Aleksandar Mijajlović

This review critically highlights recent breakthroughs in eco-friendly biosensors utilizing natural pigments, exploring their structural mechanisms and diverse applications in monitoring environmental pollutants, pH variations, and pathogen genes.

The oxygen reduction reaction (ORR) is a key process in electrochemical energy conversion technologies such as fuel cells and metal–air batteries; however, its sluggish kinetics and reliance on precious metal catalysts limit large-scale application. This review provides a comprehensive overview of recent advances in non-precious nanoscale electrocatalysts for ORR in alkaline media. Particular emphasis is placed on reaction mechanisms, including dominant pathways, kinetics, and key intermediates, as well as the advantages of alkaline electrolytes over acidic systems. The performance of various catalyst classes is systematically discussed, including transition metal-based materials (Fe, Co, Zn, Cu, and bimetallic systems) and metal-free carbon-based electrocatalysts. Special attention is given to heteroatom-doped carbon materials, carbon nanostructures, and emerging hybrid systems such as MXene-based composites. Comparative analysis highlights the relationship between catalyst composition, structure, and electrochemical performance metrics, including half-wave potential, onset potential, Tafel slope, number of electron transfer, and operational stability. Overall, non-precious catalysts demonstrate promising activity and durability, approaching that of noble metals under alkaline conditions. The insights summarized in this review guide the rational design of efficient, cost-effective ORR electrocatalysts and support the development of sustainable energy technologies.

This study investigates the morphological, compositional, and electrochemical properties of carbon materials derived from Pepsi (P) and Coca-Cola (CC) precursors, before and after chemical activation with ZnCl₂. Scanning electron microscopy revealed a lower density of surface cracks in non-activated hydrothermal carbon (NAHC) samples compared to activated carbons (ACs), indicating structural changes induced by the corrosive activation process. Particle size analysis showed an increase in average diameter after activation, particularly pronounced in CC-derived samples, which also exhibited a broader particle size distribution. Elemental mapping confirmed carbon as the dominant and homogeneously distributed element, while oxygen-containing functional groups decreased significantly after activation. Oxygen reduction reaction investigation demonstrated that all synthesized non-activated and activated samples are electrocatalytically active in alkaline solution. CC-NAHC demonstrated the lowest Tafel slope (99 mV dec⁻¹), while activated samples showed higher values, indicating slower kinetics and increased reaction limitations. Despite this, activated carbons—particularly CC-AC—displayed significantly higher diffusion-limited current densities (~−4.8 mA cm⁻² at 1600 rpm), suggesting improved mass transport and conductivity. Furthermore, electron transfer number (n) analysis indicated that P-NAHC and CC-AC follow a near four-electron ORR pathway (n ≈ 3.6–3.9).

The oxygen evolution reaction (OER), a key half-reaction in electrochemical water splitting, is limited by sluggish multi-electron transfer kinetics, starting extensive research into efficient, low-cost nanoscale electrocatalysts, particularly those based on nickel, cobalt, and iron, as well as mixed-metal, hybrid, and heteroatom-doped carbon-based metal-free systems, as presented here. Ni- and Co-based electrocatalysts show high efficiency for alkaline OER due to optimized nanostructures, surface modifications, heterostructure design, and multi-metal doping, which enhance activity, stability, and electronic properties. Their performance relies on precise atomic-level control of structure and synergistic interactions, enabling them to approach or rival noble-metal catalysts. Iron-based electrocatalysts are also promising due to their abundance, low cost, and flexible redox chemistry, forming active iron oxyhydroxide species during operation; however, their low conductivity requires structural and electronic optimization. Beyond Fe, Ni, and Co, copper-based compounds, zeolitic imidazolate framework-derived structures, and manganese phosphide–cerium oxide composites offer enhanced oxygen vacancies, tunable structures, and strong interfacial synergy. Furthermore, heteroatom-doped carbon materials incorporating nitrogen, phosphorus, or sulfur improve catalytic activity by modifying electronic structure, creating active sites, and enhancing charge transfer. Overall, careful control of composition, structure, and electronic properties enables the development of efficient, durable, and scalable noble-metal-free catalysts for OER.