Rotating Disk Electrode Research Articles

Effect of Melamine on the Oxygen Reduction Reaction in the n(111)-(111) Series of Pt3Co

Abstract Enhancement of the oxygen reduction reaction (ORR) activity is an important subject for the development of fuel cells. In this study, we prepared the n(111)-(111) series of Pt3Co single crystal electrodes using an induction heating furnace and investigated the ORR in 0.1 M HClO4 using the rotation disk electrode (RDE). The specific activity of the ORR (j k) increased monotonously with increasing terrace width n on the n(111)-(111) series of Pt3Co, showing the highest activity on Pt3Co(111). The value of j k of Pt3Co(111) was 59-fold higher than that of Pt(111) at 0.95 V(RHE). This trend was different from those of the same series of Pt, Pt3Co, and Pt3Ni reported previously. Furthermore, the ORR activity was improved on all electrodes by adding melamine to the electrolytic solution. Melamine was preferentially adsorbed at the terrace edge. Pt3Co(553) n = 5 showed the highest activity. The values of j k at 0.95 V(RHE) were 23- and 3.3-fold higher than those of Pt(553) n = 5 and Pt3Co(553) n = 5 without melamine modification, respectively. The durability of Pt3Co(553) n = 5 with melamine was improved 8.0-fold. Graphical Abstract

High-Throughput UV-Induced Synthesis and Screening of Alloy Electrocatalysts.

The combination of different elements in alloy catalysts can lead to improved activity as it provides opportunities to tune the electronic structures of surface atoms. However, the synthesis and performance screening of alloy catalysts through a vast chemical space are cost- and labor-intensive. Herein, a UV-induced, high-throughput method is reported for the synthesis and screening of alloy electrocatalysts in a fast and low-cost manner. A platform that integrates 37 mini-reaction-cells enables simultaneous UV-induced photodeposition of alloy nanoparticles with up to 37 compositions. These mini-reaction-cells further allow a transfer-free, high-throughput electrochemical performance screening. Binary (PtPd, PtIr, PdIr), ternary (PtPdIr, PtRuIr) and quaternary (PtPdRuIr) alloys have been synthesized with the activity of the binary alloys (57 compositions) for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) being screened. The predicted high performance of identified alloy compositions are subsequently validated by standard measurements using a rotating disk electrode configuration. It is found that the as-synthesized alloy nanoparticles are rich in twin boundaries and thus possess lattice strain. Density functional theory calculation implies that the high ORR activity of the screened Pt0.75Pd0.25 alloy originates from the interplay between the differentiated adsorption sites because of alloying and the strain-induced modulation of the d-band center.

CATALYST BASED ON PALLADIUM‐MODIFIED MIL‐53(AL) PYROLYSATE FOR ORR IN ALKALINE MEDIA

A catalyst for the oxygen reduction reaction (ORR) based on the metal‐organic framework material MIL‐53(Al) modified with palladium was synthesized. Its textural and morphological characteristics were studied using the method of adsorption porosimetry, SEM, TEM, energy dispersive X‐ray analysis (EDAX) , X‐ray diffraction (XRD), Raman –spectroscopy, X‐ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA‐DSC). The electrocatalytic properties of the synthesized material in ORR were studied by voltammetry using a rotating disk electrode. Corrosion resistance was studied in the CV mode. It was found that the synthesized catalyst is characterized by high corrosion resistance. The tolerance of synthetized catalyst Pyr_MIL‐53(Al)_Pd to the methanol was studied. The obtained catalyst was studied in a membrane electrode assembly (MEA) formed by spraying an ionomer suspension onto a gas diffusion layer (GDL). The synthesized Pyr‐MIL‐53 (Al)_Pd and commercial platinum (60% Pt) (HiSPEC 9100) catalysts were compared in the cathode composition, and 10% PtM (M = Ni, Mo)/CNT catalysts were used on the anode. The power density of the FC (P) was calculated based on the obtained current‐voltage curves. Based on the set of characteristics, the synthesized catalyst based on MIL‐53 (AL) doped with palladium is superior in efficiency to the commercial platinum catalyst.

Optimizing Fe-N-C Electrocatalysts for PEMFCs: Influence of Constituents and Pyrolysis on Properties and Performance

Proton exchange membrane fuel cells (PEMFCs) are promising alternative technologies with applications in stationary power systems, vehicles, and portable electronics due to their low temperature operation, fast start-up, and environmental advantages. However, the high cost of platinum-based catalysts, in particular for the oxygen reduction reaction (ORR) of the cathode side, prevents their widespread incorporation. Fe-N-C electrocatalysts have emerged as viable alternatives to platinum. In this study, different precursor components were investigated for the way that they affect the pyrolysis process, which is crucial for tailoring the final catalyst properties. In particular, carbon allotropes such as carbon Vulcan, Ketjenblack, and carbon nanotubes were selected for their unique structures and properties. In addition, various sources of iron (FeCl2, FeCl3, and K[Fe(SCN)4]) were evaluated. The influence of the pyrolysis atmosphere on the resulting Fe-N-C catalyst structures was also assessed. Through an integrated structure and surface chemistry analyses, as well as electrochemical tests with rotating disk electrode experiments in acidic media, the ORR performance and stability of these catalysts were defined. By examining the relationships between carbon sources and iron precursors, this research provides valuable information for the optimization of Fe-N-C catalysts in fuel cell applications.

Hydrogen Oxidation and Evolution Reaction on Platinum in Alkaline Electrolyte: Fixed Reference vs Overpotential and the Effect of H2 Concentration

Understanding the kinetics of the electrochemical hydrogen oxidation and evolution reaction (HOR/HER) in alkaline media is of great interest, but the underlying reaction mechanism and the dependency on the hydrogen partial pressure are still debated. In this work, we investigated the HOR/HER kinetics on a polycrystalline platinum electrode in alkaline electrolyte using the rotating disk electrode (RDE) technique. Unlike typically done, we rigorously performed data treatment and analysis on a fixed potential scale with a fixed reference potential, which has implications for the use and assessment of RDE mass-transport corrections and the definition of reaction orders. Analyzing the kinetic current curves on a fixed potential scale, we find reaction orders with respect to the hydrogen partial pressure of essentially one and zero for the HOR and HER, respectively, at larger overpotentials. In contrast, fitting the kinetic current curves in a potential range around equilibrium with a generalized Butler-Volmer model yields differing kinetic parameters and fractional reaction orders. Our findings can be explained with a potential-dependent transition in the HOR mechanism from Tafel–Volmer with a rate-determining Volmer step at small overpotentials to Heyrovsky–Volmer with a rate-determining Heyrovsky step at more positive overpotentials, with a concomitant change in the H2 reaction order from fractional to unity, thus reconciling previous propositions in the debate on the alkaline HOR/HER mechanism. Our results demonstrate the strength and advantages of using a fixed potential scale rather than the commonly employed overpotential, and we expect this approach to be beneficial also for kinetic studies of other electrocatalytic reactions.

Glucose oxidation on gold in alkaline solution: A DEMS and microkinetic modeling study

The mechanism and kinetics of the electrochemical oxidation of glucose on gold surfaces in 0.1 M NaOH solution are investigated by rotating disk electrode and differential electrochemical mass spectrometry coupled to microkinetic modeling. For this purpose, the glucose mass-transport parameters (solution viscosity and density, glucose diffusion coefficient) were determined in the relevant conditions. Koutecký–Levich analysis of rotating disk electrode current-potential curves revealed that the glucose oxidation reaction (GOR) follows a one-electron mechanism at 0.6 V vs RHE, while H2 production could be evidenced by DEMS in this potential range. This suggests that the glucose oxidation selectively produces gluconate at 0.6 V vs RHE through an electrochemical oxidative dehydrogenation (EOD) mechanism. A microkinetic model of the current-potential curves was developed, taking into account the hydrogen electrode reactions kinetics, the dissociative adsorption/desorption of glucose, and the glucose electrooxidation on gold surfaces. The dissociative adsorption of glucose was found to be the rate-determining step of the reaction. It is also revealed that the release of anodic H2 through the Tafel step is triggered by the consumption of the adsorbed reaction intermediates. The gluconate electrooxidation into further products has to be considered for potential above 0.7 V vs RHE.

Considerations for digitalisation of nickel electroforming

ABSTRACT This paper is a ‘follow-on’ from a paper previously published in this journal dealing with the laboratory to pilot scaling up approach using Industry 4.0 manufacturing methods. In particular, the paper reports a strategy for developing a model for the electroforming of nickel from a sulphamate electrolyte at laboratory scale which could subsequently provide an educated approach for transferring the process to a larger scale. At the laboratory scale, a rotating disc electrode assembly was used, which is a standard instrument to determine electrochemical parameters. Thereafter, small scale nickel discs were electroplated using this equipment, and a model of this process was developed and validated against those experimental results. These parameters were then used to actually produce electroforms in a prototype, 18 L tank system. Cross-validation between practical experiments and simulations followed which allowed for fine-tuning the model until it was consistently predicting the real process results within an acceptable error. Overall, it was found that a secondary current distribution model could be used for reasonably accurate description for the electroforming process, and could provide a quick virtual tool at a production facility.

Stability of supported Pd-based ethanol oxidation reaction electrocatalysts in alkaline media

This study evaluates the dissolution of the supported electrocatalysts Pd/C, PdSn/C, PdNb/C, and PdFe3O4/C during ethanol oxidation reaction for Alkaline Direct Liquid Fuel Cells (ADLFC) applications. A scanning flow cell (SFC) combined to an inductively coupled plasma mass spectrometry (online ICP-MS) is used to assess the dissolution stability in a broad potential window. Accelerated stress tests with and without ethanol are developed using a rotating disk electrode (RDE) with dissolution products analysis by ex-situ ICP-MS. Potential profiles simulating those experienced by the catalyst during regular fuel cell operation were used. Sn and Fe catalysts demonstrate improved activity and stability compared with the material with Pd alone. For these reasons, PdSn/C and PdFe3O4/C are suitable for ADLFC applications. Severe Nb dissolution destabilizes Pd, increasing its leaching. This work demonstrates that while additional metals and oxides can improve the alcohol oxidation kinetics of Pd, these additives’ dissolution stability must already be considered at the catalyst design stage.

Fluorine-Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship.

It is well-established that Pt-based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine-doped carbon supports, which optimize interfacial microenvironment of Pt-based catalysts and thus significantly enhance their reactive kinetics. The introduction of C-F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine-doped carbon constructs a local super-hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F-doped carbon-supported Pt catalysts exhibit over 2-fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas-involving electrocatalytic reactions.

Fluorine‐Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship

AbstractIt is well‐established that Pt‐based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine‐doped carbon supports, which optimize interfacial microenvironment of Pt‐based catalysts and thus significantly enhance their reactive kinetics. The introduction of C−F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine‐doped carbon constructs a local super‐hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F‐doped carbon‐supported Pt catalysts exhibit over 2‐fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas‐involving electrocatalytic reactions.

Electrodeposition and Optimisation of Amorphous NixSy Catalyst for Hydrogen Evolution Reaction in Alkaline Environment.

Anion exchange membrane (AEM) water electrolysers have shown their potential in green hydrogen production. One of the crucial tasks is to discover novel cost-effective and sustainable electrocatalyst materials. In this study, a low-cost Ni-S-based catalyst for hydrogen evolution reaction was prepared via a simple electrodeposition process from a modified Watts bath recipe. Physical characterisation methods suggest this deposit film to be amorphous. Optimisation of the electrodeposition parameters of the NixSy catalyst was carried out using a rotating disk electrode setup. The optimised catalyst exhibited excellent catalytical performance in 1 M KOH on a microelectrode, with overpotentials of 41 mV, 111 mV and 202 mV at 10, 100 and 1000 mA cm-2 with Tafel slope of 67.9 mV dec-1 recorded at 333 K. Long-term testing of the catalyst demonstrated steady performance over a 24 h period on microelectrode at 100 mA cm-2 with only 71 mV and 37 mV overpotential increase at 293 K and 333 K respectively. Full cell testing with the optimised NixSy as cathode and NiFe(OH)2 as anode showed 1.88 V after 1 h electrolysis at 500 mA cm-2 in 1 M KOH under 333 K with FAA-3-30 membrane.

Toward more practical site density determination in Fe-N-C catalysts for oxygen reduction reaction: NO-stripping at gas diffusion electrode setup

Evaluation of the site density of single-site iron catalysts has always been a challenge due to the complexity of those materials, and the difficulty of finding selective and simple probing methodologies. Since iron coordinating nitrogen (FeN4) is recognized as the main active site for oxygen reduction reaction (ORR), it is fundamental to develop a method to assess the content in catalysts. Several bulk-probing and surface-probing methods have been developed in the last decade each one with its pros and cons. Among others, with certain limitations, NO stripping performed at a rotating disc electrode (RDE) has become a common characterization to evaluate site density and turnover frequency of Fe-N-C due to the relatively simple procedure. At the same time, gaining information on the activity on more practical devices has brought to the spreading of gas diffusion electrodes (GDE) in half-cell configuration to mimic the cathodic compartment of a fuel cell, a leading device for those materials applications. Here we proposed a transposition of NO-stripping from RDE to GDE to evaluate the site density under more practical devices. Several parameters have been considered (i.e., pH, loading, FeN4 selectiveness) and comparisons with RDE have been done to validate the methods. A homemade Fe-N-C catalyst has been used as a benchmark and compared to Pajarito Powder catalyst and others (including metal-free) showing the validity and selectivity of the method.

Achieving Highly Durable Intermetallic PtMn3N Electrocatalyst via the Strong Metal‐N Bonds toward Oxygen Reduction Reaction

AbstractPt‐based intermetallic compounds have been considered promising electrocatalysts in the practical applications of fuel cells; however, the development of Pt‐based catalysts that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. Herein, an atomically ordered and low‐Pt intermetallic nitride (PtMn3N) catalyst are synthesized consisting of a strained Pt shell and PtMn3N core on carbon support via the KCl‐matrix protection strategy. The PtMn3N catalyst represents a high mass activity of 0.70 A mgPt−1 at 0.9 V and a specific activity degradation of 4.2% after 5000 potential cycles for the oxygen reduction reaction (ORR) in rotating disk electrode (RDE) testing, which substantially outperformed commercial Pt/C (0.25 A mgPt−1 and 17.4%). Density functional theory calculations reveal that the introduction of Mn elements to the Pt lattice is beneficial to produce appropriate compressive strain to weaken the binding energy of oxygen species and the introduction of N elements to promote the strong metal‐N interactions is conducive to alleviating the dissolution of metal atoms, allowing for displaying the prominent durability. This work provides an effective strategy of N‐doped Pt‐based intermetallic compounds to enhance the corrosion resistance of 3d transition metals and to enhance the ORR performance.

Designer Electrocatalysts for the Oxygen Reduction Reaction with Controlled Platinum Nanoparticle Locality

AbstractFor global deployment of proton exchange membrane fuel cells, achieving optimal interaction between the components of the cathode active layer remains challenging. Studies addressing the effect of nanoparticle location (inside vs outside of pores) on performance and durability mostly compare porous and nonporous carbon supports, thus coming short of decoupling nanoparticle locality from carbon support effects. To address the influence of nanoparticle locality on performance and durability, new carbon‐supported electrocatalysts with designed and distinct nanoparticle localities are presented. The developed methodology allows to place Pt nanoparticles preferentially inside or outside of the mesopores of conductive carbon supports from materials under development at Cabot Corporation. Synthesis protocols are tuned to control nanoparticle size, crystallinity, and loading; this way the effect of Pt locality can be studied for two experimental carbon supports in isolation from all other parameters. For one carbon support, Pt active surface area and activity are significantly lower when nanoparticles are placed inside the pores. In contrast, for another, more graphitic carbon support, placing nanoparticles inside or outside of the carbon pores produces no appreciable difference in active surface area and performance rotating disk electrode measurements. Given their carefully tailored structure, these catalysts provide a framework for evaluating locality‐performance‐durability relationships.

Oxygen reduction reaction on Ag nanocatalysts prepared by the microemulsion method

The microemulsion (water-in-oil) method is used to prepare novel silver catalysts onto three different carbon supports (multi-walled carbon nanotubes (MWCNT), mesoporous carbon (MC) and Vulcan carbon (C)). It allows to control the Ag particle growth and produces particles that range between 2 and 3 nm. The Ag-based catalysts are characterised by scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS). The electrocatalytic activity towards the oxygen reduction reaction (ORR) in 0.1 M KOH is studied using the rotating disc electrode method. High-angle annular dark-field (HAADF) STEM images reveal the Ag particle size range between 1.8 and 2.4 nm, and the EDS mapping shows uniform Ag distribution on the substrates used. Ag1/MC (two-step synthesis) and Ag2/MC (one-step synthesis) catalysts prepared onto the MC support show the highest mass activity of 30–37 A g−1. The two-step synthesis method is preferred to prepare Ag/MWCNT and Ag/C catalysts. The mass activity of the Ag1/MWCNT catalyst is more than two times higher than that of Ag2/MWCNT. Ag1/MWCNT shows the highest electrochemical stability after the accelerated durability test, with only a 6 mV shift in half-wave potential. The microemulsion method is very promising for the production of highly stable Ag-based catalysts with Ag particles smaller than 3 nm in size.

Mathematical modeling of hydrogen evolution by $${{{H}}}^{+}$$ and $${{{H}}}_{2}{{O}}$$ reduction at a rotating disk electrode: theoretical and numerical aspects

Mathematical modeling of hydrogen evolution by $${{{H}}}^{+}$$ and $${{{H}}}_{2}{{O}}$$ reduction at a rotating disk electrode: theoretical and numerical aspects

Be aware of the effect of electrode activation and morphology on its performance in gas diffusion electrode setups

The superior performance enhancement in catalyst research for proton exchange membrane fuel cells (PEMFC) revealed in the model rotating disc electrode (RDE) environment is rarely demonstrated in membrane electrode assemblies (MEA). The discrepancy is typically attributed to the difference in the chemical structure and morphology of the catalyst layer (CL), affecting the transport of reactants of the oxygen reduction reaction (ORR). In this study, the gas diffusion electrode (GDE) half-cell setup is used to focus on crucial aspects of CL development, especially on the activation and morphology of CLs, to gain a fundamental understanding of the development of PEMFCs. Adjusting the CL porosity by using different solvent compositions of water and isopropyl alcohol and implementing an activation method for low platinum content catalysts, we focus on understanding the contributions of macro-porosity and hydrophobicity in a liquid electrolyte-based system. We show that macro-porosity significantly influences the O2 mass transport in the CL, demonstrating increased performance with higher porosities. Coupling inductively coupled plasma mass spectrometry (ICP-MS) with the GDE half-cell setup, we propose a method for qualitative estimation of water content in the CL. Lower macro-porosity shows higher platinum dissolution, attributed to improved mass transport of dissolved ions in aqueous media.

Electrochemistry Beyond Solutions: Modeling Particle Self-Crowding of Nanoparticle Suspensions.

Nanoparticle suspensions hold promise to transform functionality of next-generation electrochemical systems including batteries, capacitors, wastewater treatment, and sensors, challenging the limits of existing electrochemical models. Classical solution-based electrochemistry assumes that charge is transported and transferred by point-like carriers. Herein, we examine the electrochemistry of a model aqueous suspension of nondissolvable electroactive nanoparticles over a wide concentration range using a rotating disk electrode. Past a concentration and rotation rate threshold, the electrochemistry deviates from solution theory with a maximum attainable current due to particle "self-crowding" where reacted particles on the electrode surface reduce the area accessible for charge transfer by unreacted particles. The observed response is rationalized with an analytical model considering the physical adsorption/desorption kinetics and interfacial transport of nondissolvable finite-size charge carriers. Experimental validation shows the model to be applicable across a range of electrode sizes and thus suitable for engineering electrochemical systems employing nondissolvable nanoparticle suspensions.

Impact of Nickel on Iridium-Ruthenium Structure and Activity for the Oxygen Evolution Reaction under Acidic Conditions.

Proton exchange membrane water electrolysis (PEMWE) is a promising technology to produce hydrogen directly from renewable electricity sources due to its high power density and potential for dynamic operation. Widespread application of PEMWE is, however, currently limited due to high cost and low efficiency, for which high loading of expensive iridium catalyst and high OER overpotential, respectively, are important reasons. In this study, we synthesize highly dispersed IrRu nanoparticles (NPs) supported on antimony-doped tin oxide (ATO) to maximize catalyst utilization. Furthermore, we study the effect of adding various amounts of Ni to the synthesis, both in terms of catalyst structure and OER activity. Through characterization using various X-ray techniques, we determine that the presence of Ni during synthesis yields significant changes in the structure of the IrRu NPs. With no Ni present, metallic IrRu NPs were synthesized with Ir-like structure, while the presence of Ni leads to the formation of IrRu oxide particles with rutile/hollandite structure. There are also clear indications that the presence of Ni yields smaller particles, which can result in better catalyst dispersion. The effect of these differences on OER activity was also studied through rotating disc electrode measurements. The IrRu-supported catalyst synthesized with Ni exhibited OER activity of up to 360 mA mgPGM -1 at 1.5 V vs RHE. This is ∼7 times higher OER activity than the best-performing IrO x benchmark reported in the literature and more than twice the activity of IrRu-supported catalyst synthesized without Ni. Finally, density functional theory (DFT) calculations were performed to further elucidate the origin of the observed activity enhancement, showing no improvement in intrinsic OER activity for hollandite Ir and Ru compared to the rutile structures. We, therefore, hypothesize that the increased activity measured for the IrRu supported catalyst synthesized with Ni present is instead due to increased electrochemical surface area.

Electrocatalytic oxidation of glycerol performed by nickel/cobalt alloys: Adding value to a common subproduct of chemical industry

The present work shows the electrocatalytical performance of nickel/cobalt alloys towards the conversion of glycerol into higher value molecules, such as glycolic acid, oxalic acid, maleic acid, diethylene glycol and ethylene glycol, identified by chromatographic technique. To do so, the alloy was directly electrodeposited on graphite electrodes, which is a simple, readily available and cheap material, the modified electrodes were characterized by scanning electron microscopy, infrared spectroscopy and electrochemical analysis. Kinetic parameters were obtained using by both Galus methodology and rotating disc electrode (RDE), obtaining the better electrocatalytic modified electrode and the total number of electrons transferred per mol of glycerol. The modified electrode labeled as Ni0.8Co0.2(OH)2 proportion has achieved the highest glycerol conversion after 4 h of reaction, approximately 99.8%, with an energy consumption of 36.38 W/mol.