Understanding Hydrogen Overpotential and Its Role in Electrochemistry

Hydrogen overpotential is a crucial factor in electrochemical processes, particularly in the context of metal electrode reactions. It refers to the extra voltage that must be applied to drive the hydrogen evolution reaction (HER) beyond its thermodynamic potential. This concept is especially relevant when considering different metals; for instance, platinum exhibits a low hydrogen overpotential due to its high exchange current density, making it favorable for various applications, including the construction of hydrogen electrodes.

Different metals have varying hydrogen overpotentials, which can significantly influence their usability in electrochemical reactions. For practical assessments, the overpotential required to evolve hydrogen at a specified current density, such as 1 mA cm–2, is often measured. Metals like gold, nickel, and iron display different characteristics, with corresponding overpotentials that range from 0.09 V for platinum to as high as 1.0 V for lead. This variability is important for applications like metal electrodeposition, where high hydrogen overpotentials can hinder metal deposition and lead to practical complications.

Beyond hydrogen evolution, the oxygen reduction reaction (ORR) also plays a significant role, particularly in the context of corrosion. The reduction of dissolved oxygen at metal surfaces generates hydrogen ions in acidic environments, while in neutral or alkaline waters, hydroxyl ions are produced. These reactions are influenced by factors such as concentration and resistance polarization, which complicate the characterization of oxygen reduction. The solubility of oxygen in water is low, making it challenging to maintain a constant supply during electrochemical processes.

Polarization characteristics are essential to understanding the relationship between applied potential and current in electrochemical systems. These characteristics can vary significantly from one electrode to another, with some metals primarily influenced by activation polarization, while others are more affected by concentration or resistance polarization. Evaluating these characteristics leads to the creation of corrosion velocity diagrams, which visually represent the corrosion rates of different metals under various conditions.

The study of hydrogen and oxygen reactions is vital in various technological applications, from batteries to corrosion prevention strategies. By understanding the principles behind hydrogen overpotential and its effects on metal surfaces, researchers and engineers can develop more efficient electrochemical systems with enhanced performance and longevity.