The Metrohm Poster Prize, established 31 years ago at the Electroanalytical Chemistry Conference (ELACH), continues to honor outstanding early-career researchers in electrochemistry. At the Electrochemistry 2024 conference in Braunschweig, Germany, three young scientists received this prestigious award. The conference, themed “Global Thinking, Local Acting,” attracted over 500 participants and served as a platform for sharing the latest research and innovations in various electrochemical fields. From over 200 poster presentations, the poster committee, composed of members of the scientific panel, selected the three most outstanding contributions.

This article presents the research of Marco Martini from Helmholtz-Institut Erlangen-Nürnberg. As doctoral candidate he focuses on the development of an electrochemical flow cell system that can be operated at elevated temperatures and pressures to more realistically investigate industrial catalyst performance. His work combines chemistry, engineering, and sustainability, utilizing innovative materials and computer-aided simulations. 

Evaluating Electrocatalysts: Overcoming Challenges at Elevated Temperatures and Pressures

The evaluation of electrocatalysts for energy conversion technologies, such as fuel cells and electrolyzers, is typically conducted on a small scale using three-electrode configurations at ambient temperature and atmospheric pressure, with the well-established Rotating Disk Electrode (RDE) setup being the most notable. However, in industrial applications, electrochemical processes often become more efficient and economically attractive when performed at elevated temperatures due to enhanced reaction kinetics, reduced overpotentials, and the potential for waste heat utilization. Consequently, more advanced experimental setups are needed to assess fundamental catalyst performance under conditions that closely resemble those in industrial operations. Despite the development of a pressurized RDE system, efforts in this direction have been limited.

Given the numerous advantages of investigating higher temperature and pressure conditions, it is curious why the scientific literature is sparse on such studies. The challenge lies in constructing an electrochemical system that simultaneously combines (electro)chemical, mechanical, and thermal stability. Practically, this requires knowledge that extends beyond the basic background of a chemist, encompassing engineering and product design skills, among others. Additionally, supporting this effort necessitates facilities like a well-equipped mechanical and electrical workshop, which many universities and most chemical laboratories cannot afford. Fortunately, the Helmholtz Institute Erlangen-Nuremberg has access to such facilities, with highly skilled personnel, including CNC machinists, electricians, and software developers, readily available to assist.

Continuous-Flow System Benefits

Given the inherent limitations of the RDE batch-type approach discussed at the beginning, a continuous-flow system was chosen for its several benefits. In a flow cell, the continuous supply of fresh electrolyte facilitates the establishment of a steady state, while reaction by-products, dissolved species, and evolved gases are consistently removed. In contrast, sealed batch systems may pose greater safety risks due to the possible build-up of explosive gas mixtures. Importantly, unlike RDE, there are no internal moving parts in a flow cell, as the electrode is fixed and mass transport is created by the electrolyte flowing past it. Fluid motion is the key parameter in flow cells for defining mass transport, as it controls the thickness of the diffusion layer: the larger the flow rate, the greater the convective mass transport, making it easier for the electroactive species to overcome a thinner diffusion layer and reach or escape from the electrode surface.

To avoid metals and therefore ionic pollution, polyether ether ketone was chosen as the constructing material for the cell, due to its optimal combination of mechanical, chemical, thermal, and machining properties. Other engineering plastics are also used in the form of tubing and fittings. The cell’s geometry design was assisted by computational fluid dynamics simulations (Figure 1), which are also combined with thermal exchange and simple reaction kinetics. The working and counter electrodes are in the form of discs made of glassy carbon or metals, such as platinum. The relative spatial organization of the working electrode (WE) and counter electrode (CE) ensures that dissolution and reaction products from the CE cannot deposit on the WE, as the WE is upstream of the CE. The disc shape enables the mechanical fitting of the electrodes to the cell bodies without the need for glues or sealants – one of the significant innovations in this work. This design allows the discs to be recoverable and easily cleaned or replaced, avoiding contamination (Figure 3).

An external housing for commercial reference electrodes (RHE and Ag/AgCl) was also developed to protect them from pressure oscillations, which could drift their potential or even mechanically destroy them.

Channel Flow Cells, like other hydrodynamic methods, can be best exploited in the so-called mass transport limited current condition. This means that the measured current is limited only by mass transport, as reaction kinetics is made extremely fast by applying a potential positive or negative enough to the equilibrium condition, causing the reaction to run extremely fast and the reactant concentration at the electrode surface to fall to zero (Figure 2).

Results and Future Directions

In conclusion, the cell was successfully heated and pressurized to 120°C and 20 bar, respectively. Measurements conducted at room temperature demonstrated the viability of testing drop-casted catalyst ink (e.g., O₂ reduction reaction, Figure 4) and the accuracy in measuring Levich slopes (Figure 5), which reflect various reaction parameters, such as the number of electrons exchanged and the diffusion coefficient.

Regarding high temperature and pressure, preliminary results on a probe system (ferro/ferricyanide couple) indicate that pressure does not affect the potential reading of the reference electrode (Figure 6, left), which is desirable. When the temperature varies from -5 to 90°C (Figure 6, right), there is a shift in both the peak current and voltage, as well as in the magnitude of the limiting current. The increased current is attributed to the increase in the diffusion coefficient and a decrease in viscosity. The shift in peak potential is a mixed effect resulting from thermodynamic changes and partially from the uncorrected potential drift caused by a temperature gradient between the cell's interior and the reference electrode. These latter effects have yet to be quantified and will be investigated more thoroughly in the future.

The next steps involve testing catalyst inks for industrially relevant reactions at higher temperatures and pressures, to investigate their behaviors under these different conditions.