Investigation On Fabrication And Electrocatalytic Water Splitting Properties Of Ni/Co Based Self-supported Nanosheet Catalysts

Electrocatalytic water splitting has been regarded as a secure and efficient approach to generate clean hydrogen-fuel energy,and it is divided into two half reactions:hydrogen evolution reaction(HER)at cathode and oxygen evolution reaction(OER)at anode.The theoretical value of overall water splitting is 1.23 V(vs RHE),but larger overpotential is needed because the actual water splitting reaction involves complex electron and ion transfer processes,resulting in slow kinetics and low efficiency.Thus,it is critical to develop efficient electrocatalysts to reduce energy consumption.Currently,the state-of-the-art catalysts for HER and OER are Pt-based and Ir/Ru-based materials,respectively.However,limited reserves and high cost hinder their large-scale applications.Consequently,earth-abundant,efficient,durable and inexpensive non-noble electrocatalysts have received much attention.Ni/Co-based two dimensional(2D)nanostructural materials(chalcogenides,oxides,hydroxides,layered dihydroxides,etc.)present obvious advantages in water splitting because of their large specific surface area and relatively open array structure.In addition,through the in-situ growth strategy,the self-supported electrodes avoid the use of binder/conducting agent,and the catalytic activity and stability can be promoted significantly.Based on the above research background,we proposed the in-situ growth strategy,and fabricated self-supported Ni/Co-based nanosheet catalysts with high performanceFirstly,the Ni foam-supported unique NiSe2 nanosheets were fabricated via the immersion-selenization strategy.The intermediate product NiC2O4·2H2O nanosheets anchored on Ni foam were first synthesized by immersion,and the unique nanosheet structure of NiSe2 originates from the inheritance and further self-assembly of NiC2O4·2H2O nanosheets in the selenization process.The XRD pattern of the selenized Ni foam presents diffraction peaks well indexed to the planes of cubic pyrite-type NiSe2 phase.SEM and TEM images show that the porous NiSe2 is composed of numerous nanosheets,the thickness of which ranges from several tens to one hundred nanometers.HRTEM and SAED results present the nano-polycrystalline nature.From the XPS results,we can know that the Ni element in the sample exists in the form of Ni?+ and Ni2+.After OER measurements,the peaks are consistent with Ni2+ and Ni3+ in the Ni spectrum,illustrating the coexistence of Ni(OH)2 and NiOOH.Electrochemical measurements show that the NiSe2/Ni electrode exhibits superior HER and OER performances compared with NiC2O4/Ni and Ni foam electrodes.Moreover,the electrolyzer was constructed using NiSe2/Ni as both cathode and anode in 1.0 M KOH electrolyte,which exhibits excellent overall water splitting performance.Sceondly,novel hybrid Ni(OH)2/FeOOH@NixFey(x:y = 2:1,1:1,1:2)nanosheet catalysts anchored on SSM were rationally designed and synthesized for improving OER performance via a scalable sputtering-alloying-dealloying-activation strategy.The NixFey-ac/SSM(x:y = 2:1,1:1,1:2)electrodes possess flexible,self-supported,composition-adjustable,and binder-free characteristics.Especially,compared with Ni1Fe1-ac/SSM and Ni1Fe2-ac/SSM electrodes,the Ni2Fe1-ac/SSM electrode displays incredible OER activity in a 1.0 M KOH electrolyte,with an overpotential of 216 mV to reach 10 mA cm-2.More importantly,the electrode went through an ultraiong durability test over 1000 h(>40 days)without obvious attenuation,exceeding most of the advanced electrocatalysts.The enhanced OER performance of the hybrid electrode can be rationalized by synergistic effect of hybridization between Ni(OH)2 and FeOOH,the optimal Ni:Fe ratio,the ultrathin nanosheet structure,large electrochemical surface area,and low activation energy.Thirdly,a novel painting-alloying-dealloying strategy was established to synthesize self-supported ?-Co(OH)2 nanosheet arrays anchored on Co plates using liquid metal as a mediator.The typical SEM and TEM images of the as-obtained Co(OH)2 product exhibit a homogeneous and well-shaped hexagonal structure.The average thickness of these hexagonal nanosheets is only?15 nm,and the mean side length is approximately 164 nm with little deviation.XPS results shows that Co element exists mainly in the form of Co2+,and partial Co3+ can be observed formed in the electrochemical activation process.The spin-orbit splitting value of Co 2p1/2 and Co 2P3/2 is 15.5 eV,further confirming the presence of Co2+ in the form of Co(OH)2.The integrated Co(OH)2/Co electrode exhibits superior electrocatalytic activity(merely 332 mV @ 10 mA cm-2)and stability towards OER in an alkaline solution.Thanks to the merit of liquid metal,both the pattern and shape of the integrated electrode can be facilely designed and regulated,without influencing the OER activity.Furthermore,using a separator between the anode and cathode,the integrated architecture of electrodes allows for the compact-type electrolyzer for overall water splitting.Finally,we report a novel sputtering-painting-alloying-dealloying strategy to synthesize CoGa LDH anchored on SSM(CoGa LDH/SSM),in which abundant oxygen vacancies are introduced into the ultrathin CoGa LDH nanosheets.The actual phase and chemical formula of the catalyst were determined by XRD,EDX analysis and Reflex simulation refining.SEM and TEM images present the ultrathin nanosheet structure.XPS results indicate the coexistence of Co2+ and Co3+.EPR measurement shows that numerous coordination-unsaturated CoO6-x octahedrons in CoGa LDH provide abundant oxygen vacancies in the CoO6-x units,which promote the catalytic activity intrinsically.The CoGa LDH/SSM electrode shows excellent electrocatalytic activity towards OER in an alkaline solution,with low overpotential of 258 mV to reach 10 mA cm-2.The stability is also worth being highlighted,and the electrode can hold its initial morphology and catalytic performance even after 70 h long-term durability.