Energy Storage Mechanism Of Fe₂O₃ As A Negative Electrode For Pseudocapacitors

Exploiting high-performance negative electrodes to achieve simultaneously both high power and energy densities is imperative direction for pseudocapacitive supercapacitors（thereafter supercapacitors）.For the existing negative electrodes,α-Fe₂O₃（thereafter Fe₂O₃）deserves to be studied because of its high theoretical specific capacitance,wide potential window,low cost,and environmental friendliness.However,the application of an Fe₂O₃electrode is largely limited by its notorious cyclic stability and fatal specific capacitance.Inspired by the four key components and their interrelationship（processing/preparation→structure→properties→performance）in materials science and engineering,this dissertation proposes that the correlation between structure and electrochemical performance should firstly be understood as a premise to deal with the challenges faced by an Fe₂O₃electrode.Herein,starting with the investigation on its structural evolution in 1 M KOH and 0.5 M Na₂SO₄,we aim to elaborate the correction between structure（including the electric double-layer（EDL）structure at solid-liquid interface）and electrochemical behaviors,and how this correlation determines the electrochemical performance of an Fe₂O₃electrode.And it is our ultimate objective that designing a more efficient strategy thereof facilitates improving the electrochemical performance of an Fe₂O₃electrode.The contents and main conclusions in this dissertation are summarized as follow:1.The energy storage mechanism of an Fe₂O₃electrode in 1 M KOH and a new strategy to enhance its cyclic stability.Nanowire-shapedα-Fe₂O₃（NW-Fe₂O₃）was prepared by using simple hydrothermal method and the following annealing in N₂.On the basis of ex-situ structural and electrochemical characterization techniques,both the energy storage and cyclic stability deterioration mechanism are unraveled.（1）We reveal that the energy storage mechanism of a NW-Fe₂O₃electrode can be described as Fe₂O₃→Fe（OH）₂（charge）→α-Fe OOH（discharge）.Meanwhile,（2）it is found that the conversion reaction between the charge product Fe（OH）₂and discharge productα-Fe OOH is the origin of the cyclic stability deterioration.Such conversion reaction is similar to the diffusional transition,leading to massive diffusion and rearrangement of atoms,as well as morphological reorganization in an electrode.Thus,the initial conversion reaction in bulk deteriorates to in surface,and the valence state of iron changes from a wide oscillation to unchanged over the cycles.Based on such observation and inspired by the diffusional and diffusionless transition in phase transition theory,（3）we proposed that the diffusionless-like transition conversion reaction,i.e.,the conversion reaction between Fe（OH）₂andδ-Fe OOH,can be a potential strategy to optimize the electrochemical performance,achieving the conversion reaction in bulk with reversible valence state change of iron,as well as bypassing any possible massive atoms rearrangement.2.Evaluation the effectiveness of strategies such as constructing nanostructures and compositing with carbonaceous materials to the problem caused by conversion reaction we revealed.Fe-based zeolitic imidazolate framework（Fe-ZIF）was synthesized via a solvent method and obtained Fe₂O₃-C through annealing in air.（1）Such method for the preparation of Fe₂O₃-C has a function of“killing two birds with one stone”,i.e.,realizing not only the constructing nanostructures but also the compositing with carbon materials.Such strategy covers two of the three strategies for optimizing the electrochemical performance of an Fe₂O₃electrode.The structural and electrochemical characterization indicate that（2）the residual carbon does improve the electrochemical performance of Fe₂O₃,however,the performance remains lower than the expectation.On the one hand,（3）the low specific capacitance indicates that not all Fe₂O₃-C take part in the reaction,as well as the observed nanosheets indicates that Fe₂O₃-C electrode also store charges via conversion reaction we exploited.On the other hand,（4）thersuch undesired results imply that the existing strategies optimizing the electrochemical performance of Fe₂O₃electrode based on the correlation between the electrical properties and electrochemical performance have their own limitation,and their effectiveness on the problem caused by the conversion reaction we unraveled should be reevaluated.Furthermore,such limitation also highlights the unique value of the diffusionless-like transition conversion reaction.3.Targeting to deeply understand the strategy of the diffusionless-like transition conversion reaction,we propose a new model to describe this new strategy.Based on the diffusionless-like transition conversion reaction between Fe（OH）₂andδ-Fe OOH,and the high specific capacitance close to the theoretical one of a NW-Fe₂O₃electrode,（1）we propose a new concept and corresponding strategy,i.e.,the bulk pseudocapacitive of electrode materials can be obtained via a diffusionless-like transition conversion reaction,named as conversion pseudocapacitance.This concept is juxtaposed with nanostructing and intercalation pseudocapacitance.（2）Aiming at developing the condition and determinant for realizing the conversion pseudocapacitance,δ-Fe OOH andβ-Co（OH）₂ as typical conversion pseudocapacitance materials and Ni（OH）₂as battery materials for comparison are chosen to investigate their electrochemical behaviors,energy storage mechanism,energy barrier of conversion reaction,and electrochemical kinetics via characterizing their electrochemical behaviors and structures,and performing density function theory（DFT）calculations.4.The energy storage mechanism of an Fe₂O₃electrode and investigation on the universality of the strategy of the diffusionless-like transition conversion reaction in 0.5M Na₂SO₄.Nanorod-shapedα-Fe₂O₃（NR-Fe₂O₃）was prepared by using a simple hydrothermal method and the following annealing in N₂.Its energy storage mechanism in 0.5 M Na₂SO₄is pinpointed via ex-situ structural characterization,and comparing to that of NW-Fe₂O₃in 1 M KOH.（1）Although both the mechanisms cover a similar conversion reaction,the conversion reaction occurred at surface and the existence of peeling-off materials only for the case in Na₂SO₄.（2）DFT calculations and structural analyses demonstrate that it is the inner Helmholtz plane（IHP）consisting of OH^-in both electrolytes leading to the same conversion reaction;whereas the concentration of OH^-with an order of magnitude reduction in the case of 0.5 M Na₂SO₄ cause the looseness of IHP,specially,along with the conversion reaction consuming of OH^-.As a result,Na⁺constituting outer Helmholtz plane（OHP）reacts with electrode directly,meanwhile there are no space for storing Na⁺within the Fe₂O₃ lattice matrix,leading to the peeling-off of active materials.Furthermore,（3）the electrochemical behaviors of NR-Fe₂O₃electrode was obtained in 13 electrolytes with different acid-base properties,as expected,showing the distinct electrochemical behaviors depending on the which kind of ions constituting IHP.DFT calculations demonstrate that specific adsorption of Fe₂O₃（110）on electrolyte ions definitely determines the composition of IHP.Different ion constituting IHP,coupled by the Coulombic force between electrode negative charged and electrolyte ions,definitely determines that NR-Fe₂O₃electrode store charges in different electrolytes via different mechanism such as EDL,conversion reaction,redox pseudocapacitance of anions in electrolyte,as well as unrealized（de）insertion.Based on such observation,we elaborate the relationship between EDL structure of Fe₂O₃electrode and its electrochemical behaviors,obviously,the structure of structure（?）electrochemical performance should be extended to EDL structure,and the correlation between EDL and pseudocapacitance at an Fe₂O₃electrode is established,although such two independent concepts are essentially different in regard with adopting a capacitance origin.Moreover,（4）our results also show that the diffusionless-like transition conversion reaction cannot function in Na₂SO₄ because of Na⁺constituting OHP will react directly with Fe₂O₃ electrode,leading to the peeling off of active materials,as well as cyclic stability deterioration.