Electrode Measurements Research Articles

COF-Assisted Construction of Steric Mass-Charge Channels to Boost Activity for High-Performance Fuel Cells.

The two-dimensional lamellar materials disperse platinum sites and minimize noble-metal usage for fuel cells, while mass transport resistance at the stacked layers spurs device failure with a significant performance decline in membrane electrode assembly (MEA). Herein, we implant porous and rigid sulfonated covalent organic frameworks (COF) into the graphene-based catalytic layer for the construction of steric mass-charge channels, which highly facilitates the activity of oxygen reduction reactions in both the rotating disk electrode (RDE) measurements and MEA device tests. Specifically, the normalized mass activity is remarkably boosted by 3.7 times to 1.56 A mgpt -1 after additions of suitable COF modifications in the RDE tests. Especially, an excellent maximum power density of 1.015 W cm-2 is realized on the MEA in H2/Air condition, representing a 22 % improvement through such constructions of steric mass-charge channels. Meanwhile, the open-circuit voltage of fuel cells demonstrates only 0.8 % reductions after 10,000 cycles of stability tests. We further extended such methodology of constructing mass-charge channels to granular PtCo and commercial Pt/C catalysts, which demonstrates a significant impetus for stimulating the catalytic activity in fuel cells.

COF‐Assisted Construction of Steric Mass‐Charge Channels to Boost Activity for High‐Performance Fuel Cells

AbstractThe two‐dimensional lamellar materials disperse platinum sites and minimize noble‐metal usage for fuel cells, while mass transport resistance at the stacked layers spurs device failure with a significant performance decline in membrane electrode assembly (MEA). Herein, we implant porous and rigid sulfonated covalent organic frameworks (COF) into the graphene‐based catalytic layer for the construction of steric mass‐charge channels, which highly facilitates the activity of oxygen reduction reactions in both the rotating disk electrode (RDE) measurements and MEA device tests. Specifically, the normalized mass activity is remarkably boosted by 3.7 times to 1.56 A mgpt−1 after additions of suitable COF modifications in the RDE tests. Especially, an excellent maximum power density of 1.015 W cm−2 is realized on the MEA in H2/Air condition, representing a 22 % improvement through such constructions of steric mass‐charge channels. Meanwhile, the open‐circuit voltage of fuel cells demonstrates only 0.8 % reductions after 10,000 cycles of stability tests. We further extended such methodology of constructing mass‐charge channels to granular PtCo and commercial Pt/C catalysts, which demonstrates a significant impetus for stimulating the catalytic activity in fuel cells.

Influence of liquid electrolyte salt nature and concentration on tortuosity measurement of battery electrode

Influence of liquid electrolyte salt nature and concentration on tortuosity measurement of battery electrode

Borohydride oxidation over Pt/C, Au/C and AuPt/C thin-film electrodes studied by rotating disk electrode and differential electrochemical mass spectrometry flow cell measurements

Borohydride oxidation over Pt/C, Au/C and AuPt/C thin-film electrodes studied by rotating disk electrode and differential electrochemical mass spectrometry flow cell measurements

Atomically Dispersed Ni-N-C Catalysts for Electrochemical CO2 Reduction.

The atomic dispersion of nickel in Ni-N-C catalysts is key for the selective generation of carbon monoxide through the electrochemical carbon dioxide reduction reaction (CO2RR). Herein, the study reports a highly selective, atomically dispersed Ni1.0%-N-C catalyst with reduced Ni loading compared to previous reports. Extensive materials characterization fails to detect Ni crystalline phases, reveals the highest concentration of atomically dispersed Ni metal, and confirms the presence of the proposed Ni-Nx active site at this reduced loading. The catalyst shows excellent activity and selectivity toward CO generation, with a faradaic efficiency for CO generation (FECO) of 97% and partial current density for CO (jco) of -9.0 mA cm-2 at -0.9 V in an electrochemical H-type cell. CO2RR activity and selectivity are also studied by rotating disk electrode (RDE) measurements where transport limitations can be suppressed. It is expected that the utility of these Ni-N-C catalysts will lie with tandem CO2RR reaction schemes to multi-carbon (C2+) products.

Anti-Tissue-Transglutaminase IgA Antibodies Presence Determination Using Electrochemical Square Wave Voltammetry and Modified Electrodes Based on Polypyrrole and Quantum Dots.

A novel electrochemical detection method utilizing a cost-effective hybrid-modified electrode has been established. A glassy carbon (GC) modified electrode was tested for its ability to measure electrochemical tTG antibody levels, which are essential for diagnosing and monitoring Celiac disease (CD). Tissue transglutaminase protein biomolecules are immobilized on a quantum dots-polypyrrole nanocomposite in the improved electrode. Initial, quantum dots (QDs) were obtained from Bombyx mori silk fibroin and embedded in polypyrrole film. Using carbodiimide coupling, a polyamidoamine (PAMAM) dendrimer was linked with GQDs-polypyrrole film to improve sensor sensitivity. The tissue transglutaminase (tTG) antigen was cross-linked onto PAMAM using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC)-N-hydroxy succinimide (NHS) chemistry to develop a nanoprobe that can detect human serum anti-tTG antibodies. The physicochemical characteristics of the synthesized nanocomposite were examined by FTIR, UV-visible, FE-SEM, EDX, and electrochemical studies. The novel electrode measures anti-tissue antibody levels in real time using human blood serum samples. The modified electrode has great repeatability and an 8.7 U/mL detection limit. Serum samples from healthy people and CD patients were compared to standard ELISA kit assays. SPSS and Excel were used for statistical analysis. The improved electrode and detection system can identify anti-tissue antibodies up to 80 U/mL.

Identification of Subsurface for Grounding Installation System Using Geoelectrical Method: Case Study of Waru Substation, Sidoarjo, East Java

In the substation planning process, a study of subsurface conditions is needed for a good grounding system. Therefore, information on soil resistivity values is needed that can be used as a reference for related parties to carry out development, which is to determine the grounding locations. This research is to identify the recommended and optimal grounding depth point in the case study of Waru Substation, Sidoarjo, East Java. A good soil for grounding installation is to have a resistivity of less than 1 Ohm. In this research, 2D geoelectric method is used to determine the subsurface resistivity value. The configuration used in this measurement uses Wenner-Schlumberger which is a combination of the Schlumberger configuration which has sensitivity to vertical layer changes and the Wenner configuration which is sensitive to horizontal layer changes. In addition, measurements were also made using the Three Point Method to measure the soil resistance value of the research area, then compared with the resistance value measured using the 2D geoelectric method. The output produced in 2D geoelectric measurements is in the form of apparent resistivity values which will then be processed using Least Square inversion to obtain 2D cross-section modeling. The value and 2D resistivity cross section are a reference for interpretation of where the appropriate depth is for grounding installation. The results of the 2D resistivity cross section show that the subsurface layer is water saturated wet clay with a resistivity value of 0 Ωm -2 Ωm and wet clay with a resistivity value of 3 Ωm - 6 Ωm. Optimal grounding installation occurs at 7 meters depth with 0 Ωm -2 Ωm resistivity in moist soil. Grounding electrode measurements that have been installed on switchyard components can validate each other with the results of 2D cross-section interpretation in determining the optimal grounding depth.

Novel Methodology of General Scaling-Approach Normalization of Impedance Parameters of Insertion Battery Electrodes – Case Study on Ni-Rich NMC Cathode: Part I. Experimental and Preliminary Analysis

Abstract Using a model system based on a Ni-rich NMC active material, we demonstrate the necessary experimental steps to perform reliable and accurate impedance measurements of active electrodes (cells) and the characterization techniques required for a meaningful analysis of the obtained impedance data. We demonstrate the practical application of a simple preliminary analysis in which mass normalization of impedance spectra together with the assumption of ideal capacitive behavior allows access to the total (chemical) insertion capacitance, C_total, of the studied active material in an electrode. We show that there is an exact quantitative relationship (equality) between C_total, and the differential capacitance, C_d, of porous insertion electrodes. The main objective is to provide experimenters with directly applicable tools and skills to develop a basic intuition for exploring and explaining the key phenomena observed in the impedance responses of porous Li-ion insertion cathodes. PART-2 will highlight the major advantages of analyzing impedance data using physics-based models. Any model that is based on elements with physical meaning and is correct should pass the “consistency test” included in the scaling relations.

Toward Spatial Control of Reaction Selectivity on Photocatalysts Using Area-Selective Atomic Layer Deposition on the Model Dual Site Electrocatalyst Platform.

Photocatalytic water splitting is a promising route to low-cost, green H2. However, this approach is currently limited in its solar-to-hydrogen conversion efficiency. One major source of efficiency loss is attributed to the high rates of undesired side and back reactions, which are exacerbated by the proximity of neighboring oxidation and reduction sites. Nanoscopic oxide coatings have previously been used to selectively block undesired reactants from reaching active sites; however, a coating encapsulating the entire photocatalyst particle limits activity as it cannot facilitate both half-reactions. In this work, area selective atomic layer deposition (AS-ALD) was used to selectively deposit semipermeable TiO2 films onto model metallic cocatalysts for enhancing reaction selectivity while maintaining a high overall activity. Pt and Au were used as exemplary reduction and oxidation cocatalyst sites, respectively, where Au was deactivated toward ALD growth through self-assembled thiol monolayers while TiO2 was coated onto Pt sites. Electroanalytical measurements of monometallic thin film electrodes showed that the TiO2-encapsulated Pt effectively suppressed undesired H2 oxidation and Fe(II)/Fe(III) redox reactions while still permitting the desired hydrogen evolution reaction (HER). A planar model photocatalyst platform containing patterned interdigitated arrays of Au and Pt microelectrodes was further assessed using scanning electrochemical microscopy (SECM), demonstrating the successful use of AS-ALD to enable local reaction selectivity in a dual-reaction-site (photo)electrocatalytic system. Finally, interdigitated microelectrodes having independent potential control were used to show that selectively deposited TiO2 coatings can suppress the rate of back reactions on neighboring active sites by an order of magnitude compared with uncoated control samples.

Surface Roughness-Dependent Corrosion: Implications for Cochlear Implant Reliability

Abstract The corrosion behavior of platinum electrodes in cochlear implant (CI) systems is a critical factor affecting their long-term functionality. This study investigated the influence of the surface roughness of the platinum electrodes on their corrosion behavior. The results of in vitro experiments indicate that higher surface roughness tends to accelerate corrosion by providing numerous initiation sites. Although the electrochemical measurements of platinum electrodes with different surface roughnesses showed only slight differences in terms of shifts in potentials and higher corrosion current densities, data from the literature suggests an enhanced charge capacity with rougher surfaces. These findings contribute to an understanding of the failure mechanisms of CIs and can ultimately help to improve the design and durability of CI systems.

X-Ray CT and XAS Analysis of NMC811 Cycled at Various Rates

LiNi0.8Mn0.1Co0.1O2 (NMC811) is attracting attention as high capacity cathode materials for lithium-ion batteries, which potentially increases specific capacity and reduces cost by replacing the rare metal cobalt with nickel. However, NMC811 has an issue with cycle capability compared to the conventionally used LiCoO2 (LCO). Various factors contribute the degradation of NMC811. Although the crystal structure evolution, such as cation mixing and oxygen loss1, surface reconstruction2 and intraparticle crack formation induced by charge-discharge cycles were proposed3, previous studies tend to focus on the specific factors. In addition, the effect from high rate charge-discharge has not been analyzed. In this study, the cycle capability of NMC811 was examined at various rates. The particle morphology and the surface chemical state after charge-discharge cycle were examined X-ray computed tomography (CT) and soft X-ray absorption spectroscopy (XAS), respectively.Composite electrodes were prepared by mixing cathode active material (NMC811 or LCO), conductive additive (acetylene black) and binder (polyvinylidene fluoride) in a mass ratio of 8:1:1. Current collector was an aluminum foil. 2032-type coin cells were assembled, using 1 M LiPF6 in a mixture of ethylene carbonate and ethyl methyl carbonate in a 3:7 v/v as electrolyte, and lithium metal foil as the counter electrode. Galvanostatic charge-discharge measurements were conducted at cut-off potential 3.2-4.3 V at a rate of 0.1C and 2C. After the cycle test, the coin cells were disassembled, and the electrode sheets were washed with dimethyl carbonate and then vacuum dried. X-ray CT measurements of the disassembled electrodes were performed at the beamline BL20XU of SPring-8. O K-edge XAS measurements were performed at the beamline BL-2 of the Ritsumeikan University SR Center.When comparing the results of charge-discharge cycle tests over 50 cycles at 0.1C and 2C rates using NMC811 and LCO, a significant degradation was observed for NMC811 cycled at 2C rate. X-ray CT analysis compared the initial state of the particles with the state after 50 charge-discharge cycles. Only surface cracks were observed in the LCO, whereas cracks extending to the interior were observed in the NMC811 after 50 cycles. In addition, an increase in microcracks on the particle surface was observed at the electrode after 2C cycles for NMC811. O K-edge XAS obtained from the total electron yield mode reflects the chemical state of the particle surface. In the electrode of NMC811 after 50 cycles at 2C, the peak derived from active materials disappeared and the peak from surface film increased. These findings indicate that higher cycle rates in NMC811 induce mechanical stress on the particles, leading to cracking and the formation of a resistive layer, which ultimately cause significant capacity degradation.

Investigation of the Reaction Mechanism of Graphite Fluoride Rechargeable Batteries

Introduction With the accelerated development of next-generation rechargeable batteries, graphite fluoride with high energy density and excellent discharge potential flatness into rechargeable batteries has recently become to attract attention as an active material. In particular, lithium-graphite fluoride rechargeable battery using graphite fluoride derived from natural graphite has been reported in a previous study, which has increased the feasibility of this system.1 The charge and discharge reactions of this system can be considered as follows.2 (CxF)n + nLi+ + ne- ⇆ xnC + nLiF (1)In charging process, carbon is fluorinated and graphite fluoride is formed. However, it has problems such as extremely fast degradation and increased potential hysteresis compared to lithium-ion batteries. These problems may be caused by the low conductivity and high dissociation energy of lithium fluoride. Another possible cause is the amorphous carbon formed when graphite fluoride is discharged, which is unfavorable for graphite fluoride compounds (F-GICs) formation. However, no detailed studies about F-GIC forming mechanism have been carried out, and further research is required.In this study, experiments were carried out using graphite fluoride composite electrodes. Furthermore, we used the composite electrode mixed with carbon and lithium fluoride as a model of discharge product of graphite fluoride. From these experiments, the aim is to clarify the electrochemical reaction mechanism of this system by investigating the influence of the size of lithium fluoride and the crystallinity of carbon on the electrochemical properties. Experimental methods First, in order to investigate the influence of lithium fluoride crystal size, graphite fluoride (CF) obtained by fluorinated scary graphite composite electrodes were prepared. The electrodes were first discharged to 1.5 V at a current density of 50 mA g- 1. Then, the electrodes were left at OCV for 1 hour or 1 week to prepare electrodes with different crystallinity of lithium fluoride.Second, in order to investigate the influence of the crystallinity of carbon, ball-milled and non-ball-milled natural graphite (SNO-15) were prepared and mixed with lithium fluoride, respectively, to produce carbon and lithium fluoride composite electrodes named WBMCL and WNBMCL electrodes, respectively.Two-electrode type coin cell was used for the electrochemical measurements. The composite electrodes were used as the working electrode and lithium metal was used as the counter electrode. The electrolyte was 5.8 mol dm- 3 LiBF4/sulfolane (C4H8O2S). Charge-discharge measurements were carried out at a current density of 50 mA g- 1 and in a voltage range of 1.5-5.3 V. Results and discussion (Effect of LiF size)XRD measurements were performed on cells left at OCV for 1 hour and 1 week after the first discharge of the CF composite electrode, showing that lithium fluoride crystal growth was more advanced on the electrode left for 1 week after discharge. To investigate the effect of lithium fluoride crystal size, charge-discharge experiments were carried out using these electrodes with different lithium fluoride crystal sizes. Charge-discharge curves are shown in Figure 1 (a). It was confirmed that there was not significant difference in reversible capacity between these two electrodes, and that the size of the lithium fluoride had no adverse effect on the charging of the lithium-graphite fluoride rechargeable battery. This result indicated the fact that partially dissolved fluoride ions from lithium fluoride react with carbon, rather than directly dissociating and reacting with lithium fluoride, which has a high dissociation energy.(Effect of crystallinity on the system)XRD measurements confirmed that ball-milled carbon was amorphous. To investigate the effect of carbon crystallinity, charge-discharge measurements of WBMCL and WNBMCL electrodes were carried out. Results are shown in Figure 1(b), (c). It was confirmed that the WBMCL composite electrode had a larger capacity. From these results, it can be inferred that graphite with high crystallinity is not suitable for charging lithium-graphite fluoride rechargeable batteries, and that amorphous carbon is more advantageous.In summary, on the charge-discharge reaction of lithium-graphite fluoride rechargeable batteries, the effect of carbon crystallinity was found to be more significant than the crystallite size of lithium fluoride. References 1) Ito, C. Lee, Y. Miyahara, S. Yamazaki, T. Yamada, K. Hiraga, T, Abe, and K. Miyazaki, Chem. Mater., 34, 8711-8718, (2022).2) Zhan, S. Yang, Y. Wang, Y. Wang, L. Ling, K, and X. Feng, Adv. Mater. Interfaces., 1, 4, (2014). Figure 1

(Invited) Enhancement of Li-Ion Intercalation Kinetics in High-Concentration Electrolytes

Highly concentrated electrolyte (HCE) solutions containing over 3 mol dm–3 of Li salts have attracted attention recently owing to their unique physicochemical and electrochemical properties such as high thermal and electrochemical stability, unusual Li+ ion transport processes, and good compatibility with next-generation batteries containing Li anodes. Certain HCEs enhance the charge-transfer reaction rate at the electrode/electrolyte interface in Li-ion batteries. The solvation structure of Li+ in HCEs significantly affects the electrochemical interfacial reaction kinetics. In the organic electrolytes of lithium-ion batteries, the desolvation process of Li+ ion at the electrode/electrolyte interface is the rate-limiting process of the charge transfer reaction.1 Our group has found that the rate of charge transfer reaction of Li+ intercalation electrode is affected not only by Li+ solvation structure but also by the activity of Li+ (aLi+)and viscosity in the electrolyte.2 In this study, the charge transfer resistance (Rct) of the LiMn2O4 (LMO)/electrolyte interface in the electrolyte with various Li salt concentrations was evaluated to determine the effect of Li+ activity and viscosity on the charge transfer kinetics. LMO thin-film electrodes were prepared using the polyvinylpyrrolidone (PVP) sol-gel method. The electrochemical measurements of the LMO thin-film electrode were performed using a three-electrode cell. Li metal was employed as both the counter and reference electrodes and a LMO thin film as the working electrode. The mixtures composed of LiN(SO2CF3)2 (LiTFSA) and monoglyme (G1) were used as model electrolytes. Electrochemical impedance spectroscopy (EIS) was conducted at 30 oC and 3.9 VLi to determine Rct. In the range of concentration less than 1.8 mol/L ([LiTFSA]/[G1] =1:4 in molar ratio), Rct decreases with increasing LiTFSA concentration, while it increases at higher than 1.8 mol L-1. The minimum Rct at 1.8 mol L-1 can be attributed to the combined effects of electrolyte viscosity and Li+ activity in the electrolyte. Further results including the reaction kinetics in other electrolytes also will be presented. Acknowledgements: This study was partially supported by the JSPS KAKENHI (Grant No. JP22H00340 and JP 23K17370).

(Invited) Confinement Effects on the Redox Reaction of Aromatic Molecules

Introduction Aromatic compounds have been studied as electrode materials for redox flow batteries and electrochemical flow capacitors. Electrochemical flow capacitors, consisting of carbon-based flowable electrodes, incorporate the advantageous characteristics of flow batteries and supercapacitors. Recently, pseudocapacitive flowable electrodes have been reported to achieve enhanced energy densities. The graphitic walls of the slit-shaped micropores in carbon materials have a unique interaction potential energy. Herein, we demonstrate the anomalous redox behavior of aromatic compounds physisorbed in the slit-shaped micropores of activated carbon (AC).1 In addition, we introduce the insight of the confinement effects.2 Experimental 4,5-Dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (BQDS) was used as a redox active molecule. A flowable electrode was prepared by simply dispersing carbon material in 1 mM BQDS + 0.5 M H2SO4. Electrochemical measurements of the flowable electrodes were conducted using a rotating disk electrode connected to an automatic polarization system. A mirror-polished glassy carbon rod (diameter, 6 mm) was used as the working electrode, a Pt mesh as the counter electrode, and a reversible hydrogen electrode (RHE) as the reference electrode. Before the electrochemical measurements, break-in potential cycling between 0 and 1200 mV vs. RHE was conducted using a de-aerated flowable electrode at a scan rate of 200 mV s−1 for 100 cycles while rotating the working electrode (1600 rpm). The redox behavior of the flowable electrode was investigated by potential cycling at scan rates of 500–2 mV s−1. Before cyclic voltammetry, the working electrode dispersion was stirred at 2500 rpm for 2 min, and the measurement was commenced after 1 min to obtain reproducible data. In addition, the effect of the micropore geometry of AC on the adsorption-controlled redox reaction was investigated.2 Results and Discussion The electrochemical behavior of various aromatic compounds was studied to demonstrate the proof-of-concept of a micropore-confined electrode reaction. The peak-potential separation (DE p)of BQDSH2/BQDS redox reaction was 330 mV at 5 mV s-1. The value of DE p decreased to 30 mV when carbon black (CB) with negligible microporosity was dispersed in the electrolyte. Remarkably, DE p = 0 mV was observed when micropore-rich AC is dispersed in the electrolyte. Based on the X-ray scattering analysis and HRMC simulations, we found that the micropore-confined aromatic compounds behave as adsorbed species in an adsorption-based system. The charge/discharge performance of a two-electrode cell was investigated. AQDS/AC and BQDS/AC were employed as negative and positive electrodes, respectively. The features of the charge/discharge curves of the cell changed from battery-type to pseudocapacitor-type when these electrodes were used. This behavior is attributed to the rapid charge-transfer step of micropore-confined molecules compared with that of unconfined molecules.In conclusion, we found that the adsorption-controlled process of the redox reaction for BQDS could be achieved using slit-shaped graphitic micropores with widths less than 1 nm in AC. Acknowledgments This work was partially supported by the Electrotechnology of Chubu, Kondo Memorial Foundation, and Tokyo Ohka Foundation for The Promotion of Science and Technology. References D. Takimoto, K. Suzuki, R. Futamura, T. Iiyama, S. Hideshima, and W. Sugimoto, ACS Appl. Mater. Interfaces, 14, 31131 (2022).D. Takimoto, K. Suzuki, S. Hideshima, and W. Sugimoto, Electrochemistry, 91, 077006 (2023).

Epitaxy Promotes OER Stability of IrO2-Coated SnO2 Nanoparticles in PEM Electrolysis

The transition from fossil fuel-based and CO2-intensive energy production to a future sustainable energy-based economy requires solutions for efficient energy conversion and storage for later re-electrification of green energy from a renewable source such as solar or wind power. One possibility is to convert energy from sustainable sources into chemical energy in the form of hydrogen by electrolysis of water. To meet this challenge, Proton Exchange Membrane (PEM) electrolysis is a powerful, state-of-the-art technology that uses scarce iridium-based catalysts to promote the oxygen evolution reaction (OER). To improve the sustainability and economics of the PEM process, it is necessary to minimize the use of rare and precious iridium to reduce the overall density of iridium-based catalysts for the OER.Here, we present different strategies for the synthesis of low-total iridium OER catalysts. In addition to the choice of support material and support morphology, we also consider the coating strategy to produce highly active and durable catalysts.The coating of corrosion-resistant high surface area metal oxide supports of various morphologies with an ultrathin layer of amorphous iridium hydroxide by a simple wet chemical synthesis at low temperature and ambient pressure or by a hydrothermal approach leads to the formation of interconnected amorphous hydrous iridium oxide particles. In a subsequent step, the amorphous phase is oxidized at elevated temperatures using an Adams fusion salt melt. This process allows a controllable phase transformation and crystallization to form a layer of interconnected partially crystalline IrOx nanoparticles of ≈1.5-2 nm domain size on the surface of the supporting metal oxides, thus providing an electrical percolation path in the catalyst layer.The appropriate choice of support material in terms of crystal lattice parameters can moderate the crystal growth of the iridium oxide in terms of epitaxy and significantly increase the stability due to tight anchoring to the support. The final core-shell nanostructures comprise highly active iridium oxide with total Ir weight fractions as low as 7at% on SnO2 nanoparticles, compared to a state-of-the-art benchmark catalyst with 56at% Ir supported on TiO2 nanoparticles. Transmission electron microscopy (TEM) imaging was used to elucidate the effect of different core-shell designs, particularly on iridium coating and crystallinity. Rotating disk electrode (RDE) measurements were performed to evaluate the electrochemical properties and assess the activity of respective catalyst as well as the cycling stability. Further, membrane electrode (MEA) polarization curves suggest enhanced stability of epitaxial catalyst layers.

Structural and Compositional Tuning of Platinum Alloy Nanowires for Enhanced Oxygen Reduction Reaction

Polymer electrolyte fuel cells (PEFCs) have gained considerable attention due to their high energy density, high conversion efficiency, and wide operating temperatures. The main challenges encountered in the commercialization of PEFCs are the sluggish kinetics and low durability of the cathodic oxygen reduction reaction (ORR) catalyst. The conventional electrocatalyst, platinum (Pt) nanoparticles (2~5nm) dispersed on carbon support, suffer insufficient catalytic activity and low durability owing to enlargement in particle size due to particle coalescence or Ostwald ripening during PEFC operations. In this scenario, the facile fabrication of advanced functional catalysts with improving ORR activity and stability is significant for PEFCs.Among the various catalysts reported so far in the literature, Pt is an effective metal for catalyzing the ORR reaction due to its intrinsic catalytic performance and stability, however, it's scarcity and high cost limit its large-scale practical application. Compared to nanoparticle counterparts, the anisotropic one-dimensional nanostructure has been explored for higher activity due to its high flexibility, outstanding conductivity, and thermal stability.1–3 Several strategies have been adopted in the literature to enhance the catalytic activity and the Pt utilization efficiency. One aspect is the atomic scale tuning of the Pt chemical environment through proper alloying with bimetallic or trimetallic elements. In addition, it is essential to construct favorable structures in alloy systems with maximum exposed active sites.In this work, we synthesized a unique nanostructure of Pt alloy nanowires (PtNiCo NW) through a solvothermal method by reducing the Pt, Ni, and Co precursors in a solvent mixture with reducing agents and surfactants (oleyl amine with W(CO)6, glucose and CTAB). The atomic scale tailoring of the Pt chemical environment of the catalyst was achieved by varying the alloy composition. The TEM observation indicates that structural changes occurred by changing the reaction conditions of the catalysts; initially, smooth nanowires are formed with a diameter of 2 nm (Fig. 1a) followed by the formation of the rough surface due to an increase in the reduction of Ni over the Pt NWs over time. A porous nanostructure of Pt with Ni (Fig 1b) was fabricated through an acid treatment process.The electrochemical surface area and ORR activities of the PtNiCo NWs on carbon support were estimated from rotating disk electrode measurements in 0.1 M HClO4 electrolyte solution. Compared with the smooth PtNiCo NW catalyst, the porous PtNiCo NW catalyst with a rough surface exhibited a higher mass activity of 1.6 ± 0.1 Amg-Pt−1 and an excellent specific activity of 3.7 ± 0.2 mAcm-Pt−2. The higher specific activity would arise from the increased roughness on the Pt surface through the formation of porous structure and the optimized surface-active sites on the catalyst surface through composition tuning.AcknowledgmentThis presentation is based on the results obtained from the project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.References M. Li et al, Science, 2016, 354 (6318), 1414–1419.X. Tian et al, Science, 2019, 336 (6467), 850–856.K. Jiang et al, Sci. Adv., 2017, 3(2), e160170. Fig 1. TEM images of Pt-based nanowires having (a) smooth and (b) rough-porous structures. Figure 1

Highly-Coordinated Iron Complex with Multidentate Ligands for Aqueous Redox Flow Battery Electrolytes

Redox flow batteries (RFBs) possess a longer cycle life than other secondary batteries owing to their limited electrode deterioration upon charge/discharge cycling. Commercial RFBs use vanadium ions as active materials, resulting in a high cost. Therefore, more abundant and less expensive iron-based complexes are desirable as catholyte active materials for RFBs. However, to develop stable iron-based aqueous electrolytes, precipitation of ferric hydroxides under a mild alkaline condition should be suppressed. One potential strategy is to use polydentate ligands that stabilizes the iron coordination structure owing to a chelating effect. For example, a heptadentated [Fe(DTPA)]2 − (DTPA: diethylenetriaminepentaacetate) shows an excellent cycle stability of 0.029% per cycle as an RFB posolyte.[2] In this study, we further improve the cycle stability of iron-based complexes using an octadentate tetraazacyclododecatetraacetate (DOTA) ligand.The target complex, K[Fe(DOTA)], was prepared by adding FeCl3•6H2O to an aqueous solution of deprotonated DOTA. Single-crystal X-ray structural analysis reveals an eight-coordinated structure of the iron complex. Cyclic voltammetry shows a redox peak at 0.37 V (vs. SHE) for K[Fe(DOTA)], which is higher by 0.088 V than that for K2[Fe(DTPA)] and by 0.12 V than that for K[Fe(EDTA)] (Figure 1). Levich plots obtained by rotating desk electrode measurements reveal the diffusion coefficient of 9.9×10−7 cm2 s−1 and the electron transfer rate constant of 5.0×10−3 cm s−1 for K[Fe(DOTA)] , which are comparable to those of K2[Fe(DTPA)]. UV-Vis spectroscopy shows the aqueous solubility of 0.57 mol L−1 for K[Fe(DOTA)], which is lower than that of K2[Fe(DTPA)] (1.3 mol L−1). The low solubility of K[Fe(DOTA)] should arise from large lattice energy owing to its symmetric structure. Charge/discharge measurements using an H-type cell with 0.10 mol L−1 KCl aqueous solution reveal that K[Fe(DOTA)] exhibits a greater capacity retention rate of 99.973% per cycle than K2[Fe(DTPA)] (99.333%/cycle) (Figure 2). Reference: [1] H. J. Schugar, C. Walling, R. B. Jones, H. B. Gray, J. Am. Chem. Soc. 1967, 89, 3712.[2] S. E. Waters, B. H. Robb, M. P. Marshak, ACS Energy Lett. 2020, 5, 1758. Figure 1

Modeling the Metal Poisoning of PEM Water Electrolyzers

Proton Exchange Membrane (PEM) water electrolyzers have high requirements according to the feed water quality (ultra-pure water), since different contaminations, e.g. metal ions, have negative effects on the electrochemical performance and durability. However, due to different reasons the feed water will not be free from metal ions over the whole operation time and the contaminants can be accumulating over time. Consequently, it is important to understand the underlying processes and to describe the metal poisoning.Metal poisoning is well known, but not fully investigated yet. However, this topic will be investigated in more detail due to the importance and according to recent conference talks and discussions like at the International Energy Agency Task 30 Electrolysis group. Already in 2013 Zhang et al. showed a clear effect of sodium ions on the performance, whether the sodium ions were fed to anode or cathode side [1]. Due to reference electrode measurements they could show that the performance loss can be assigned to the cathode. Furthermore, strong changes of the pH were measured [1]. Other metal ions also affect the performance and durability, but in different manner, Becker et al. published a comprehensive review to the impact of impurities on water electrolysis [2]. The different mechanisms for metal impurities are summarized to i) substitution of protons of the ionomer and consequently leading to a loss of the proton conductivity, ii) deposition on the cathode catalyst, which lead to a reduced active surface and increase in activation overpotentials and iii) causing negative side reaction as the Fentons reactions that lead to further chemical degradation effects [2]. However, it is not easy to describe and quantify these effects. Due to the lack of detailed knowledge and above all due to the complexity, there are only few existing modeling works on this topic, such as the work of Schalenbach et al. who modeled the sodium ion transport of poisoned Nafion membranes [3].To improve the understanding and provision of physical, numerical descriptions we are focused in this contribution on the model-based description of metal poisoning on the electrochemical performance of PEM water electrolyzers. Therefore, we use a comprehensive, multiphysical model that include charge, energy and different mass balances. We include literature and own theories into this model framework to describe the metal poisoning effect. The model is validated by literature and experimental in-house results to evaluate which effects and mechanisms are important. This will lead to a better understanding and description of the effect of metal poisoning.

Investigating the Impact of Various Binder Contents and Compression on Graphite Electrode Thickness Change Measurements

Volume change of active materials during the insertion of Li+-ions into the electrode’s host structure in lithium-ion batteries (LiBs) has become an increasingly researched topic. Reversible and irreversible thickness changes occur during operation because of active material volume change upon lithiation/delithiation and SEI growth. In LiBs, particularly the volume change of the negative active material can cause mechanical disintegration of the composite coating [1], causing loss of active material. LiBs are usually operated in a compressed state maintained by geometrical boundary conditions to ensure uniform contact between electrode layers. Therefore, the thickness changes can cause increased stack pressure and, subsequently, microstructural changes [2] and thus affect the performance and lifetime of the battery [3]. To better understand the aforementioned phenomena, accurate thickness measurements are required.Measuring the thickness change of a cell stack does not provide accurate information about the individual thickness change of the two electrodes. Therefore, single-electrode electrochemical dilatometry (ECD) can help to better understand the impact of the individual electrodes on cell stack thickness change. Due to the common use of a commercially available setup (EL-Cell, ECD-3-nano, Germany) in literature, most ECD experiments were conducted at a pressure of ~ 16 kPa onto a Ø10 mm working electrode (WE). Only in some publications higher pressures > 0.1 MPa were applied during single electrode measurements [4]. The applied pressure in lab-scale coin cell setups and state-of-the-art LiBs is usually significantly higher (0.1 – 0.5 MPa) than that of single-electrode ECD investigations. Therefore, the question arises whether the pressure has an impact on the measured thickness change.Thus, this work integrates a wave spring washer into a commercially available single-electrode ECD Setup (ECD-3-nano, El-Cell GmbH, Germany) to apply additional pressure onto the WE, without requiring geometrical modifications of the setup. The integration of the wave spring washer increases the applied pressure from 16 kPa to > 100 kPa. Graphite electrodes with different binder types and contents are investigated since distinct measurement artifacts were observed for electrodes with certain binder compositions in preliminary experiments. Electrodes without additional pressure show a first-cycle lithiation thickness increase of up to 30% (see Figure 1), which is much higher than the active material volume change of graphite obtained from XRD data [5] would suggest. For all examined electrodes at lower pressure the first and tenth cycle lithiation thickness change is more than 50 % higher compared to the setup at high pressure. The mechanism causing the discrepancy between measurements at the two pressure levels is identified as the curvature of the electrode edges at low pressures present in commercially available ECDs. Therefore, the setup at increased pressure enables a meaningful investigation of the influence of the binder content on irreversible and reversible thickness change for the electrodes under investigation using more realistic mechanical boundary conditions. Measurements show that irreversible thickness change decreases with increasing binder content. Possibly the higher binder content helps to prevent restructuring of the particles during cycling due to higher mechanical strength of the electrode. Delithiation thickness change is similar among electrodes with normal to high binder contents. Only electrodes with a very low binder content have a significantly lower reversible thickness change.Overall, this study shows that the influence of pressure onto the WE should be considered for future ECD-experiments and setups. This work presents an easy to implement solution to apply additional pressure onto the WE for a common commercially available ECD-setup.

Shape-Controlled Iridium Catalysts for OER in PEM Electrolysis

Hydrogen is a clean energy carrier that holds the potential to power future transportation.1 One way to generate green hydrogen is proton exchange membrane (PEM) electrolysis with renewable electricity, but this process is limited by the sluggish oxygen evolution reaction (OER).2, 3 Iridium (Ir) catalysts have balanced activity and durability for OER.4, 5 However, due to the scarcity and high cost, the structures of the Ir catalysts must be properly controlled to improve the catalyst performance and reduce Ir loading.6, 7 Currently, most Ir catalysts are nanoparticles without well-defined crystal facets, and it remains largely unknown what Ir surface structure is the most effective for OER.4 Here, we present the facet-selective growth of Ir catalysts by using various shape-controlled seeds. These shape-controlled Ir catalysts exhibit high Ir atom usage efficiency due to the formation of only a few atomic layers of Ir shells. The activity and durability of these shape-controlled Ir catalysts towards OER are evaluated by rotating disk electrode measurements and tested in a PEM electrolyzer. This work demonstrates the possibility to further improve Ir catalyst performance by precise control over the arrangements of catalyst surface atoms. Acknowledgments We acknowledge the funding support from the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL) (20240061DR) and the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE). References (1) Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43-51.(2) Wang, Y.; Pang, Y.; Xu, H.; Martinez, A.; Chen, K. S. PEM Fuel cell and electrolysis cell technologies and hydrogen infrastructure development – a review. Energy Environ. Sci. 2022, 15, 2288-2328.(3) An, L.; Wei, C.; Lu, M.; Liu, H.; Chen, Y.; Scherer, G. G.; Fisher, A. C.; Xi, P.; Xu, Z. J.; Yan, C.-H. Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment. Adv. Mater. 2021, 33, 2006328.(4) Wang, S.; Shen, T.; Yang, C.; Luo, G.; Wang, D. Engineering Iridium-Based Oxygen Evolution Reaction Electrocatalysts for Proton Exchange Membrane Water Electrolyzers. ACS Catal. 2023, 13, 8670-8691.(5) Gao, J.; Liu, Y.; Liu, B.; Huang, K.-W. Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts. ACS Nano 2022, 16, 17761-17777.(6) Alia, S. M.; Pylypenko, S.; Neyerlin, K. C.; Kocha, S. S.; Pivovar, B. S. Activity and durability of iridium nanoparticles in the oxygen evolution reaction. ECS Trans. 2015, 69, 883-892.(7) Alia, S. M.; Shulda, S.; Ngo, C.; Pylypenko, S.; Pivovar, B. S. Iridium-Based Nanowires as Highly Active, Oxygen Evolution Reaction Electrocatalysts. ACS Catal. 2018, 8, 2111-2120.