Synthesis,Characterization And Electrocatalytic Performance Of Biomass-derived Carbon-Based Hybrid Materials

Oxygen Reduction Reaction?ORR?and Oxygen Evolution Reaction?OER?are key reactions in metal-air batteries and fuel cells,and their cathode catalysts determine the power density of the device.Although precious metals?such as Pt?and metal oxides?such as RuO₂and IrO₂?are the most effective and widely used catalysts for ORR and OER,their high cost,low methanol tolerance and limited stability significantly hinder their use in large-scale energy installations.Therefore,the development of low-cost alternative catalysts with high activity and durability is extremely important for ORR and OER.The porous carbon materials with high conductivity and hierarchical porous nanostructures play a key role in penetrating of electrolyte and mass/ion transport,resulting in highly enhanced electrochemistry performances.Therefore,porous carbon materials havebeen widely used in many fields,including energy storge,batteries and so on.The interconnected porous structure can not only shorten the transport distance of ions in the carbon material,but also provide a continuous and fast electronic transport path.In addition,the interconnected porous structure ensures porous carbon materials a higher conductivity and better mechanical stability.Thus,there is an urgent need to develop a cost-effective process for high catalytic activityporous carbon materials for use in practical energy conversion devices.In this paper,a series of novel and efficient biomass-based porous carbon electrocatalysts were constructed.The details are as follows:1.Enzyme is explored to effectively hydrolyze the partial cellulose in bulk raw wood to form a large number of nanopores,which helps to maximally expose the inner parts of raw wood to sufficiently dope nitrogen onto carbon skeletons during the subsequent pyrolysis process.The resulting carbons exhibit excellent catalytic activity with respect to the oxygen reduction and oxygen evolution reactions.As-fabricated cellulose-digested,carbonized wood plates are mechanically strong,have high conductivity,and contain a crosslinked network and natural ion-transport channels and can be employed directly as metal-free electrodes without carbon paper,polymer binders,or carbon black.When used as metal-free cathodes in zinc-air batteries,they result in a specific capacity of 801 mA h g^-1 and an energy density of 955 Wh kg^-1,with the long-term stability of the batteries being as high as 110 h.2.A wood based N-doped 3D porous carbon with a high specific surface area was prepared from eycalyptus by combining hydrothermal carbonization with ammonia pyrolysis.During the hydrothermal cabonization process,small molecular compuonds produced by hydrolysis of lignocellulose can be polymerized and self-assembled on the surface of the wall of wood,which can improve the specific surface area of the samples(form 731 m² g^-1 to 1036m² g^-1).After being further carbonized at 1000 ^oC in NH₃,the 3D self-surporting porous carbon material with a larger surface area(1036 m² g^-1)and nitrogen content of 3.33%was obtained.The as-prepared sample exhibited an excellent oxygen reduction reaction performance with an onset potential of 0.988 V vs.RHE,tafel slope of 61 mV dec^-1,electron transfer numbers of 3.85-3.95,which is attributed to its large specific surface area,hierarchical pore structure and abundant active sites.The zinc-air batteryusing the above sample as a cathode,reveals a specific capacity of 790 mAh g^-1 coressopnding to the energy densitity of 1003.3 Wh kg^-1 and a long-term stability.In addition,the self-supported 3D bulk catalyst can be directly used as a cathode in zinc-air baterry.3.Atomically dispersed Fe-N₄ species anchoring on porous carbon nanosheets?denoted as SA-FeCNS?with a high surface area(S_BET up to 1436 m² g^-1)and hierarchical porous structures was fabricated through a facile template-free approach by using hydrochar nanosheets obtained from the hydrothermal carbonization of hemicellulose as the carbon source.GO has emerged as an attractive morphology-directing agent.It is believed that the2D nanosheet structures of catalyst can favorite easy molecular and ionic diffusion throughout the highly porous architecture to the reactive surface and facilitate the rapid transport of electrons along the interconnected carbon nanosheets during electrocatalysis.The as-prepared material demonstrates excellent catalytic activities towards the ORR with greatly improved activity,long-term stability,and better tolerance to methanol in alkaline medium.Furthermore,we assembled a Zn-air battery using atomically dispersed Fe in the cathode for ORR catalysis and achieve attractive performance.4.Atomically dispersed Fe atoms anchored on N,S-doped porous carbon nanosheets?denoted as SA-Fe/NCS?was fabricated.Sodium lignosulfonate,as a carbon precursor,can also act as chelating and space isolation agent of Fe ions to promote their full transformation to single Fe atoms without forming Fe nanoparticles.The freeze-dried complex is then mixed with ammonium chloride as a nitrogen source.The confined Fe ions were directly reduced by carbonization of sodium lignosulfonate to yield the isolated Fe atoms on the N,S-doped carbon nanosheets without post-acid treatment.During pyrolysis,ammonia?NH₃?decomposed from ammonium chloride can subsequently react with metal atoms and carbon source to form M-N_x-C moieties,taking over the protection and preventing metal atoms from aggregation.As a control,Fe-based nanoparticles were observed when using evaporate method due to the aggregation of iron hydroxide produced by the hydrolysis of uncoordinated iron ion.In addition,a pleasant surprise is that the formation of NaCl in the precursor can act as the porosity-inducing templates and only need to be washed.The experimental results demonstrated high ORR activity and long-term operational stability of the catalysts in alkaline electrolyte solutions.Furthermore,we assembled a Zn-air battery using atomically dispersed Fe in the cathode for ORR catalysis and achieve attractive performance.5.A biomass-based nitrogen-doped free-standing hollow carbon nanofiber film was fabricated from lignin,polyacrylonitrile?PAN?and polymethyl methacrylate?PMMA?mixture via electrospinning,followed by carbonization and thermal annealing process.The resulting hollow carbon nanofiber film possesses a high surface area of 922.4 m² g^-1,hierarchically porous structure,and high nitrogen content.As compared with the benchmark Pt/C catalyst,the carbon nanofiber film has a remarkable ORR activity,with an identical onset and half-wave potential,a nearly two times higher kinetic current at low over potential region,comparable number of electron transfer,and significantly improved working stability and tolerance to fuel crossover.Moreover,the hollow carbon nanofiber film showed high activity for the OER.A Zn-air battery device with this hollow carbon nanofiber film as catalyst showed a superior specific capacity of 763 mAh g^-1 and an energy density of 947 Wh kg^-1.Moreover,flexible all-solid-state rechargeable Zn-air battery with the hollow carbon nanofiber film exhibited an excellent mechanical and cycling stability.