Temperature-Dependent Dissolution of Polycrystalline Platinum in Sulfuric Acid Electrolyte

Balancing Activity and Stability through Compositional Engineering of Ternary PtNi-Au Alloy ORR Catalysts

Achieving the optimal balance between cost-efficiency and stability of oxygen reduction reaction (ORR) catalysts is currently among the key research focuses aiming at reaching a broader implementation of proton-exchange membrane fuel cells (PEMFCs). To address this challenge, we combine two well-established strategies to enhance both activity and stability of platinum-based ORR catalysts. Specifically, we prepare ternary PtNi-Au alloys, where each alloying element plays a distinct role: Ni reduces costs and boosts ORR activity, while Au enhances stability. A systematic comparative analysis of the activity-stability relationship for compositionally tuned PtNi-Au model layers, prepared by magnetron co-sputtering, was conducted using a diverse range of complementary characterization techniques and electrochemistry, supported by density functional theory calculations. Our study reveals that a progressive increase of the Au concentration in the Pt50Ni50 alloy from 3 to 15 at % leads to opposing catalyst activity and stability trends. Specifically, we observe a decrease in the ORR activity accompanied by an increase in catalyst stability, manifested in the suppression of both Pt and Ni dissolution. Despite the reduced activity compared to PtNi, the PtNi-Au alloy with 15 at % Au still exhibits nearly three times the activity of monometallic Pt. It also demonstrates a significantly improved dissolution stability relative to that of the PtNi alloy and even monometallic Pt. These findings provide valuable insights into the intricate balance between activity and stability in multimetallic ORR catalysts, paving the way for the design of cost-effective and durable materials for PEMFCs.

Scanning Gas Diffusion Electrode Setup for Real-Time Analysis of Catalyst Layers

The scanning gas diffusion electrode (S-GDE) half-cell is introduced as a new tool to improve the evaluation of electrodes used in electrochemical energy conversion technologies. It allows both fast screening and fundamental studies of real catalyst layers by applying coupled mass spectrometry techniques such as inductively coupled plasma mass spectrometry and online gas mass spectrometry. Hence, the proposed setup overcomes the limitations of aqueous model systems and full cell-level studies, bridging the gap between the two approaches. In this proof-of-concept work, standard fuel cell electrodes are investigated at elevated oxygen reduction reaction current densities, while dissolved Pt x+ ions in the electrolyte and gaseous CO2 in the outlet gas stream are detected to track platinum dissolution and carbon corrosion, respectively. Relevant current densities of up to 0.75 A cm-2 are demonstrated. The electrochemically active surface area, oxygen reduction reaction activity, and Pt dissolution rates are quantified and benchmarked to the values obtained in the conventional stationary GDE half-cell. Moreover, it is found that Pt dissolution is suppressed when O2 is purged into the catalyst layer. Overall, this work demonstrates the feasibility of fast fuel cell electrode screening obtaining, complementary to electrochemical, mass spectrometry data necessary in fundamental studies on structure/performance relationships under actual reaction conditions. While Pt/C, in relevance to its fuel cell application, is used in this study, the proposed setup can be applied in water electrolysis, CO2 conversion, metal-air batteries, and other neighbor technologies.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Towards a realistic prediction of catalyst durability from liquid half-cell tests

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Elucidation of Electrochemically Induced but Chemically Driven Pt Dissolution

Securing the electrochemical durability of noble metal platinum is of central importance for the successful implementation of a proton exchange membrane fuel cell (PEMFC). Pt dissolution, a major cause of PEMFC degradation, is known to be a potential-dependent transient process, but its underlying mechanism is puzzling. Herein, we elucidate a chemical Pt dissolution process that can occur in various electrocatalytic conditions. This process intensively occurs during potential perturbations with a millisecond timescale, which has yet to be seriously considered. The open circuit potential profiles identify the dominant formation of metastable Pt species at such short timescales and their simultaneous dissolution. Considering on these findings, a proof-of-concept strategy for alleviating chemical Pt dissolution is further studied by tuning electric double layer charging. These results suggest that stable Pt electrocatalysis can be achieved if rational synthetic or systematic strategies are further developed.

Optimal Pt-Au Alloying for Efficient and Stable Oxygen Reduction Reaction Catalysts

Stabilization of cathode catalysts in hydrogen-fueled proton-exchange membrane fuel cells (PEMFCs) is paramount to their widespread commercialization. Targeting that aim, Pt-Au alloy catalysts with various compositions (Pt₉₅Au₅, Pt₉₀Au₁₀, and Pt₈₀Au₂₀) prepared by magnetron sputtering were investigated. The promising stability improvement of the Pt-Au catalyst, manifested in suppressed platinum dissolution with increasing Au content, was documented over an extended potential range up to 1.5 V_RHE. On the other hand, at elevated concentrations, Au showed a detrimental effect on oxygen reduction reaction activity. A systematic study involving complementary characterization techniques, electrochemistry, and Monte Carlo simulations based on density functional theory data enabled us to gain a comprehensive understanding of the composition-activity-stability relationship to find optimal Pt-Au alloying for maintaining the activity of platinum and improving its resistance to dissolution. According to the results, Pt-Au alloy with 10% gold represent the most promising composition retaining the activity of monometallic Pt while suppressing Pt dissolution by 50% at the upper potential limit of 1.2 V_RHE and by 20% at devastating 1.5 V_RHE.

Stability challenges of carbon-supported Pt-nanoalloys as fuel cell oxygen reduction reaction electrocatalysts

Carbon-supported Pt-based nanoalloys (CSPtNs) as the oxygen reduction reaction (ORR) electrocatalysts are considered state-of-the-art electrocatalysts for use in proton exchange membrane fuel cells (PEMFCs). Although their ORR activity performance is already adequate to allow lowering of the Pt loading and thus commercialisation of the fuel cell technology, their stability remains an open challenge. In this Feature Article, the recent achievements and acquired knowledge on the degradation behaviour of these electrocatalysts are overviewed and discussed.

Importance of Chemical Activation and the Effect of Low Operation Voltage on the Performance of Pt-Alloy Fuel Cell Electrocatalysts

Pt-alloy (Pt-M) nanoparticles (NPs) with less-expensive 3d transition metals (M = Ni, Cu, Co) supported on high-surface-area carbon supports are currently the state-of-the-art (SoA) solution to reach the production phase in proton exchange membrane fuel cells (PEMFCs). However, while Pt-M electrocatalysts show promise in terms of increased activity for oxygen reduction reaction (ORR) and, thus, cost reductions from the significantly lower use of expensive and rare Pt, key challenges in terms of synthesis, activation, and stability remain to unlock their true potential. This work systematically tackles them with a combination of electrocatalyst synthesis and characterization methodologies including thin-film rotating disc electrodes (TF-RDEs), an electrochemical flow cell linked to an inductively coupled plasma mass spectrometer (EFC-ICP-MS), and testing in 50 cm² membrane electrode assemblies (MEAs). In the first part of the present work, we highlight the crucial importance of the chemical activation (dealloying) step on the performance of Pt-M electrocatalysts in the MEA at high current densities (HCDs). In addition, we provide the scientific community with a preliminary and facile method of distinguishing between a "poorly" and "adequately" dealloyed (activated) Pt-alloy electrocatalyst using a much simpler and affordable TF-RDE methodology using the well-known CO-stripping process. Since the transition-metal cations can also be introduced in a PEMFC due to the degradation of the Pt-M NPs, the second part of the work focuses on presenting clear evidence on the direct impact of the lower voltage limit (LVL) on the stability of Pt-M electrocatalysts. The data suggests that in addition to intrinsic improvements in stability, significant improvements in the PEMFC lifetime can also be obtained via the correct MEA design and applied limits of operation, namely, restricting not just the upper but equally important also the lower operation voltage.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Understanding the Crucial Significance of the Temperature and Potential Window on the Stability of Carbon Supported Pt-Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts

The present research provides a study of carbon-supported intermetallic Pt-alloy electrocatalysts and assesses their stability against metal dissolution in relation to the operating temperature and the potential window using two advanced electrochemical methodologies: (i) the in-house designed high-temperature disk electrode (HT-DE) methodology as well as (ii) a modification of the electrochemical flow cell coupled to an inductively coupled plasma mass spectrometer (EFC-ICP-MS) methodology, allowing for highly sensitive time- and potential-resolved measurements of metal dissolution. While the rate of carbon corrosion follows the Arrhenius law and increases exponentially with temperature, the findings of the present study contradict the generally accepted hypothesis that the kinetics of Pt and subsequently the less noble metal dissolution are supposed to be for the most part unaffected by temperature. On the contrary, clear evidence is presented that in addition to the importance of the voltage/potential window, the temperature is one of the most critical parameters governing the stability of Pt and thus, in the case of Pt-alloy electrocatalysts, also the ability of the nanoparticles (NPs) to retain the less noble metal. Lastly, but also very importantly, results indicate that the rate of Pt redeposition significantly increases with temperature, which has been the main reason why mechanistic interpretation of the temperature-dependent kinetics related to the stability of Pt remained highly speculative until now.

Particle Size Effect on Platinum Dissolution: Practical Considerations for Fuel Cells

The high costs of polymer membrane electrolyte fuel cells (PEMFCs) remain a roadblock for a competitive market with combustion engine vehicles. The PEMFC costs can be reduced by decreasing the size of Pt nanoparticles in the catalyst layer, thereby increasing the Pt dispersion and utilization. Furthermore, high-power performance loss due to O₂ transport resistance is alleviated by decreasing the particle size and increasing dispersion. However, firm conclusions on how Pt particle size impacts durability remain elusive due to synthetic difficulties in exclusively varying single parameters (e.g., particle size and loading). Therefore, here the particle size of Pt nanoparticles was varied from 2.0 to 2.8 and 3.7 nm while keeping the loading constant (30 wt %) on a Vulcan support using the two-step surfactant-free toolbox method. By studying the electrochemical dissolution in situ using online inductively coupled plasma mass spectrometry (online ICP-MS), mass-specific dissolution trends are revealed and are attributed to particle-size-dependent changes in electrochemically active surface area. Such degradation trends are critical for the start/stop of PEMFCs and currently require the implementation of potential control systems in consumer vehicles. Additionally, shifts in the onset of anodic dissolution and also oxidation to more negative potentials with decreasing particle size were observed. These results indicate a similar mechanism of anodic dissolution related to place-exchange when moving from extended polycrystalline Pt to nanoparticle scales. The negative shifts in the onset as the particle size decreases highlight a practical limitation for PEMFCs during load/idle conditions: without further material improvements, which inhibit Pt dissolution, reduction in costs and improvement in high-power performance via increased Pt utilization and dispersion will not be possible by decreasing particle sizes further.

Evolution of the PtNi Bimetallic Alloy Fuel Cell Catalyst under Simulated Operational Conditions

Comprehensive understanding of the catalyst corrosion dynamics is a prerequisite for the development of an efficient cathode catalyst in proton-exchange membrane fuel cells. To reach this aim, the behavior of fuel cell catalysts must be investigated directly under reaction conditions. Herein, we applied a strategic combination of in situ/online techniques: in situ electrochemical atomic force microscopy, in situ grazing incidence small angle X-ray scattering, and electrochemical scanning flow cell with online detection by inductively coupled plasma mass spectrometry. This combination of techniques allows in-depth investigation of the potential-dependent surface restructuring of a PtNi model thin film catalyst during potentiodynamic cycling in an aqueous acidic electrolyte. The study reveals a clear correlation between the upper potential limit and structural behavior of the PtNi catalyst, namely, its dealloying and coarsening. The results show that at 0.6 and 1.0 V_RHE upper potentials, the PtNi catalyst essentially preserves its structure during the entire cycling procedure. The crucial changes in the morphology of PtNi layers are found to occur at 1.3 and 1.5 V_RHE cycling potentials. Strong dealloying at the early stage of cycling is substituted with strong coarsening of catalyst particles at the later stage. The coarsening at the later stage of cycling is assigned to the electrochemical Ostwald ripening process.

The Role of Surface Hydroxylation, Lattice Vacancies and Bond Covalency in the Electrochemical Oxidation of Water (OER) on Ni-Depleted Iridium Oxide Catalysts

The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atom efficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: (1) use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, (2) use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and (3) use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments.

Dissolution of Platinum Single Crystals in Acidic Medium

Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4 . Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS) the transient dissolution was detected on-line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111).

Electrochemical On-line ICP-MS in Electrocatalysis Research

Electrocatalyst degradation due to dissolution is one of the major challenges in electrochemical energy conversion technologies such as fuel cells and electrolysers. While tendencies towards dissolution can be grasped considering available thermodynamic data, the kinetics of material's stability in real conditions is still difficult to predict and have to be measured experimentally, ideally in-situ and/or on-line. On-line inductively coupled plasma mass spectrometry (ICP-MS) is a technique developed recently to address exactly this issue. It allows time- and potential-resolved analysis of dissolution products in the electrolyte during the reaction under dynamic conditions. In this work, applications of on-line ICP-MS techniques in studies embracing dissolution of catalysts for oxygen reduction (ORR) and evolution (OER) as well as hydrogen oxidation (HOR) and evolution (HER) reactions are reviewed.

In Situ Electrochemical AFM Imaging of a Pt Electrode in Sulfuric Acid under Potential Cycling Conditions

Understanding the electrochemical behavior of Pt at the solid/liquid interface is of significant importance for the development of efficient electrochemical devices, such as fuel cells and water electrolyzers. In this work, the evolution of the surface morphology of a polycrystalline platinum under potential cycling conditions was investigated by in situ electrochemical atomic force microscopy (EC-AFM). After 50 cycles between 0.05 and 1.8 V in 0.1 M H2SO4, the Pt surface is coarsened and nanoparticles of several nanometers appear on the surface. The critical upper and lower potentials for the formation of nanoparticles are found to be 1.8 and 0.8 V, respectively. The in situ AFM observation coupled with Cyclic Voltammerty reveals the periodic disappearance and reappearance of the nanoparticles, based on which the formation of nanoparticles is attributed to the deposition of dissolved Pt from solution, and a model for the nanoparticle formation is proposed. While the formation of a thick oxide layer is a prerequisite, the reduction process is found to have a strong influence on Pt nanoparticle formation as well. This investigation provides a visualization of the Pt electrode surface under electrochemical control in a large potential window, enabling a broader understanding of the Pt electrode roughening mechanisms.

In-situ electrochemical activation designed hybrid electrocatalysts for water electrolysis

Developing transition metal-based electrocatalysts with rich active sites for water electrolysis plays important roles in renewable energy fields. So far, some strategies including designing nanostructures, incorporating conductive support or foreign elements have been adopted to develop efficient electrocatalysts. Herein, we summarize recent progresses and propose in-situ electrochemical activation as a new pretreating technique for enhanced catalytic performances. The activation techniques mainly comprise facile electrochemical processes such as anodic oxidation, cathodic reduction, etching, lithium-assisted tuning and counter electrode electro-dissolution. During these electrochemical treatments, the catalyst surfaces are modified from bulk phase, which can tune local electronic structures, create more active species, enlarge surface area and thus improve the catalytic performances. Meanwhile, this technique can couple the atomic, electronic structures with electrocatalysis mechanisms for water splitting. Compared to traditional chemical treatment, the in-situ electrochemical activation techniques have superior advantages such as facile operation, mild environment, variable control, high efficiency and flexibility. This review may provide guidance for improving water electrolysis efficiencies and hold promising for application in many other energy-conversion fields such as supercapacitors, fuel cells and batteries.

Potential-Cycling Synthesis of Single Platinum Atoms for Efficient Hydrogen Evolution in Neutral Media

Single-atom catalysts (SACs) have exhibited high activities for the hydrogen evolution reaction (HER) electrocatalysis in acidic or alkaline media, when they are used with binders on cathodes. However, to date, no SACs have been reported for the HER electrocatalysis in neutral media. We demonstrate a potential-cycling method to synthesize a catalyst comprising single Pt atoms on CoP-based nanotube arrays supported by a Ni foam, termed PtSA-NT-NF. This binder-free catalyst is centimeter-scale and scalable. It is directly used as HER cathodes, whose performances at low and high current densities in phosphate buffer solutions (pH 7.2) are comparable to and better than, respectively, those of commercial Pt/C. The Pt mass activity of PtSA-NT-NF is 4 times of that of Pt/C, and its electrocatalytic stability is also better than that of Pt/C. This work provides a large-scale production strategy for binder-free Pt SAC electrodes for efficient HER in neutral media.

Environmentally Friendly Carbon-Preserving Recovery of Noble Metals From Supported Fuel Cell Catalysts

The dissolution of noble-metal catalysts under mild and carbon-preserving conditions offers the possibility of in situ regeneration of the catalyst nanoparticles in fuel cells or other applications. Here, we report on the complete dissolution of the fuel cell catalyst, platinum nanoparticles, under very mild conditions at room temperature in 0.1 M HClO4 and 0.1 M HCl by electrochemical potential cycling between 0.5-1.1 V at a scan rate of 50 mV s(-1) . Dissolution rates as high as 22.5 μg cm(-2) per cycle were achieved, which ensured a relatively short dissolution timescale of 3-5 h for a Pt loading of 0.35 mg cm(-2) on carbon. The influence of chloride ions and oxygen in the electrolyte on the dissolution was investigated, and a dissolution mechanism is proposed on the basis of the experimental observations and available literature results. During the dissolution process, the corrosion of the carbon support was minimal, as observed by X-ray photoelectron spectroscopy (XPS).

Dissolution of Platinum in the Operational Range of Fuel Cells

One of the most important practical issues in low-temperature fuel-cell catalyst degradation is platinum dissolution. According to the literature, it initiates at 0.6-0.9 V_RHE, whereas previous time- and potential-resolved inductively coupled plasma mass spectrometry (ICP-MS) experiments, however, revealed dissolution onset at only 1.05 V_RHE. In this manuscript, the apparent discrepancy is addressed by investigating bulk and nanoparticulated catalysts. It is shown that, given enough time for accumulation, traces of platinum can be detected at potentials as low as 0.85 V_RHE. At these low potentials, anodic dissolution is the dominant process, whereas, at more positive potentials, more platinum dissolves during the oxide reduction after accumulation. Interestingly, the potential and time dissolution dependence is similar for both types of electrode. Dissolution processes are discussed with relevance to fuel-cell operation and plausible dissolution mechanisms are considered.

Mass spectrometry in the home and garden

Identification of active components in a variety of chemical products used directly by consumers is described at both trace and bulk levels using mass spectrometry. The combination of external ambient ionization with a portable mass spectrometer capable of tandem mass spectrometry provides high chemical specificity and sensitivity as well as allowing on-site monitoring. These experiments were done using a custom-built portable ion trap mass spectrometer in combination with the ambient ionization methods of paper spray, leaf spray, and low temperature plasma ionization. Bactericides, garden chemicals, air fresheners, and other products were examined. Herbicide applied to suburban lawns was detected in situ on single leaves 5 d after application.