Determination Of Real Surface Area Research Articles

Impact of Intermittent Operation on the Lifetime and Performance of a PEM Water Electrolyzer

In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen.1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm-2 geo) and low (0.1 Acm-2 geo) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrOx 2, the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrOx is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes.3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2).4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrOx.5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrOx. Interestingly, the polarization curve recorded directly after IrOx reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re-precipitation occurs. In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM-WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM-WE durability. In addition, PEM-WE load cycles to OCV may serve as an accelerated aging test. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013, 38, 4901-4934.Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017, Abstr. # I01F-1648.Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974, 49, 217-226.Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 220, 83-100.Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014, 161, F876-F882. Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min-1 H2O (anode)/ 50 mL min-1 H2 (cathode) for an MEA with ~1.6 mgIr cm-²MEA anode and ~0.3 mgPt cm-²MEA cathode loading using a Nafion® 212 (~50 µm) membrane Figure 1

Oxygen reduction on Pd nanoparticle/multi-walled carbon nanotube composites

The electrochemical reduction of oxygen was studied on multi-walled carbon nanotube-supported palladium nanoparticle (PdNP/MWCNT) modified glassy carbon (GC) electrodes. The catalyst materials were prepared using two different methods and their electrocatalytic activity towards oxygen reduction was explored in both acid and alkaline media using the rotating disk electrode (RDE) method. Cyclic voltammetry was employed for CO stripping experiments and for the determination of real surface area of carbon nanotube–supported Pd nanoparticles. The surface morphology of PdNP/MWCNT nanocomposites was examined by transmission electron microscopy (TEM). Oxygen electroreduction kinetics were compared with those of bulk palladium electrodes. Enhanced electrocatalytic activity of PdNP/MWCNT modified GC electrodes was observed. The Tafel behaviour of oxygen reduction was similar for PdNP/MWCNT and bulk Pd electrodes. The RDE data analysis showed that the reduction of oxygen on the PdNP/MWCNT electrocatalysts studied followed a four-electron pathway.

On the real surface area of smooth solid electrodes

The determination of real surface area of the electrodes is very important for calculation of various parameters characterizing electrochemical processes. It has been noticed that real surface area determined by the electrochemical methods depends on the method used and on the experimental conditions. In this work the real surface area of the gold polycrystalline electrode is presented, determined by various electrochemical methods in aqueous and nonaqueous solutions with varying concentration of nonadsorbing electrolyte. In conclusion it is confirmed that the so-called roughness factor depends on the method used. The values of roughness factor determined for polycrystalline gold electrode using methods based on Gouy-Chapman theory are dependent on diffuse layer thickness via electrolyte concentration or solvent dielectric constant. These results are discussed in comparison with the theoretical predictions.

Determination of the surface area of Pd catalysts by underpotential deposition of copper

Copper adsorption (underpotential deposition) was used for determination of real surface area of palladium. The method is compared with other electrochemical methods based on hydrogen and oxygen adsorption.

Determination of real surface area of palladium electrodes. Electrochemical oxidation of thin palladium films on gold. Comments

Determination of real surface area of palladium electrodes. Electrochemical oxidation of thin palladium films on gold. Comments

Hydrogen adsorption on platinum, iridium and rhodium electrodes at reduced temperatures and the determination of real surface area

Cyclic voltammograms of platinum and iridium electrodes in 5 M sulphuric acid at −72°C display much greater separation of the hydrogen adsorption and evolution currents than at 20°C. This enables the saturation hydrogen coverage and the real surface area of an electrode to be determined directly from the voltammogram. The adsorption of hydrogen on rhodium is not sufficiently rapid to maintain equilibrium on a voltammogram at reduced temperatures, and the saturation hydrogen coverage cannot be obtained directly. The accuracy of determination of surface area for the three metals from measurements at 20–25°C is discussed. The voltammograms for platinum electrodes at −72°C do not show the “third anodic hydrogen peak” observed at 20°C. The processes which have been suggested to account for this peak are discussed.

Hydrogen adsorption on platinum, iridium and rhodium electrodes at reduced temperatures and the determination of real surface area

Cyclic voltammograms of platinum and iridium electrodes in 5 M sulphuric acid at −72°C display much greater separation of the hydrogen adsorption and evolution currents than at 20°C. This enables the saturation hydrogen coverage and the real surface area of an electrode to be determined directly from the voltammogram. The adsorption of hydrogen on rhodium is not sufficiently rapid to maintain equilibrium on a voltammogram at reduced temperatures, and the saturation hydrogen coverage cannot be obtained directly. The accuracy of determination of surface area for the three metals from measurements at 20–25°C is discussed. The voltammograms for platinum electrodes at −72°C do not show the “third anodic hydrogen peak” observed at 20°C. The processes which have been suggested to account for this peak are discussed.