| The use of a large amount of fossil fuel,such as coal,oil,and natural gas causes serious environmental pollution and greenhouse effect,which do harm to the body health of human beings and the sustainable development of the economy.In 2020,Chinese government proposed the "double-carbon" target,meaning "carbon peaking"and "carbon neutralization".Therefore,it is urgent to build a clean,low-carbon,safe,and efficient modern energy system that is mainly based on renewable energy.However,renewable energy resources,such as wind energy and solar energy,cannot provide a stable and continuous energy supply,due to their intermittent property,meaning uneven distribution in space and time.Energy storage devices,which store excessive energy and provide energy supply when in need,can compensate for the intermittent property and thus plays a key role in the development of renewable energy resources.Among kinds of energy storage devices,lithium-ion batteries are used in commercial energy storage for their high energy density and cycling stability.However,the safety of lithium-ion batteries and the shortage of lithium resources limit their application prospects in large-scale energy storage grids.Zn-based batteries,which use Zn metal and aqueous solutions as the negative electrode and the electrolyte,respectively,have attracted intensive interest due to their intrinsic advantages,such as low price,high energy density,and safety.In this dissertation,to improve the stability of the Zn metal electrodes,the influence of mass transfer on the Zn metal electrode performance is concentrated.Firstly,the performance of the Zn metal electrode in different electrolytes was tested to verify the difference of performance and critical issues caused by transport properties and reaction mechanisms.Based on this,the transport process of the Zn metal deposition was studied,focusing on the oriented uneven deposition that caused battery short-circuit.From the perspectives of mass and charge transfer,the oriented uneven deposition behavior of Zn metal electrodes was analyzed in detail to reveal the transport mechanisms.According to the revealing mechanisms,the flowing electrolyte was introduced into the static battery as an effective strategy to suppress the oriented uneven deposition behavior via enhancing internal mass transfer.Besides,the relationship between flow rate and operation condition,and the effect of the flow field on Zn electrode performance were systematically investigated.Finally,the enhanced mass transfer effects on the discharge performance of Zn-air flow full batteries were systematically investigated by extending the investigation of the Zn metal electrode.The specific investigations and innovations are as follows:1.In the different electrolyte systems,transport properties and reaction mechanisms are different,which leads to different performance and issues.The performance of the Zn metal electrode in the different electrolytes was analyzed systematically.The results show that electrode reaction kinetics are urgently required to be improved in the neutral/slightly acid electrolytes while the stability and reversibility of Zn metal electrodes in the alkaline electrolytes restrict the Zn metal electrode performance.2.The transport process of Zn during electrochemical deposition was investigated via in-situ observations and material characterization,focusing on the oriented uneven Zn deposition that caused battery short-circuit.Moreover,a twodimensional Zn deposition model coupling electrochemistry and mass transfer was established based on the experiments to reveal the oriented uneven Zn deposition mechanisms from the perspective of mass and charge transfer.The obtained mechanisms are as follows:firstly,the nucleation with a large amount of deposition Zn and "dead Zn" detached from the Zn metal electrode substrate and accumulated at the bottom of the electrode in the presence of gravity,which led to the initial oriented uneven Zn distribution,in which charge and zincate ions are more likely to be rich on the protrusions at the bottom of the Zn metal electrode.More charge and absorbed zincate ions corresponded to large current densities and electrochemical reaction rates.With the increment of plating/stripping cycles,oriented uneven Zn deposition became more and more severe and thereby led to the short circuit.Besides,the hydrogen bubbles generated by the hydrogen evolution reaction accelerated the oriented uneven deposition.3.To improve the stability of Zn metal electrodes,flowing electrolyte was introduced into the static batteries to suppress oriented uneven deposition via enhancing internal mass transfer.Besides,the relationship between flow rate and operation condition,and the effect of the flow field on Zn electrode performance were systematically investigated.The oriented uneven deposition was alleviated and the Zn electrode stability was improved when introducing the flowing electrolyte.The higher current density should be corresponded to a higher flow rate to improve the Zn electrode stability.Besides,the direction of the inlet is changed from the side to the bottom to take full advantage of the flowing electrolyte.Under this circumstance,the Zn metal symmetric batteries could operate stably over 18000 cycles(1200 h).4.The discharge performance of Zn-air batteries in the presence of enhanced transport was investigated by extending the investigations of the Zn metal electrode.The impact of the enhanced transfer on the discharge performance of zinc-air batteries was investigated through experimental tests.Based on this,a three-dimensional model coupling electrochemistry and mass transfer was established to reveal the mechanisms behind the changes in the discharge performance of zinc-air batteries.The results showed that the peak power density and discharge capacity were improved by 10%and 23%in the presence of the flowing electrolyte,which originated from the enhanced mass transfer of hydroxide ions and zincate ions,respectively,according to the model calculation. |
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