Applying a voltage to metal electrodes in contact with aqueous electrolytes results in the electrolysis of water at voltages above the decomposition voltage and plasma formation in the electrolyte at much higher voltages referred to as contact glow discharge electrolysis (CGDE). While several studies explore parameters that lead to changes in the I-U characteristics in this voltage range, little is known about the evolution of the structural properties of the electrodes. Here we study this aspect on materials essential to electrocatalysis, namely Pt, Au, and Cu. The stationary I-U characteristics are almost identical for all electrodes. Detailed structural characterization by optical microscopy, scanning electron microscopy, and electrochemical approaches reveal that Pt is stable during electrolysis and CGDE, while Au and Cu exhibit a voltage-dependent oxide formation. More importantly, oxides are reduced when the Au and Cu electrodes are kept in the electrolysis solution after electrolysis. We suspect that H 2 O 2 (formed during electrolysis) is responsible for the oxide reduction. The reduced oxides (which are also accessible via electrochemical reduction) form a porous film, representing a possible new class of materials in energy storage and conversion studies.

Nanoporous Au (NPG) films have promising properties, making them suitable for various applications in (electro)catalysis or (bio)sensing. Tuning the structural properties, such as the pore size or the surface‐to‐volume ratio, often requires complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from a bare Au electrode. In a first approach a Au oxide film is prepared by high voltage (HV) electrolysis in a KOH solution, which is then reduced either electrochemically or in the presence of H2O2. The resulting NPG structures and their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H2O2‐containing KOH solution, as well as the applied voltage and temperature during HV electrolysis. Secondly, the NPG film can be prepared directly by applying voltages that result in anodic contact glow discharge electrolysis (aCGDE). By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.

Plasma in-liquid by means of anodic contact glow discharge electrolysis (aCGDE) is a growing research field allowing the selective modification of the electrode and the electrolyte. The aim of this proof of concept study is to demonstrate that auxiliary electrochemical electrodes placed in the vicinity of the plasma electrode can be modified by aCGDE (ignited at the anode by applying a DC voltage between the driving electrodes). Furthermore, we illustrate in how far such auxiliary electrodes can be used as a probe to detect products (in particular, [Formula: see text], [Formula: see text], and [Formula: see text]) formed in the solution by aCGDE via electrochemical techniques. In this work, aCGDE is achieved by applying a voltage of 580 V to a small Pt wire (plasma electrode) versus a large stainless steel counter electrode. An auxiliary Pt electrochemical working electrode, operated in a three electrode configuration, is placed at different distances from the plasma working electrode. Depending on the distance, we find small changes in the working electrode structure. More importantly, we will show that, in principle, the local [Formula: see text] concentration in the electrolyte can be monitored operando. After aCGDE, the concentration changes with time and depends on the distance from the plasma electrode.

Applying an external potential difference between two electrodes leads to a voltage drop in an ion conducting electrolyte. This drop is particularly large in poorly conducting electrolytes and for high currents. Measuring the electrolyte potential is relevant in electrochemistry, e.g., bipolar electrochemistry, ohmic microscopy, or contact glow discharge electrolysis. Here, we study the course of the electrolyte potential during high voltage electrolysis in an electrolysis cell using two reversible hydrogen electrodes as reference electrodes, placed at different positions in the electrolyte. The electrolysis is performed with a Pt working and stainless steel counter electrode in a KOH solution. A computational COMSOL model is devised which supports the experimentally obtained potential distribution. The influence of the cell geometry on the electrolyte potentials is evaluated. Applying the knowledge of the potential distribution to the formation of a Au oxide surface structure produced during high voltage electrolysis, we find that the amount of oxide formed is related to the current rather than the applied voltage.

Nanoporous Au (NPG) films often show distinctly different properties than bare Au electrodes, which make them suitable for various applications in (electro)catalysis or (bio)sensing. A great deal of effort has gone into finding suitable preparation techniques that can be used to target structural properties, such as the pore size or the surface-to-volume ratio. Many of the methods described for preparing these NPG films require complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from bare Au electrodes. One approach is to prepare in a first step a Au oxide film by high voltage (HV) electrolysis in a KOH solution, which in a second step is reduced either electrochemically or in the presence of H₂O₂. The resulting NPG structures as well as their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H₂O₂-containing KOH solution, as well as the applied voltage and temperature during the HV electrolysis. The NPG film can also be prepared directly by applying electrolysis voltages that result in anodic contact glow discharge electrolysis (aCGDE) over an extended period of time. By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.