Synthesis And Application In Hydrogen Production And Utilization Of Hierarchical Micro/Nanostructures Of NI/CO Based Electrocatalysts

This dissertation focuses on the electrocatalysis for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the application of hydrogen production and utilization. To improve the electrocatalytic performances, the extrinsic (microstructure) and intrinsic (electronic configuration) properties of nanomaterials are designed and controlled, and synthesize the Ni-based HER electrocatalyst, NiCo-based OER electrocatalyst, and Co-based ORR electrocatalyst, respectively. The specific works are as follows:(1) A hierarchical micro/nanostructured Ni/C electrocatalyst with high HER activity has been designed and synthesized to change the extrinsic (microstructure) and intrinsic (electronic configuration) properties of the material. To obtaining an Ni/C electrocatalyst with a high specific surface area and Ni nanoparticles dispersed well, an NiAl-layered double-hydroxide nanosheet array@graphene oxide (NiAl-LDH@GO) as precursor was synthesized by the hydrothermal method, and key factors and process for forming of NiAl-LDH@GO were investigated. The obtained NiAl-LDH@GO was used as the precursor to synthetize an N-doped carbon@Ni-Al2O3 nanosheet array@GO composite (N-C@Ni-Al2O3@GO) by coating with dopamine followed by calcination. N-C@Ni-Al2O3@GO has a high specific surface area with the Ni nanoparticles in the composite dispersed well and the sizes of Ni nanoparticles are small, which lead to the exposure of more active sites for electrocatalysis. N-C@Ni-Al2O3@GO is used as a non-noble metal electrocatalyst for HER in alkaline medium (1 mol·L-1 NaOH), and exhibits high electrocatalytic activity with low onset overpotential (75 mV), low Tafel slope b (115.1 mV·decade-1), and large exchange current density j0 (3.8×10-5 A·cm-2). N-C@Ni-Al2O3@GO exhibts the best HER performance in comparsion with Ni-A12O3@GO and N-C@GO which have the same morphology with N-C@Ni-Al2O3@GO. Accroding to the XPS results, the binding energies of Ni 2p in N-C@Ni-Al2O3@GO are higher than those in Ni-A12O3@GO by 0.2～0.3 eV, and the binding energies of N 1s in N-C@Ni-Al2O3@GO are lower than those in N-C@GO by 0.2-0.4 eV, which reveals strong interaction between the Ni nanoparticles and the N-C coating layer. The N-C coating layer reduces the electron density of Ni atoms and polarizes the Ni atoms into Ni (5+) (Lewis acid), which facilitates the adsorption of the H2O molecule (Lewis base, which has O lone pair electrons) and attraction of electrons from the electrode to reduce the activation energy of the Volmer step, which improves the electrocatalytic activity of N-C@Ni-Al2O3@GO for HER in alkaline media. In addition, the N-C coating layer and the XC-72 additive can form an electrically conductive network, which is helpful for the transfer of electrons from the electrode to the Ni nanoparticles, thereby the electrochemical reaction (Volmer step) could be sped up and the electrocatalytic activity could be improved.(2) A hierarchical micro/nanostructured NiCo-based electrocatalyst with high OER activity and stability has been designed and synthesized to change the extrinsic (microstructure) property of the material. Hierarchical hollow urchins of NiCo2O4 (HU-NiCo2O4) were synthesized by thermal decomposition of short basic nickel/cobalt carbonate nanofibers grown on the surface of the sulfonated polystyrene latex templates. In order to explore the effect of the special morphology of HU-NiCo2O4 for OER activity, nanoparticle aggregates of NiCo2O4 (NA-NiCo2O4) as the control sample were also studied. The test results of the electrocatalytic performance show that HU-NiCo2O4 has higher OER activity and stability than NA-NiCo2O4:the onset overpotential (ηonset) of HU-NiCo2O4 is lower than that of NA-NiCo2O4 by 33.8 mV; the current density of HU-NiCo2O4 is higher than that of NA-NiCo2O4; at the high overpotential region, polarization curve of NA-NiCo2O4 deviates from the linear region larger than that of HU-NiCo2O4 by 13.8 mV; the anodic current of HU-NiCo2O4 electrode after 1200th cycles displays comparable value with that after the 1st cycle. The superior OER performance of HU-NiCO2O4 can be attributed to its unique morphology. The good dispersion of NiCo2O4 particles in HU-NiCo2O4 and the thin and porous HU-NiCo2O4 shell expose the active sites, enable the electrolyte to effectively infiltrate the catalyst and promote oxygen to escape rapidly from the surface of the catalyst for continuing catalysis processes, consequently, HU-NiCo2O4 displays excellent OER electrocatalytic activity in an alkaline medium. Additionally, the thin and porous shell connect the cavities to form the well-connected three-dimensional through pore structure, which is beneficial towards efficient oxygen diffusion and consequently highly advantageous towards the high stability of the electrode.(3) A hierarchical micro/nanostructured Co-based electrocatalyst with high ORR stability at all pH values has been designed and synthesized to change the extrinsic (microstructure) property of the material. Co9Sg/S and N co-doped C (Co9S8/SN-C) composite was synthesized by calcining a precursor dried from a solution containing Co2+ and m-aminobenzenesulfonate anion after the complete reaction between CoCO3 and m-aminobenzenesulfonic acid (mole ratio=1:2) in deionized water. M-aminobenzenesulfonate anion provides sulfur source, carbon source and nitrogen source, which greatly simplifies the preparation process. In addition, the high valence sulfur in m-aminobenzenesulfonate anion is stable and safe in air or water. Co9S8/SN-C is a kind of irregular bulk material, in which Co9S8 nanoparticles are well wrapped around by SN-C nanofibers network. Co9S8/SN-C was evaluated as electrocatalyst for ORR in an alkaline medium (0.1 mol·L-1 KOH, pH=13), in an acid medium (0.5 mol·L-1 H2SO4, pH=1) and in a neutral medium (0.1 mol·L-1 PBS (phosphate buffered solution), pH=7) on RDE (RRDE). Co9S8/SN-C manifests good electrocatalytic activity and enhanced stability at all pH values. Additionally, this facile method can be a general strategy to prepare Ni4S3/SN-C, Ni9S8/SN-C, MnS/SN-C, and ZnS/SN-C.