The impact of dissolved reactive gases on platinum dissolution in acidic media (2023)

Since the 1980s, single-crystal Pt electrodes with well-defined surface structures have been deemed stable under mild electrochemical conditions (e.g., in the potential region of electric double layers, underpotential deposition of hydrogen, or mild hydrogen evolution/OH adsorption) and have served as model electrodes for unraveling the structure-performance relation in electrocatalysis. With the advancement of in situ electrochemical microscopy/spectroscopy techniques, subtle surface restructuring under mild electrochemical conditions has been achieved in the last decade. Surface restructuring can considerably modify electrocatalytic properties by generating/destroying highly active sites, thereby interfering with the deduction of the structure-performance relation. In this review, we summarize recent progress in the restructuring of well-defined Pt(-based) electrode surfaces under mild electrochemical conditions. The importance of the meticulous structural characterization of Pt electrodes before, during, and after electrochemical measurements is demonstrated using CO adsorption/oxidation, hydrogen adsorption/evolution, and oxygen reduction as examples. The implications of present findings for correctly identifying the reaction mechanisms and kinetics of other electrocatalytic systems are also briefly discussed.

Isopropanol (IPA) can be used as a rechargeable electrofuel. In this approach, IPA is oxidized to acetone (ACE) in a direct alcohol fuel cell and the formed ACE is subsequently back-converted to IPA in a heterogeneously catalyzed process. To study the electrochemical reaction mechanisms of the IPA oxidation at the molecular level, appropriate and well-defined model electrocatalysts are necessary. In this work we prepare such model electrocatalysts by surface science methods in ultra-high vacuum (UHV). The catalysts consist of well-defined platinum nanoparticles on carbon supports. As carbon support, we use flat highly ordered pyrolytic graphite (HOPG) and thin (20 nm) magnetron sputtered carbon films on a polycrystalline gold substrate. In a first step, we characterize the model electrocatalysts and investigate their stability in-situ with complementary methods, i.e. by electrochemical scanning tunneling microscopy (EC-STM), electrochemical on-line inductively coupled plasma mass spectrometry (ICP-MS) and CO stripping experiments followed by electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We determined a stability window ranging from -0.65 V_RHE to 1.15 V_RHE for both sample types, independent of the presence or absence of IPA in the electrolyte. In the second step, we study the oxidation of IPA on tPt nanoparticles using differential electrochemical mass spectrometry (DEMS) and EC-IRRAS. The onset of IPA oxidation is observed at 0.3 V_RHE. ACE is formed with high selectivity, while we identify traces of CO₂ as the only side-product formed at higher potentials. However, we do not observe any formation of adsorbed CO. A direct comparison of these results with previous work on Pt(111) suggests that low coordinated Pt sites and size effects play a subordinate role for IPA oxidation on Pt electrocatalysts.

Proton Exchange Membrane Fuel Cells (PEMFCs) have the perspective to intensely decrease global emission through environmentally-friendly potential. This review paper summarizes the degradation of platinum catalyst layer that has become a significant issue in the improvement of PEMFCs. The review intends to categorise and provide a clear understanding between disintegration and agglomerate that occurs during platinum degradation. In each process, different degradation mechanisms and their migration processes are presented. The improvement in platinum degradation as a function of increasing the performance of PEMFC is established. Prospects for addressing platinum degradation through the exploration of further experimental and numerical research are recommended. Lastly, this paper through recommendation attempts to prevent platinum degradation and reduces high costs associated with the replacement of catalysts in the PEMFCs.

Some electrochemical systems are known to have higher average efficiency when operated under an oscillatory regime. Given the compromise between activity and stability, the stability of electrochemical interfaces in a self-organized, oscillatory state must be taken into account. Here we evaluate the electro-oxidation of methanol and formic acid on platinum under regular and oscillatory conditions, and study the stability by following the Pt dissolution rates in situ with a stationary probe rotating disk electrode (SPRDE) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). Generally speaking, as the electro-oxidation reaction proceeds, the platinum dissolution rate increases considerably. To guarantee Pt stability, the potential must be kept below 1.0Vvs.RHE. Interestingly, no dissolution is detectable when the electrode potential undergoes temporary self-organization, ensuring a stable and active interface.

The influence of the oxygen reduction reaction on the oxidation of Pt(111) is studied by surface X-ray diffraction. The oxygen reduction reaction does not significantly influence the place-exchange process during the initial stages of oxidation and there is no change in the onset potential and kinetics.

Journal of Power Sources, Volume 364, 2017, pp. 215-225

Three-dimensional (3D) crumpled reduced graphene oxide supported Pt-Ir alloys that served as bifunctional oxygen catalysts for use in untized regenerative fuel cells were synthesized by a facile spray pyrolysis method. Pt-Ir catalysts supported on rGO (Pt-Ir/rGOs) were physically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) to observe change in composition by heat treatment, alloying, and morphological transition of the catalysts. Their catalytic activities and stabilities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) conditions were electrochemically investigated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), potential cycling and hold tests on the rotating disk electrode (RDE). Pt-Ir/rGO with no post heat-treatment (Pt-Ir/rGO_NP) showed a lower activity for ORR and OER although metal nanoparticles decorated on the support are relatively small. However, Pt-Ir/rGO showed remarkably enhanced activity following heat treatment, depending on temperature. Pt-Ir/rGO heat-treated at 600°C after spray pyrolysis (Pt-Ir/rGO_P600) exhibited a higher activity and stability than a commercially available Pt/C catalyst kept under the ORR condition, and it also revealed a comparable OER activity and durability versus the commercial unsupported Ir catalyst.

Electrochemistry Communications, Volume 96, 2018, pp. 53-56

Photocorrosion is one of the main challenges in photoelectrochemical water splitting. Traditional methods for degradation assessment, such as chronoamperometry or electrolyte/electrode post-analysis, provide limited information on the degradation mechanisms and kinetics. To address this issue, a new setup, which is based on a light source, an electrochemical cell, and an on-line inductively coupled plasma mass spectrometer (ICP-MS), has been developed. The first results on the photocorrosion of a commercial WO₃ powder on Au are demonstrated in this work. It is shown that, in the absence of light, WO₃ is stable in a wide potential range. On the other hand, in the presence of light, it dissolves proportionally to the anodic photocurrent. The latter is explained by the formation of aqueous tungsten complexes with the electrolyte that are thermodynamically more stable than WO₃. As can be anticipated from these initial results, the novel method will enable the characterization of a wide range of photoelectrochemical materials, and thus eventually lead to the development of long-term stable devices.

Journal of Power Sources, Volume 245, 2014, pp. 1035-1045

Operation of polymer electrolyte membrane fuel cells (PEMFC) at higher cell temperatures accelerates Pt dissolution in the catalyst layer. In this study, a Pt dissolution accelerated stress testing protocol involving the application of a potentiostatic square-wave with 3s at 0.6V followed by 3s at 1.0V was developed to test fuel cell membrane electrode assemblies (MEAs). The use of this Pt dissolution protocol at three different temperatures (40°C, 60°C and 80°C) was investigated for the same membrane electrode assembly composition. Impedance analysis of the membrane electrode assemblies showed an increase in polarization resistance during the course of the accelerated stress testing. Polarization analysis and electrochemical active surface area (ECSA) loss measurements revealed evidence of increased cathode catalyst layer (CCL) degradation due to Pt dissolution and deposition in the membrane as the cell temperature was raised. Scanning electron microscope (SEM) images confirmed the formation of Pt bands in the membrane. A diagnostic expression was developed to estimate kinetic losses due to oxygen reduction using the effective platinum surface area (EPSA) estimated from cyclic voltammograms. The results indicated that performance degradation occurred mainly due to Pt loss.

Journal of Power Sources, Volume 490, 2021, Article 229568

Addition of an oxygen evolution reaction (OER) catalyst is a materials approach to mitigate the impacts of potential reversal caused by fuel starvation. In this study Iridium oxide (IrO_x) supported on graphitized Vulcan (GV) black (56wt% Ir) was added as an OER catalyst into the anode of a membrane electrode assembly (MEA) (0.1 mg_Ir/cm²). When exposed to intermittent 5-, 10- and 30- minutes of starvation, with 10min recovery periods, the reversed cell potential was clamped at −0.8V, mitigating severe carbon degradation. The intermittently starved MEAs regained >95% of their initial performance. After the first event, the performance loss was significant at 4% (0.2 A/cm²) with increases in ohmic resistance, thereafter the performance remained relatively stable. Using SEM and EIS, the increased ohmic resistance was attributed to deformation and contraction of the membrane and ionomer reconfiguration which impacted proton conductive pathways. Thinning of the anode was unavoidable, contributing to contact resistance and decreased performance. The Pt and IrO_x/GV catalyst remained relatively stable when subjected to multiple short periods of fuel starvation. The IrOx/GV MEA was reversal tolerant and provided insight into the degradation processes which occur during periodic and prolonged fuel starvation.

Electrochimica Acta, Volume 246, 2017, pp. 654-670

Iridium based materials are one of the most active electrocatalysts used for the oxygen evolution reaction (OER) in polymer electrolyte membrane (PEM) water electrolysers. To increase the utilization, the iridium electrocatalyst is typically dispersed on a high-surface area support material. This results in less iridium being required and consequently reduced catalyst cost. In this work, six metal carbides and oxides were characterized and evaluated as supports for iridium electrocatalyst. The supports studied included: tantalum carbide (TaC), niobium carbide (NbC), titanium carbide (TiC), tungsten carbide (WC), niobium oxide (NbO₂), and antimony-doped tin oxide (Sb₂O₅-SnO₂).

Journal of Catalysis, Volume 396, 2021, pp. 387-394

Due to the high demand for renewable and infrastructure compatible energy conversion and storage technologies, research on organic fuel cells receives increasing interest again recently. Organic fuels such as alcohols provide an attractive avenue to overcome the drawbacks of hydrogen as an energy carrier. Particularly interesting are secondary alcohols that almost exclusively form ketones as the final oxidation product, as they can be utilized in “zero emission” concepts without CO₂ as a by-product. The state-of-the-art electrocatalyst in secondary alcohol oxidation is Pt-Ru, which demonstrates low onset potentials for the oxidation of the most facile secondary alcohol isopropanol. Yet, the achievable current densities are still relatively low and decrease rapidly due to the formed product acetone, which can poison the catalyst surface over time. Therefore, there is an inevitable need for the development of novel electrocatalyst materials circumventing these challenges. In this study, we employ a high-throughput electrochemical approach coupled to on-line inductively-coupled plasma mass spectrometry to map the composition-dependent activity and stability of Pt_xIr_100-x alloy electrocatalysts toward the electro-oxidation of isopropanol. The activity and stability of magnetron sputtered Pt_xIr_100-x material libraries are studied in 0.1M HClO₄ both in the absence and presence of isopropanol. The highest current densities are achieved for the sample containing the least amount of Ir (3.4 at.%), with a continuous decrease with the increasing amount of Ir. The alloys are inactive towards the oxidation of isopropanol when the amount of Ir exceeded 80 at%. The presence of isopropanol also has a notable effect on stability: while dissolution rates do not change in the case of pure Pt and Ir, a significant increase in stability is observed for the Pt_xIr_100-x thin-film samples at all applied upper potential limits. This is explained by the strong adsorption of acetone on the surface of the catalyst that inhibits the formation of surface oxides.