Preparation Of Non-noble Metal Multi-level Structural Materials And Their Electrocatalytic Properties

It is urgently necessary to reduce the dependence on fossil energy in current energy system,which requires us to further enrich the types of renewable energy and expands its proportion in the new energy composition.To achieve large-scale and substantial application of renewable energy,it demands us not only to reduce the cost of renewable energy,but also to increase the "availability" of renewable energy.Those demands have stimulated the continuous development of mobile electrochemical energy conversion,storage technologies and devices.In recent years,fuel cells have become one of the hottest item in many electrochemical energy conversion technologies with the rising research of hydrogen energy.In addition to the extensively studied oxyge-hydrogen gas fuel cells,metal(lithium,zinc,magnesium)air batteries and direct sodium borohydride(hydrazine,ammonia borane)fuel cells exhibit promising value and application advantages due to the relatively portable and easy storage characteristics of their fuel.In this thesis,zinc air batteries and direct sodium borohydride fuel cells were adopted as potential application devices;Electrocatalytic oxygen reduction reaction(ORR)and sodium borohydride oxidation reaction(BOR)were investigated as the key electrochemical reactions;Development of low-cost and efficient electrocatalyst for ORR and BOR electrode materials are the material carrier.The main contents are as followsCurrently,ORR on the cathode of fuel cells generally featured with the problems of sluggish kinetics,large overpotential,strong dependence on precious metals catalysts and inhibited oxygen diffusion and mass transfer,which cannot be ignored within design electrode materials.In this section,we successfully synthesize a series composite of transition metal and nitrogen co-doped honeycomb porous carbon(MN-HPC)by freeze drying&carbonization method.Cheap and available glucose and dicyandiamide were used as carbon-nitrogen sources.The low-cost metal salts,containing Fe,Co,Bi,Cu and Mn,were used as non-precious metal precursor.The NaCl crystalline was carried as an easily removable template.The thickness of carbon wall was only several nanometers and the pore diameter was around 250 nm according to the SEM and TEM results.Impressively,the MN-HPC with well-through porous structure facilitate the loading of abundant active sites and providing the path of mass transfer.Moreover,the ORR performance of optimal BiFeN-900 in three electrode system demonstrated its competitive properties with advanced on-set potential(0.985 V),half-wave potential(0.841 V)and limiting current density(-5.56-1 cm-2),comparable or even superior to those of commercial 20 wt.%Pt/C.Furthermore,when BiFeN-900 was assembled into cathode of zinc air/oxygen battery,the discharge voltage could reach at 1.475 V,1.27 V and 1.14 V at open circuit state(0),10 and 100 mA cm-2.Finally,the discharge stability test indicated that output voltage at 10 mA cm-2 could maintains at 1.28 to 1.25 V in 24 hours,and the voltage recovered by only replacing zinc anode.The zinc-air battery using optimal BiFeN-HPC cathode not only shows excellent discharge capability with high voltage and high current but also outstanding stability.Although the CuN doped porous carbon material have shown great potential to surpass FeNC and Pt/C catalysts,the agglomerated copper-based active sites still restrict its performance in ORR.At same time,the derived metal-organic-frameworks(MOFs)carbon materials though can make use of foreign atoms to coordinate and dilute active metal nodes sites ascribed to the periodic properties of MOFs.However,tightly packed MOFs polyhedrons leads to the low exposure of active sites and difficult in mass transfer.In order to solve above obstacle,we introduce a novel methodology to in-situ deposite CuZnMOFs onto carbon wall of HPC.Experimental results reveal that CuZnMOFs shows a uniform small size about 30 nm.By optimizing the synthesis parameters,such as molar ratio of Cu and Zn at 1:5,mass ratio of CuZnMOFs and HPC at 1:1,firstly calcination at 500? for two hours and then elevated at 900?with 2?/min,the optimal CuN/HPC-900(1:1)show excellent limiting current density of 6.15mA cm-2,high electron transfer number of 3.96 and positive peak potential at 0.828V in cyclic voltammetry.Furthermore,the ZAB equipped with CuN/HPC-900(1:1)cathode exhibits open circuit potential with 1.55V,discharge voltage arrived at 1.31V at 10mA cm-2 and peak power density attain 168 mW cm-2 with high discharge current density on 225 mA cm-2 Finally,the long-term stability test indicates discharge voltage only decrease from 1.27 V to 1.22 V after 960 cyclic charging-discharging operation with a small decay of 3.94%.In conclusion,this highly steady stability properties can be fully applied in practical zinc air battery.The direct borohydride fuel cells(DBHFCs)do not require the use of gaseous H2,using NaBH4 as fuel at anode which is easier to store and replenish.However,the overpotential of the sodium borohydride oxidation reaction(BOR)is quite high,because the BOR process involves 8-electron transfer and multiple intermediates,resulting in sluggish kinetics.The development of electrocatalysts with high activity towards BOR is still a major challenge.In this section,the NiMoN-base catalysts with hierarchical structure was fabricated on the 3D porous nickle foam substrate through hydrothermal step and high temperature pyrolysis nitridation method.The hierarchical structure NiMo@N/NF prepared under optimized conditions exhibits comparable BOR performance,which shows a low working potential of 0.039 V(vs.RHE)at the high current density of 100 mA cm-2.On the other hand,NiMo@N/NF electrode also shows a working potential of-0.148 V at 100 mA cm-2 and great stability as the HER electrocatalysts.At the current density of 100 mA cm-2,the working potential gap of NiMo@N/NF towards BOR anode and HER cathode(BOR-HER)is just 0.187 V,suggesting that this electrode is kind of great bifunctional electrocatalysts for BOR and HER.Employing NiMo@N/NF as cathode of the DBHFCs,the open circuit potential could achieve 1.05 V and the discharge voltage is as high as 0.832 V at 10 mA cm-2 in the catholyte of NaOH solution.While when the H2SO4 solution is used as catholyte,the open circuit potential is 1.84 V and the power density could reach 89.3 mW cm-2 at 80v.The above results indicate that NiMo@N/NF with hierarchical structure is an excellent BOR with low-overpotential and high-performance.The high performance of NiMo@N/NF towards BOR could be attribute to:(1)the urchin-like micropillars hierarchical structure was in-situ fabricated on 3D porous Nickle foam substrate,which could greatly improve the specific surface area of catalysts,provide sufficient active sites,facilities the diffusion of electrolyte and escape of gas;(2)the synergistic effect of NiMo N could modify the electronic structure electrocatalyst,which would improve binding capacity of BH4-and intermediates with catalysts and thus enhance the intrinsic activity of catalysts;(3)the great electron conductivity of self-supported NiMo@N.