Preparation And Performance Study Of Nitrogen And Phosphorus Double-doped Carbon-supported Cobalt-based Oxides Based On Oxygen Reduction And Evolution Reaction

The deepening energy and environmental crisis has accelerated which people's urgent need to develop low-cost,portable,and highly reliable new energy technologies.Reversible fuel cells and reversible metal-air batteries are considered as promising energy conversion devices,due to they have the advantages of low price,clean environment and high energy density.However,these rapid and stable electrical equipment are limited to the slow rate of oxygen reduction reaction?ORR?and oxygen precipitation reaction?OER?on the oxygen electrode.Platinum and ruthenium double electrocatalysts show high electrocatalytic activity for ORR and OER,but their practical application are limited to their shortcomings of scarce earth reserves,high price and poor cycle stability.Therefore,the development of cheap and efficient dual-function catalysts is extremely important for promoting the practical application of reversible fuel cells and reversible metal-air batteries.In order to construct an efficient non-precious metal bifunctional electrocatalyst,this work is used the sodium chloride as template,at the same time,used Ethylenediaminetetraacetic acid disodium salt?EDTA-2Na?and phytic acid?PA?as carbon,nitrogen and phosphorus sources.A dual-function electrocatalyst with both ORR and OER catalytic activity,which is cobalt oxide are supported on N,P doped porous structure.the porous structure of carbon framework and its ORR electrocatalytic performance are changed by adjusting the ratio of EDTA-2Na and PA.The cobalt metal ions are co-complexed with EDTA-2Na and PA to achieve uniform loading of cobalt oxide nanoparticles to improve the OER catalytic performance.The research content of this work includes the following three aspects.?1?In order to increase the surface area of the catalyst and the number of catalytic active sites,this work successfully prepared a cobalt and nitrogen-doped carbon?Co/NCT?electrocatalyst by using the green and environmentally friendly sodium chloride as template.Co/NCT has a three-dimensional structure of ultra-thin nanosheets and a large specific surface area.This structure is beneficial to fully expose the rich catalytic active sites,promote the transport of electrons and protons,and thus improve its ORR catalytic performance.Compared with Co/NC samples synthesized without sodium chloride template,Co/NCT has greatly improved ORR performance,showing higher half-wave potential?0.824 V vs.RHE?and limiting diffusion current density(4.27 mA cm^-2),with good ORR electrocatalytic activity.In addition,it follows the catalytic ORR transfer path of nearly four electrons and shows excellent resistance to methanol poisoning and catalytic stability.?2?In order to exert the synergistic effect between heteroatoms to improve the catalyst's ORR and OER catalytic performance,in this work,PA and EDTA-2Na were used as complexing agents to prepare N,P double-doped carbon-supported cobalt oxide composite electrocatalysts?Co_xO_y/NPCT?.The results show that Co_xO_y/NPCT has higher ORR and OER electrocatalytic performance than the only nitrogen doped Co_xO_y/NCT and only phosphorus doped Co_xO_y/PCT samples.In 0.1 M KOH electrolyte,the catalytic ORR half-wave potential of Co_xO_y/NPCT is 0.835 V?vs.RHE?,which is only 14 mV different from the commercial Pt/C?0.849 V?.In addition,it follows the four-electron transfer path of catalytic ORR,and has higher stability and better methanol tolerance than Pt/C.In terms of catalytic OER,the overpotential of Co_xO_y/NPCT at a current density of 10 mA cm-2 is 370 mV,which has a lower Tafel slope and a larger electrochemically active area than Co_xO_y/NCT and Co_xO_y/PCT.This is mainly due to the synergistic effect between nitrogen and phosphorus and the increase in the number of active sites.In addition,the study found that changes in the ratio of EDTA-2Na and PA doping have a great influence on the morphology,structure and composition of the electrocatalyst.?3?In order to further optimize the doping ratio of nitrogen and phosphorus to improve the ORR and OER catalytic performance of the catalyst,this work explored the influence of the ratio of EDTA-2Na and PA on the micro-morphology and catalytic performance of the synthesized catalyst.Structural studies have shown that when the proportion of PA increases,the resulting catalyst gradually transforms from a ultra-thin carbon nanosheet layer to a structure in which thicker carbon nanosheets and carbon particles are stacked on top of each other;in composition,the form of Co also transitions from metallic Co to CoO,Eventually transformed into amorphous Co.The electrochemical performance study shows that when the molar ratio of EDTA-2Na to PA is 9:1,the prepared electrocatalyst Co_xO_y/NPCT-2 has the best bifunctional catalytic performance of ORR and OER.In the catalytic ORR test of 0.1M KOH,Co_xO_y/NPCT-2 showed a half-wave potential of 0.841 V,which was close to the test value of the commercial Pt/C(E_1/2=0.849 V)catalyst,and showed a ratio of Pt/C More excellent stability and methanol tolerance.In the OER catalytic performance test of 1 M KOH electrolyte,the overpotential of Co_xO_y/NPCT-2 at a current density of 10 mAcm-2 is 360 mV,which is close to the test value of commercial RuO₂;in addition,Co_xO_y/NPCT-2 The Tafield slope is 77.1 mV dec-1,and has a high electrochemically active area and excellent stability.The potential difference between the half-wave potential of ORR and the potential of OER at 10mA cm-2 in Co_xO_y/NPCT-2 in 1 M KOH electrolyte is only 0.77 V,which is lower than most literature reports on non-precious metal carbon-based electrocatalysts value.