Miroslava Varničić

Fuel cells, batteries, and supercapacitors represent the most efficient and promising technologies for electrochemical energy conversion and storage. However, their production and practical application are primarily constrained by the high cost of materials (65% - 75%), such as noble metals, including platinum, which is commonly used as an electrocatalyst for the oxygen reduction reaction (ORR). Therefore, the investigation of novel materials is crucial for the development of alternative solutions that can compete with existing technologies. The main objective of this study is to investigate ORR electrocatalysts based on cobalt/lanthanum oxides (Co/La material) as alternatives to noble metals for the cathodic reaction. The synthesized material was fabricated as an electrode on a graphite disk with a surface area of 0.196 cm² and tested for the oxygen reduction reaction using standard electrochemical techniques, namely cyclic and linear sweep voltammetry. The results indicate that the hybrid material is a promising electrocatalyst for ORR, achieving a current density of -3.5 mA cm-². The calculated number of exchanged electrons for the Co/La material suggests a four-electron oxygen reduction mechanism and high electrocatalytic activity towards ORR.

This study examines the kinetics and mechanism of the oxygen reduction reaction (ORR) on a polycrystalline rhodium electrode (Rh(poly)) in acidic and alkaline media, using rotating disc electrode measurements. This study found that the ORR activity of the Rh(poly) electrode decreases in the order of 0.1 M NaOH > 0.1 M HClO₄ > 0.05 M H₂SO₄ concerning the half-wave potentials. The Tafel slopes for ORR on Rh(poly) in the cathodic direction are 60 and 120 mV dec⁻¹ at low and high overpotentials, respectively, in perchloric acid and alkaline solutions. However, strongly adsorbed sulfate anions hinder the ORR on Rh(poly) in sulfuric acid, leading to higher Tafel slopes. The highest ORR activity of Rh(poly) in an alkaline media suggests the promoting role of the specifically adsorbed OH⁻ anions and RhOH. In all cases, ORR on Rh(poly) proceeds through the 4e-series reaction pathway.

This research focuses on the rational design of porous enzymatic electrodes, using horseradish peroxidase (HRP) as a model biocatalyst. Our goal was to identify the main obstacles to maximizing biocatalyst utilization within complex porous structures and to assess the impact of various carbon nanomaterials on electrode performance. We evaluated as-synthesized carbon nanomaterials, such as Carbon Aerogel, Coral Carbon, and Carbon Hollow Spheres, against the commercially available Vulcan XC72 carbon nanomaterial. The 3D electrodes were constructed using gelatin as a binder, which was cross-linked with glutaraldehyde. The bioelectrodes were characterized electrochemically in the absence and presence of 3 mM of hydrogen peroxide. The capacitive behavior observed was in accordance with the BET surface area of the materials under study. The catalytic activity towards hydrogen peroxide reduction was partially linked to the capacitive behavior trend in the absence of hydrogen peroxide. Notably, the Coral Carbon electrode demonstrated large capacitive currents but low catalytic currents, an exception to the observed trend. Microscopic analysis of the electrodes indicated suboptimal gelatin distribution in the Coral Carbon electrode. This study also highlighted the challenges in transferring the preparation procedure from one carbon nanomaterial to another, emphasizing the importance of binder quantity, which appears to depend on particle size and quantity and warrants further studies. Under conditions of the present study, Vulcan XC72 with a catalytic current of ca. 300 µA cm⁻² in the presence of 3 mM of hydrogen peroxide was found to be the most optimal biocatalyst support.

The main objective of this research was a systematic development of advanced micro/nanostructured materials based on the most used metal-oxides for ORR and metal-oxides with an extremely low-loading of Pt for comparison. Hybrid composites compared were: MnO₂, La₂O₃, mixed lanthanum manganese oxides (LMO), and mixed lanthanum manganese oxides with reduced platinum load (LMO-Pt). The influence of the reduced amount of noble metal, as well as single oxide activity toward ORR, was analyzed. The complete electrochemical performance of the hybrid materials has been performed by means of CV, LSV, and EIS. It was shown that all synthesized catalytic materials were ORR-active with noticeable reduction currents in O₂ saturated 0.1 M KOH. The ORR behavior indicated that the La₂O₃ electrode has a different mechanism than the other tested electrode materials (MnO₂, LMO, and LMO-Pt). The EIS results have revealed that the ORR reaction is of a mixed character, being electrochemically and diffusion controlled. Even more, diffusion is of mixed character due to transport of O₂ molecules and the chemical reaction of oxygen reduction. O₂ diffusion was shown to be the dominant process for MnO₂, LMO, and LMO-Pt electrolytic materials, while chemical reaction is the dominant process for La₂O₃ electrolytic materials.