Nano-structured Cerium Oxide-based Composite Materials:Preparation And Electrocatalytic Properties Investigation

Energy storage and conversion devices provide feasible solutions for solving a series of environmental pollution and energy crisis brought by economic globalization.Zinc-air battery has attracted wide attention owing to their high energy density,abundant reserves,environmental friendliness and low cost.However,the slow kinetics and large overpotential of oxygen reduction(ORR)and oxygen evolution reactions(OER)on the air electrode severely limit the commercial application of zinc-air batteries.So far,noble metal-based catalysts(Pt,Ru O₂,Ir O₂ and their alloys)with high catalytic activity suffer from limited reserves,high cost and poor stability,limiting their large-scale commercial promotion and application in the zinc-air battery field.Hence,the development of low-cost,high-efficiency,stable ORR and OER electrocatalysts plays a key role in improving the efficiency of energy production technology.Recently,CeO₂ as a transition metal oxide,has become a hot material in the field of catalysis research due to its abundant oxygen vacancy defects,reversible surface oxygen ion exchange and flexible transition between Ce³⁺and Ce⁴⁺states.However,its poor conductivity greatly restricts the development of single component electrocatalyst.Compounding with conductive nitrogen-doped carbon has been developed to overcome these drawbacks.The main research contents are listed as follows:(1)Incorporation of CeO₂ NPs in zeolitic imidazolate framework-8(ZIF-8)followed by high-temperature pyrolysis leads to generate CeO₂@N-doped carbon(CeO₂@N-C)materials.The introduction of the CeO₂ nanoparticles greatly enhances the catalytic activity of the ZIF-8-derived carbon materials in the ORR owing to the presence of Ce³⁺and oxygen vacancies,and the maintained skeleton of the original ZIF-8.At the same time,ZIF-8 transforms into an N-doped carbon layer with uniform mesoporous structure at high temperature.Due to the synergistic cooperation effect,CeO₂@N-C exhibits excellent ORR catalytic activity in both acidic and alkaline electrolyte.In 0.1 M KOH aqueous solution,the onset and half-wave potentials of CeO₂@N-C are 1.003 and 0.908 V,respectively,which are comparable to those of Pt/C catalysts.Furthermore,it exhibites much better stability and methanol tolerance than Pt/C,indicating its great potential in energy conversion.(2)In the presence of phytic acid,a layer of polyaniline is uniformly coated on the surface of CeO₂(CeO₂@PANI)via in situ polymerization.Subsequently,CeO₂@PANI is pyrolyzed at high temperature to prepare hollow structured N,P co-doped carbon-coated CeO₂/Ce PO₄ nanoparticles.The introduction of another heteroatom besides N makes the structure of carbon become more disordered,thus generating more defect active sites.At the same time,the synergy effect makes the catalyst exhibit excellent ORR activity,comparable to the performance of commercial Pt/C catalysts.Furthermore,it presents a high durability and strong methanol tolerance,revealing its great potential for application in energy conversion.(3)A novel bifunctional electrocatalyst with CeO₂ and Co nanoparticles embedded in N-doped carbon(CeO₂/Co@N-C)is achieved via pyrolyzing CeO₂@ZIF-67particles.In alkaline solution,CeO₂/Co@N-C shows an excellent electrocatalytic performance with an ORR onset potential of 0.998 V,while the overpotential of OER is 474 m V at 10 m A cm^-2.The liquid rechargeable zinc–air battery assembled by using CeO₂/Co@N-doped carbon as the air cathode shows excellent charge/discharge performance and a maximum power density of 102.69 m W cm^-2.Furthermore,the rechargeable zinc–air battery exhibits stable cycling performance after 70 hours and round-trip efficiency of ca.59%.(4)Using a self-template method,the carbon microspheres prepared under hydrothermal conditions are used to construct an efficient and stable trifunctional catalyst through two-step calcination,where the CeO₂/Co nanoparticles are evenly embedded in nitrogen-doped carbon hollow microsphere with multi-shell nanostructure(CeO₂/Co@NCH).As an electrocatalyst for the ORR,it exhibits satisfactory onset potential and half-wave potential,as well as superior durability and methanol tolerance.Meanwhile,it illustrates high activity towards OER in 0.1 M KOH electrolyte,which only requires overpotential of 470 m V to drive the current density of 10 m A cm^-2.Moreover,the rechargeable zinc-air battery using the CeO₂/Co@NCH as the air cathode electrocatalyst presents a high round-trip efficiency after 170 hours.As bifunctional electrocatalyst for overall water splitting in alkaline solution,it needs only 1.74 V to afford 10 m A cm^-2 current density with excellent long-term stability.Furthermore,a water splitting cell is integrated with rechargeable zinc-air batteries by taking advantage of the OER/ORR and OER/HER performance of the sample,and the results illustrate its good possibility to be a good candidate for the application in renewable and sustainable energy conversion.