Design And Fabrication Of Oxygen-electrocatalytic Functional Electrospun-carbon-fibers For Flexible Zinc-air Batteries

Due to their high theoretical energy density,clean reactive substances and relatively low-cost,metal-air batteries are considered to be one of the most ideal candidates for next generation wearable energy storage.However,the lack of flexible effective oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?electrochemical catalytic electrodes is the main bottleneck restricting the large-scale application of metal-air batteries in wearable electronic devices.Therefore,it's of great significance to fabricate efficient bi-functional electrocatalysts with high catalytic activity and self-supported flexible feature for designing and developing of flexible metal-air batteries.In recent years,electrospun-carbon-fibers?ECF?,with high specific surface area,favourable conductivity,and good mechanical properties,maintain unique advantages as self-supported catalysts or three-dimensional self-supporting skeleton structure to support catalysts,and can be directly applied to wearable metal-air batteries.Herein,four kinds of oxygen-catalitic ECFs were designed and fabricated in this thesis,specifically,they are:multiple channel ECFs,carbon nanotube decorated ECFs,porous carbon flake decorated ECFs and composite structure decorated ECFs.The novel fabrication is based on electrospinning techque and in-situ growth of active materials on prepared electrospun carbon fibers.The preparation processes were optimized by controlling precursors and the physical and chemical properties of different structure ECFs were also characterized and compared to select the preferred samples.Finally,the flexible zinc-air battery fabricated with the as-prepared electrocataysts perform low overpotentials,high discharge voltages and satisfactory stabilities,demonstrating their promising applications for energy storage in wearable electronic devices.The specific research results are as follows:?1?Fabrication of ECFs with various structures and their characterizations comparisoni)Optimized preparation and preferred samples of various structural ECFs:?1?Cellulose acetate?CA?is used as template to fabricate ECFs with multi-channel structures?MC-ECF?.The regulation of the channels structures is achieved by controlling the CA propotion in precursors.The BET surface area(S_BET),mesopore volume(V_meso)and micropore volume(V_micro)are increasing with the increase of CA propotion.The preferred sample showed S_BET of 116.40 m²/g and ORR/OER bi-functional overpotential of?E=0.97 V.The formation mechanism of channel structures is analysised through morphology and BET characterizations;?2?Based on the recipe of MC-ECFs,a method for preparing one-dimensional carbon nanotube surface-modified electrospun carbon fiber?CNT-ECF?was developed using melamine as solid carbon sources.The structure of the obtained product was optimized by changing the ratio of melamine to carbonization precursor.The S_BET of the material showed a trend of increasing first and then decreasing with the increase of melamine adding ratio.The V_meso and V_micro did not change significantly,and the optimized melamine adding ratio was 5:1.Through the characterization and electrochemical performance tests,the preferred sample was screened out with the S_BET of 232.3 m²/g and?E of 0.801 V.In this series,melamine is used as solid carbon source for the growth of carbon nanotube,which is safer and easier to operate than traditional gaseous carbon sources?acetylene,ethylene?.The possible formation mechanism of carbon nanotubes was analyzed by X-ray diffraction?XRD?and microscopic morphology;?3?A surface-modified electrospun carbon fiber with polygonal porous flake carbon material decorated structure?PCF-ECF?was prepared by using metal organic framework?MOF?as pore-forming template.By controlling the impregnation growth time of MOF,the material was structurally controlled and optimized.With the prolongation of solution immersion growth time,the S_BET and V_meso of the obtained carbon fibers are increased,and the V_micro remained unchanged.It was found that the MOF structure stopped growing after 60 minutes of impregnation;the pre-oxidation process would cause the collapse of the MOF structure;the direct carbonization precursor could maintain the original morphology of MOF.The preferred sample(S_BET 341.6 m²/g,?E=0.806 V)was screened by the bi-functional oxygen catalytic performance test.The formation mechanism of MOF on the surface of PAN electrospun fibers was analyzed by XRD and morphological characterization.ii)Based on the optimal recipes of the above three ECFs,the preparation process of oxygen-catalysis electrospun carbon fiber is summarized as follows:electrospinning solution preparation?electrospinning??solution immersion pretreatment???pre-oxidation??carbonization?sample.Through process comparison,PCF-ECF,which requires solution impregnation pretreatment step and does not require pre-oxidation step,has minimal energy consumption.MC-ECF and CNT-ECF do not need solution impregnation steps,thus have the advantage of efficient preparation.By comparing the correlation between S_BET and oxygen catalytic performance,it is found that the larger the S_BET of the three structural ECFs,the better oxygen catalytic performances were achieved.CNT-ECF exhibits the biggest correlation slope,indicating that this material can obtain oxygen catalytic performance comparable to the other two structures with smaller S_BET.By XRD and X-ray photon spectroscopy?XPS?,it is found that the main crystall active materials of the three materials are the same;pyridinium nitrogen,graphite nitrogen,cobalt-nitrogen and cobalt particles activated graphite structure are the main active sites in three materials.While cobalt-nitrogen structure can not form in MC-ECF.Characterization of oxygen catalytic properties showed that CNT-ECF and PCF-ECF had the best ORR and OER activities,respectively;CNT-ECF exhibited the best?E of 0.801 V;CNT-ECF structure has the highest ORR electron transfer number?>3.8?;PCF-ECF has the lowest OER Tafel slope?92mV/dec?.Through characterization and comparison,it is found that the surface modification structure of electrospun carbon fiber is more conducive to the improvement of its oxygen catalytic performance,and effect is limited by using the method of manufacturing internal pores.?2?Optimized preperation of composite structure ECFs and comparison between composite structure and single structural ECFsi)The MOF grown PAN electrospun fiber membrane was used as the carbonization precursor,and melamine was added in the carbonization stage to prepare surface-modified electrospun carbon fiber with carbon nanotube and porous carbon flack composite structure?CNT/PCF-ECF?.The material was optimized by changing the adding ratio of melamine.The S_BET of the material increases first and then decreases with the increase of the proportion of melamine in precursors.The V_meso decreases greatly when the melamine adding ratio exceeds 10:1 due to the collapse of the PCF structure,and the optimized melamine adding ratio is 10:1.The selected sample(specific S_BET=470.7 m²/g,?E=0.753 V)was screened by the bi-functional oxygen catalytic performance test.It was found that the formation mechanism of the composite structure was related to the exsistence of cobalt in the MOFs and the adding of melamine as carbon nanotube source.ii)Compared with the preparation parameters of CNT-ECF and PCF-ECF,the preparation process of CNT/PCF-ECF has less energy consumption;the large-scale preparation is subject to the solution impregnation pretreatment process.Comparing the correlation between the S_BET and oxygen catalytic performance,it is found that the larger the S_BET of CNT/PCF-ECF,the better the bifunctional catalytic performance is achieved.It is also found that CNT/PCF-ECF exhibits a minimum correlation slope compared to those of single structural material,indicating that the intrinsic activity of the active site gradually becomes the main factor effect the bi-functional oxygen catalytic perfoemance of the material in the interval of high specific surface area.Comparison of the physical and chemical properties of the material revealed that CNT/PCF-ECF obtained larger S_BET,V_meso and more active sites contents than those of single structures.The comparison of oxygen catalytic performance found that CNT/PCF-ECF exhibited higher ORR catalytic activity than that of CNT-ECF and better OER catalytic performance than that of PCF-ECF.In addition,CNT/PCF-ECF also exceeded the commercial precious metals Pt/C and Ir/C in terms of ORR and OER overpotential,respectively,and showed good catalytic performances.CNT/PCF-ECF has ORR electron transfer number close to 4 and OER Tafel slope?60 mV/dec?lower than Ir/C,which shows the advantage of the composite structure for improving the catalytic performance of electrospun carbon fiber.The above results show that the composite of CNT and PCF single structures on the surface of electrospun carbon fibers exhibits synergistic effect.?3?Preparation and performaces of flexible zinc-air battery based on electrospun carbon fiber catalytic cathodei)Preparation of fully flexible zinc-air battery:?1?In order to solve the problem of the rigidity caused by zinc foil,a commercial carbon fiber cloth was used as the electrode substrate skeleton,and a flexible metal zinc anode was prepared by electroplating;?2?Directly using the synthesized electrospun carbon fiber aggregate material as the catalytic positive electrode can omit the traditional catalyst pulping process and avoids the use of non-conductive organic binder;?3?The zinc-air battery is prepared by separator-electrolyte-flexible zinc electrode integrated design and stitched package.The resulting zinc air battery has similar flexibility to textiles.ii)The performance of the prepared ECFs catalytic electrodes in fully flexible zinc-air battery devices was compared by cycling performance tests at the current density of 1 mA/cm².Among them,The zinc-air battery based on CNT/PCF-ECF shows the lowest overpotential??V=0.68 V?and obtained the best battery performances.In addition,the CNT-ECF-based zinc-air battery has a similar discharge voltage?1.11 V?,a lower charge potential?1.90 V?,and better overpotential performance??V=0.79?than that of Pt/C-based electrode,indicating that CNT-ECF also has the utility for replacing commercial Pt/C electrodes.Finally,the CNT/PCF-ECF-based zinc-air battery exhibits good bending behavior and has a volumetric energy density of 63.19 mWh/cm³ and specific capacity of 530.17 mAh/g_zn.This battery performance compare favourably to those of recently reported flexible zinc-air batteries.The prepared fully flexible zinc-air battery can power external electronic components under wearing condition,and exhibits good practical values.