Spectroscopy Measurements Research Articles

Quantum-Confined Lifshitz Transition on Weyl Semimetal Td-MoTe2.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal Td-MoTe2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

Analysis of a plasma reactor performance for direct nitrogen fixation by use of three-dimensional simulations and experiments

This study utilizes state-of-the-art in-situ measurements and advanced three-dimensional simulations to investigate the operation of a microwave plasma reactor for nitrogen fixation at a high subatmospheric pressure under various power inputs. It is shown that the system should be treated as a warm plasma due to negligible differences in vibrational and rotational temperatures and that it is required to account for chemical non-equilibrium. Complex flow field and significant temperature and concentration gradients are observed, emphasizing the need for three-dimensional simulations to accurately describe the interactions between mass, heat, and momentum transport and chemical reactions in non-equilibrium conditions. The innermost plasma regions reach temperatures close to 6000 K, while wall temperatures remain low due to presence of a low-temperature swirling flow field around the hot core. Analysis reveals that the swirling flow field around the hot core provides intrinsic partial quenching in regions with large temperature gradients which counteract the return to chemical equilibrium and enrich the flow surrounding the hot core. The inner flow exhibits a recirculation zone, stagnation point, and a transition into an increasingly parabolic flow profile closer to the reactor outlet. The agreement between the numerical modeling and the measurements is strong. In-situ Raman spectroscopy measurements confirm the calculated temperature field inside the reactor. Thermocouple measurements on the reactor wall are also in good agreement with the simulation. Additionally, Fourier-transform infrared spectroscopy measurements agree very well with the simulated amount of nitrogen oxide exiting the reactor at different microwave power inputs. The understanding and modeling capabilities are expected to support the development of future reactor designs that can facilitate higher yields.

Synthesis of cadmium(II) complexes with polyamines: Crystal structure, theoretical calculation, photocatalytic, DNA interaction and biological studies

Three cadmium(II) complexes of N-(2-aminoethyl)-1, 2-ethanediamine (DETA) (1), N-(3-aminopropyl)-1, 3-propanediamine (DPTA) (2) and N,N'-(ethane-1,2-diyl)bis(propane-1,3-diamine) (DPETA) (3) have been synthesized and characterized by single crystal X-ray crystallography, UV–visible spectroscopy, IR and conductivity measurements. Complexes 1 and 3 show octahedral geometry while complex 2 show five coordinated square pyramidal geometry. Two molecules of DETA ligand are coordinated to the Cd(II) ion in complex 1. But one molecule each of DPTA and DPETA ligands and two bromide ions are coordinated to the Cd(II) ions to form square pyramidal (2) and octahedral structure (3). DFT results show that complex 3 is the most favoured complex thermodynamically while complex 1 is the least. The complexes exhibit good photocatalytic activity with reference to the intensity of organic dye methylene blue. The complexes were investigated for DNA interaction by absorption and fluorescence spectroscopy suggesting both hypochromism (internal contact or intercalative interaction) and hyperchromism (external contact or electrostatic binding). The cadmium (II) complexes show growth inhibitory activity against four pathogenic bacteria, two gram +ve, viz., Staphyloccus aureus, Bacillus subtilis and two gram –ve, viz., Escherichia coli, Salmonella typhi. Albumin denaturation assay and Heat Induced Haemolysis were studied for the complexes for anti-inflammatory activity.

Facile one-pot synthesis and characterization of ZnSe/HPMC nanocomposites

Herein, we report the facile synthesis of ZnSe/HPMC (Hydroxypropyl Methyl Cellulose) nanocomposites by in-situ method. The biodegradable HPMC polymer acts as capping molecule as well as polymer matrix. The optical and structural properties of ZnSe/HPMC nanocomposites were studied by optical absorption, photoluminescence (PL) spectroscopy, FTIR, XRD and TEM measurements. Size of the ZnSe nanoparticles was tuned by controlling the pH of the reaction. As prepared ZnSe nanoparticles were spherical in shape, homogeneously distributed in the polymer and XRD patterns exhibit face centred cubic phase. The present method is a novel approach for the synthesis of ZnSe nanoparticles using HPMC as a capping molecule as well as matrix to develop nanocomposite.

Ultrahigh-mobility microbial cytochrome nanowires generate power from humidity.

Mixed electronic-ionic conductors are crucial for various technologies, including harvesting power from humidity in a durable, self-sustainable, manner unrestricted by location or environment1,2. Biological proteins have been proposed as mixed conductors for 50 years3,4. Recently, Geobacter sulfurreducens pili filaments have been claimed to act as nanowires to generate power5,6. Here, we show that the power is generated by G. sulfurreducens-produced cytochrome OmcZ nanowires that show 20,000-fold higher electron conductivity than pili7. Remarkably, nanowires show ultrahigh electron and proton mobility (>0.25 cm2/Vs), owing to directional charge migration through seamlessly-stacked hemes and a charged, hydrogen-bonding surface, respectively. AC impedance spectroscopy and DC conductivity measurements using four-probe van der Pauw and back-gated field-effect-transistor devices reveal that humidity increases carrier mobility by 30,000-fold. Cooling halves the activation energy, thereby accelerating charge transport. Electrochemical measurements identify the voltage and mobilities required to switch pure electronic conduction to mixed conduction for power generation. The high aspect ratio (1:1000) and hydrophilic nanowire surface captures moisture efficiently to reduce oxygen reversibly, generating large potentials (>0.5 V) necessary to sustain high power. Our studies establish a new class of biologically-synthesized, low-cost and high-performance mixed-conductors and identify key design principles for improving power output using highly-tunable electronic and protein structures.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Enhancement-mode ZnGa2O4-based Phototransistor with high UV–visible rejection ratio grown by metalorganic chemical vapor deposition

In this study, a high-crystallinity zinc gallium oxide (ZnGa2O4) epilayer of about 80 nm thickness was successfully grown using metalorganic chemical vapor deposition. X-ray diffraction, atomic force microscopy, UV–visible spectroscopy measurements, and X-ray photoelectron spectroscopy were used to investigate the quality of the epilayer. Transmission line measurement analysis revealed the electrical properties of the ZnGa2O4 epilayer: a sheet resistance of 276 MΩ/□, a transfer length of 0.28 μm, and a specific contact resistivity of 21.63 Ω/mm2. A phototransistor made of ZnGa2O4 was designed with a channel width of 250 μm and a channel length of 20 μm. The source-drain electrodes were fabricated using a Ti/Al/Ni stacking technique. The gate dielectric comprised 30 nm thick Al2O3, and the gate contact was made of Ni. The device possessed a higher threshold voltage value of about 4.8 V and an impressive On/Off current ratio of 106. Under 240 nm UV light exposure, the phototransistor exhibited a transition from enhancement mode to depletion mode, leading to a remarkable shift in the threshold voltage from 5 V to approximately −16 V. Moreover, the responsivity and rejection ratio at 240 nm was 128.5 A/W and 105 at a gate voltage of −10 V. The study shows that an increasing thickness of the ZnGa2O4 epilayer from 80 nm to 135 nm results in a depletion mode transistor. The higher concentration of about twenty percent oxygen vacancies resulted in a longer fall time for the phototransistor. These results underscore the significant potential of ZnGa2O4 in facilitating deep UV-based detection applications.

Explainable Feature Engineering for Multi-Modal Tissue State Monitoring Based on Impedance Spectroscopy.

One of the most promising approaches to food quality assessments is the use of impedance spectroscopy combined with machine learning. Thereby, feature selection is decisive for a high classification accuracy. Physically based features have particularly significant advantages because they are able to consider prior knowledge and to concentrate the data into pertinent understandable information, building a solid basis for classification. In this study, we aim to identify physically based measurable features for muscle type and freshness classifications of bovine meat based on impedance spectroscopy measurements. We carry out a combined study where features are ranked based on their F1-score, cumulative feature selection, and t-distributed Stochastic Neighbor Embedding (t-SNE). In terms of features, we analyze the characteristic points (CPs) of the impedance spectrum and the model parameters (MPs) obtained by fitting a physical model to the measurements. The results show that either MPs or CPs alone are sufficient for detecting muscle type. Combining capacitance (C) and extracellular resistance (Rex) or the modulus of the characteristic point Z1 and the phase at the characteristic frequency of the beta dispersion (Phi2) leads to accurate separation. In contrast, the detection of freshness is more challenging. It requires more distinct features. We achieved a 90% freshness separation using the MPs describing intracellular resistance (Rin) and capacitance (C). A 95.5% freshness separation was achieved by considering the phase at the end of the beta dispersion (Phi3) and Rin. Including additional features related to muscle type improves the separability of samples; ultimately, a 99.6% separation can be achieved by selecting the appropriate features.

Corrosion protection investigations of oxide‐silane composite coating on hot dip aluminized steel

AbstractThe hot dip aluminized ASTM 1035 steels (Al‐coated steel) underwent anodic oxidization, and various thicknesses of Al2O3 coating were applied to the surface of the Al‐coated steels (Al–Al2O3‐coated steel). The corrosion resistance of the Al–Al2O3‐coated steel was investigated using potentiodynamic tests, electrochemical impedance spectroscopy tests and measurement of galvanic current density in a bimetallic cell. It was found that the anodic oxidation resulted in the formation of a porous Al2O3 coating on the surface of the Al‐coated steel, improving both its corrosion resistance and galvanic corrosion resistance when coupled with carbon fiber reinforced nylon 6 composite (Cf/PA6). Silane treatment effectively sealed the pores within the Al2O3 coating, resulting in improved corrosion protection for Al–Al2O3‐coated steel due to the exceptional insulation, impermeability and hydrophobic properties of silane coating. Compared to the Al‐coated steel/carbon fiber reinforced polymer (CFRP) couple and Al–Al2O3‐coated steel/CFRP couple, stable galvanic current density decreased by approximately 75% and 45%, respectively.

(Invited) Internal Photoemission (IPE) Spectroscopy Measurement of Interfacial Energy Barriers in Operating Metal/Insulator/Metal (MIM) and Metal/Ferroelectric/Metal (MFM) Devices

Metal-insulator-metal (MIM) and Metal/Ferroelectric/Metal (MFM) devices find application as high speed tunnel diodes for rectenna based energy harvesting and sensing of IR radiation, hot-electron transistors, single electron transistors, resistive random access memory (RRAM), back end of line (BEOL) capacitors, and ferroelectric tunnel junction (FTJ) diodes. Precise knowledge of band-offsets / energy barriers (φBn) at metal/insulator interfaces is critical for understanding, predicting, and optimizing charge transport in these devices.The Schottky-Mott rule, which neglects charge transfer across interfaces, states that the difference between the Fermi level of a metal (EF) and the conduction band of an insulator should vary simply with the metal vacuum work function (ΦM,vac) and the insulator electron affinity (χi) so that φBn = ΦM,vac - χi. In induced gap state theory, charge transfer at intrinsic interface traps creates an interfacial dipole that drives the EF towards the charge neutral level of the insulator (ECNL,i) the energy at which the dominant character of the interface states switches from donor-like to acceptor-like. A metal on an insulator will thus behave as if it has an effective work function (ΦM-eff) where ΦM,eff = ECNL,I + S(ΦM,vac - ECNL,i), where S = dφBn / dΦM,vac.1 Empirically, S = 1 / (1 + 0.1(εopt - 1)2), so that as the optical dielectric constant of the insulator (εhf) increases, S decreases from an ideal 1 towards 0 so that the insulator more effectively "pins" EFM at ECNL-i.2 Despite some success of MIGS theory, it is difficult to calculate or determine ECNL for a given material and it is observed that actual φBn's often deviate substantially from predictions. Indeed, φBn's depends on material processing, extrinsic effects (such as interfacial and near-interfacial trapped charge arising from point defects), dipoles (due to interfacial chemical reactions), and scavenging of oxygen from the insulator. Therefore, it is necessary to directly measure φBn for the specific metal/insulator or metal/ferroelectric combination in use. Internal photoemission (IPE) spectroscopy is an electro-optical technique that allows measurement of φBn at both metal interfaces in operating MIM & MFM devices.3-5 IPE is the only technique capable of measuring barrier heights, in-situ, in operating device structures. To date, relatively little work has been reported on IPE of these devices.6-12 In this invited talk, IPE spectroscopy measurements are described and measurements are presented of φBn's in MIM structures between insulators (Al2O3, HfO2, SiO2, NiO, CoOx) and ferroelectric HfZrOx deposited via atomic layer deposition (ALD) and various metals including ALD Ru, sputtered thin film amorphous metals (ZrCuAlNi, TaWSi, and TaNiSi), and benchmark metals (TaN, Al, and Au). Comparisons with electrical measurements on the same devices are found to be qualitatively consistent with φBns determined via IPE, even when inconsistent with Schottky model predictions, pointing to various non-idealities present in the materials and their interfaces and demonstrating the power of IPE for understanding and optimizing MIM & MFM device operation. C. Yeo, T.J. King, and C. Hu, J. Appl. Phys. 92, 7266 (2002).Mönch, Phys. Rev. Lett. 58, 1260 (1987).K. Adamchuk and V. V. Afanas’ev, Prog. Surf. Sci. 41, 111 (1992).V. Afanas’ev and A. Stesmans, J. Appl. Phys. 102, 081301 (2007).V. Nguyen, O. A. Kirillov, and J. S. Suehle, Thin Solid Films 519, 2811 (2011).Afanas’ev, N. Kolomiiets, M. Houssa, and A. Stesmans, Phys. Status Solidi A 215, 1700865 (2017).V. Afanas’ev et al., Appl. Phys. Lett. 98, 132901 (2011).A. Jenkins, T. Klarr, D. Z. Austin, W. Li, N. V. Nguyen, and J. F. Conley Jr., Phys. Status Solidi RRL – Rapid Res. Lett. 12, 1700437 (2018).A. Jenkins, J. M. McGlone, J. F. Wager, and J. F. Conley, Jr., J. Appl. Phys. 125, 055301 (2019).A. Jenkins, T. Klarr, J. M. McGlone, J. F. Wager, and J. F. Conley Jr., ECS Trans. 85, 729 (2018).A. Jenkins, K.E.K. Holden, S.W. Smith, M.T. Brumbach, M.D. Henry, C. Weiland, J.C. Woicik, S.T. Jaszewski, J.F. Ihlefeld, and J.F. Conley. ACS AMI 13, 14634 (2021).L. Peterson, T. Mimura, J. Ihlefeld, and J.F. Conley Jr., "Internal Photoemission (IPE) Spectroscopy Measurement of Electrode Energy Barriers in Pristine, Woken, and Poled Ferroelectric HZO MFM Devices," in the American Vacuum Society (AVS) Workshop for Innovative Nano Devices and Systems (WINDS) Book of s, Kona, HI, Dec. 2023.

Construction of Ag3PO4/g-C3N4 Z-Scheme Heterojunction Composites with Visible Light Response for Enhanced Photocatalytic Degradation.

Ag3PO4/g-C3N4 photocatalytic composites were synthesized via calcination and hydrothermal synthesis for the degradation of rhodamine B (RhB) in wastewater, and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS). The degradation of RhB by Ag3PO4/g-C3N4 composites was investigated to evaluate their photocatalytic performance and cyclic degradation stability. The experimental results showed that the composites demonstrated notable photocatalytic activity and stability during degradation. Their high degradation efficiency is attributed to the Z-scheme transfer mechanism, in which the electrons in the Ag3PO4 conduction band and the holes in the g-C3N4 valence band are annihilated by heterojunction recombination, which greatly limits the recombination of photogenerated electrons and holes in the catalyst and enhances the activity of the composite photocatalyst. In addition, measurements of photocurrent (PC) and electrochemical impedance spectroscopy (EIS) confirmed that the efficient charge separation of photo-generated charges stemmed from strong interactions at the close contact interface. Finally, the mechanism for catalytic enhancement in the composite photocatalysts was proposed based on hole and radical trapping experiments, electron paramagnetic resonance (EPR) analysis, and work function evaluation.

Limitations and Opportunities of Self-Supported Co-Based Anodes for Pure Water-Fed Anion Exchange Membrane Water Electrolyzers

Hydrogen has been regarded as an energy source with a potential to be used in various sectors, including transportation, power generation and industrial processes1-3. Producing hydrogen via water electrolysis using green electricity will be necessary for a sustainable future. Proton exchange membrane water electrolysis (PEMWE) is one of mature and leading electrolysis technologies, while anion exchange membrane water electrolysis (AEMWE) is regarded as a new, emerging technology4. However, PEMWE system suffers from their expensive cost and scarcity of iridium-based oxygen evolution reaction (OER) catalysts, having no alternative OER catalysts. Compared to PEMWE, AEMWE can be pursued with non-platinum group metal (non-PGM) thanks to its alkaline environment4,5.Here, non-PGM Co-based catalyst coated anodes were prepared using a simple hydrothermal reaction followed by heat treatment. Different phases and crystallinity of cobalt-based anodes were prepared and tested in pure water-fed AEMWE devices. To investigate the catalyst-ionomer/membrane interfaces at the anodes, the loadings of Co3O4 catalyst and ionomer topcoats were controlled. The cell performance in polarization curves was correlated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements. The comparison of anodes operated with pure water feed and with supporting electrolyte was made to highlight the importance of catalyst-ionomer interface in pure water-fed AEMWE. Lastly, our self-supported Co3O4 anode outperformed a commercial Co3O4 NP ink-based anode, projecting a potential of direct growth of catalyst materials on PTLs in pure water-fed AEMWE.Reference(1) Agyekum, E. B.; Nutakor, C.; Agwa, A. M.; Kamel, S. A Critical Review of Renewable Hydrogen Production Methods: Factors Affecting Their Scale-Up and Its Role in Future Energy Generation. Membranes 2022, 12 (2).(2) Qusay, H.; Sameer, A.; Aws Zuhair, S.; Hayder, M. S.; Marek, J. Green hydrogen: A pathway to a sustainable energy future. Int. J. Hydrogen Energy 2024, 50, 310-333.(3) Joonsik, H.; Krisha, M.; HeeJin, C. A review of hydrogen utilization in power generation and transportation sectors: Achievements and future challenges. Int. J. Hydrogen Energy 2023, 48 (74), 28629-28648.(4) Lee, S. A.; Kim, J.; Kwon, K. C.; Park, S. H.; Jang, H. W. Anion exchange membrane water electrolysis for sustainable large-scale hydrogen production. Carbon Neutralization 2022, 1 (1), 26-48.(5) Juchan, Y.; Myeong Je, J.; Xiaojun, Z.; Yoo Sei, P.; Jooyoung, L.; Sung Mook, C.; Yadong, Y. Non-precious electrocatalysts for oxygen evolution reaction in anion exchange membrane water electrolysis: A mini review. Electrochem. Commun. 2021, 131, 107118.

Decomposition of Electrolyte Solutions at Positive Electrodes

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the high voltage (>4.5 V) reactivity of commonly used electrolyte solutions is still poorly understood; electrolyte decomposition at the positive electrode is generally believed to proceed via electrochemical oxidation of the organic carbonate solvent,1 while more recent work has revealed an alternative pathway involving reactive lattice oxygen species originating from the positive electrode (i.e., singlet oxygen).2 Unfortunately, we still have a limited understanding of these reactions, the products that form, and which chemical structural units in the electrolyte solution make them reactive - all of which make it challenging to assess and mitigate the impact of electrolyte decomposition on the battery’s lifetime.In this work, operando gas measurements and solution NMR were combined to study the electrochemical and chemical decomposition of widely used electrolyte salts, solvents, and additives. First, the electrochemical stability of the electrolyte components was reviewed against C65 electrodes in a standard H-cell setup: all solvents reveal little decomposition up to 5.5 V (vs Li/Li+), consistent with previous electrochemical measurements.3,4 Based on the experimentally detected soluble and gaseous products, the electrochemical decomposition mechanism is inferred. Subsequently, the chemical stability against reactive oxygen species (i.e., singlet oxygen; 1O2, peroxide anion; O2 2 −, and superoxide radical anion, O2 −) was explored. All carbonate solvents demonstrated instability against reactive oxygen species. As before, the experimentally detected gaseous and soluble products were used to infer the decomposition mechanisms. The proposed mechanisms were verified using 17O and 18O isotopic labeling for NMR spectroscopy and mass spectrometry measurements, respectively. The findings on the reactivity between the electrolyte components and reactive oxygen species provide a deeper understanding of electrolyte decomposition processes occurring at high voltage positive electrodes (e.g., Li-rich layered and disordered rocksalt oxides, high-voltage spinels, and polyanionic materials). Moreover, these insights will support the development of more stable electrolyte formulations, further extending the lifetime of lithium-ion batteries.(1) Imhof, R.; Novak, P. Oxidative Electrolyte Solvent Degradation in Lithium-Ion Batteries: An In Situ Differential Electrochemical Mass Spectrometry Investigation. J Electrochem Soc 1999, 146 (5), 1702–1706.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries. J Electrochem Soc 2017, 164 (7), A1361–A1377. https://doi.org/10.1149/2.0021707jes.(3) Zhang, X.; Soc, J. E.; Zhang, X.; Kostecki, R.; Richardson, T. J.; Pugh, J. K.; Ross, P. N. Electrochemical and Infrared Studies of the Reduction of Organic Carbonates Electrochemical and Infrared Studies of the Reduction of Organic Carbonates. Journal of The Electrochemical Society 2001, 148 (12), A1341–A1345. https://doi.org/10.1149/1.1415547.(4) Moshkovich, M.; Cojocaru, M.; Gottlieb, H. E.; Aurbach, D. The Study of the Anodic Stability of Alkyl Carbonate Solutions by in Situ FTIR Spectroscopy, EQCM, NMR and MS. Journal of Electroanalytical Chemistry 2001, 497, 84–96. https://doi.org/http://dx.doi.org/10.1016/S0022-0728(00)00457-5.

The Effect of Cation Contamination on the Water Transport for PEM Water Electrolysis

Large-scale hydrogen production by PEM water electrolysis (PEMWE) in the energy sector requires a deep understanding of the system behaviour in failure cases and possible recovery procedures. This includes the purification of the anolyte feed water because the contamination with ions can cause the PEMWE stack to breakdown.[1] For instance, the presence of iron trace cations or other metal cations in the feed water leads to a performance loss due to higher ohmic resistance of the cell and higher iR-free voltages.[2] [3] Moreover, cation contamination might influence the water transport in a PEMWE cell. In principle, the transfer of the water molecules to the cathode is proportional to the proton flux from the cathode to the anode. The proportionality factor is defined as water transport coefficient and is also temperature dependent.[4] In case of the mono- and multivalent cations as trace elements in the feed water, the water loading of the membrane, which is expressed by water molecules per sulfonic acid group of the membrane, λ, decreases over time and leads to successive increase in the ohmic resistance. [5] However, the influence of cation contamination on the water transport in PEMWE is poorly understood to date.In this work, we investigated the effect of cation contamination in the anode feed water on the water crossover for PEMWE. Cation contamination experiments were carried out at a constant current density by adding different concentrations such as K2SO4, Na2SO4, etc. to the anode feed water. All electrochemical measurements were carried out in an in-house test bench equipped with a single cell of 5 cm2 geometric electrode area and potentiostat with booster. A commercially available catalyst coated perfluorosulfonic acid membranes (loading of 0.3 mg cm-2 geo Pt/C and 1 mgcm-2 geo IrOx) was used. The anode and cathode compartments were kept at atmospheric pressure and cell temperature of 80°C. To determine the water crossover from anode to cathode at different current densities, the cathode exhaust was cooled in a heat exchanger and measured by a gravimetric method. [4] Firstly, a conditioning procedure was performed with purified feed water until steady cell performance was achieved. The cell performance was evaluated using polarization curves, electrochemical impedance spectroscopy (EIS) and water crossover measurements. The last two are of particular interest, because they provide information about membrane humidification. The Tafel slope and mass transport overvoltage were established from the polarization curves and high frequency resistance (HFR). We observed a significant rise of the cell voltage by adding cations to the anode water feed within few hours. The analysis of Tafel slope and EIS data also reveal an increase of ohmic and mass transport resistances. In these experiments, we were able to correlate the HFR results with the water crossover as a function of the nature of the cation and its concentrations. Our data shows that the specific water crossover per current density decreases as a consequence of cation contamination. The switch back to purified feed water allows us to monitor the dynamics of the performance recovery process obtained from EIS and water crossover measurements. Additional measurements of the polarization curves and HFR were used to distinguish between reversible and irreversible degradation processes due to the cation contamination.Altogether, we present the effect of cation contamination on the water transport processes and provide deeper insights into the mass transport and performance losses for PEMWE.

(Invited) Zwitter Ion-Promoted Hybrid Sepiolite Membranes for Energy Applications

Functionalized natural clays exhibit significant potential as electrode materials, solid-state electrolytes and separators in energy storage and conversion devices. Features that evoke interest in clay minerals are their (1) porous structures and adsorption properties, (2) tunable surface areas, (3) remarkable thermal and mechanical stabilities, (4) abundant natural reserves, and (5) cost. By tweaking such features along with functionalization with desired molecules, they can be used as an ideal candidate for solid-state electrolyte and battery separators. Naturally occurring clays possess layered silicate structures with interchangeable, intercalated ions and tunable chemical properties. Among them, sepiolite, which is a hydrated magnesium silicate, is one of the most appealing clays having a high-aspect ratio. The blocks and channels in sepiolite with discontinuity in the external silica sheet expose a significant number of surface-silanol groups directly accessible to functionalize with organic and inorganic species. Moreover, its exceptionally high surface area and high cation exchange capacity (CEC) compared to other clays enables easy functionalization, thereby imparting exceptional mechanical durability and material properties that can be used for myriads of applications.In this work a sustainable, cost-effective, environmentally benign, carbon-neutral, and bio-friendly sepiolite clay composites have been developed using betaine and ionic liquid without the use of polymers, metals, or carbon-based precursors.Betaine-modified sepiolite clay composites have been synthesized to fabricate flexible and mechanically robust membranes. These membranes are characterized using powder X-ray diffraction, ATR-FTIR, XPS, surface area analyses, rheology, and imaging techniques (e.g., SEM). Morphological analyses reveal sepiolite possesses needle-shaped morphology in a highly aggregated state. The addition of betaine disaggregates such bundles, forming a 3D network with unique “clay anemone” structures. The XRD patterns are consistent with the literature reflecting no change in the d-spacing and corresponding 2θ value (d100 2θ = 7.4 ͦ) before and after betaine treatment. This provides strong evidence of modification limited to the external surface. ATR-FTIR spectra of the sepiolite-betaine composite membranes show the presence of 935, 1330,1391, and 1616 cm-1 frequencies that correspond to C-N-C, N-C-H, C-N and C=Ostretching respectively. This suggests the successful functionalization of sepiolite clay by betaine through an intercalation mechanism. Further, the electrochemical properties of betaine-modified sepiolite membranes are investigated by electrochemical impedance spectroscopy (EIS) measurements to study the ion dynamics of the hybrid membranes. The room temperature ionic conductivities of these membranes are measured using lithium perchlorate (LiClO4) in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solvent (1:1). The concentration of LiClO4 is varied to monitor the change in ionic conductivity and ease of ion transfer through the membranes. The highest ionic conductivity is observed in 0.1M LiClO4 concentration which is superior to the ionic conductivity of sepiolite or hybrid materials with carbon-based precursors, metal ions, quantum dots, or polymers. Viscoelastic properties of the sepiolite slurries with varying concentrations of betaine are also studied to investigate the interaction mechanism between the sepiolite-betaine system and correlate the impact of these interactions on the structure and ionic conductivity of the hybrid sepiolite membranes. Contrary, the IL-sepiolite membranes exhibit superior ionic conductivity compared to betaine-sepiolite membranes, which gives a distinct advantage in the physicochemical and electrochemical properties. The composites are synthesized with varying concentrations of ionic liquid-to-clay ratios and pressed into pellets for conductivity measurements. The highest ionic conductivity of 1 x 10-2 S/cm is achieved by modifying the clay with 10 wt. % IL specimens. This helps to improve the ionic conductivity of the IL-clays. The XRD spectra of sepiolite-IL composites show almost no change in the 2 theta values and peak intensities compared to the pristine sepiolite, confirming the crystalline structure of the sepiolite clay is intact after functionalization with IL. The ATR-FTIR confirms the IL-sepiolite modification with the appearance of a new band at 1345 cm-1 that is attributed to -CH stretching frequencyfrom the alkyl chains of IL present in the clay structures.We have developed a new approach using earth-abundant silicate-based materials to synthesize hybrid zwitterion-modified sepiolite clay membranes. Such materials are not only flexible and mechanically durable, but they also exhibit enhanced ionic conductivity and show exceptional thermal and chemical durability at temperatures above 300 °C and with most aggressive organic solvents, making them a superior candidate for an ergonomic and eco-friendly membrane-based separator for energy devices and gas absorption. Current work is in progress to understand the fundamentals underlying the unique structure-property relationship of hybrid sepiolite, zwitterion-silicate interactions, and conductivity variance with sepiolite architectures, thereby exploiting newer and better properties for application in catalysis, flexible electronics, and biomedical fields. Figure 1

(Invited) Nitrogen-Doped High-Entropy Alloy as Efficient and Stable Electrocatalyst

The development of Pt-based catalysts for use in fuel cells that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. The high-entropy alloy (HEA) is one of the most promising candidates for electrocatalysts, owing to their unique properties involving the high entropy effect, lattice distortion effect, sluggish diffusion effect, and “cocktail” effect.1 Alternatively, nitrogen (N)-doping effects have been well documented and demonstrated in our previous studies, which further fine-tune the adsorption energetics of intermediates and improve the corrosion resistance by the pinning effect of metal-nitrogen bonds.2-3 Combining the HEA and N-doping strategies, the tailored lattice and interstice atomic sites lead to the formation of chemically stable multiple metal-nitrogen (M-N) bonds and the generation of the possible synergistic effect on promoting ORR performance. Herein, we report a N-doped high-entropy alloy (HEA) electrocatalyst that consists of a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (denoted as N-Pt/HEA/C). The N-Pt/HEA/C catalyst showed high mass activity of 1.34 A mgPt -1 at 0.9 V for the oxygen reduction reaction (ORR), which substantially outperformed commercial Pt/C and most of the other binary/ternary Pt-based catalysts. The N-Pt/HEA/C catalyst also demonstrated excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) testing. Using operando X-ray absorption spectroscopy (XAS) measurements and theoretical calculations, we revealed that the enhanced ORR activity of N-Pt/HEA/C originated from the optimized adsorption energy of intermediates, consequent of the tailored electronic structure formed upon the N-doping. Furthermore, we showed that the multiple metal-nitrogen bonds formed synergistically improved the corrosion resistance of the 3d transition metals and enhanced the ORR durability.References Yao, Y.; Dong, Q.; Brozena, A.; Luo, J.; Miao, J.; Chi, M.; Wang, C.; Kevrekidis, I. G.; Ren, Z. J.; Greeley, J.; Wang, G.; Anapolsky, A.; Hu, L., High-entropy nanoparticles: Synthesis-structure-property relationships and data-driven discovery. Science 2022, 376 (6589), eabn3103.Zhao, X.; Xi, C.; Zhang, R.; Song, L.; Wang, C.; Spendelow, J. S.; Frenkel, A. I.; Yang, J.; Xin, H. L.; Sasaki, K., High-Performance Nitrogen-Doped Intermetallic PtNi Catalyst for the Oxygen Reduction Reaction. ACS Catalysis 2020, 10 (18), 10637-10645.Zhao, X.; Cheng, H.; Song, L.; Han, L.; Zhang, R.; Kwon, G.; Ma, L.; Ehrlich, S. N.; Frenkel, A. I.; Yang, J.; Sasaki, K.; Xin, H. L., Rhombohedral Ordered Intermetallic Nanocatalyst Boosts the Oxygen Reduction Reaction. ACS Catalysis 2020, 11 (1), 184-192.

Advanced Nanostructured Textile Catalysts for Enhanced Hydrogen Production in PEM Water Electrolysis: An Overview and Recent Developments

As the global focus shifts towards a zero-emission society, hydrogen has emerged as a critical element in various industries, particularly in green hydrogen production. A promising method for this is Polymer Electrolyte Membrane Water Electrolysis (PEMWE), which uses pure water and electricity to produce high-purity hydrogen. However, the efficiency of PEMWE is highly dependent on the effectiveness of its catalysts, particularly in the context of the catalyst-coated membranes (CCMs) employed in these systems. Conventional preparation of these CCMs involves multiple steps, including synthesis and application of a catalyst ink, which often leads to catalyst agglomeration and requires high Ir (Iridium) loadings to maintain uniformity and performance.In our latest study, we have developed a novel approach using ultralight conductive IrO2 nanostructured textiles (NST) as a catalyst layer for PEMWE. This innovative method successfully circumvents the issue of electrically isolated inactive catalysts, a common problem with lower Ir amounts in traditional catalyst layers. By employing a process of sputtering IrO2 onto water-soluble polymer nanofibers, we have achieved facile CCM preparation and demonstrated unprecedented long-term hydrogen evolution with ultra-low Ir mass loadings at high rates. Our study revealed that the high performance of these CCMs is derived from their low resistivity and unique porous three-dimensional structure.The preparation of NST involves dissolving polyvinylpyrrolidone (PVP) in methanol, followed by electrospinning this solution onto a metal-foil collector. The resulting PVP nanofibers then undergo a sputtering process to deposit IrO2, forming the nanostructured textiles. This method eliminates the need for wet chemical routes and labor-intensive processes typically associated with conventional catalyst layer preparations, thus offering a more environmentally friendly and scalable alternative.Our performance evaluations of these NST CCMs, with varying Ir mass loadings, have shown significant advancements. The operational cell voltages of CCMs containing as little as 0.05 mg-Ir cm−2 were suppressed across all investigated current densities compared to conventional CCMs. Notably, operational voltages remained below 2 V even at the highest current density of 7 A cm−2. This level of performance is unprecedented for CCMs with such low catalyst amounts. Additionally, the long-term stability of these CCMs was remarkable, with operational times exceeding 1500 hours without significant increases in voltage, showcasing their potential for practical applications.An essential aspect of our study was the analysis of the electrical properties and the surface structure of the IrO2 NSTs. We observed that the electrical conductivity of these textiles was significantly higher than that of conventional catalyst layers. This high conductivity, coupled with the unique interconnected structure of the textiles, ensures efficient electrical contact between nano catalysts and the current collector, thus avoiding the formation of isolated catalysts. Our analyses revealed that the IrO2 NSTs have unique surface terminations and a high number of undercoordinated surface Ir atoms, contributing to their high intrinsic catalytic activity.Further insights were gained through operando X-ray absorption spectroscopy (XAS) measurements, which showed that the IrO2-nanostructured textiles possess a large number of D-band holes and are intrinsically highly active. This finding is crucial as it links the unique chemical state of the textiles to their high activity in oxygen evolution reactions (OER), a key process in PEMWE.Our recent developments in nanostructured textile catalysts for PEMWE mark a significant leap in hydrogen production technology. By demonstrating high-rate hydrogen production at low operational voltages and with significantly lower Ir mass loadings compared to conventional CCMs, we have addressed major challenges in the field. These advancements not only enhance the efficiency and sustainability of hydrogen production but also open avenues for applying this technology to other electrochemical conversion cells, paving the way for broader applications and further research. In this talk, we discuss the latest activity on NST, and highlight the challenges and opportunities for hydrogen production.

On-Board Diagnostics for Li-Ion Batteries Using Electrochemical Impedance Spectroscopy (EIS)-Based Health Estimation Models

There exists a growing need for standardized On-Board Diagnostics (OBD) for electric vehicles [1] to provide accurate health metrics and guarantees to both consumers and manufacturers.Previous work has shown the powerful LIB capacity-based State-of-Health (SoH) estimation capability of Electrochemical Impedance Spectroscopy (EIS) measurements and data-driven models [2] [3]. Since EIS measurements are dependent not only on SoH but also State-of-Charge (SoC) and temperature [4], it is important that the measurements are conducted after the cell reaches an equilibrium, and that these other variables are also tracked. Although EIS measurements are somewhat quicker than some traditional capacity-determination experimental methods, the time taken for such measurements is not insignificant. Therefore, building a pipeline to determin the EIS frequency measurements most important for SoH estimation is an important task in developing a suitable EIS-based OBD. By exploring a frequency range between 0.1 and 200 Hz, we study EIS measurements related to diffusion and charge transfer processes in LIB operation [5], with each process being partially distinguishable due to their varying timescales.In this work, using EIS data collected from 5Ah LIBs with an NMC-111 cathode and a graphite anode at various SoCs (0, 25, 50, 75 and 100%) and cell lifetime (0, 10, 20, 40 and 90 days) as input features, we develop sequential, data-driven health estimation models for LIBs. The 22 cells used for this analysis have been aged over a period of 90 days in two different ways: either through active cycling at different C-rates (0.2 and 1C) and temperatures (0, 25 and 40⁰C), or passive “calendar aging” where the cells are left without use at a specific temperature and SOC. Using feature attribution techniques (Shapley values, feature occlusion, etc.), we find the most influential of the frequency ranges in the EIS measurements that relate strongly with cell performance degradation. To develop a streamlined and efficient SoH estimation framework, we formulate an optimization problem to find the EIS experimental design in terms of frequency ranges that delivers maximum accuracy in SoH estimation at different points in the lifetime of the cell. These streamlined and optimized EIS experimental designs and SoH estimation models can be used directly in the development of rapid and efficient on-board diagnostic tools.

Insights into the Structure and Activity of Bimetallic Au/Cu2O Catalysts during CO2 Electroreduction to C2 products

Despite its promising performance for C2+ product formation in electrochemical reduction of CO2 (CO2RR), Cu-based catalysts exhibit a relatively poor selectivity towards important chemical intermediates such as ethylene and ethanol. Up to 16 different products can typically be formed during CO2RR.1 Cu is the only element that can form C-C bonds at an appreciable rate.2 Therefore, it is critical to better understand the active sites of Cu-based electrocatalysts, as it can lead to better catalyst design with improved selectivity towards desired products. A common approach is to investigate the reactivity of well-defined surface terminations of Cu, which can be achieved by employing shape-controlled (nano)particles.3 Here, we expanded on such studies by evaluating the impact of Au on well-defined Cu2O surfaces for the electrochemical reduction of CO2. This promoter metal was chosen, because it can reduce CO2 to CO, which is widely accepted as the key intermediate for C-C coupling.2,4 In this study, 40 nm Cu2O nanocubes that predominantly expose the (100) facet were synthesized with a ligand-free method. The (100) facet is more active in the production of ethylene at the expense of unwanted methane formation, which takes place predominantly at the (111) facet.3 The surface of the nanocubes was decorated with Au nanoparticles by a galvanic replacement reaction. HAADF-STEM images of the catalysts with various Au loadings are shown in Figure 1a-d. After addition of Au nanoparticles, the shape of the Cu2O nanocubes was preserved. However, the Au nanoparticles were highly dispersed at low loadings (1 mol % Au) whereas Au clusters were observed at higher loadings (10 mol % Au). This result was in line with synchrotron X-ray diffraction (XRD) results, whereby the Au(111) reflection appears for the 10Au/Cu2O sample. Besides, the formation of a Au-Cu alloy and CuO phase was confirmed as well. The highly dispersed nature of the Au nanoparticles at low loadings was further established by Au 4f X-ray photoelectron spectroscopy (XPS), whereby a shift towards higher binding energies was observed as compared to the samples with higher Au loadings. The electrocatalytic performance of the catalysts was assessed over a range of potentials in neutral electrolyte (0.1 M KHCO3) in H-cell configuration. With highly dispersed Au as promotor, we found that the formation of ethylene and ethanol was significantly enhanced (Figure 1e-f). More specifically, the Faradaic effiency towards ethanol increased by roughly 1.5 times by introducing 1 mol % Au to the Cu2O nanocubes. Surprisingly, the 10Au/Cu2O sample displayed the lowest Faradaic efficiency for C2 products. We believe that the dispersion of Au on the Cu2O surface plays an important role in the product formation, whereby CO can be formed at Au sites and diffuse to Cu sites in proximity for further reduction to C2 products (CO spillover). To further study structure-activity relationships, we performed in-situ X-ray absorption spectroscopy (XAS) measurements. Figure 1g shows the normalized Cu K-edge X-ray absorption near-edge structure (XANES) of the Cu2O and Au/Cu2O samples in the final state. The evolution of the Cu(0), Cu(I) and Cu(II) fractions was followed by Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis. This analysis revealed that the final oxidation state of the catalysts is different. The Cu(I)/Cu(0) ratio shows the following trend 1Au/Cu2O > 5Au/Cu2O > Cu2O > 10Au/Cu2O. Based on these observations, we hypothesize that the oxidation state of Cu affects the product distribution and in particular, the formation of oxygenates. The presence of Cu(I) species and the formation of Cu(0) species during CO2 electroreduction was further confirmed with in-situ XRD measurements. Finally, we performed quasi in-situ XPS measurements to study the reduction of the surface of the catalysts. Acknowledgements This publication is part of the research programme 'Reversible Large-scale Energy Storage' (RELEASE) with project number 17621 which is financed by the Dutch Research Council (NWO).

Corrosion of Copper-Nickel Alloys in Seawater-Based Electrolytes

Copper nickel materials are used for their corrosion resistance due to the protective film that forms in seawater. The development of this film over time depends on the exposure condition, and it is critically important to understand the aging process for CuNi components used in marine systems to ensure passivity under sulfide contaminated conditions. Here we study the formation of the protective film over time and its effect on corrosion performance. Electrochemical impedance spectroscopy (EIS) measurements during exposure are used to develop a model of film growth on CuNi 70/30 alloys. These electrochemical measurements are coupled with surface analytical techniques such as glow discharge-optical emission spectroscopy (GD-OES) and X-ray photoelectron spectroscopy (XPS) to characterize the composition of the surface oxide. GD-OES results show that as the protective layer forms, nickel mainly remains at the metal surface and the outer layers are copper enriched oxides and chlorides. There is also incorporation of various elements from the seawater within the film. The rate of this film growth is correlated to the resulting decrease in corrosion rate. Different exposure conditions, such as under flow, are investigated.The translation of these protective film mechanisms to additively manufactured CuNi materials is also of interest, to ensure similar performance with these materials. Evaluating corrosion performance of these newly developed materials involves careful comparison to existing knowledge of film growth and potential for breakdown.