 1.53 V,1.64 V and 55 mV dec^–1 for OER and–0.18 V,–0.31 V and 78 mV dec^–1 for HER respectively.Similarly,Co–CoO@3DHPG catalyst show onset potential,overpotential(10 mA cm^–2)and Tafel are 1.59 V,1.68 V and 65 mV dec^–1 for OER and–0.26 V,–0.40 V and 85 mV dec^–1 for HER respectively.Electrocatalysts show good stability after four hours continuously.This simple synthetic method will open a new way for cheap and efficient electrocatalysts of other types of non–precious metals for OER and HER.5.Energy and environmental affairs and better results of Ni and Co and their oxide gain the attention for the combine study as a single electrocatalyst.The simultaneous bimetallic Ni–NiO/Co–CoO@3DG electrocatalyst were developed by using a cheap source of resin as 3DG source.The as–prepared bimetallic electrocatalyst illustrates highly porous morphology with an ultrathin–wall of 3DG.The highly porous morphology and wide surface area of the as–synthesized sample may facilitate the electrochemical performances.The electrochemical studies of the electrocatalyst were examined in KOH?1 M?solution.The bimetallic electrocatalyst delivers the current densities of 10 mA cm^–2 at a low overpotential of–0.35 V and 1.64 V?vs RHE?for HER and OER respectively.The electrocatalyst exhibited the Tafel value of 123.4 and 85.4 mV dec^–11 for HER and OER with ECSA value of 15.9 mF cm^–2respectively.