Addressing stability challenges of using bimetallic electrocatalysts: the case of gold?palladium nanoalloys

Bimetallic catalysts are known to often provide enhanced activity compared to pure metals, due to their electronic, geometric and ensemble effects. However, applied catalytic reaction conditions may induce restructuring, metal diffusion and dealloying. This gives rise to a drastic change in surface composition, thus limiting the application of bimetallic catalysts in real systems. Here, we report a study on dealloying using an AuPd bimetallic nanocatalyst (1 : 1 molar ratio) as a model system. The changes in surface composition over time are monitored in situ by cyclic voltammetry, and dissolution is studied in parallel using online inductively coupled plasma mass spectrometry (ICP-MS). It is demonstrated how experimental conditions such as different acidic media (0.1 M HClO4 and H2SO4), different gases (Ar and O-2), upper potential limit and scan rate significantly affect the partial dissolution rates and consequently the surface composition. The understanding of these alterations is crucial for the determination of fundamental catalyst activity, and plays an essential role for real applications, where long-term stability is a key parameter. In particular, the findings can be utilized for the development of catalysts with enhanced activity and/or selectivity

Operando Insights on the Degradation Mechanisms of Rhenium doped Molybdenum Disulfide Nanocatalysts for Electrolyzer Applications

MoS2 nanostructures are promising catalysts for proton-exchange-membrane (PEM) electrolyzers to replace expensive noble metals. Their broadscale application demands high activity for the hydrogen evolution reaction (HER) as well as good durability. Doping in MoS2 is commonly applied to enhance the HER activity of MoS2-based nanocatalysts, but the effect of dopants in the electrochemical and structural stability is yet to be discussed. Herein, we correlate operando electrochemical measurements to the structural evolution of the materials down to the nanometric scale by identical location electron microscopy and spectroscopy. Different degradation mechanisms at first electrolyte contact, open circuit stabilization and HER conditions are identified for MoS2 nanocatalysts with and without Rhenium doping. Our results demonstrate that doping in MoS2 nanocatalysts can not only improve their HER activity, but also their stability. Doping of MoS2-based nanocatalysts is validated as a promising strategy to follow for the continuous improvement of high performance and durable PEM electrolyzers

Palladium electrodissolution from model surfaces and nanoparticles

Palladium (Pd) is considered as a possible candidate as catalyst for proton exchange membrane fuel cells (PEMFCs) due to its high activity and affordable price compared to platinum (Pt). However, the stability of Pd is known to be limited, yet still not fully understood. In this work, Pd dissolution is studied in acidic media using an online inductively coupled plasma mass spectrometry (ICP-MS) in combination with an electrochemical scanning flow cell (SFC). Crucial parameters influencing dissolution like potential scan rate, upper potential limit (UPL) and electrolyte composition are studied on a bulk polycrystalline Pd (poly -Pd). Furthermore, a comparison with a supported high -surface area catalyst is carried out for its potential use in industrial applications. For this aim, a carbon supported Pd nanocatalyst (Pd/C) is synthesized and its performance is compared with that of bulk poly -Pd. Our results evidence that the transient dissolution is promoted by three main contributions (one anodic and two cathodic). At potentials below 1.5 VRHE the anodic dissolution is the dominating mechanism, whereas at higher potentials the cathodic mechanisms prevail. On the basis of the obtained results, a model is thereafter proposed to explain the transient Pd dissolution.(C) 2017 Elsevier Ltd. All rights reserved

Oxygen evolution activity and stability of iridium in acidic media. Part 1. – Metallic iridium

A better understanding of iridium dissolution is important in the elucidation of the general mechanism of noble metals corrosion and in the design of more durable iridium based applied materials. In the current work iridium dissolution has been addressed by a complementary electrochemical and mass spectrometric technique based on a scanning flow cell (SFC) and inductively coupled plasma mass spectrometry (ICP-MS). Time- and potential-resolved iridium dissolution profiles are recorded and analyzed. It is found that during the anodic treatment dissolution is increasing constantly with potential. Anodic dissolution decreases with time, which is attributed to the buildup of a passivating oxide layer. In case a significantly low reductive potential is applied to the oxidized electrode, an additional dissolution process is observed. It is concluded that like other members of the noble metals group, e.g. gold and platinum, thoroughly studied before, dissolution of iridium is initiated by a change in the iridium oxidation state during the initial formation or reduction of a thin compact anhydrous oxide layer. Both transitions metal/oxide and oxide/metal during oxidation and reduction, respectively, result in dissolution. Thus, dissolution is a transient process. The Ir(III)/Ir(IV) transition in the thick hydrous oxide layer, responsible for iridium oxide electrochromism, do not lead, however, to any significant change in dissolution signal. At even higher anodic potentials, further destabilization is caused by a change in the oxidation number (increase and decrease) of iridium cations during oxygen evolution reaction (OER). At studied potentials OER on the metallic electrodes, covered by a thin compact anhydrous oxide layer, has Tafel slope of ca. 66 mV dec$^{−1}$, which is comparable to literature data on bulk IrO$_{2}$ electrodes and, probably, implies on similarities in the OER mechanism on these materials

Oxygen evolution activity and stability of iridium in acidic media. Part 2. – Electrochemically grown hydrous iridium oxide

Hydrous iridium oxide is a well-known material for its electrochromism and electrocatalytic activity, e.g. in oxygen and chlorine evolution reactions (OER) and (CER). Poor durability during the OER is, however, usually considered as a major drawback. In the current work dissolution of hydrous oxide, prepared from metallic iridium by applying a potential cycle protocol of different number of cycles, has been investigated using a scanning flow cell (SFC) inductively coupled plasma mass spectrometer (ICP-MS) based setup. It is shown that in the potential region preceding the OER, dissolution behavior of such electrodes is very similar to that of metallic iridium discussed in Part I. It is suggested that at anodic potentials the process is controlled by oxidation of metallic iridium underneath a hydrous oxide, with formation of a thin compact anhydrous oxide layer sandwiched between metal and hydrous oxide. The application of a reductive potential results in the reduction of the compact oxide layer and also leads to dissolution. At these potentials some dissolution of hydrous oxide itself is postulated. Decomposition of iridium (V) oxyhydroxide with formation of molecular oxygen and Ir(III) complexes is suggested at higher anodic potentials during OER. We hypothesize that formation of the soluble Ir(III) complex or complexes and their dissolution is responsible for the observed variation of dissolution with potential in the whole studied potential window. Based on the experimental results and an extended literature overview, a new mechanism of OER triggered dissolution is proposed. The difference in activity and stability of electrochemically prepared hydrous oxides and, usually more stable, “dry” oxides is suggested to be a consequence of different OER mechanisms on these materials

Stability limits of tin-based electrocatalyst supports

Tin-based oxides are attractive catalyst support materials considered for application in fuel cells and electrolysers. If properly doped, these oxides are relatively good conductors, assuring that ohmic drop in real applications is minimal. Corrosion of dopants, however, will lead to severe performance deterioration. The present work aims to investigate the potential dependent dissolution rates of indium tin oxide (ITO), fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) in the broad potential window ranging from -0.6 to 3.2 V-RHE in 0.1 M H2SO4 electrolyte. It is shown that in the cathodic part of the studied potential window all oxides dissolve during the electrochemical reduction of the oxide - cathodic dissolution. In case an oxidation potential is applied to the reduced electrode, metal oxidation is accompanied with additional dissolution - anodic dissolution. Additional dissolution is observed during the oxygen evolution reaction. FTO withstands anodic conditions best, while little and strong dissolution is observed for ATO and ITO, respectively. In discussion of possible corrosion mechanisms, obtained dissolution onset potentials are correlated with existing thermodynamic data

Electrochemical dissolution of gold in presence of chloride and bromide traces studied by on-line electrochemical inductively coupled plasma mass spectrometry

Electrochemical dissolution of gold in the presence of halide ions is traditionally considered as a model case in metal corrosion and passivation studies. Due to a relatively low sensitivity of the conventional techniques, such studies were predominantly limited to electrolytes with high concentrations of halides. The current work employs a special electrochemical scanning flow cell with online inductively coupled plasma mass spectrometry (SFC-ICP-MS) that allows overcoming those limitations, enabling the quantification of gold dissolution in the presence of chloride and bromide traces (1 μM ≤ [Cl−/Br−] ≤ 100 μM). Moreover, time- and potential-resolved gold dissolution profiles under different potential programs are provided. Based on the experimental results, a tentative mechanism of gold dissolution based on the catalyzing role of hydroxyl and halide anions is discussed

Activity and Stability of Electrochemically and Thermally Treated Iridium for the Oxygen Evolution Reaction

Iridium is the main element in modern catalysts for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE), which is predominantly due to its relatively good activity and tolerable stability in harsh PEMWE conditions. Limited abundance of iridium, however, poses limitations on widespread applications of these devices, in particular in the large scale conversion and storage of renewable energy. In this work we investigate if the electrocatalytic performance of iridium can be fine-tuned by thermal treatment of catalysts at different temperatures. The OER activity and the dissolution of two different iridium electrodes, viz. (a) flat metallic iridium surfaces prepared by electron beam physical vapor deposition (EBPVD) and (b) electrochemically prepared porous hydrous iridium oxide films (HIROF) are studied. The range of applied annealing temperatures is 100°C–600°C, with a general trend of decreasing activity and increasing stability the higher the temperature. Numerous peculiarities in the trend are however observed. These are discussed considering variations of oxide structure, morphology and electronic conductivity

The Electrochemical Dissolution of Noble Metals in Alkaline Media

In this study, the electrochemical transient dissolution of polycrystalline silver, gold, iridium, palladium, platinum, rhodium, and ruthenium is examined in 0.05 M NaOH alkaline electrolyte as a function of electrode potential. An inductively coupled plasma mass spectrometer connected to an electrochemical flow cell is used for online detection of the metals dissolution rates. Broad potential windows starting from the hydrogen and going to the oxygen evolution reaction (OER) potentials are used to study the dissolution. The measured dissolution data, such as onsets of dissolution are analyzed and compared with available thermodynamic data. For most metals, at potentials, at which thermodynamics predict metal/solute or metal/oxide transitions, an initiation of the dissolution process is observed. It is suggested that dissolution during metal/oxide transitions is a purely kinetic effect that reflects the solubility of unstable transient oxides. Such oxides can also be formed during the oxygen evolution reaction. The latter fact is used to explain metals dissolution in the region of OER