Thin-film Rotating Disk Electrode Research Articles

(Invited) 25 Years of the Thin-Film Rotating Disk Electrode Technique

Twenty-five years ago, in summer 1998, the first paper introducing the thin-film rotating disk electrode (TFRDE) method was published in the Journal of the Electrochemical Society [1]. At this time, we demonstrated that by preparing a thin layer of a supported Pt/Vulcan catalyst mixed with a small amount of Nafion could reliably be coated on top of a glassy carbon disk, electrochemical reactions like the hydrogen oxidation reaction could be measured on an RDE taking advantage of the well-known mass-transport conditions of an RDE. By keeping the layer thickness in the range of 1 micrometer, additional mass-transport resistances induced by the catalyst layer/Nafion film became negligible. That is, all equations valid on an RDE could be applied without modification. Further work on the oxygen reduction reaction by using the TF Rotating Ring-Disk electrode allowed detecting peroxide formation on a supported catalyst [2]. Both publications grounded the path for catalyst development in fuel cell. The method has been used since then as a standard method for catalyst screening and characterization in both academia and industries. In addition, many researchers improved the TFRDE method within their research. The application span of the TFRDE developed from fuel cell research, oxidation of small organic molecules, to work on Li-O2 batteries, water electrolysis, and CO2 reduction.This presentation will offer a journey over the last 25 years and the application of the TFRDE technique in catalyst development and characterization.[1] T.J. Schmidt, H.A. Gasteiger, G.D. Stäb, P. Urban, D.M. Kolb, R.J. Behm Characterization of High-Surface-Area Electrocatalysts Using a Rotating Disk Electrode Configuration J. Electrochem. Soc. 145 (1998) 2354-2358 [2] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm Oxygen Reduction on a High-Surface Area Pt/Vulcan Carbon Catalyst: A Thin-Film Rotating Ring-Disk Study J. Electroanal. Chem. 495 (2001) 134-145 [3] T.J. Schmidt, H.A. Gasteiger Rotating Thin-Film Method for Supported Catalysts, in Handbook of Fuel Cells: Fundamentals, Technology and Applications, Vol. 2 Part 3, W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), John Wiley & Sons, New York, 316-333 (2003)

Spontaneous ORR Activity Drop of Fe-N-C Catalysts upon Atmospheric Storage

Iron-Nitrogen-Carbon (Fe-N-C) catalysts are promising materials to replace the scarce and expensive platinum-group materials in proton exchange membrane fuel cells (PEMFC) cathodes. Fe-N-C catalysts can be prepared by multiple ways [1-5], and each synthesis method can be fine-tuned to produce single Fe atoms coordinated to nitrogen atoms (FeNx), identified as the most active sites toward the oxygen reduction reaction (ORR). However, despite these notable efforts, a rapid drop in ORR activity is noticed within the first hours of operation in thin-film rotating disk electrode (RDE) (liquid electrolyte) or in PEMFC systems (solid electrolyte) [6]. Unravelling the degradation mechanisms at stake and developing strategies to improve the durability of Fe-N-C catalysts has thus become an important objective for researchers worldwide [7].In this work, we provide evidences for spontaneous ageing of Fe-N-C catalysts synthesized using different synthetic approaches, and thus featuring distinct chemical, textural and structural properties. For all materials, a pronounced drop of ORR activity is measured, indicating that the “spontaneous ageing” phenomenon is universal. The drop of ORR mass activity over time may reach as high as 90 % of the initial value, and relates to a drop of the active site density and/or the turnover frequency. These adverse effects can be mitigated using storage under dry and oxygen-free atmosphere.

(Invited) Experimental and Data Analysis Platforms to Better Understand the Corrosion and Discharge of Powdered Zn

Zinc anodes have been a cornerstone of many commercial and pre-commercial battery technologies including primary and secondary alkaline cells, Zn-air batteries and Zn-ion batteries. To maximize the performance of devices at both the lab and commercial scales, moderate surface area (mm-sized) Zn powder is used – either immobilized onto a diffusion electrode or incorporated into a slurry. Unlike nano-sized materials, micron-sized materials are poorly characterized by traditional electrochemical techniques like thin-film rotating disk electrodes, because the roughness introduced by the relatively large particles disrupts the ideal fluid flow near the surface, negating the mathematical description of the electrode behavior. This leads many researchers to try and characterize the Zn behavior in more application-like cells. The downside of doing this is that there is a strong influence of the counter electrode, electrolyte solution and the efficacy of the specific individual in assembling the cells. Therefore, there is a need to develop new platforms that have the ability to determine the electrochemical properties of powdered Zn – most notably achievable capacity and corrosion current – and to develop analytical models to describe their behavior.Over the past several years, our group has been developing new platforms to understand how Zn behaves during discharge and charge, including 1-D cells and cells that mimic the geometry of a primary alkaline battery [1]. However, those platforms have their limitations. In this talk, another new cell, coined the “gravity cell” will be discussed. The gravity cell has a small well where Zn (either as raw powder or in slurry form) is gravity deposited onto an indium foil current collector. Typically, a gel electrolyte is placed on top of the Zn bed, followed by liquid electrolyte, which also contains the counter and reference electrodes. The gravity cell is used to both discharge the Zn powder and to measure its corrosion characteristics. This can be done as a function of mass loading, which introduces limitations into the behavior as the Zn discharges from the Zn/gel interface towards the current collector. This type of discharge, limited by Ohmic resistance, requires new mathematical frameworks to extract the desired parameters. This talk will focus on the design of the cell as well as the derivation of the mathematical framework. Then, data will be shown using several types of Zn powder both in the presence and absence of surfactants that aim to reduce the corrosion rate.

Periodic anti-phase boundaries and crystal superstructures in PtCu3 nanoparticles as fuel cell electrocatalysts

To reduce the cost of the electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, supported metal alloy nanoparticles are commonly used to decrease the amount of the scarce and expensive Pt. Since the structure of the nanoparticles has a crucial influence on the catalytic properties, advanced characterization methods help elucidate these relationships and ultimately enable more rational synthesis. This work focuses on investigating the presence of one particular planar defect, namely periodic anti-phase boundaries in carbon-supported PtCu3 nanoparticles. The studying of this structural phenomenon was approached with several characterization tools. Ex-situ X-ray diffractograms were used in conjunction with computer simulations and Rietveld analyses to reliably determine the modulated unit cell containing anti-phase boundaries, while their temperature-dependent formation and disappearance were observed with high-temperature X-ray powder diffraction. In addition, their presence was also confirmed by electron diffraction and atomically resolved scanning transmission electron microscopy. Furthermore, the average neighborhood of Cu and Pt atoms was confirmed using extended X-ray absorption fine structure. Finally, the electrocatalytic activity for the oxygen reduction reaction for composites with and without anti-phase boundaries was determined using a thin-film rotating disk electrode, and the performance was found to correlate with the degree of alloy ordering but not with the presence of anti-phase boundaries. Overall, this study represents a significant step towards better control over the atomically precise synthesis of advanced functional metallic materials by providing valuable insight into the formation of planar defects in metallic alloy nanoparticles and their impact on the structure-property relationships of electrocatalysts.

Mechanistic Study of Fast Performance Decay of PtCu Alloy-based Catalyst Layers for Polymer Electrolyte Fuel Cells through Electrochemical Impedance Spectroscopy.

In the past, platinum-copper catalysts have proven to be highly active for the oxygen reduction reaction (ORR), but transferring the high activities measured in thin-film rotating disk electrodes (TF-RDEs) to high-performing membrane electrode assemblies (MEAs) has proven difficult due to stability issues during operation. High initial performance can be achieved. However, fast performance decay on a timescale of 24 h is induced by repeated voltage load steps with H2/air supplied. This performance decay is accelerated if high relative humidity (>60% RH) is set for a prolonged time and low voltages are applied during polarization. The reasons and possible solutions for this issue have been investigated by means of electrochemical impedance spectroscopy and distribution of relaxation time analysis (EIS-DRT). The affected electrochemical sub-processes have been identified by comparing the PtCu electrocatalyst with commercial Pt/C benchmark materials in homemade catalyst-coated membranes (CCMs). The proton transport resistance (Rpt) increased by a factor of ~2 compared to the benchmark materials. These results provide important insight into the challenges encountered with the de-alloyed PtCu/KB electrocatalyst during cell break-in and operation. This provides a basis for improvements in the catalysts' design and break-in procedures for the highly attractive PtCu/KB catalyst system.

Reduced graphene oxide as efficient carbon support for Pd-based ethanol oxidation catalysts in alkaline media

The sluggish kinetics of the ethanol oxidation reaction (EOR) and the related development of low-cost, highly active and stable anode catalysts still remains the major challenge in alkaline direct ethanol fuel cells (ADEFCs). In this respect, we synthesized a PdNiBi nanocatalyst on reduced graphene oxide (rGO) via a facile synthesis method. The prepared composite catalyst was physicochemically characterized by SEM, STEM, EDX, ICP-OES and XRD to analyze the morphology, particle distribution and size, elemental composition and structure. The electrochemical activity and stability towards EOR in alkaline media were examined using the thin-film rotating disk electrode technique. The results reveal well-dispersed and strongly anchored nanoparticles on the rGO support, providing abundant active sites. The PdNiBi/rGO presents a higher EOR activity and stability compared to a commercial Pd/C ascribed to a high ECSA and synergistic effects between Pd, Ni and Bi and the rGO material. These findings suggest PdNiBi/rGO as a promising anode catalyst in ADEFC applications.

How Effective Is Graphitization of Biomasses for the Carbon Stability of Pt/C ORR Catalysts?

Catalysts for the oxygen reduction reaction (ORR) in PEM fuel cells are commonly constituted of Pt-based nanoparticles and a carbon support originating from fossil resources. In order to employ a more sustainable carbon support, activated sawdust was chosen in this study. This was firstly steam-activated at 750 °C and then thermally treated at elevated temperatures up to 2800 °C and reducing conditions at 1100 °C. Various physical characterization methods were applied to systematically relate treatment parameters to surface and structural properties of the carbon material. Deposition of small Pt nanoparticles on the biochar-based supports yielded in ORR active catalysts which were analyzed by thin-film rotating disc electrode measurements. The activity and stability towards the ORR of these novel catalysts was compared to a commercial raw oil-based Pt/C and the influence of support modification on the ORR performance was discussed.

Toward the Continuous Production of Multigram Quantities of Highly Uniform Supported Metallic Nanoparticles and Their Application for Synthesis of Superior Intermetallic Pt-Alloy ORR Electrocatalysts.

A fast and facile pulse combustion (PC) method that allows for the continuous production of multigram quantities of high-metal-loaded and highly uniform supported metallic nanoparticles (SMNPs) is presented. Namely, various metal on carbon (M/C) composites have been prepared by using only three feedstock components: water, metal–salt, and the supporting material. The present approach can be elegantly utilized also for numerous other applications in electrocatalysis, heterogeneous catalysis, and sensors. In this study, the PC-prepared M/C composites were used as metal precursors for the Pt NPs deposition using double passivation with the galvanic displacement method (DP method). Lastly, by using thin-film rotating disc electrode (TF-RDE) and gas-diffusion electrode (GDE) methodologies, we show that the synergistic effects of combining PC technology with the DP method enable production of superior intermetallic Pt–M electrocatalysts with an improved oxygen reduction reaction (ORR) performance when compared to a commercial Pt–Co electrocatalyst for proton exchange membrane fuel cells (PEMFCs) application.

Graphene Derivatives As Carbon Support Material in Alkaline Direct Ethanol Fuel Cells

The increasing global demand for energy and the associated rise in CO2 emissions due to the use of fossil fuels require the development and use of renewable energy sources. In this context, fuel cells are identified as one of the key technologies for clean energy production. Nevertheless, there are three critical technical barriers for fuel cells that limit their commercialization: performance (activity), durability (stability) and cost. Alkaline fuel cells, particularly alkaline direct ethanol fuel cells (ADEFCs), have attracted much attention in recent decades because ethanol fuel is easy to handle, store, and transport, has low cost, and is environmentally friendly.ADEFCs are limited by their slow kinetics, especially the electrochemical reaction at the anode. Currently, no catalyst is available that cleaves the carbon-carbon bond of ethanol, resulting in a CO2 efficiency of 100% (C1 pathway). The ethanol oxidation reaction (EOR) in alkaline media is incomplete and predominantly follows the C2 pathway. Ethanol is oxidized by hydroxide ions to acetate as the main oxidation product and 4e- instead of 12e-, CO2 and water are released during the complete oxidation. During EOR, acetate is adsorbed on the catalyst surface as the active species. Nevertheless, various reaction intermediates deactivate the catalyst due to poisoning of the active sites, thus reducing the EOR performance. Palladium is currently the most suitable catalyst to replace platinum in ADEFCs. Palladium precursor salts are also cheaper than platinum salts and Pd/C catalysts show higher EOR activity (ethanol oxidation reaction) than Pt/C under alkaline conditions. This can be attributed to the higher oxophilic properties and the inherent ability of Pd to cleave C-C bonds. This positive effect also results in higher stability and lower susceptibility to poisoning of the active sites of the Pd catalyst during EOR. In addition, many non-precious metal co-catalysts are available for palladium, which further reduce the material cost while improving the catalytic activity, by-product tolerance and stability for alkaline EOR [1,2].The development of new anode catalysts with high activity and stability towards alkaline EOR is essential to improve the performance of direct ethanol fuel cell. Carbon materials, especially N-doped or reduced graphene oxides (rGO), are a promising catalyst support due to their low cost, high electrical conductivity, large surface area, and reasonable stability [3]. The deposition of the metal nanoparticles on the graphene oxide support generates a higher active surface area of the catalysts, which increases the electrocatalytic activity while reducing the cost.In the current studies, palladium-based catalysts are synthesized with different graphene oxide (GO) support materials (N-doped GO and rGO) by the modified instant method. The prepared catalysts were characterized ex-situ by the thin film rotating disk electrode technique using a standard three-electrode setup. To determine the electrochemical active surface area and EOR activity, cyclic voltammetry and EOR measurements were recorded in N2 purged potassium hydroxide and alkaline ethanol solution (1 M), respectively. Chronoamperometry was performed to investigate the stability of the catalysts. The cyclic voltammograms (CV) of the two catalysts compared show minor differences in peak current density, onset potential and by-product tolerance for the alkaline EOR. Further details will be discussed in the poster presentation.The authors acknowledge the financial support by the Austrian Science Fund (FWF): I 3871-N37 and by the Slovenian Research Agency (ARRS) through the Research Funding Programme P1-0175 and Bilateral Research Funding Project N2-0087.[1] B.Cermenek, J.Ranninger, V.Hacker; In Ethanol: Science and Engineering, A.Basile, A.Iulianelli, F.Dalena, N.T.Veziroglu, Eds.; Elsevier Inc.: Amsterdam, 2019, 383-405.[2] B.Cermenek, J.Ranninger, B.Feketeföldi, I.Letofsky-Papst, N.Kienzl, B. Bitschnau, V. Hacker, Nano Research, 2019, 12 (3), 683-693.[3] M. Nosan, M. Löffler, I. Jerman, M. Kolar, I. Katsounaros, and B. Genorio, ACS Appl. Energy Mater. 2021 Figure 1

Nano-Engineering of High Performance Pt-Alloy Intermetallics

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).1 However, while Pt-M electrocatalysts show promise in terms of increased activity for oxygen reduction reaction (ORR) and thus, cost reductions related with a significantly lower use of expensive and rare Pt, many key challenges remain at unlocking their true potential.This work systematically tackles several of these key challenges with a combination of electrocatalysts synthesis and characterization methodologies, namely thin-film rotating disc electrode (TF-RDE), electrochemical flow cell coupled to inductively coupled plasma mass spectrometer (EFC-ICP-MS) as well as the membrane electrode assembly (MEA). For instance, intermetallics as a sub-class of Pt-M electrocatalysts, holds promise at improving their intrinsic stability, however, usually at the sacrifice of the electrochemically active surface area (ECSA). In relation to this, we show a production pathway based on the proprietary double passivation with galvanic displacement (GD) method 2,3 as an intrinsically better methodology for deposition of Pt NPs, combining both the intermetallic structure and very high ECSA in the same electrocatalyst material. This is possible due to the intrinsically better mechanism of Pt NP deposition on carbon substrates. Whereas Pt NP synthesis and deposition is sequential in nature (2. step process) when using conventional deposition methods, in the case of double passivation with GD method Pt NPs crystallize directly out of the carbon support and combining these two crucial steps into a single one (Scheme 1). Secondly, we highlight the decisive importance of the chemical activation (de-alloying) step on the performance of Pt-M electrocatalysts in the MEA, namely at high current densities (HCDs). In addition, we provide the scientific community the necessary tools to properly evaluate their suitability of de-alloyed (chemically activated) Pt-M electrocatalysts using a much simpler and affordable TF-RDE methodology by using the well-known CO-electrooxidation.

Electron-Conductive Three-Dimensional Inorganic Nanocrystals As a New Catalyst Support for PEM Fuel Cell Cathode

State-of-the-art polymer electrolyte membrane fuel cells (PEMFC) cathode catalysts are platinum or platinum alloy nanoparticles loaded on a high surface area carbon support. Although these catalysts have been successfully implemented in passenger fuel-cell electric vehicles (FCEVs) (e.g., the Toyota Mirai), these catalysts are facing many performance and durability challenges that limit their applications in medium-duty and heavy-duty transportation. Among these challenges is the corrosion of the carbon support, which represents a source of efficiency loss (i.e., maximum voltages must be “clipped” to prevent catalyst degradation) and limitation of PEMFC lifetime. Corrosion of the carbon support affects catalyst utilization and the transport properties of the cathode as it can result in loss of electrical connection to the platinum alloy nanoparticles and destruction of electrode porosity.1-4 This talk introduces a new class of PEM fuel cell cathode catalyst supports called electron-conductive three-dimensional inorganic nanocrystals. The highly-conductive nanocrystals are fabricated by a chemical vapor deposition (CVD) assisted route using a home-built vertical plug-flow fused-quartz reactor. The developed synthesis method can produce gram scale amounts and operates in batch mode. Platinum nanoparticles have been introduced to the surface of newly- developed supports using different synthetic routes in order to demonstrate the excellent support durability after being subjected to different synthetic treatments. The ORR activity, catalyst performance durability, and catalyst support durability have been evaluated by means of the thin-film rotating disk electrode (TF-RDE) technique. The platinum content in the prepared catalysts has been measured using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF). The morphology, crystal structure, and spatial distribution of the metal nanoparticles have been characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS), respectively. The electronic structure of the conductive 3-D inorganic nanocrystals has been identified by means of electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA) to be graphene-like with sp2 hybridized bonds.AcknowledgementsThis work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.References F. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7, 1127 (2016).Subbaraman, D. Strmcnik, A. P. Paulikas, V. R. Stamenkovic, and N. M. Markovic, Chemphyschem, 11, 2825 (2010).Cullen, David A., K. C. Neyerlin, Rajesh K. Ahluwalia, Rangachary Mukundan, Karren L. More, Rodney L. Borup, Adam Z. Weber, Deborah J. Myers, and Ahmet Kusoglu. "New roads and challenges for fuel cells in heavy-duty transportation." Nature Energy (2021): 1-13.

X-Ray Absorption Spectroscopy Studies of Ir-Oxide Based Oxygen Evolution Catalysts Revisited

Hydrogen plays a pivotal role in the future decarbonization of the energy sector [1], but its production in a sustainable way is crucial for it to have a positive environmental impact. Due to its high power density, quick start-up and response time as well as pressurized hydrogen delivery, polymer electrolyte membrane water electrolysis (PEMWE) is perfectly suited to facilitate H2 production from renewable electricity [2]. However, at large scale application, the use of scarce and expensive, Ir-based catalysts in PEM water electrolyser anodes limits the proliferation of this technology [3, 4]. For this reason, significant research activities are being conducted to better understand the oxygen evolution reaction (OER) mechanism on benchmark IrO2 based catalysts, as to further improve catalyst design and decrease the efficiency losses related to this reaction.When trying to elucidate a reaction mechanism, the use of operando and/or in situ techniques is often beneficial, and thus in last decades the OER mechanism on Ir-based materials has been extensively investigated through operando / in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [5-10]. Despite enormous efforts in this research area, there is still a lack of consensus regarding the Ir oxidation state under OER conditions and the nature of the OER-active sites. In line with this, in this study we employed operando XAS to investigate the Ir oxidation state in commercial IrO2 (Umicore®) catalyst under OER polarization curve conditions. These XAS measurements were performed both in transmission and fluorescence modes; for the former, electrodes with loadings ≈ 40 fold higher than those used in rotating disk electrode (RDE) measurements were required in order to achieve appropriate XAS data quality. In fluorescence mode, however, thinner electrodes with only four times higher loadings than in RDE could be investigated. Prior to the operando XAS measurement, the utilization of these IrO2 catalyst layers in the operando flow cell (FC) was assessed to ensure that the acquired spectroscopic data was representative of the processes occurring under real conditions. Achieving a good catalyst layer utilization was especially important in the case of the thick electrodes used in transmission XAS measurement, since their resistance is much higher compared to that in thin film RDE measurements. The so obtained results were additionally affected by beam damage of the Nafion ionomer used as the catalyst layer binder. All these factors can further lead to data misinterpretation. Regarding the beam damage, reducing the beam flux helped to prevent this effect and allowed us to obtain reliable results. Additionally, the use of thin catalyst layers in the operando FC and its combination of time-resolved XAS (so called “quickXAS”) in fluorescence mode led to novel insight on the effects of evolved O2 bubbles on the spectroscopic results and subsequent interpretation thereof.In summary, this contribution will provide an overview of common pitfalls and good practices encountered when conducting operando XAS studies of OER-electrocatalysts.

Defect-Rich Α-MnO2 – Revealing the Optimum Mn4+/Mn3+ Cation Defect Density for High Electrocatalytic Activity

The Oxygen Reduction (ORR) and Oxygen Evolution Reaction (OER) are of great importance for a variety of electrochemical energy storage and conversion applications, i.e., fuel cells, rechargeable metal-air batteries, and electrolyzers. To efficiently enable both oxygen reactions, catalysts demonstrating high (bifunctional) electrocatalytic activity are of crucial importance.[1,2] Though precious metal catalysts, i.e., carbon supported platinum, ruthenium, iridium, or alloys thereof, are considered as the state-of-the-art in terms of catalytic activity towards the ORR and OER, respectively, their scarcity and high costs have given rise to extensive research and development in the past decade focusing on lower cost alternatives as well as strategies to improve catalytic activity.[3] Among a variety of catalyst materials, manganese oxides have gained significant interest as potentially suitable alternatives as they are inexpensive, readily available and reasonably catalytically active.[4,5] More important, tunnel-type α-MnO2, which is built of double chains of MnO6 octahedra arranged at its edges and corners, demonstrates electrocatalytic activity rivaling commercial precious-metal catalysts.[6,7] Besides various structural and physicochemical properties identified to be relevant for electrocatalysis, manganese cation defects along with oxygen vacancies created from an alternation of the Mn oxidation state, viz., from Mn4+ to Mn3+, are considered as true catalytically active sites.[8,9] Though a few studies attempted to correlate Mn4+/Mn3+ cation defects and electrocatalytic activity, the optimum defect density still remains unclear.[10–12] The aim of this work is to unravel the true nature and relationship between the electrocatalytic activity and cation defects within α-MnO2. Electrocatalytic activity with respect to the ORR and OER, i.e., half-wave potentials, overpotentials and Tafel slopes, of α-MnO2 prepared from different synthesis routes and conditions as well as commercial manganese oxides are investigated using the Thin-film Rotating Disk Electrode (TF-RDE) technique. Mn4+/Mn3+ cation defects of various manganese oxides were evaluated by means of X-ray photoelectron spectroscopy (XPS). Through a successive variation of synthesis conditions the relationship of Mn4+/Mn3+ cation defect formation on the electrocatalytic activity of α-MnO2 is evaluated and the optimum Mn4+/Mn3+ cation defect density for highest electrocatalytic activity identified.

(Invited) How to Expand the Scope of Cyclic Voltammetry with the Thin-Film Rotating (Ring) Disk Electrode to Investigate Oxygen Reduction Reaction Electrocatalysts

The performance of electrochemical energy conversion and storage (EECS) devices is modulated by a cascade of phenomena that include: (i) the redox processes taking place at the electrodes; (ii) the transport of charge, with a particular reference to the migration of ions through the electrolyte; and (iii) the transport of mass involved in the provision of reactants/removal of products to/from the electrodes. These phenomena are intimately correlated with one another and with all the different features of the functional materials adopted in the fabrication of the specific EECS device. Thus, it is often very complex to unravel the different contributions to overall system performance. In addition, it is not practical to pursue a purely “trial-and-error” approach where all the possible combinations of functional materials are used to fabricate and test full-fledged EECS prototypes, as this would entail too massive an investment in terms of both resources and time.For this reason, there is a great need to implement experimental approaches able to elucidate the details of specific electrochemical features (e.g., reaction kinetics) of the various functional materials used in EECS devices, at the same time minimizing the cloaking effects due to: (i) the formation of complex interfaces with other components of the device; and (ii) spatial and temporal inhomogeneity in the motion of both charged and neutral species through the system. The resulting information is critical in the development of advanced functional materials bestowing to the EECS device a performance and durability beyond the state of the art.Perhaps the most successful and widely adopted of such approaches is cyclic voltammetry conducted with a thin-film rotating disk electrode or ring-disk electrode (CV-TF-R(R)DE) [1]. CV-TF-R(R)DE is typically used to screen quickly the performance and the details of the reaction mechanism of electrocatalysts (ECs) for the oxygen reduction reaction (ORR). The latter process is widely exploited in a broad variety of EECS devices including fuel cells (FCs) and metal-air batteries. The ORR is particularly critical since it often bottlenecks the operation of the entire system. Even though the CV-TF-R(R)DE method is very popular and broadly used in the scientific community, most researchers do not use all its remarkable capabilities.Here it is shown how to implement both innovative and more traditional (but no longer widespread) approaches to exploit CV-TF-R(R)DE to its full potential, with the purpose to study the most relevant electrochemical features of ORR ECs and quickly identify the most promising candidates for application in FCs or other electrochemical devices [2]. Particular emphasis is devoted to analysis techniques meant to study in detail the ORR kinetic features of vastly different ECs, allowing for quantitative performance comparisons at the same high level of accuracy without risking serious distortions due to an excessive impact of corrections during data analysis. It is also shown how to adopt the CV-TF-R(R)DE approach to understand quantitatively whether an ORR EC exhibits improved morphology/mass transport features in comparison with a Pt/C benchmark. In summary, it is elucidated how to expand significantly the scope of CV-TF-R(R)DE studies with respect to the level typically achieved in the state of the art, taking into consideration both kinetic and morphology/mass transport features by means of a simple morphokinetic (MK) correlation map. Acknowledgements The research leading to the results reported in this work has received funding from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells e HELPER” funded by the University of Padova.

Carbon black functionalized by grafting of Azo‐generated‐radicals as electrocatalyst support for the oxygen reduction reaction

AbstractFunctionalization of the carbon support is a promising strategy for lowering platinum loadings at the cathode of proton exchange membrane fuel cells. Here, nitrogen‐containing moieties are introduced directly onto carbon black by a simple and easily tunable grafting method initiated by the thermal decomposition of azo compounds in water. X‐ray photoelectron spectroscopy (XPS) and elemental analysis confirm the successful functionalization. After deposition of Pt nanoparticles onto the carbon supports, the electrocatalysts are characterized in thin film rotating disk electrode (RDE) configuration towards the acidic oxygen reduction reaction (ORR). The grafting does not influence the Pt intrinsic ORR activity when the thin film does not include ionomer. However, the functionalization modifies the ionomer/carbon material interaction, leading to higher apparent ORR activity when the ionomer is included in the thin film. Moreover, accelerated stress testing shows durability enhancement as the result of grafting.

Operando X-ray Absorption Spectroscopic Study on the Influence of Specific Adsorption of the Sulfo Group in the Perfluorosulfonic Acid Ionomer on the Oxygen Reduction Reaction Activity of the Pt/C Catalyst

The influence of specific adsorption of the sulfo group in the perfluorosulfonic acid ionomer, Nafion, on the oxygen reduction reaction (ORR) of a carbon-supported Pt/C catalyst using a thin-film rotating disk electrode was investigated. The relationship between the catalyst activity and coating of the Pt/C catalyst with Nafion was quantitatively evaluated through electrochemical measurements, operando X-ray absorption spectroscopy (XAS), and CO stripping voltammetry. Activity of the Pt/C catalyst decreased with increasing ionomer-to-carbon weight ratios. To date, the effect of specific adsorption on a catalyst has been investigated using CO stripping voltammetry. However, quantitative evaluation of specific adsorption at the ORR potential (0.50–1.0 V) has not been performed yet. We quantitatively evaluated the specific adsorption during the ORR by measuring the 5d orbital vacancy of the Pt/C catalyst using operando XAS. The difference in the electronic structures of Pt in the high potential range, with and without the ionomer, was successfully established.

Improving Electrochemical Activity of Rhombic Dodecahedral Pt3(Ni,X) Nanoparticles Using Transition Metal Additions

Large scale use of polymer electrolyte membrane fuel cell (PEMFC) technology remains in its infancy owing to high costs, performance and durability challenges. Novel platinum-based electrocatalysts have been developed to improve catalytic activity and overall durability in acidic environments. Of note is the synthesis of binary Pt3Ni in a rhombic dodecahedral shape with a Pt skin.[1] These nanoparticles demonstrate very high specific/mass activities and are comparably durable over 10,000 potential cycles. The objective of this work is to further improve on the Pt3Ni rhombic dodecahedral nanoframes using ternary additions Pt3(Ni,X), where X denotes an additional transition metal.Previous work[2] has been able to recreate the Pt3Ni rhombic dodecahedron under different synthesis conditions. These nanoparticles were used in combination with nitrogen-doped carbon support to examine the effects of surface nitrogen as anchoring sites. Examination using XPS, electrochemical CO stripping, and SEIRAS shows that the nitrogen sites alter the d-band center of surface platinum in addition to creating a more homogeneous distribution of the nanoparticles.The addition of a ternary element allows for additional fine tuning of the d-band center of platinum near the particle surface. This leads to an increase in overall activity over binary systems when examining electrocatalyst volcano plots. In addition, ternary elements could lead to a reduction in the rate of nickel leeching over repeated electrochemical cycles.DFT studies[3] examined the adsorption energy of ternary Pt3(M,X) where M is another transition metal different than X. The model arranged the ternary surfaces into intermetallic, Pt skin, and Pt skin with subsurface intermetallic layers. From this work, Rankin recommended the investigation of Pt skin ternary systems using Pd, Rh, Ru, Ir, and Os; as well as a Pt-Ni-Cu system with a subsurface intermetallic layer.Ternary Pt3(Ni,X) nanoparticle systems have been investigated previously but have not been directly synthesized into a rhombic dodecahedral shape. Thus, the scope of this work is to investigate Pt3(Ni,X) systems where X includes Cu, Pd, Rh, Ru, Ir, and Os. Using similar synthesis methods,[2] rhombic dodecahedral nanoparticles will be created with a Pt skin. Examination will follow using Scanning Tunneling Electron Microscopy (STEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), electrochemical CO stripping, and thin film rotating disk electrode (RDE) measurement.Successful candidates will be added with modified carbon supports (nitrogen doping, additions of polybenzimidazole (PBI), modifications of pore structure) with the goal of improving local mass transport at the support surface. Further annealing may also be investigated to create ordered intermetallic layers under the Pt skin which have been shown to improve activity. This work could lead to significant improvements into electrocatalyst activity and durability which are necessary to implement PEMFC technology onto a global scale.

Electrocatalyst Performance at the Gas/Electrolyte Interface under High-Mass-Transport Conditions: Optimization of the "Floating Electrode" Method.

The thin-film rotating disk electrode (TF-RDE) is a well-developed, conventional ex situ electrochemical method that is limited by poor mass transport in the dissolved phase and hence can only measure the kinetic response for Pt-based catalysts in a narrow overpotential range. Thus, the applicability of TF-RDE results in assessing how catalysts perform in fuel cells has been questioned. To address this problem, we use the floating electrode (FE) technique, which can facilitate high-mass transport to a catalyst layer composed of an ultralow loading of catalyst (1-15 μgPt cmgeo-2) at the gas/electrolyte interface. In this paper, the aspects that have critical effects on the performance of the FE system are measured and parametrized. We find that, in order to obtain reproducible results with high performance, the following factors need to be taken into account: system cleanliness, break-in procedure, hydrophobic agent, ionomer type, and the measurements of catalyst surface area and loading. For some of these parameters, we examined a range of different approaches/materials and determined the optimum configuration. We find that the gas permeability of the hydrophobic agent is an important factor for improving the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) performance. We provide evidence that the suppression of the HOR and ORR introduced by the Nafion ionomers is more than a local mass transport barrier but that a mechanism involving the adsorption of the sulfonate on Pt also plays a significant role. The work provides intriguing insights into how to manufacture and optimize electrocatalyst systems that must function at the gas/electrolyte interface.

Hidden in plain sight: unlocking the full potential of cyclic voltammetry with the thin-film rotating (ring) disk electrode studies for the investigation of oxygen reduction reaction electrocatalysts

Cyclic voltammetry conducted with a thin-film rotating disk electrode, or ring-disk electrode (CV-TF-R(R)DE) is a very popular ‘ex situ’ tool for the rapid screening of electrocatalysts for their activity in oxygen reduction reaction. Despite its popularity and broad use, in most instances only a small part of the information that could be accessed by CV-TF-R(R)DE is actually used by scientists in their research. This work outlines both innovative and more traditional (but half-forgotten) ways of using CV-TF-R(R)DE to its optimal or full potential in the ongoing quest to study the most relevant features of oxygen reduction reaction electrocatalysts and quickly identify the most promising candidates for their applications in fuel cells or other electrochemical devices.

Full wave analysis of stripping chronopotentiometry at scanned deposition potential (SSCP): Obtaining binding curves in labile heterogeneous macromolecular systems for any metal-to-ligand ratio

Abstract The different deposition potentials applied in Scanned Stripping ChronoPotentiometry (SSCP) probe different values of the free metal and complex concentrations at the electrode surface. The knowledge of these concentrations gives access to a relevant window of the binding curve of a metal to a homogeneous or heterogeneous ligand. Here, a suitable mathematical treatment for the determination of these surface concentrations, when using a mercury thin film rotating disk electrode, is reported. The proposed procedure does not require ligand excess conditions and takes advantage of the knowledge of the free metal ion concentration in the bulk solution provided by the technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). It is experimentally shown that the information derived from the SSCP points (i.e. at different deposition potentials in a unique solution) is consistent with the speciation results yielded by the technique AGNES in pertinent solutions prepared within a range of metal-to-ligand ratios. Successful examples with polystyrene sulfonate, Laurentian Fulvic acid and a peat humic acid are reported. It is concluded that, by running consecutively SSCP and AGNES in a single solution, speciation information corresponding to different compositions (i.e. those locally generated at the electrode surface by the end of the deposition step) can be retrieved, this enabling an easy determination of the binding curve.