Potassium Compound-assistant Synthesis Of Biomass-derived Porous Carbons For Use In Electrochemical Energy Storage

Supercapacitors(SCs)have attracted wide attention due to high power density and long lifetime.However,their large-scale applications are confined by their low energy density.As a class of newborn EES devices for next generation,hybrid Li-ion capacitors and Na-ion capacitors(LICs/NICs)exhibit not only high power and long lifetime,but also higher energy density than SCs.It is important to design and develop advanced electrode materials to construct high-performance SCs and LICs/NICs.Porous carbons are recognized as promising electrode materials for SCs and LICs/NICs due to their appealing properties,including good electronic conductivities,large specific surface areas,high chemical stabilities and easy preparation.Resourceful and green biomass is one of the precursors to prepare porous carbons owing to their natural microstructures and unique compositions.It is of great significance to transform biomass into porous carbon electrode materials with low cost and high performance by structural design and compositional control,thus realizing the development of high-performance SCs and LICs/NICs.As by-products of fish processing,fish scale and skin are commonly discarded as wastes.However,abundant collagen within fish scale and skin contains rich C,N,O and S atoms,which can be used as precursor to prepare heteroatom-doped carbons.The collagen hydrolysates extracted from fish scale and skin can be evenly mixed with water-soluble templates or activators in aqueous solution to realize structural and compositional control of porous carbons.Moreover,the inorganic hydroxyapatite(HA)crystals uniformly distributed within the fish scale can serve as natural templates and activators to indirectly control the structures and compositions of porous carbons.In this paper,we used fish scale or skin as precursors and proposed simple and feasible strategies to develop advanced porous carbons with different structures and compositions with the assistant of KOH and potassium salts.Furthermore,we analyzed the effects of potassium compounds on the structures and compositions of the obtained porous carbons.The obtained porous carbons were employed as electrode materials of symmetric SC or LIC/NIC according to their structural and compositional features.The electrochemical energy storage behaviors of the porous carbon electrodes were analyzed in detail.Additionally,high-performance symmetric SCs or LICs/NICs were constructed based on the porous carbon electrodes.The works and results are summarized as follow:(1)Hierarchically porous carbon bulks(HPCs)with N,O heteroatoms were prepared using fish scale as precursor by the synergetic effect of KOH and HA.Two HPCs were obtained by adjusting the KOH dosage and pyrolysis temperature.The HPC pyrolyzed at 550? possessed higher heteroatom content(4.3 at.%for N,18.8 at.%for O)and relatively lower specific surface area(594 m2 g-1)than the HPC pyrolyzed at 800?(with larger specific surface area of 3286 m2 g-1,N content of 2.7 at.%,and O content of 7.6 at.%).These two HPCs were employed as anode and cathode materials,respectively.By virtue of hybrid energy-storage features induced by interconnected hierarchical porosity and rich heteroatoms,the HPC electrodes exhibited good performance in half cells.Consequently,the assembled NICs based on the HPC electrodes delivered high energy density(103.2 Wh kg-1),high power density(15.9 kW kg-1)and long cycling lifetime(81.1%of the initial capacitance at 3.5 A g-1 over 2500 cycles).(2)To further control the structures and compositions of porous carbons,KOH solution was applied to transform fish scale collagen into water-soluble hydrolysates.After lyophilization,the collagen hydrolysates were coated on the KOH-derived compounds in the shape of micron-sized flowers by self-assembly process.The multi-heteroatoms doped ultrathin porous carbon nanosheet with micropore-dominant porosity(PCNS-600)was obtained after pyrolyzing the lyophilization products at 600? with the assistance of potassium compound templates and activators.The PCNS-600 was employed as electrode material for symmetric SC in aqueous electrolyte owing to its structural and compositional features.In a three-electrode system,the rich surface heteroatoms of the PCNS-600 electrode(14.64 at.%for O,5.05 at.%for N,and 0.92 at.%for S)not only supplied a large EDLC through providing a large efficient charge-storage surface area due to an improved electrode wettability to aqueous electrolyte,but also introduced a high pseudocapacitance due to,redox reactions.Moreover,PCNS-600 electrode simultaneously showed high gravimetric/volumetric capacitive performance because of its relatively high compacting density(1.06 g cm-3)resulting from the micropore-dominant 2D nanosheet structure(with a specific surface area of 962 m2 g-1).More interestingly,the ultrathin 2D porous nanosheet structure(with a thickness of 3-5 nm)of the PCNS-600 electrode accelerated the electrolyte-ion transport during the charge and discharge process,resulting in good rate capability.Consequently,the assembled symmetric SC based on the PNCS-600 electrodes delivered integrated high volumetric energy(12.8 Wh L-1)and power densities(10.8 kW L-1)as well as remarkable cycling performance without apparent capacitance decay after 20 000 cycles at 5 A g-1.(3)To confirm universality of the preparation method in the above work,fish skin with abundant collagen was used as precursor for porous carbons.The obtained porous carbon(MPCNS)also possessed a micropore-dominant carbon nanosheet structure with rich surface heteroatoms(8.18 at.%for N,14.02 at.%for O,and 2.25 at.%for S),high specific surface area(1017 m2 g-1),and relatively high compacting density(1.06 g cm-3).The constructed symmetric SC based on the MPCNS electrode exhibited high volumetric energy density(12.5 Wh L-1)and power density(9.67 kW L-1)as well as superior cycling performance(without obvious decay after 20 000 cycles at 5 A g-1).(4)To further regulate the porosity of porous carbon nanosheet,KCl solution was used to extract collagen hydrolysate from fish scale.By freeze-drying the KCl solution with collagen hydrolysate,ice templates created numerous dendrite-like canals and the collagen hydrolysate was coated on KCl nanoparticles that were uniformly dispersed along the canals.After pyrolysis at 750?,a mesopore-dominant carbon nanosheet(NOPCNS)with a thickness of?4 nm was obtained and possessed numerous N and O heteroatoms(8.2 at.%for N,8.4 at.%for O).The KCl not only served as template to create mesopore-dominate nanosheet structure,but also suppressed N atom loss due to nano-confinement effect.Additionally,the KCl/ice double templates could be easily removed out and collected for recycling,revealing efficient and eco-friendly properties of the preparation method.The obtained NOPCNS was employed as anode material for Li-ion storage due to its unique structure and composition.The NOPCNS anode showed surprising rate capability(249 mAh g-1 at 50 A g-1)and cycling performance(without decay after 10,000 cycles)in half cells.Consequently,a hybrid LIC based on the NOPCNS anode outputted high energy density(184 Wh kg-1),ultrahigh power density(78.1 kW kg-1),and ultralong lifetime(with energy retention of 70%after 10 000 cycles).(5)Based on the above works,we proposed several principles for the design of advanced carbon electrode materials for SCs,LICs and NICs:(i)Hierarchically porosity and porous nanosheet structure can facilitate electrolyte ion diffusion,resulting in good rate performance.(ii)Heteroatom doping can not only induce defects to increase active sites for electrolyte ion storage,but also expand the carbon interlayer distance to facilitate ion insertion/extraction.(iii)The specific heteroatoms are beneficial to improvement of electrical conductivities,wettabilities,and other physicochemical properties of porous carbon materials,therefore enhancing reversible capacitances,rate capabilities,and cycle performance.(iv)High specific surface area is necessary for electrode materials of SCs and cathode materials of LICs and NICs but not necessary for anode materials of LICs and NICs since too high specific surface area would lead to very low initial coulombic efficiency of anodes.