Construction Of Nickel Based Nanomaterials And Study On The Electrocatalytic Water Splitting Performance

Hydrogen is a clean,renewable,and high energy density energy carrier,but there is no natural hydrogen on Earth.Electrocatalytic water splitting is an important method for preparing high-purity hydrogen at room temperature,including two half-reactions of hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Slow kinetics leads to higher overpotentials in the two processes,which impedes the large-scale development of the water electrolysis for hydrogen.The preparation of an efficient,inexpensive,and stable catalyst as an electrode material is the key to solving the above problems.In general,a reasonable open three-dimensional micro-nanostructure design can avoid the use of high molecular polymer binders like Nafion,which can improve conductivity,increase the active sites of the catalyst,and facilitate gas evacuation,and further enhance the catalytic performance of the electrode.In this paper,a series of non-platinum?low platinum content?micro-nanocatalysts are constructed by a simple and repeatable method using foamed nickel or carbon fiber cloth as a current collector,based on inexpensive transition metals such as nickel and iron,and the highly efficient electrocatalytic water splitting systems are designed.The main research work is as follows:A nickel foam?NF?supported two-dimensional Ni₂P nanosheet catalyst?Ni₂P NSs-NF?with edge sites was prepared by chemical deposition method and sintering method for electrocatalytic HER.During the phosphating process,many in-plane nanopores are produced on the Ni₂P nanosheets.On the one hand,a layered porous network of Ni₂P nanosheet arrays with open nanostructures can promote electrolyte permeation and hydrogen molecule release.On the other hand,many active edges on Ni₂P help to expose more hydrogen evolution active sites and promote the HER process.The electrode exhibits efficient electrocatalytic activity under both alkaline and acidic conditions.The theoretical calculation of the hydrogen adsorption energy of each crystal plane based on density functional theory?DFT?shows that the?211?crystal plane of Ni₂P can be the most active sites of HER.The nickel foam supported Ni₃S₂ nanosheet electrodes?Ni₃S₂ NSs-NF?with abundant edge sites were designed based on the synthetic strategy of Ni₂P NSs-NF.The Ni₃S₂ NSs-NF electrode requires only an overpotential of 244 m V to drive the OER at 10 m A·cm^-2,which is comparable to the commercial precious metal catalyst Ru O₂-NF?241 m V?.Based on the XRD and XPS analysis of Ni₃S₂ NSs-NF before and after operating at 10 m A·cm^-2 for 10 h,it can be inferred that under the OER reaction state,the Ni₃S₂-Ni OOH heterojunction was formed in situ on the surface of the electrode material.Compared with Ni₃S₂,the density of states?DOS?at the Fermi level of Ni₃S₂-Ni OOH is enhanced by DFT calculation,which has higher electron transport capability and can promote the OER reaction.Based on the previous research,the iron-nickel double-metal hydroxide precursor was first synthesized on the nickel foam by a simple hydrothermal method,and then phosphating with red phosphorus at 500°C to synthesize the nickel foam supported two-dimensional nickel-iron phosphide?Fe Ni-P/NF?.The body of Fe Ni-P/NF is Fe Ni-P crystal,and the surface is covered with a several-nanometers thin layer of amorphous Ni?Fe?phosphate/oxide,which can prevent the internal crystal from being further oxidized during the OER process and promote the electron transport,further enhance the OER activity.The water splitting device was assembled with Fe Ni-P/NF as a bifunctional electrode.When the current density is 10m A·cm^-2,the cell voltage is stable at 1.57 V.The experiment was further carried out for 100 h at 100 m A·cm^-2,and the voltage curve was stable and basically lower than 1.85 V,indicating that Fe Ni-P/NF is a very efficient and stable bifunctional electrode with a huge potential for alkaline water splitting for hydrogen production industry.In order to further improve the electrocatalytic performance,the carbon fiber cloth supported Ni Fe based layered double hydroxides?Ni Fe LDH/CC?was prepared by hydrothermal method,and then two high-efficiency electrocatalysts were prepared based on the Ni Fe LDH/CC matrix for HER and OER,respectively.For the HER catalyst,ultrafine Pt sub-nanometer clusters with an average size of 0.59 nm were immobilized on two-dimensional?2D?Ni Fe LDH nanosheets by a one-step wet chemical reduction method.The highly uniformly dispersed platinum sub-nanometer clusters can expose more active sites,shorten the electron transport path,and greatly reduce the amount of platinum.In addition,the Ni Fe LDH accelerates the dissociation of water,produces a hydrogen intermediate,and then adsorbs on nearby Pt atoms.In particular,the strong interaction between the platinum sub-nanometer clusters and the two-dimensional Ni Fe LDH nanosheets can effectively prevent the agglomeration of the platinum sub-nanometer clusters.Therefore,the Pt-Ni Fe LDH/CC electrode with ultra-low Pt content?1.56 wt%?has a ultra-low overpotential of 28m V at the current density of 10 m A·cm^-2,which is comparable to the commercial Pt/C catalyst?Com-Pt/C?electrode.For the OER catalyst,a three-dimensional edge-rich(Ni_0.77Fe_0.23)Se₂/CC was obtained by sa imple selenization treatment of Ni Fe LDH/CC.The introduction of many nanopores increases the active sites and enhances the reaction kinetics of OER.The(Ni_0.77Fe_0.23)Se₂/CC electrode only needs an overpotential of 228 m V driving a current density of 10 m A·cm^-2,which is superior to the commercial Ru O₂ catalyst?Com-Ru O₂,251 m V?.Therefore,the assembled two-electrode Pt-Ni Fe LDH/CC||(Ni_0.77Fe_0.23)Se₂/CC electrocatalytic water splitting system has good performance,requiring only a low voltage of 1.57 V to drive a current density of 30 m A·cm^-2.Thin Ni Fe LDH vertically grown on the surface of micron-sized nickel-coated foamed nickel?Ni Fe LDH/Ni@NF?was prepared by the electrodeposition-hydrothermal method,and the preparation conditions were optimized.When the nickel film was electrodeposited for1000 s,the raw material of Ni:Fe ratio in hydrothermal reaction was 4:1,and the hydrothermal reaction time was 16 h,the obtained material has the best OER performance,which was recorded as Ni₄Fe₁ LDH-16h/Ni-1000s@NF.Employed the electrodeposited nickel coated foamed nickel skeleton as a matrix,the Ni₄Fe₁ LDH nanosheet grown is more vertical on direction,thinner in thickness,higher in height and larger in electrochemical active surface area than the foamed nickel directly as a matrix,thereby effectively improving the OER performance.Thermoelectric generator driven water splitting system?TEG-WS?was established employed the Ni₄Fe₁ LDH-16h/Ni-1000s@NF as the anode material,the Pt-Ni₄Fe₁ LDH-16h/Ni-1000s@NF as the cathode material.When the temperature difference is 210 K,the TEG-WS system efficiency is 2.23%.Because the efficiency of the commercial thermoelectric generator used in the experiment is low and the experimental temperature is not high,the total efficiency of the system is quite low,it still indicates a thermo-electric-hydrogen production pathway,which provides a new idea for clean energy production.