Preparation And Performance Of Biochar-based Electrocatalyst For Oxygen Reduction By Molten Salt Method

Biochar derived from biomass has become an indispensable carbon material as the catalyst for the new clean energy technologies due to the wide source of raw materials,large reserves and renewability,and possesses a great significance for meeting the challenges from the deterioration of ecological environment and energy crisis faced by human society.The preparation,properties and applications of the biochar have been extensively studied for a long time.A variety of preparation methods such as pyrolysis method,hydrothermal method,pyrolysis activation method,template method,ion thermal carbonization method,molten salt pyrolysis method and microwave treatment method have been reported.The results show that catalytic performances of the prepared non-metallic biochar-based catalysts for oxygen reduction reaction(ORR)is much lower than that of commercial Pt/C,and the electrocatalytic performances of metal-biochar-based catalysts are only close to or slightly better than that of commercial Pt/C,which cannot meet the needs of commercial application.In this paper,we will focus on the development of novel biochar-based electrocatalysts with high effectivity by using simple,feasible and environmentally friendly preparation methods.According to the composition and structure of biomass,an efficiently combined and optimized method(pyrolysis-heating treatment)based on molten salt pyrolysis and heating-treatment was used to prepare the biochar-based catalysts,which possesses the high density of active sites and the high ability of electron and mass transport resulting from the templating,etching and dispersion functions of molten salt and the higher degree of graphitization at the higher temperature.The research results show that the prepared biochar-based catalysts exhibit an excellent ORR catalytic performance and provide theoretical and practical basis for promoting the commercial application of biochar-based materials in new energy technology.The results of the research can be classified as follow.1.The pomelo peel(the unprocessed biomass)biochar-based catalysts was prepared by pyrolysis-heating treatment method.In this method,the templating,etching and dispersion functions of molten salt and the higher degree of graphitization at the higher temperature can avoid the agglomeration of pyrolysis products and create the high density of active sites and the high ability of electron and mass transport in the pomelo peel biochar.The half-wave potential of ORR over Peel-700-1000 prepared by pyrolysis in Na Cl-KCl molten salt at 700?and then heat-treated at 1000?is 0.83 V vs.RHE in 0.1 M KOH,lower than that over Pt/C by10 m V,but higher than that of non-metallic pomelo peel biochar-based ORR catalyst reported in literature.The half-wave potential of ORR over catalyst PM-700-1000obtained by adding melamine during heat treatment at 1000?is 0.88 V vs.RHE in0.1 M KOH and is 40 m V higher than that over Pt/C and higher than that on all ORR catalysts prepared with biomass as precursors reported in the literature.The zinc-air battery(ZAB)assembled with PM-700-1000 has the peak power density of 211 m W cm^-2 and a specific capacity of 807 m Ah g _Zn^-1,which is better than the ZAB assembled with commercial Pt/C(126 m W cm^-2,684 m Ah g _Zn^-1)and non-metallic carbon-based catalytic materials reported in the literature.It demonstrates the superiority of the molten salt pyrolysis-heat treatment method and the possibility of biochar materials to replace the precious metal Pt in clean energy technology.2.The N-doped biochar-based catalysts were prepared by pyrolyzing the unprocessed biomass peanut shells in molten KCl and then heat-treatment.The staggered texture and channel structure in the peanut shells are conducive to the formation of pore structures in the prepared biochar-based catalysts.The peanut biochar-based catalysts PS-800-1000 prepared by pyrolysis in molten KCl at 800°C and heat-treatment at 1000°C has the large specific surface area,high defect density and high degree of graphitization.The half-wave potential of ORR over PS-800-1000is 0.84 V vs.RHE,which is comparable to that of commercial Pt/C,and higher than all the peanut shell-derived biochar-based catalysts reported in the literature.ZAB based on PS-800-1000 has the higher peak power density(141 m W cm^-2),specific capacity(767 m Ah g_zn^-1)and better charge-discharge cycle stability than zinc-air battery based on Pt/C.Therefore,the biochar catalysts prepared from peanut shells by molten KCl pyrolysis and heat-treatment is expected to be widely used in energy conversion/storage devices.3.The cellulose extracted from biomass is composed of C,H and O,belongs to the processed biomass and the largest biopolymer in nature.The cellulose biochar(C-750-1000)containing C,O,and H elements was prepared by pyrolysis at 750?in the mixed Li Cl-KCl molten salt and heat-treatment at 1000?.The dispersibility and corrosiveness of Li Cl-KCl molten salt endows the pyrolysis products of cellulose with polyporous structure and more defect active sites.The ORR catalytic performance of C-750-1000 is significantly higher than that of the cellulose-derived catalysts reported in the literature,but the half-wave potential of ORR on it is 70 m V lower than that on Pt/C.The catalyst CN-750-1000 obtained by adding melamine as the N source in the heat treatment process has more nitrogen-containing active sites and larger specific surface area,and of which the ORR catalytic performance has also been significantly improved.The half-wave potential of ORR on CN-750-1000 is 30m V higher than that of Pt/C and also superior to other cellulose-derived metal-and non-metal-biochar-based catalysts reported in the literature.The peak power density(202 m W cm^-2)and specific capacity(785 m Ah g_zn^-1)of ZAB based on CN-750-1000are higher than that based on Pt/C(126 m W cm^-2,684 m Ah g_zn^-1)and other ZABs based on non-metallic carbon-based catalysts reported in the literature.It is of great significance to use natural cellulose as a raw material for preparation of the biochar-based electrocatalysts,which can reduce the cost of energy technology and promote the sustainable development of clean new energy technology.4.Chitosan is a processed biomass composed of C,H,O,and N,and the N content is about 7 wt%.By pyrolyzing chitosan in molten KCl at 800°C and then heat treatment at 800°C,the highly efficient ORR electrocatalyst CK-800-800 with foam-like structure was prepared(C refers to chitosan,K refers to molten KCl,800 is the pyrolysis and heat treatment temperature(?)).In alkaline medium,the ORR catalytic activity of CK-800-800(E_half-wave=0.86 V vs.RHE),stability and methanol resistance are better than those of commercial Pt/C catalysts and other non-metallic biochar-based catalysts reported in the literature.Compared with the zinc-air battery based on Pt/C and most non-metallic carbon-based catalysts,the zinc-air battery assembled with CK-800-800 has higher peak power density(178 m W cm^-2)and specific capacity(780 m Ah g_zn^-1)and cycle stability.It shows that chitosan biochar material has the potential to replace Pt/C catalyst in energy storage/conversion technology,and has a broad practical prospect.5.Chitin(C₈H₁₃O₅N)_n is the second largest biopolymer in nature and composed of C,H,O,and N.Its nitrogen content is slightly lower than that of chitosan,but its thermal stability is higher than chitosan,which will be conducive to the preparation of biochar-based catalysts with high nitrogen content.The chitin was pyrolyzed in molten salt Na Cl-KCl at 750°C,and then the pyrolysis product is heat-treated at1000°C to obtain chitin biochar(Chitin-750-1000)with N content of 4.07 at%and BET specific surface area of 1279 m² g^-1.The onset potential of ORR over Chitin-750-1000 in alkaline medium is 1.06 V vs.RHE,and the half-wave potential is0.88 V vs.RHE,which are higher than those over Pt/C by 50 m V and 40 m V respectively,and higher than those over metal-and non-metal-biochar-based catalysts reported in the literature.Chitin-750-1000 assembled zinc-air battery(ZAB)exhibits the higher peak power density(223 m W cm^-2),specific capacity(811 m Ah g_zn^-1)and energy density(1026 Wh kg_zn^-1)than ZAB assembled with Pt/C,which is also higher than those of ZABs based on non-metal carbon-based catalysts reported in the literature.These results prove that Chitin-750-1000 has been the most active non-metallic electrocatalyst for ORR so far.The above research results show that the molten salt pyrolysis-heat treatment method is a simple and effective method for preparing highly efficient biochar-based electrocatalysts.By this method,all biomass including the unprocessed biomass and the processed biomass can be used as raw materials to prepare the biochar-based electrocatalysts with an excellent catalytic performance.At the same time,it also fully demonstrates that biochar-based materials have the possibility of replacing the precious metal Pt in new energy technologies,which is of great importance to the realization of sustainable development of society.