Izboljšanje učinkovitosti pseudokondenzatorjev na osnovi Co in Ni

The increasing demand for clean and reliable energy has driven the search for efficient energy storage systems to complement intermittent renewable sources such as solar and wind. While batteries offer high energy density and supercapacitors deliver high power, there is still a need for materials that combine both properties to achieve optimal performance in advanced energy storage applications. Pseudocapacitive materials that utilize fast redox reactions, such as cobalt and nickel hydroxide, offer great promise for energy storage applications, but their performance is strongly dependent on structural features that influence ion diffusion and redox activity. In this thesis, we systematically explored the impact of composition and structural parameters on the electrochemical properties of Co- and Ni-based hydroxide materials. The structural properties of Co(OH)2 and Ni(OH)2 significantly influence their electrochemical performance. Hydrothermally synthesized Co(OH)2 prepared at lower temperatures exhibits a hydrotalcite-like structure with larger basal spacing and more stacking defects, which promote ion diffusion and enhance redox reactions, resulting in higher capacitance. In contrast, higher synthesis temperature leads to the formation of cobalt carbonate hydroxide with restricted ion accessibility and reduced pseudocapacitive properties. Similarly, for Ni(OH)2, the incorporation of carbonate increases with the temperature of hydrothermal synthesis. Using hexamethylenetetramine (HMT), it is possible to prepare hydroxide with minimal carbonate content in the lower temperature range, where HMT decomposition is incomplete. While using urea as a precipitating agent, the carbonate incorporation cannot be avoided. The presence of carbonate in Ni(OH)2 hinders electrochemical performance. These findings underscore the importance of structural features, particularly minimizing carbonate content, for optimizing pseudocapacitors' performance. To enhance the voltage window and improve capacitance, we incorporated redox-inactive M3+ ions (Al3+, Ga3+, and In3+) into the Co(OH)2 and Ni(OH)2, aiming to modify the electronic structure and consequently suppress the oxygen evolution reaction (OER). While CoAl- and NiAl-LDH showed promising results, a mismatch in cation size of larger Ga3+ and In3+ led to poorly crystalline powders. Nevertheless, the suppression of OER activity was successful for all the samples. However, carbonate intercalation was identified as a limiting factor for capacitance for all M3+ incorporated hydroxides, which we avoid in the next steps of materials optimization. An alternative approach to enhance electrochemical performance and capacitance involved expanding the basal plane distance of layered double hydroxides (LDHs) by the intercalation of larger organic anions. Specifically, ethyl sulfate (ES) and n-octyl sulfate (OS) were incorporated into CoAl-LDH by coprecipitation, increasing the interlayer spacing. CoAl-LDH materials facilitated suppression of the OER, widened the voltage window and improved capacitance. While ES showed increased enhancement of capacitance, there was also a large amount of carbonate ions present in the structure, indicating ongoing anion exchange. The OS ion exhibited superior performance among all prepared materials in this work. Furthermore, it showed resistance to exchange with carbonate, suggesting its potential for improved cycle life. Finally, the formation of a nanocomposite by incorporating gold nanoparticles into CoAl-LDH was explored. Through the intercalation of the terephthalate anion (TA), the basal plane distance of the LDH was expanded, providing a suitable environment for gold nanoparticle growth without disrupting the LDH structure. The TA anions were exchanged with the tetrachloroaurate anions, followed by the growth of gold nanoparticles within the basal space. The resulting CoAl-TA-LDH/Au nanocomposite exhibited a significant increase in surface area and charge storage capacity, with performance surpassing best-performing Co(OH)2. This enhancement is attributed to the synergistic effects of the gold nanoparticles, which facilitated improved ion diffusion and enhanced the Co redox behavior.