Amplifying The Performance Of Prussian Blue Derived Iron-Nitrogen-Doped Carbon For Power Generation Of Air-Cathode Microbial Fuel Cells

Energy exhaustion and environmental pollution are the two most pressing problems facing mankind today.Microbial fuel cell（MFC）is considered as a promising way of wastewater treatment because it can degrade organic matters in wastewater and generate electricity to recover energy.Air-cathode MFC is considered to be one of the most practical devices because it can directly utilize oxygen in the air.However,oxygen is affected by multiple-electron reduction reaction（ORR）path and the reaction kinetics is slow,which seriously limits the efficiency of MFC.Iron-nitrogen-doped carbon（Fe-N-C）has been regarded as the most promising alternative of Pt due to its low cost and high performance.Among them,Prussian blue（PB）is often used as a precursor and template for the preparation of functionalized Fe-N-C materials because of its low preparation cost,high yield,inherent Fe-N bond,adjustable composition and morphology as well as abundant regular structures.However,PB has low carbon/nitrogen content and high metal content,in which the calcined metal particles seriously block the pore structure of rare carbon,resulting in poor specific surface area,low nitrogen content and few exposed active sites of the derived Fe-N-C.Currently,the modification researches about PB-derived Fe-N-C are still relatively few and mainly focus on designing composite precursor of PB with high-porous carbon materials to improve its specific surface area,but the results in current reports seem to be not satisfactory.The main reason lies in that in the synthesis of PB,the rapid coprecipitation reaction causes crystal grow fast,leading to overmuch PB particles and their size are also too large to load onto other nanoscale substrates.Besides,the adhesion between the two phases is weak,so PB is difficult to disperse evenly.Nowadays,it is still a big challenge to simultaneously improve the specific surface area,increase nitrogen content and enhance the inherent activity of PB-derived Fe-N-C to achieve or even exceed the activity of Pt catalysts.In this paper,based on surface structure,electronic structure and their cooperation,we modify PB-derived Fe-N-C catalysts to improve the number of exposed active sites and their inherent activities.Besides,we also explore the effect of structural modification on ORR catalysis,which is expected to provide a new insight into enhancing their performance in MFC.Among them,the modification of surface structure includes two studies.One is preparing Zn-Fe Prussian Blue analogue by introducing zinc cation to replace the iron cation in PB.Zn-Fe PBA is conductive to increase the porosity due to the low-boiling zinc would volatilize at high temperature.Another study is loading Zn-Fe PBA onto the leaf-like Zn-based zeolite imidazole framework（Zn-ZIF-L）skillfully by using a ligand exchange method to overcome the shortcomings of crystal growth and poor adhesion.The specific surface area,pore volume and nitrogen content of the derived Fe-N-C are successfully improved by the two terms.In addition,the cobalt-iron alloy can intensify the electric dipole and change the electronic structure compared with the iron-based carbon materials,which has been reported to show higher ORR catalytic activity.Based on the advantages of ligand exchange method and the conductive CoFe component in cobalt iron Prussian blue analogue（Co-Fe PBA）,Co-Fe PBA is loaded on the leaf-like cobalt based zeolite imidazole framework（Co-ZIF-L@Co-Fe PBA）,and a novel cobalt-iron bimetal nitrogen-doped carbon（CoFe-N-C）material is obtained after calcination,and its electronic structure is successfully modified.Finally,considering that zinc metal and porous ZIFs can effectively improve the surface structure and nitrogen content,and cobalt iron alloy shows a strong inherent activity,we skillfully realize the heterogeneous loading of Co-Fe PBA on polyhedral ZIF-8 by shell substitution with the ligand exchange method as the premise.The ORR activity and MFC performance of the derived CoFe-N-C with surface structure and electronic structure modifications are substantially improved.Meanwhile,a series of physicochemical characterization and density functional theory（DFT）calculation are performed to analyze the mechanism of electrocatalytic ORR.The main conclusions of this paper are as follows:（1）The Fe cation in PB is replaced by Zn cation to synthesize spherical Zn-Fe PBA.A novel Fe and N co-doped carbon porous nanosphere（FFC/NG）with a specific surface area different from that of PB-derived Fe-N-C（PB-1000）was obtained after carbonization.The results display that the mesoporous ratio of FFC/NG increases,and its specific surface area is 7.3 times of PB-1000 and its pore volume is9.1 times of PB-1000,effectively increasing exposured active sites in surface.The increased pore volume promotes more oxygen to the surface,and the increased exposure of active sites also accelerates the oxygen reduction rate,thus enhancing the efficiency of MFC.The oxygen reduction activity of FFC/NG is better than that of PB-1000,and the power density of FFC/NG is 1913 m W·m^-2.It is not only higher than PB-1000(1100 m W·m^-2),but also higher than the commercial Pt/C(1206m W·m^-2).The results indicate that the surface structural modification based on zinc volatilization can effectively improve the performance of MFC.（2）Considering the strong coordination of Fe（CN）₆^3-in Zn-Fe PBA,a ligand exchange method is proposed in this study,in which Fe（CN）₆^3-is used to replace the deprotonated 2-methylimidazole ligand of Zn-ZIF-L to effectively achieve combination of the two phases（Zn-ZIF-L@Zn-Fe PBA）.After carbonization,a novel Fe-N-C material（C-Zn-ZIF-L@Zn-Fe PBA）is obtained.The results show that Zn-Fe PBA and Zn-ZIF-L are bonded closely by the shared zinc coordination center,and the ligand exchange delays the rapid crystallization of Zn-Fe PBA on the surface,avoiding the reduction of specific surface area caused by the excessive content of Zn-Fe PBA in the composite precursor.Two-dimensional Zn-ZIF-L can effectively disperse Zn-Fe PBA nanoparticles and reduce their aggregation.Fe₃C generated during the carbonization process of Zn-Fe PBA at high temperature can catalyze Zn-ZIF-L to form intertwined carbon nanotubes,which enhances the specific surface area and graphitization degree of the catalyst and provides additional active sites for oxygen reduction.The maximum power density of C-Zn-ZIF-L@Zn-Fe PBA is 2511m W·m^-2,which is higher than those derivatives of Zn-Fe PBA and Zn-ZIF-L,and is2.1 times of the commercial Pt/C.The results indicate that the surface structural modification based on constructing composite metal-organic-frameworks precursor can further improve the efficiency of MFC（3）Using Co-ZIF-L as the maternal MOF and Fe（CN）₆^3-as the daughter ligand,by adjusting the solvent and reaction time,Co-ZIF-L is gradually exchanged into Co-Fe PBA.The results show that the Co-Fe alloy,Co and N co-doped carbon materials(Co_0.7Fe_0.3@Co-NC-1)is successfully prepared after high-temperature carbonization by using the heterogeneous metal-organic frameworks precursor（Co-ZIF-L@CoFe PBA）obtained with reaction time of 1 h in water solution.Physical and chemical characterization shows that the Co-ZIF-L in Co-ZIF-L@CoFe PBA developes into a rich porous structure and also provides a large spatial structure for the adhesion of CoFe.Moreover,the Co particles provide additional active sites for ORR.Co_0.7Fe_0.3-NC itself has highly active CoFe but its specific surface area is low,while the combination with the porous Co-NC derived from Co-ZIF-L makes up for this deficiency.This synergistic effect is incomparable by single Co-NC and single Co_0.7Fe_0.3-NC.As expected,the oxygen reduction activity of Co_0.7Fe_0.3@Co-NC-1 is not only higher than that of Fe-N-C derived from PB,but also higher than that of Co-NC and CoFe-NC catalysts derived from Co-ZIF-L and CoFe PBA,respectively.The maximum power density is 2486±56 m W·m^-2.The results indicate that the electronic structural modification based on bimetal optimization is effective.（4）Taking a core-shell dizeolite-imidazole-framework（ZIF-8@ZIF-67）as the template and performing ligand exchange reaction with Fe（CN）₆^3-,the ZIF-67 shell is successfully replaced by Co-Fe PBA to prepare the polyhedral precursor of Co-Fe PBA loaded onto ZIF-8（ZIF-8@Co-Fe PBA）.After carbonization,an electrocatalyst（NC-CoFe NC-RC）with structures of rambutan-like Co-Fe alloy,nitrogen co-doped carbon hollow polyhedron covered with carbon nanotubes on surface is obtained.The electrochemical activity of NC-CoFe NC-RC is demonstrated to be stronger than that of ZIF-8 derived NC and ligand-exchanged ZIF-67 derived CoFe-N-C material,reflecting the synergistic modification effect of surface structure and electronic structure.The maximum power density of NC-CoFe NC-RC is 2627±53 m W·m^-2.In electricity generation applications,two MFC series can drive 1.3 V electronic watch and phenol electric adsorption device with phenol removal up to 92.3%.In general,this study proposes strategies such as zinc volatilization and compositing metal-organic frameworks（MOFs）,which effectively improve the ORR activity and MFC power output of PB-derived Fe-N-C air cathode electrocatalyst,and also provides a new method and insight for the development of MOF@MOF heterogeneous structures with various morphologies and components,which is expected to be widely used in other fields of energy storage and conversion,the environment and so on.