Preparation And Properties Of Corn Straw Based ORR/OER Electrocatalysts

Oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）are the most important reactions on the cathode of fuel cell and metal-air cell,playing decisive roles in the performance and stability of the cell.Generally,commercial Pt/C and IrO₂ are considered to be the effective ORR and OER catalysts.However,their large-scale application are seriously hindered because of their high price,scarce reserves,poor stability,and other shortcomings.As a kind of abundant and renewable agricultural waste on the earth,corn stalk has been considered as an ideal carbon precursor due to they have the advantages of low cost,high carbon content,and easy functionalization.Therefore,in this paper,bifunctional OER/ORR electrocatalytic materials with excellent performance were prepared by different methods using corn straw as carbon precursor.The details are as follows:（1）Using corn straw as raw material,ZnCl₂ as activator and isolator,boric acid,ammonium chloride,and cobalt nitrate as B,N,and Co sources,respectively,the bifunctional N,B and Co co-doped electrocatalysts（SA-Co,Fe,B/NHPC-800）were prepared by hydrothermal reaction at 180℃and carbonization at 800℃.It was found that the addition of ZnCl₂ can effectively prevent the aggregation of Co and Fe in the carbonization process at high temperatures,and at the same time significantly increase the specific surface area and improve its electrocatalytic performance.Benefiting from the synergistic effect of N,B,Fe,and Co,SA-Co,Fe,B/NHPC-800 catalyst exhibited excellent ORR catalytic activity（0.857V vs.RHE）,which was superior to commercial Pt/C catalyst（0.834 V vs.RHE）.At the same time,the catalyst also showed good OER activity,the overpotential was 383 m V at the current density of 10 m A cm^-2,and the Tafel slope of OER was 100.6 m V dec^-1,which was much lower than the commercial IrO₂(145.99 m V dec^-1).（2）A bifunctional electrocatalyst（Pre-B,Co/NHPC-CaCl₂）with excellent performance was prepared by a simple one-step carbonization process using CaCl₂ as activator and corn straw as carbon precursor.As a mild activator,CaCl₂ is widely distributed in the network structure of corn straw in the process of high-temperature carbonization.Under the repeated washing of hydrochloric acid and water,CaCl₂ was dissolved and a large number of nano-pore structures were produced.Meanwhile,the nitrogen content of the catalytic material was increased（8.79%）.Benefiting from the activation of CaCl₂ and the synergistic effect of N,B,Fe,and Co,the corn straw-based catalyst showed outstanding ORR catalytic activity（0.848 V vs.RHE）and excellent OER catalytic activity(E_j=10=1.623V).It also showed excellent ORR stability（the stability test in 20000s only shows 7.62%decay）.（3）A bifunctional catalyst（N,B,F@Co-CNF）with a highly dispersed and hierarchical porous structure was prepared by carbonization at 800℃without an extra activator.The N,B,F@Co-CNF catalyst was synthesized by adding boric acid,cobalt nitrate,and polytetrafluoroethylene（PTFE）to form a uniform hydrogel and then freeze-dried.The prepared N,B,F@Co-CNF catalysts have good ORR catalytic performance（0.845 V vs.RHE）.At the same time,nanocellulose contains a large number of hydroxyl groups,which is beneficial to the loading of metal cobalt and can prevent the accumulation of metal cobalt at high temperatures.More importantly,we found that the doping of F element can greatly promote the cracking degree of catalytic materials,increase the specific surface area and improve the graphitization degree.The addition of boric acid is beneficial to increase the metal cobalt content in the material.With the combined action of nanocellulose and boric acid,the prepared catalyst has excellent OER catalytic activity(E_j=10=1.598V).