Electrochemical and morphological characterization of the Interface at negative electrodes in aqueous metal-ion batteries "Gas Evolution&electrodepostion Efficiency"

Abstract

Providing a sufficient amount of energy is a primary problem for current and future societies. To achieve this goal, it is essential to expand the use of renewable energy sources such as the sun and wind, as soon as possible. These energy sources are inherently intermittent. Thus, appropriate grid-scale energy storage systems are required to store high amount of energy in a very short period of time. One possible choice for stationary applications in which volumetric and gravimetric energy densities are not primary factors is aqueous metal-ion batteries. Recently, aqueous zinc-ion batteries based on copper hexacyanoferrate with an average potential of 1.73 V have been developed. The main limiting factors for this new family of batteries include a low electrodeposition efficiency and hydrogen evolution on the negative electrode. These problems are related to the use of zinc as the negative electrode, in which the electrochemical reduction potential is sufficiently low that, at least thermodynamically, hydrogen evolution becomes favorable. Hydrogen evolution negatively influences the electrodeposition efficiency of the electrode and performance of the battery, hindering the power density, and lowering the overall energy efficiency. Therefore, decreasing the level of hydrogen evolution and increasing the electrodeposition efficiency are of primary importance in this type of battery. In the first part of this dissertation, a new electrochemical cell was designed for in-operando characterization of gas evolution in batteries via differential electrochemical mass spectrometry (DEMS). Different parameters such as the position, size, and shape of the electrodes, flow of the carrier gas, contact between the current connectors and electrodes, sealing of the cell, and setup to run the DEMS measurements were all discussed. The performance of the cell for DEMS measurements was validated by investigating the gas evolution at the graphite electrode in an organic electrolyte. To do so, cyclic voltammetry (CV) was combined with the DEMS method. Moreover, the ability of the cell to perform electrochemical impedance spectroscopy (EIS) measurements was confirmed. Subsequently, the cell was used to study hydrogen evolution on negative electrodes in an aqueous zinc-ion battery based on Prussian blue derivatives. To accomplish this goal, galvanostatic cycling with potential limitation (GCPL) was combined with the DEMS method. The results showed that increasing the concentration of the electrolyte could suppress the level of hydrogen evolution. The second part of this dissertation studied the morphology of zinc electrodeposition on the negative electrode at different current densities. Several constant current densities were applied and the surface morphology investigated via SEM and color 3D laser microscopy. The results showed that below the limiting current density, no preferential growth was observed on iv the surface of the electrode. However, above the limiting current density, large hexagonal crystals were formed all over the surface. Thereafter, the effect of Branched Polyethyleneimine (BPEI) as an electrolyte additive on the zinc electrodeposition mechanism was studied. It was determined that the presence of BPEI in the range of 30 ppm inside the electrolyte could suppress the growth of hexagonal crystals and stabilize the electrodeposition efficiency of zinc. The third part of this dissertation focused the application of layered double hydroxide (LDH) as a substrate for zinc electrodeposition. To do so, GCPL was used and the electrodeposition efficiency of zinc on different substrates serving as the negative electrode was examined. The results showed that the appropriate ratio of zinc to LDH as a substrate considerably enhanced the efficiency of zinc electrodeposition in 500 mM of zinc sulfate, from 88% to 98%. Moreover, LDH suppressed the intense potential drop at the beginning of the reduction reaction that could be attributed to the elimination of hydrogen evolution. LDH, in an appropriate combination with zinc, was determined to be a very good alternative for use as a negative electrode in aqueous zinc-ion batteries.