Catalyst Dissolution Research Articles

Unlocking OER potential: Tailoring layered double perovskites through self-reconstruction

The sluggish rate of the anode oxygen evolution reaction (OER) is a major bottleneck for green hydrogen production, making a room-temperature alkaline water electrolyser critical to unlocking the full potential of this technology. Using a perovskite oxide as an electrochemical matrix and forming a self-assembled oxyhydroxide (OOH) layer on the surface via a self-reconstruction activation process (SRAP) is a promising strategy to enhance OER catalytic activity. However, the mechanistic details and long-term impact of SRAP, especially its relationship to catalytic activity, oxygen collection efficiency, and catalyst dissolution, are not fully understood. To address this knowledge gap, we used layered double perovskite (LDP) PrBa0·75Ca0·25Co1·5Fe0·5O5+δ(PBCCF) and PrBa0·75Ca0·25Co1·5Fe0·4Ni0·1O5+δ (PBCCFN) as model electrochemical catalysts and then triggered SRAP. Subsequently, chronoamperometry (CA), rotating ring disk electrode (RRDE) and in-situ electrochemical quartz crystal microbalance (EQCM) were used to study the effect of the Ni-doping on PBCCF SRAP in terms of catalytic activity, oxygen collection efficiency and catalyst stability. The results showed that the OER catalytic current of the LDP PBCCF and PBCCFN was enhanced via SRAP and the oxygen collection efficiency can be maintained, suggesting that the increase in catalytic current is attributed to OER catalytic activity increase rather than electrochemical catalyst dissolution. Additionally, the effect of the catalytic activity enhancement was more significant in PBCCFN. More importantly, the stability of the LDP PBCCF and PBCCFN after SRAP are higher than simple perovskite Ba0·5Sr0·5Co0·8Fe0·2O3-δ (BSCF) because of the increase in the redeposition rate. The observations suggest that using LDP PBCCF and PBCCFN as matrices followed by triggering SRAP can provide valuable insights for designing LDP based OER electrocatalysts.

Effect of CO2 deficiency on the performance of membrane electrode CO2 electrolyzer

To mitigate greenhouse effects, carbon dioxide reduction reaction (CO2RR) has been used as an efficient means of carbon reduction. In CO2 electrolyzer, CO2 deficiency can happen and degrade the reaction efficiency. Herein, an efficient and long-lived formic acid three-cell electrolyzer is used to study the effect of CO2 deficiency, by operating the electrolyzer from full CO2 supply to CO2 deficiency. In addition, the effects of various CO2 fluxes and concentrations on the electrolyzer current, acid concentration and lifetime are investigated. This study quantitatively reveals the impact of CO2 deficiency on current density, product selectivity, and electrolyzer lifetime. The findings indicate that current density decreases by 12.74 %, while CO2 conversion efficiency drops by 92.5 %, demonstrating a significant reduction in the reactivity of CO2 conversion to formate ions. Conversely, the hydrogen evolution reaction is enhanced. Prolonged CO2 deficiency (below 13 ml/min) can also lead to catalyst degradation, including separation and dissolution within the cathode catalyst layer, ultimately diminishing overall performance. Compared with the CO2 flux, the CO2 concentration exerts a more pronounced influence. To ensure the electrolysis efficiency, the carbon dioxide concentration should not be less than 80 %.

Degradation and Protection Mechanisms of Platinum-Based Catalysts in Hydrogen-Bromine Flow Batteries

Cost-effective and large-scale electrical energy storage is pivotal in facilitating the assimilation of renewable energy sources like wind and solar, as well as enhancing the resilience of the future electricity grid. Hydrogen-Bromine flow battery (HBFB) technology is a candidate for a low cost and flexible system. Bromine is abundantly available on Earth. In addition, the reaction between bromine and bromide is characterized by rapid kinetic properties, thus no platinum group metals (PGMs) are required on the bromine side of the cell as a catalyst to sustain a high current density of >500 mA/cm2. Moreover, hydrogen-based flow batteries can be coupled with hydrogen pipelines & electrolyzers to boost the integration of new energy systems.However, the selection of bromine brings technical challenges that can limit the commercial deployment of such HBFB systems. We have proven a lifetime of more than 8,000 hours of continuous cycling at the lab scale. During this proof-of-concept validation, we found that bromine species (liquid) crosses through a dense proton exchange membrane to the hydrogen side of the cell. On this side, a Platinum-based catalyst layer is present to promote hydrogen oxidation-evolution reactions (HOR/HER). The crossover liquid interacts first with the catalyst layer before it crosses through the gas diffusion layer and exits the membrane electrode assembly (MEA). Chemical interactions between the crossover liquid and the catalyst can lead to catalyst migration, Ostwald ripening, or even permanent catalyst dissolution. This is a concern and must be addressed. The root of this concern is that the Platinum-based catalyst is highly reactive (high surface-active area respective to the loading, m2/g), and also the crossover bromine species are likely corrosive (highly acidic, pH <0, and contain polybromides). Therefore, chemical interaction cannot be fully avoided. Nevertheless, the cathodic protection reduces this interaction, which prolongs the lifetime of the electrochemical system. The cathodic protection consists of an applied positive potential and the presence of hydrogen gas on the catalyst nanoparticles. The catalyst degradation together with cathodic protection needs to be better understood and quantified to gain the confidence that the HBFB system fundamentally can last for more than 10 years of continuous operation. This confidence is essential to deliver a minimum viable product (MVP) of HBFB, e.g. on a 100 kW – 1 MW scale, to the market.To address the aforementioned scientific challenges, this work studies the liquid crossover phenomena and the interaction between the crossover liquid and platinum-based catalyst in depth. Liquid crossover rate, concentration, and bromine speciation from working electrochemical cells are quantified at different states of charge (SoCs) and different modes (charge or discharge). This information is used to set up further ex-situ experiments to understand and determine the chemical interaction between crossover liquid and catalyst. Electrochemical ex-situ tests for Pt dissolution measurement such as determination of the electrochemically active surface area (ECSA), are performed before and after soaking hydrogen electrodes in different concentrations of hydrogen bromide solutions that mimic the crossover liquid. These results are then combined with in-situ tests, where the catalyst dissolution rate is quantified in actual crossover liquid and electrolyte by the inductively coupled plasma (ICP) method. Finally, post-mortem analyses on the catalyst layer based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. will give quantitative and qualitative insights into the impact of cycling and liquid crossover on the catalyst performance, morphology, crystal structure, and chemical state of the catalyst.The goal of this work is to quantify the extrapolated lifetime of HBFB systems within a high confidence level. Then conclude if this technology is fundamentally sound and can be scaled up to an MVP to be deployed to the market. As a low cost energy storage solution, the HBFB’s lifetime should be at least 5 years and preferably more than 10 years to attract commercial attention. Moreover, the collective knowledge gained in this work about the catalyst degradation mechanisms will spark further ideas and guide the next generation of the system and its operating conditions to reduce the degradation rate. These presented ideas may also shine a light on catalyst degradation and protection mechanisms in other electrochemical applications that operate in corrosive conditions.Figure 1 Membrane electrode assembly architecture, incl. electrochemical reactions and species transport phenomena in the charge and discharge modes. Figure 1

Development of an Onboard Urea Electrolyser to Reduce NOx Emissions in Vehicles By Enhancing the Selective Catalytic Reduction with the Produced H2 and NH3

In the persistent pursuit of reducing nitrogen oxide emissions from diesel exhaust, challenges arise in achieving optimal performance at lower temperatures, especially in urban driving conditions. This research focuses on a pragmatic approach – development of an onboard urea electrolyser using diesel exhaust fluid (DEF) as the hydrogen-rich carrier. The primary goal is to enhance the NOx reduction efficiency at temperatures below 200oC by providing the electrolyser products – hydrogen and ammonia [1]. The reason behind this innovation lies in recognising DEF as a viable electrolyte, offering both safety and a promise of performance increase due to splitting of urea instead of water. Beyond the contribution to NOx reduction, this approach aligns with the broader societal and political imperatives of integrating cleaner energy solutions within the automotive sector. However, the development of a functional urea electrolyser involves certain complexities. The key considerations are the material stability under operating conditions, anode catalyst dissolution in DEF, mass transport limitations, and the reduction of relatively high cathode overpotentials. A notable aspect of our research involves addressing these challenges in scenarios both with and without potassium hydroxide alongside DEF, requiring a nuanced understanding of the electrochemical processes involved. Additionally, weight, packaging, and system integration play an important role in the development of an onboard urea electrolyser.This study systematically explores these challenges to devise a comprehensive understanding of the underlying electrochemical processes involved as well as the integration and operation strategy. The research methodology adopts single cell tests for which activation and measurement protocols have been established to achieve representative and comparable performance curve recordings and electrochemical impedance spectroscopy (EIS) data. Additionally, to various operating conditions of the electrolyte, the cell components screening adds to a considerable effort of this study. We explore the influence on the materials and design of the bipolar plates (BP) and porous transport layers (PTL); electrode performance and durability as well as the membrane selection. Within this study we consider anion exchange membranes (AEM) and bipolar membranes (BPM) for the performance comparison. Adopting both catalyst-coated membrane (CCM) and catalyst-coated substrate (CCS) approaches enables for examination of electrode preparation techniques and their influence on performance of the electrolysis cell. The initial results are promising, especially when employing 1 M KOH in the DEF electrolyte at 70oC, reaching 0.85 A cm-1 at 2 V. The respective performance curve is presented in Figure 1., together with a diagram of the electrolyser system integration into an existing Diesel exhaust line.This research represents a practical stride towards realising the denitrification of transportation sector especially in city cycle driving when the exhaust gases temperature is too low for the state-of-the-art SCR systems. By addressing identified challenges and optimising key parameters, our study lays a robust foundation for the scalable implementation of a functional urea electrolyser for transport applications and a basic understanding of the electrochemical processes behind the performance and stability of such a system.Figure 1.Depicts a recorded polarisation curve of a single cell using an anion exchange membrane (AEM) and nickel catalyst anode fed DEF and 1 M KOH as an electrolyte at 70oC. Below, the suggested integration of the electrolyser into latest generation Diesel exhaust aftertreatment system. It includes diesel oxidation catalyst (DOC), selective Diesel particulate filter (sDPF), hydrogen supported denitrification catalyst ‘H2-deNOx’ and finally a selective catalytic reduction (SCR) step.Reference: Esser, E., Kureti, S., Heckemüller, L., Todt, A. et al., "Low-Temperature NOx Reduction by H2 in Diesel Engine Exhaust," SAE Int. J. Adv. & Curr. Prac. in Mobility 4(5):1828-1845, 2022, https://doi.org/10.4271/2022-01-0538. Figure 1

Electrocatalytic Selenate Transformation to Tunable Products Using Affordable Catalysts

Anthropogenic selenium (Se) pollution has profound ecosystem and human health impacts. Compared to selenite or Se(IV), selenate or Se(VI) is chemically inert and more challenging for selective removal from the complex aquatic environment.1 Current Se(VI) removal efforts seek intensive resource and energy input to drive Se(VI) transformation, and are subject to environmental and operating conditions.2 The potential solution for sustainable Se(VI) transformation involves catalytic reduction of Se(VI). In current catalytic approaches, the reduction of Se(VI) is achieved using either abiotic photocatalysts (e.g., TiO2) or biotic enzymes (e.g., nitrate reductase).3 Here, we explored the opportunities of electrified Se(VI) transformation with functionalized catalysts to manage aquatic selenate pollution. Se(VI) can be removed through phase transformation into solid Se(0) that could cover reactive surfaces and prevent further catalysis, or gaseous Se(-II) that is toxic and needs to be captured. Alternatively, the reduction of Se(VI) into reactive Se(IV) allows exposure of active interfaces for continuous electrochemcial transformation. Therefore, a controlled reduction of Se(VI) to Se(IV) would be ideal in aquatic Se(VI) management and potential Se recovery. To the best of our knowledge, controlled Se(VI) transformation has not been fully achieved in the literature. In this study, we evaluated five affordable catalysts on graphite cathodes, including TiO2,3 underpotentially deposited Cu (UPD Cu),4 underpotentially deposited Cd (UPD Cd), Co, and CuFe nanoarrays.5 Among these five candidates, characteristic Se(VI) reduction peaks were identified for TiO2, UPD Cu, and UPD Cd through cyclic voltammetry. Less than 5% of 1-mM Se was removed using UPD Cu and UPD Cd, based on 24-hour chronoamperometry tests at corresponding potentials. Unfortunately, ppm-level Cu or Cd were detected in the treated effluent. In contrast, more than 90% of Se(VI) was removed using TiO2 as the catalyst after 24 hours. TiO2 was more robust and stable in electrocatalytic reactions with minimal catalyst dissolution and, therefore, was selected for further electrocatalytic Se(VI) reduction studies.During the TiO2-catalyzed reduction, Se(VI) would first transform into Se(IV) at a relatively positive potential. A maximum of 97% Se(VI) was found to be converted to Se(IV) in a 6-hour electrocatalytic Se(VI) reduction at -0.18 V. Decreasing electrode potential to -0.35 V, solid deposits of Se(0) appeared on a TiO2-modified graphite cathode. With a more negative electrode potential, further Se(0) reduction to Se(-II) was identified. The findings spotlight the opportunities to control Se(VI) transformation and harvest tunable products per removal demand and strategies. The effect of operational conditions was investigated at various TiO2 loading and solution pH. Higher TiO2 loading and lower solution pH favored the electrocatalytic Se(VI) transformation. The removal kinetics and performance were then studied under optimal operational conditions at -0.45 V, where Se(0) was dominant among Se(VI) reduction products, and -0.70 V, where Se(-II) was abundant. Kinetics analysis revealed that electrocatalytic Se(VI) reduction at -0.70 V was roughly four times faster as compared to that at -0.45 V. A total of 88.7 ± 2.3% of Se(VI) was removed after 24 hours of electrocatalysis at -0.70 V. In contrast, 48.2 ± 9.3% of Se(VI) was removed after 24 hours of electrocatalysis at -0.45 V. These results warrant further evaluation of the regenerability and lifespan of TiO2-modified graphite electrodes, an optimized electrode/reactor design, and an integrated Se removal/recovery system.

PGM-Free Oxygen Evolution Reaction Catalyst for PEM Water Electrolysis – a Mechanistic Study on Activity and Durability for Practical Applications

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential. Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve add a significant cost barrier to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost.For PEMWE application, a PGM-free anodic catalyst must be highly active to match the performance of Ir. Furthermore, it must be stable against oxidative acid corrosion. At Argonne National Laboratory, we recently developed a new PGM-free OER of a nanofibrous cobalt spinel catalyst co-doped with lanthanum and manganese (LMCF). [1] The catalyst was synthesized from the precursor of cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. The Co-ZIF was mixed with a polymer solution and electrospun into nanofiber before being converted to fibrous cobalt spinel under low temperature oxidation to remove all the organics and carbon. The new PGM-free catalyst demonstrated a low overpotential of 353 mV (RHE) at 10 mA/cm2 and a low OER degradation over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at its anode at ~2.0 mg/cm2 demonstrated a current density of 2000 mA/cm2 at 2.47 volts (Nafion® 115 membrane) or 4000 mA/cm2 at 3.00 volt (Nafion® 212 membrane). The membrane electrode was also subjected to accelerated-stress-test (AST) in both voltage cycling and galvanostatic run at different current densities. Low degradation rates under both ASTs were observed.In addition to catalyst design and synthesis, we also conducted extensive structural characterizations, stability measurement, as well as computational modeling. Our in situ study found that the OER process involves significant lattice oxygen movement, leading to lowering instead of raising the cobalt oxidation state during the electrolysis. The measurement of the stability number, combined with computational Pourbaix diagram, elucidated the importance of the operating potential in reducing catalyst dissolution. These findings offer in-depth understanding and direction of future development for PGM-free OER catalyst in PEM water electrolyzer. Acknowledgement: This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewal Energy, Hydrogen and Fuel Cell Technologies Office, and by Laboratory Directed Research and Development funding of Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DEAC02-06CH11357. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.[1] “La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis” Lina Chong, Guoping Gao, Jianguo Wen, Haixia Li, Haiping Xu, Zach Green, Joshua D. Sugar, A. Jeremy Kropf, Wenqian Xu, Xiao-Min Lin, Hui Xu, Lin-Wang Wang, Di-Jia Liu, Science 380, 609–616 (2023)

A Cu doped RuO2 catalyst for efficient and durable acidic water oxidation

AbstractProducing hydrogen by water electrolysis in acidic system is essential for advancing the proton‐exchange membrane water electrolyzer (PEMWE) and maintaining the global sustainability. However, excavating highly active and durable catalysts for anodic oxygen evolution reaction (OER) in acid medium remains a great challenge due to the sluggish kinetics of OER and the severe catalyst dissolution. In this work, we developed a Cu doped RuO2 catalyst (Cu‐RuO2@CP) through simple drop casting and low temperature pyrolysis procedures. The incorporation of Cu into RuO2 executes complicated responsibilities: improving OER activity by raising more oxygen vacancies and accelerating the charge transfer between solid‐liquid interface; strengthening the long‐term durability via suppressing the over‐oxidation of RuO2 into soluble RuO4; lowering the catalyst cost by replacing with less expensive Cu. Hence, the Cu‐RuO2@CP exhibits excellent activity and stability in 0.5 M H2SO4 electrolyte, requiring a low overpotential of 200 mV to achieve 10 mA cm−2 and preserving stability for almost 500 h. Moreover, the overpotential is as low as 397 mV even when operating under an industrial‐level current density of 1000 mA cm−2, paving a new way to develop high performance OER electrocatalyst in acidic environment to promote the large‐scale utilization of PEMWE.

Efficient selenate removal from impaired waters with TiO2-assisted electrocatalysis

Aquatic selenium (Se) oxyanions have profound ecosystem and human health impacts, necessitating their conversion and immobilization into elemental Se(0) to mitigate the aquatic Se pollution. While thermodynamically favorable, this transformation encounters kinetic limitations, especially for selenate (SeO42−) or Se(VI). To lower the activation barrier, we investigated the electrocatalytic Se(VI) transformation using five affordable catalysts on graphite cathodes, including TiO2, underpotentially deposited Cu (UPD Cu), underpotentially deposited Cd (UPD Cd), Co, and CuFe. Among these five catalysts, we identified characteristic Se(VI) reduction peaks for TiO2 through cyclic voltammetry. Other catalysts removed less than 5% of 1-mM Se(VI) in 24-h chronoamperometry tests while leaching ppm-level metal cations in the treated water. In contrast, TiO2 as the electrocatalyst could remove more than 80% of 1-mM Se(VI) with negligible catalyst dissolution. Mechanistic investigations revealed a six-electron Se(VI)/Se(0) reduction pathway at -0.30 V (vs. Ag/AgCl), resulting in red Se(0) deposits on the TiO2-coated graphite cathode. Further potential decrease to more negative than -0.45 V led to Se(-II) formation, triggering cathodic Se(0) dissolution and surface regeneration. Electrochemical impedance spectroscopy indicated that Se(VI) reduction was optimal with a moderate TiO2 loading of 0.55 mg cm−2 and acidic environments (pH=1.0∼2.5), achieving an optimized removal of 88.7 ± 2.3% under -0.70 V and an energy input of 3.6 kWh kg−1 Se. These findings lay the foundation for efficient selenate removal from impaired waters. Future efforts should evaluate catalyst performance over time and refine electrode and reactor designs to improve efficiency.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites.

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Towards improved online dissolution evaluation of Pt-alloy PEMFC electrocatalysts via electrochemical flow cell - ICP-MS setup upgrades

Electrochemical flow cell coupled with an inductively coupled plasma mass spectrometer (EFC-ICP-MS) is a powerful electroanalytical technique to monitor in-situ dissolution of metallic electrocatalysts and to understand mechanism of degradation under operating conditions. Its utilisation has witnessed a notable increase in the electrocatalyst field in the last decade where it has been extensively used to study the stability of platinum group metals (PGMs) under oxygen reduction and oxygen evolution reaction conditions. Online ICP-MS has allowed the scientific and industrial community to optimise the activity and stability of PGMs thanks to a better understanding of the complex metal corrosion processes. Among the different setups, the electrochemical flow cell design is the most common as it is based on a commercially available design. Nonetheless, besides different materials and different electrochemical protocols, the impact of the geometry and various parameters of the setup on the recorded dissolution signal has not been studied until now. Such parameters can influence the results obtained with an EFC-ICP-MS and thus the interpretation of the dissolution mechanism and/or stability assessment. Hereby, we demonstrate that the length of the tubing between the outlet of the cell and the inlet of the ICP-MS impacts the resolution of the PtCo catalyst dissolution peaks. This, in turn, facilitates studies where the detection of extremely low concentrations is necessary, such as under a very narrow potential window. Similarly, a reduced internal volume of the cell restricts Pt redeposition, contributing to a more precise evaluation of stability. These claims were supported by dynamic continuum mechanics modelling of the ion concentration in a model EFC. Finally, we provide guidelines and advice to properly measure dissolution with an electrochemical cell coupled with ICP-MS.

Simulated Start-Stop and the Impact of Catalyst Layer Redox on Degradation and Performance Loss in Low-Temperature Electrolysis

Stress tests are developed that focus on anode catalyst layer degradation in proton exchange membrane electrolysis due to simulated start-stop operation. Ex situ testing indicates that repeated redox cycling accelerates catalyst dissolution, due to near-surface reduction and the higher dissolution kinetics of metals when cycling to high potentials. Similar results occur in situ, where a large decrease in cell kinetics (>70%) is found along with iridium migrating from the anode catalyst layer into the membrane. Additional processes are observed, however, including changes in iridium oxidation, the formation of thinner and denser catalyst layers, and platinum migration from the transport layer. Increased interfacial weakening is also found, adding to both ohmic and kinetic loss by adding contact resistances and isolating portions of the catalyst layer. Repeated shutoffs of the water flow further accelerate performance loss and increase the frequency of tearing and delamination at interfaces and within catalyst layers. These tests were applied to several commercial catalysts, where higher loss rates were observed for catalysts that contained ruthenium or high metal content. These results demonstrate the need to understand how operational stops occur, to identify how loss mechanisms are accelerated, and to develop strategies to limit performance loss.

Early Failure Detection for CO2 Reduction Electrolyzers Via in-Line Electrochemical Impedance Spectroscopy

The electrochemical CO2 reduction reaction (CO2RR) presents the opportunity to produce chemical feedstocks and fuels directly from CO2. Recent achievements in energy efficiency and current density are approaching the targets required for commercial viability; however, stability remains more than two orders of magnitude below the required 80,000 h. In CO2RR there is a challenge to distinguish the cause of failure among many possibilities. Failure can result from initial setup (e.g. over or under compression of the membrane and electrodes), gradual degradation of components (e.g. cathode and anode catalyst restructuring and dissolution), accumulation of products or byproducts (e.g. alcohols or salt accumulation), or immediate failures (e.g. a hole in the membrane or a short circuit). These failure modes must be addressed to reach the targeted stability needed for the wide-scale adoption of this technology. Early identification and mitigation of these failures would increase the electrolyzer lifetime and the commercial viability of CO2RR electrolyzers.Current techniques to characterize failures of the electrolyzer rely on post-mortem analysis of cell components. The analysis of individual components after operation may not accurately reflect their state in operando. There are techniques that are capable of analyzing the cell in operation (e.g. XAS, Raman, and TEM), but these typically require modified cell architectures and are operated at lower reaction rates. To effectively diagnose failure, the analysis technique should be applied to electrolyzers operating under realistic conditions.Here we implement an in-line real-time electrochemical impedance spectroscopy (EIS) technique to monitor CO2RR electrolyzers during operation. EIS is a non-destructive technique that can provide continuous information on the performance of specific components within the electrolyzer. We characterize common failure modes, such as poor compression, salt formation, catalyst degradation, and short circuits, and their identifying changes to the EIS response. We extract key electrochemical parameters from the EIS response and use them to develop a framework to identify and prevent the most common failure modes. Among other applications, this framework allowed for the detection of anode catalyst degradation 11 h before other indicators, such as cell voltage or product selectivity. This technique and framework can be applied to monitor CO2 electrolyzers as they are scaled and stacked.

(Invited) The Stability of Nanomaterials in Electrochemical Energy Conversion, Illustrated on Selected Examples

Energy is the most valuable resource for human activity and the foundation for all human progress. Thereby, electrochemical processes are considered a crucial technology for our future society's energy landscape. Catalysts are fundamental components of these processes, and the ability to control their structure and composition during synthesis is essential for obtaining high-performance and stable catalysts while preserving critical resources. The stability of materials that can endure reaction conditions over extended periods is of immense significance. In nanoparticle systems, typical deactivation mechanisms include catalyst agglomeration, dissolution, coalescence, Ostwald ripening, support corrosion, or poisoning.[1] In this conference contribution, I will discuss the prerequisites for stable electrocatalysts using selected examples. I will demonstrate how the catalyst's environment and support material can prevent degradation based on our recent observations.[2-5][1] Y. Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, E. F. Holby, D. Morgan, Top. Catal., 46, 285-305, 2007, J. C. Meier, I. Katsounaros, C. Galeano, H. J. Bongard, A. A. Topalov, A. Kostka, A. Karschin, F. Schüth, K. J. J. Mayrhofer, Energy & Environmental Science 2012, 5, 9319-9330.[2] D. Göhl, H. Rueß, A. M. Mingers, K. J. J. Mayrhofer, J. M. Schneider, M. Ledendecker, J. Electrochem. Soc., 169, 011502, 2022[3] D. Göhl, A. Garg, P. Paciok, K. J. Mayrhofer, M. Heggen, Y. Shao-Horn, R. E. Dunin-Borkowski, Y. Román-Leshkov, M. Ledendecker, Nat. Mater., 19, 287-291, 2020[4] D. Göhl, H. Rueß, M. Pander, A. R. Zeradjanin, K. J. Mayrhofer, J. M. Schneider, A. Erbe, M. Ledendecker, J. Electrochem. Soc., 167, 021501, 2020[5] M. Ledendecker, S. Krick Calderón, C. Papp, H. P. Steinrück, M. Antonietti, M. Shalom, Angew. Chem. Int. Ed., 54, 12361-12365, 2015

Real-Time Mass Spectrometry for Simultaneous Monitoring of Electrode Dissolution and Reaction Product Distribution during Organic Electrosynthesis

An organic electrosynthesis is an emerging tool for modern chemical manufacturing fitting the environmental and sustainability agenda. It utilizes polarized electrode surfaces as “traceless” oxidants or reductants, whereas its heterogeneous nature and exceptional tunability enable easy access to radical intermediates and novel mechanistic pathways. In the line of further energy input improvement, there is a growing demand for appropriate catalytic materials devoted to organic molecule transformations, for which the activity and selectivity have so far been most studied. Although the investigation of electrodes’ and electrocatalysts’ stability in the field of electrochemical energy conversion is well-established, there is still a lack of attention to the symmetrical studies for organic electrosynthesis. Nevertheless, electrode material losses shorten the reactor’s lifetime and can cause metal cathodic re-depositions, severely altering reaction product selectivity and final product contaminations. Although some reports on the ex-cell metal dissolution analysis are available, no information about its mechanism can be gained in such a way. To highlight the issue numerically, an analytical method to characterize rates and mechanisms of catalyst dissolution within a real-time response-enabled potential-, temperature-, or time-resolved parametrization of the electrocatalyst is highly demanded.Herein we present a system to study organic electrosynthetic protocols under dynamic operations to quantitatively monitor the electrode dissolution aligned with reaction products during organic molecules transformations. The instrument comprises an electrochemical flow cell (EFC) with exchangeable working electrodes, the outlet of which is interfaced with an online quadrupole mass spectrometer (OQMS) for the analysis of extracted by solid membrane gaseous products and an inductively coupled plasma mass spectrometer (ICP-MS) for the detection of dissolved metals. The system is exemplified by a benchmark Kolbe electrolysis over platinum Pt anode, commonly considered a stable and inert electrode material. Based on the literature, methanol has been used as a solvent of choice, while partially neutralized acetic acids with different bases were studied as objects. The methodology shed some light on understanding the role of platinum oxide formation on the electrode surface and can be used to study time- and potential-resolved catalyst stability and the possibility for dissolved transition metal-catalyzed electroorganic transformations.

Nickel-Doped Cobalt Oxyhydroxide Nanowires Coupled Polyaniline Functionalizedcarbon Cloths As Multifunctional Electrocatalysts for Hydrogen/Oxygen Evolutionreactions at All pH Levels

Hydrogen represents a carbon-free and sustainable energy carrier. Electrochemical water splitting via hydrogenevolution reaction (HER) and oxygen evolution reaction (OER) is a green approach for the hydrogen production.Nevertheless, unfavorable thermodynamics and sluggish kinetics limit the water splitting efficiency. While thebenchmark catalysts such as Pt@C and RuO2/C catalysts display remarkable catalytic activity, the inferiordurability remains a major concern. Further, scarcity and high demand result in sharp rise in cost, hindering theirlarge-scale commercialization. Herein we rationally design the synthesis of nickel-doped cobalt oxyhydroxidenanowires grown on PANi functionalized CF (Ni@CoOOH@PANi-CF) as multi-functional electrocatalyst for HERand OER. In this design, the interfacial PANi layers substantially improve the surface functionality of the CFsubstrate and thereby facilitates strong coupling interaction with the Ni@CoOOH catalyst, which influences thecharge transport kinetics across the catalyst/substrate interface and also maintains the structural integrity of theoverall catalyst throughout the cyclic and long-term durability tests. Furthermore, the Ni@CoOOH provideabundant active sites for the dissociation of water, while the PANi-CF facilitates the adsorption of H+ ions fromthe intermediates and promotes its convention to hydrogen molecules, which accelerates the HER effectively.Due to these intriguing features, the Ni@CoOOH@PANi-CF manifested remarkable HER efficiency with lowoverpotential (35, 30, and 100 mV mV at 10 mA cm-2) in acidic, alkaline, and neutral electrolytes. Besides, theNi@CoOOH@PANi-CF demonstrated batter durability even in acidic electrolyte. As a key functionality, PANilayers protected the carbon surface from corrosive reactions and suppressed the active catalyst dissolution,resulting in high tolerance for both HER and OER at all pH conditions.

Origin and Formation Mechanism of Carbon Shell-Encapsulated Metal Nanoparticles for Powerful Fuel Cell Durability

Proton exchange membrane fuel cells (PEMFCs) face technical issues of performance degradation due to catalyst dissolution and agglomeration in real-world operations. To address these challenges, intensive research has been recently conducted to introduce additional structural units on the catalyst surface. Among various concepts for surface modification, carbon shell encapsulation is known to be a promising strategy since the carbon shell can act as a protective layer for metal nanoparticles. As an interesting approach to form carbon shells on catalyst surfaces, the precursor ligand-induced formation is preferred due to its facile synthesis and tunable control over the carbon shell porosity. However, the origin of the carbon source and the carbon shell formation mechanism have not been studied in depth yet. Herein, this study aims to investigate carbon sources through the use of different precursors and the introduction of new methodologies related to the ligand exchange phenomenon. Subsequently, we provide new insights into the carbon shell formation mechanism using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Finally, the thermal stability and electrochemical durability of carbon shells are thoroughly investigated through in situ transmission electron microscopy (in situ TEM) and accelerated durability tests.

Degradation analysis of polymer electrolyte membrane water electrolyzer with different membrane thicknesses

A constant current density of 3.0 A/cm2 was applied to PEMWE assembled with three commercial Nafion membranes (NR212, N115, and N117) for 144 h to investigate the influence of membrane thickness on the cell durability. The results show that N115-cell exhibits an improvement in performance after 144 h by the decrease of ohmic resistance caused by the topology adaptation. The cells with NR212 and N117 showed voltage increases, and the most significant degradation could be observed in the NR212-cell. The highest exposing potential of N117-cell accelerated the passivation of the porous transport layer (PTL) and the catalyst dissolution, consequently increasing ohmic losses and kinetic losses. The remarkable performance losses of the NR212 originated from severe ohmic degradation. This phenomenon was attributed to the pinhole formation on the membrane and the resulting decrease in the electrical conductivity of the iridium catalyst layer due to the reduction of iridium oxide catalyst.

Developing Reversal-Tolerant Anode for PEFCs by Introducing Iridium-Based Layered Structure

On the anode side of the polymer electrolyte fuel cell (PEFC), hydrogen fuel can be blocked from reaching the catalyst layer due to anode water flooding which can occur at high humidity, low current density, and rapid load changing conditions [1-3]. This can cause the cell voltage reversal phenomena where anode potential increases relative to the cathode potential resulting in severe carbon corrosion, Platinum catalyst dissolution and agglomeration, and then causing performance loss and eventually cell failure. Currently, control based strategies such as voltage monitoring or flushing excess water have been suggested to overcome this issue [4,5], but unfortunately this increases the complexity and cost of the overall system. In order to greatly reduce system cost, more passive, material-based based solutions such asdevelopment of reversal tolerant anodes (RTAs) are necessary. RTAs include a water electrolysis catalyst such as Ir, Ru, IrO2, RuO2, etc, that can preferentially promote and sustain water electrolysis reaction during cell reversal in order to delay or even prevent the anode from reaching voltages higher than ~1.7 V, [6] resulting in a passive and hence cost-effective solution for the fuel starvation problem.The standard Pt/C+IrO2 RTA catalyst was first studied to investigate its cell reversal characteristics, and several current shortcomings were identified. We found that the RTA underwent significant carbon-based performance degradation even during the water electrolysis stage, and the oxygen evolution reaction (OER) catalyst, IrO2, was found to be deactivated after every subsequent cell reversal experiments. Furthermore, a comparative study of the standard Pt/C+IrO2 RTA with other structures such as Pt/C+Ir/C and Ir(IrOx)/Pt/C hybrid catalyst revealed a new cause of performance degradationbased on Platinum and Iridium particle alloying. Therefore, it is important to manage both the carbon-based degradation and alloying-based degradation in order to make RTAs feasible, and thus tackle the fuel starvation problem.Therefore introduction of layered structure was introduced in RTA catalyst, as explained in Fig. 1. In this study two types of layered RTA catalysts were explored towards addressing these problems. Layered RTA catalysts, LRC1, was made by spray-printing IrO2 (0.24 mgIrO2/cm2) first and then Pt/C (0.3 mgPt/cm2) on the Nafion 212 membrane as shown in Fig.1 (b). In LRC2 case, Cvulcan layer was spray-printed between IrO2 and Pt/C layers as described in Fig.1 (c). Compared to the standard RTA, LRC1 showed improved OER activity and higher performance after the fuel starvation durability test. The LRC2 structure was able to suppress alloying based degradation almost completely. Our findings contribute greatly to the understanding of the fuel starvation based degradation phenomena, and will help in designing better RTAs with improved performance and durability. Fig. 1 Schematic diagram showing (a) standard mixed RTA: Pt/C+IrO2, (b) LRC1: Pt/C -> IrO2, (c) LRC2: Pt/C -> Cvulcan -> IrO2 References [1] Hong, B. K., Mandal, P., Oh, J.-G. & Litster, S. On the impact of water activity on reversal tolerant fuel cell anode performance and durability. J. Power Sources 328, 280–288 (2016).[2] Knights, S. D., Colbow, K. M., St-Pierre, J. & Wilkinson, D. P. Aging mechanisms and lifetime of PEFC and DMFC. J. power sources 127, 127–134 (2004).[3] O’Rourke, J., Ramani, M. & Arcak, M. In situ detection of anode flooding of a PEM fuel cell. Int. J. Hydrog. Energy 34, 6765–6770 (2009). [4] Kurnia, J. C., Sasmito, A. P. & Shamim, T. Advances in proton exchange membrane fuel cell with dead-end anode operation: A review. Appl. Energy 252, 113416 (2019).[5] Chen, Y.-S., Yang, C.-W. & Lee, J.-Y. Implementation and evaluation for anode purging of a fuel cell based on nitrogen concentration. Appl. Energy 113, 1519–1524 (2014).[6] Ralph, Thomas R., Sarah Hudson, and David P. Wilkinson. "Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes." ECS Transactions1.8 (2006): 67. Figure 1

A Structural Model for Transient Pt Oxidation during Fuel Cell Start-up Using Electrochemical X-Ray Photoelectron Spectroscopy

Potential spikes during the start-up (SU) and shutdown (SD) of fuel cells are a major cause of platinum (Pt) electrocatalyst degradation, which limits the lifetime of the device. The electrochemical oxidation of Pt that occurs on the cathode during the potential spikes plays a key role in this degradation process. However, the composition of the oxide species formed, as well as their role in catalyst dissolution remains unclear. In this study, we employ a special arrangement of XPS (X-ray Photoelectron Spectroscopy), in which the Pt electrocatalyst is covered by graphene, making the in situ examination of the Pt oxidation/reduction under wet conditions possible. We use this assembly to investigate oxidation state changes of Pt within fuel cell relevant potential window. We show that above 1.1 VRHE, a mixed Ptδ+/Pt2+/Pt4+ surface oxide is formed, with an average oxidation state that gradually increases as the potential is increased. By comparing a model based on the XPS data to the oxidation charge measured during potential spikes, we show that our description of Pt oxidation is also valid during the transient conditions of fuel cell SU/SD. This is due to the rapid Pt oxidation kinetics during the pulses. As a result of the irreversibility of Pt oxidation, some remnants of oxidized Pt remain at typical fuel cell operating potentials after a pulse. Figure 1

An Open-Source Continuum Model for Anion-Exchange Membrane Water Electrolysis

Anion-exchange membrane (AEM) electrolysis has the potential to produce green hydrogen at low cost by combining the advantages of conventional alkaline electrolysis and proton-exchange membrane electrolysis. The alkaline environment in AEM electrolysis enables the use of less expensive catalysts such as nickel, whereas the use of a solid polymer electrolyte enables differential pressure operation. Recent advancements in AEM performance and lifetime have spurred interest in AEM electrolysis, but many open research areas remain, such as understanding the impacts of water transport in the membrane and salt content in the electrolyte on cell performance and degradation. Furthermore, integrating electrolyser systems into renewable energy grids necessitates dynamic operation of the electrolyser cell, which introduces additional challenges. Computational modelling of AEM electrolysis is ideally suited to tackle many of these open questions by providing insight into the transport processes and electrochemical reactions occurring in the cell under dynamic conditions.In this work, an open-source, transient continuum modelling framework for anion-exchange membrane (AEM) electrolysis is presented and applied to study electrolyzer cell dynamic performance. The one-dimensional cell model contains coupled equations for multiphase flow in the porous transport layers, a parameterized solution property model for potassium hydroxide electrolytes, and coupled ion and water transport equations to account for water activity gradients within the AEM. The model is validated with experimental results from an AEM electrolyser cell. We find that pH gradients develop within the electrolyte due to the production and consumption of hydroxide, which can lead to voltage losses and cell degradation. The influence of these pH gradients on potential catalyst dissolution mechanisms is explored and discussed. Finally, initial studies of transient operation will be presented.This work has been performed in the frame of the CHANNEL project. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under grant agreement No 875088. This Joint undertaking receives support from the European Union's Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research. Some of this work has been performed within the MODELYS project "Electrolyzer 2030 – Cell and stack designs" financially supported by the Research Council of Norway under project number 326809.