Electrochemical Atomic Force Microscopy Research Articles

Transpassive Metal Dissolution vs. Oxygen Evolution Reaction: Implication for Alloy Stability and Electrocatalysis.

Multi-principal element alloys (MPEAs) are gaining interest in corrosion and electrocatalysis research due to their electrochemical stability across a broad pH range and the design flexibility they offer. Using the equimolar CrCoNi alloy, we observe significant metal dissolution in a corrosive electrolyte (0.1 M NaCl, pH 2) concurrently with the oxygen evolution reaction (OER) in the transpassive region, despite the absence of hysteresis in polarization curves or other obvious corrosion indicators. We present a characterization scheme to delineate the contribution of OER and alloy dissolution, using scanning electrochemical microscopy (SECM) for OER-onset detection, and quantitative chemical analysis with inductively coupled-mass spectrometry (ICP-MS) and ultraviolet visible light (UV/Vis) spectrometry to elucidate metal dissolution processes. In situ electrochemical atomic force microscopy (EC-AFM) revealed that the transpassive metal dissolution on CrCoNi is dominated by intergranular corrosion. These results have significant implications for the stability of MPEAs in corrosion systems, emphasizing the necessity of analytically determining metal ions released from MPEA electrodes into the electrolyte when evaluating Faradaic efficiencies of OER catalysts. The release of transition metal ions not only reduces the Faradaic efficiency of electrolyzers but may also cause poisoning and degradation of membranes in electrochemical reactors.

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries.

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Molecularly imprinted electrochemical biosensor for thrombin detection by comparing different monomers.

Aim: Investigating molecularly imprinted polymers (MIPs) in electrochemical biosensors for thrombin detection, an essential protein biomarker. Comparing different monomers to showcase distinct sensitivity, specificity and stability advantages. Materials & methods: Dopamine, thionine and ethanolamine serve as monomers for MIP synthesis. Electrochemical methods and atomic force microscopy characterize sensor surfaces. Performance is evaluated, emphasizing monomer-specific electrochemical responses. Results: Monomer-specific electrochemical responses highlight dopamine's superior signal change and stability over 30days. Notably, a low 5pg/ml limit of detection, a broad linear range (5-200pg/ml) and enhanced selectivity against interferents are observed. Conclusion: Dopamine-based MIPs show promise for high-performance electrochemical thrombin biosensors, suggesting significant applications in clinical diagnostics.

An electrochemical immunosensor based on hybrid self-assembled monolayers for rapid detection of Bombyx mori nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly contagious and poses a serious threat to sericulture production. Because there are currently no effective treatments for BmNPV, a rapid and simple detection method is urgently needed. This paper describes an electrochemical immunosensor for the detection of BmNPV. The immunosensor was fabricated by covalently immobilizing anti-BmNPV, a biorecognition element, onto the surface of the working gold electrode via 11-mercaptoundecanoic acid (MUA)/β-mercaptoethanol (ME) hybrid self-assembled monolayers. Electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM) were used to characterize the electrochemical performance and morphology of the immunosensor, respectively. Under optimum conditions, the developed immunosensor exhibited a linear response to BmNPV polyhedrin in the range of 1 × 102–1 × 108 fg/mL, with a low detection limit of 14.54 fg/mL. The immunosensor also exhibited remarkable repeatability, reproducibility, specificity, accuracy, and regeneration. Normal silkworm blood was mixed with BmNPV polyhedrin and analyzed quantitatively using this sensor, and the recovery was 92.31 %–100.61 %. Additionally, the sensor was used to analyze silkworm blood samples at different time points after BmNPV infection, and an obvious antigen signal was detected at 12 h post infection. Although this result agreed with that provided by the conventional polymerase chain reaction (PCR) method, the electroanalysis method established in this study was simpler, shorter in detection period, and lower in material cost. Furthermore, this innovative electrochemical immunosensor, developed for the ultra-sensitive and rapid detection of BmNPV, can be used for the early detection of virus-infected silkworms.

Ionic Solvent Shell Drives Electroactuation in Organic Mixed Ionic-Electronic Conductors.

The conversion of electrochemical processes into mechanical deformation in organic mixed ionic-electronic conductors (OMIECs) enables artificial muscle-like actuators but is also critical for degradation processes affecting OMIEC-based devices. To provide a microscopic understanding of electroactuation, the modulated electrochemical atomic force microscopy (mEC-AFM) is introduced here as a novel in-operando characterization method for electroactive materials. The technique enables multidimensional spectroscopic investigations of local electroactuation and charge uptake giving access to the electroactuation transfer function. For poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based microelectrodes, the spectroscopic measurements are combined with multichannel mEC-AFM imaging, providing maps of local electroactuation amplitude and phase as well as surface morphology. The results demonstrate that the amplitude and timescales of electroactuation are governed by the drift motion of hydrated ions. Accordingly, slower water diffusion processes are not limiting, and the results illustrate how OMIEC microactuators can operate at sub-millisecond timescales.

Activationless Electron Transfer of Redox-DNA in Electrochemical Nanogaps.

Our recent discovery of decreased reorganization energy in electrode-tethered redox-DNA systems prompts inquiries into the origin of this phenomenon and suggests its potential use to lower the activation energy of electrochemical reactions. Here, we show that the confinement of the DNA chain in a nanogap amplifies this effect to an extent to which it nearly abolishes the intrinsic activation energy of electron transfer. Employing electrochemical atomic force microscopy (AFM-SECM), we create sub-10 nm nanogaps between a planar electrode surface bearing end-anchored ferrocenylated DNA chains and an incoming microelectrode tip. The redox cycling of the DNA's ferrocenyl (Fc) moiety between the surface and the tip generates a measurable current at the scale of ∼10 molecules. Our experimental findings are rigorously interpreted through theoretical modeling and original molecular dynamics simulations (Q-Biol code). Several intriguing findings emerge from our investigation: (i) The electron transport resulting from DNA dynamics is many times faster than predicted by simple diffusion considerations. (ii) The current in the nanogap is solely governed by the electron transfer rate at the electrodes. (iii) This rate rapidly saturates as overpotentials applied to the nanogap electrodes increase, implying near-complete suppression of the reorganization energy for the oxidation/reduction of the Fc heads within confined DNA. Furthermore, evidence is presented that this may constitute a general, previously unforeseen, behavior of redox polymer chains in electrochemical nanogaps.

Nanoscale Visualization of Lithium Plating/Stripping Tuned by On‐site Formed Solid Electrolyte Interphase in All‐Solid‐State Lithium‐Metal Batteries

AbstractThe interfacial processes, mainly the lithium (Li) plating/stripping and the evolution of the solid electrolyte interphase (SEI), are directly related to the performance of all‐solid‐state Li‐metal batteries (ASSLBs). However, the complex processes at solid‐solid interfaces are embedded under the solid‐state electrolyte, making it challenging to analyze the dynamic processes in real time. Here, using in situ electrochemical atomic force microscopy and optical microscopy, we directly visualized the Li plating/stripping/replating behavior, and measured the morphological and mechanical properties of the on‐site formed SEI at nanoscale. Li spheres plating/stripping/replating at the argyrodite solid electrolyte (Li6PS5Cl)/Li electrode interface is coupled with the formation/wrinkling/inflating of the SEI on its surface. Combined with in situ X‐ray photoelectron spectroscopy, details of the stepwise formation and physicochemical properties of SEI on the Li spheres are obtained. It is shown that higher operation rates can decrease the uniformity of the Li+‐conducting networks in the SEI and worsen Li plating/stripping reversibility. By regulating the applied current rates, uniform nucleation and reversible plating/stripping processes can be achieved, leading to the extension of the cycling life. The in situ analysis of the on‐site formed SEI at solid‐solid interfaces provides the correlation between the interfacial evolution and the electrochemical performance in ASSLBs.

Nanoscale Visualization of Lithium Plating/Stripping Tuned by On-site Formed Solid Electrolyte Interphase in All-Solid-State Lithium-Metal Batteries.

The interfacial processes, mainly the lithium (Li) plating/stripping and the evolution of the solid electrolyte interphase (SEI), are directly related to the performance of all-solid-state Li-metal batteries (ASSLBs). However, the complex processes at solid-solid interfaces are embedded under the solid-state electrolyte, making it challenging to analyze the dynamic processes in real time. Here, using in situ electrochemical atomic force microscopy and optical microscopy, we directly visualized the Li plating/stripping/replating behavior, and measured the morphological and mechanical properties of the on-site formed SEI at nanoscale. Li spheres plating/stripping/replating at the argyrodite solid electrolyte (Li6 PS5 Cl)/Li electrode interface is coupled with the formation/wrinkling/inflating of the SEI on its surface. Combined with in situ X-ray photoelectron spectroscopy, details of the stepwise formation and physicochemical properties of SEI on the Li spheres are obtained. It is shown that higher operation rates can decrease the uniformity of the Li+ -conducting networks in the SEI and worsen Li plating/stripping reversibility. By regulating the applied current rates, uniform nucleation and reversible plating/stripping processes can be achieved, leading to the extension of the cycling life. The in situ analysis of the on-site formed SEI at solid-solid interfaces provides the correlation between the interfacial evolution and the electrochemical performance in ASSLBs.

Organic Thin Films Enable Retaining the Oxidation State of Copper Catalysts during CO2 Electroreduction.

A key challenge in electrocatalysis remains controlling a catalyst's structural, chemical, and electrical properties under reaction conditions. While organic coatings showed promise for enhancing the selectivity and stability of catalysts for CO2 electroreduction (CO2RR), their impact on the chemical state of underlying metal electrodes has remained unclear. In this study, we show that organic thin films on polycrystalline copper (Cu) enable retaining Cu+ species at reducing potentials down to -1.0 V vs RHE, as evidenced by operando Raman and quasi in situ X-ray photoelectron spectroscopy. In situ electrochemical atomic force microscopy revealed the integrity of the porous organic film and nearly unaltered Cu electrode morphology. While the pristine thin film enhances the CO2-to-ethylene conversion, the addition of organic modifiers into electrolytes gives rise to improved CO2RR performance stability. Our findings showcase hybrid metal-organic systems as a versatile approach to control, beyond morphology and local environment, the oxidation states of catalysts and energy conversion materials.

Mapping the nanoscale elastic property modulations of polypyrrole thin films in liquid electrolyte with EC-AFM.

Linking structure to mechanical and elastic properties is a major concern for the development of novel electroactive materials. This work reports on the potential-induced changes in thickness and Young modulus of a substrate supported, perchlorate doped polypyrrole thin film (<100 nm) investigated with electrochemical atomic force microscopy (AFM) under in situ conditions. This was accomplished by nanomechanical mapping of potentiodynamically electropolymerized polypyrrole film in electrolyte solution with AFM during redox cycling. The polypyrrole film thickness and Young modulus follow the electrical potential nearly linearly, increasing due to solvent and ion influx as the film is oxidized, and decreasing during reduction. Our measurements also confirm the presence of a potential-independent, passive swelling which is accompanied by softening of the film, likely caused by osmotic effects. Additionally, the heterogeneous distribution of the Young modulus can be directly traced to the typical nodular surface topography of polypyrrole, with the top of the nodular area possessing lower modulus, thus highlighting the complex relationship between topography and elastic properties.

Unraveling the mechanism of methyl acetate additive for reinforcing the solid electrolyte interface on graphite anodes

A stable ultrathin SEI with an inorganic/organic bilayer structure is in situ formed with addition of methyl acetate, the formation and evolution process are investigated by in situ scanning electrochemical microscopy and atomic force microscopy.

Revealing the synergistic effect of LiF and Li3N in solid electrolyte interphases for stable lithium metal batteries using in situ electrochemical atomic force microscopy

Specific inorganic components (e.g., pure LiF-, Li3N- and LiF/Li3N composites) were obtained in solid electrolyte interphases (SEIs) for lithium metal batteries and synergistic functional mechanism of the LiF and Li3N in SEIs were fully investigated.

Inkjet Printed Ti3C2 Electrodes for Anode-Free Zinc-Ion Battery

Additive manufacturing techniques are coming to the fore in many technological fields as cheaper and more versatile manufacturing routes alternative to the physical deposition methods. In particular, when miniaturized, thin-film or patterned structures are needed, Inkjet printing (IJP) has been widely demonstrated as suitable techniques to fabricate coated electrodes and microdevices for many applications including energy storage 1. In this regard, Zinc-ion batteries (ZIBs) are a promising alternative to the traditional lithium-ion batteries due to the abundance and low cost of zinc. In ZIBs, a metallic Zn chips is usually employed as the anode and a variety of materials, such as metal oxides or polymers, as the cathode. Zinc-ion batteries have the potential for high energy density and long cycle life, however they suffer of the well-known issue of Zn dendritic growth and poor efficiency of the plating-dissolution process 2. Ti3C2 MXene have been recently demonstrated as a high-performance “zincophilic” substrate for smooth and efficient zinc with high reversibility 3,4.In this work, we investigate inkjet-printed Ti3C2 MXene thin coatings as an ideal substrate for anode-free ZIBs. Stable aqueous Ti3C2 MXene inks were formulated and successfully inkjet printed on different substrates. The zinc metal nucleation phenomena on MXene 2D sheets were investigated by electrochemical potentiodynamic polarization techniques as well as galvanostatic ones, comparing them to a zinc metallic foil. Ex-situ electron scanning microscopy (SEM) analyses were employed to observe the plated Zn morphology after cycling at both low and high current density. Taking advantage of the smooth inkjet-printed Ti3C2 MXene coatings, electrochemical atomic force microscopy (EC-AFM) was used for the first time to perform in-operando investigation of the Zn plating process, allowing to reveal the early stage Zn plating mechanism on the as-printed Bibliography C. Li, F. Bu, Q. Wang, and X. Liu, Advanced Materials Interfaces, 9, 2201051 (2022).K. Wang, ACS Omega, 5, 10225–10227 (2020).J. M. Park et al., Journal of Energy Chemistry, 76, 187–194 (2023).Z. Gong et al., Journal of Colloid and Interface Science, 625, 700–710 (2022).

(Invited) In Situ Atomic Force Microscopy and X-Ray Spectroscopy of Electrocatalysts

In situ characterization offers an effective way to study and quantify chemical, structural and morphological changes of electrocatalysts as well as to capture the kinetics of transformations and dissolution. For fundamental understanding of electrochemically induced transformations, such in situ experiments require well-defined materials (ideally single crystalline) as they can provide precise potential-dependent quantities that can be used to accurately assess theoretical models and predictions. In this regard, the study of materials under water oxidizing conditions represents a particular challenge due to multiple pathways for the degradation of the surface and bulk and complexity of high-quality material preparation.In this talk, I will discuss how a combination of in situ microscopy techniques can provide insight into the surface and bulk behavior of electrocatalysts during electrocatalytic water oxidation (OER). Specifically, I will show how operando electrochemical atomic force microscopy (EC-AFM) can be used to record nanoscale changes in the surface morphology during anodic corrosion of a perovskite surface. I will also discuss how operando EC-AFM can be combined with operando transmission X-ray microscopy and scanning electrochemical cell technique to probe the evolution and reactivity of single particles of Co(OH)2. In the end, I will cover key bulk and surface processes that occur in oxides under oxidative conditions as well as possible future development of in situ techniques that can help us probe such transformations.

Surface Modification of Li Metal Anodes Using Ag Nanoparticles and PVDF: An EC-AFM Investigation

Lithium metal has captured interest as an attractive anode material for next-generation batteries due to its extremely large theoretical specific capacity (approximately 3800 mAh/g) and low reduction potential. However, challenges related to uncontrolled lithium plating and stripping, unwanted side reactions, and uneven lithium growth on the surface (top growth) contribute to safety concerns and poor cycling performance, hindering the widespread application of lithium metal batteries [1]. In this study, we conducted electrochemical atomic force microscopy (EC-AFM) to investigate the lithium deposition behavior of silver nanoparticles (Ag NPs) on lithium metal substrate surfaces. Kelvin probe force microscopy (KPFM) revealed that Ag NPs possess a lower surface potential than the lithium substrate surface, providing reaction sites for lithium deposition during the plating process. This leads to the formation of a lithium-silver (LiAg) solid solution instead of pure lithium metal, effectively mitigating the issues associated with uncontrolled Li plating. The LiAg alloy phase exhibited improved reversibility compared to bare lithium metal, resulting in improved cycling performance [2]. Additionally, we found that coating the lithium metal surface with a layer of polyvinylidene fluoride (PVDF) promotes bottom-up lithium deposition while physically suppressing dendrite formation. As an ionically conductive but electronically resistant polymer, PVDF allows Li+ transport while suppressing lithium deposition on the top surface [3], reducing safety issues associated with dendrite formation during lithium deposition. Our EC-AFM results reveal the morphological evolution of Ag NPs and PVDF on lithium metal during the plating and stripping process, offering preliminary insights into the underlying mechanisms. Based on our investigations, we propose that the combination of Ag NPs and PVDF could potentially be an effective choice for modifying lithium metal surfaces. This approach aims to enable controlled lithium plating and stripping, as well as bottom-up lithium deposition, with the goal of improving safety and cycling performance for anodes in the next generation of high-energy-density lithium metal batteries, focusing on improving safety and achieving long life cycle.[1] H. Park et al, Adv. Energy Mater. 2021, 11, 2003039.[2] S. Jin et al, J. Am. Chem. Soc. 2020, 19, 8818.[3] J. Yun et al, ACS Energy Lett. 2020, 5, 3108.

Morphology of RuO2(110) Films during Electrochemistry

RuO2 is a well-known electrocatalyst for the oxygen evolution reaction (OER), a critical electrochemical reaction in the water electrolyzer. The behavior of RuO2 during the OER is, however, a subject of debate, in particular, the corrosion mechanism. In this contribution, we report an electrochemical AFM (EC-AFM) study on well-defined RuO2 films grown using molecular beam epitaxy (MBE). We track the morphology of step edges prior, during, and after the OER in acid and alkaline electrolytes to gain insights into the corrosion behavior. Our EC-AFM experiment shows the potential range of RuO2 corrosion and hints at the role of strain and defects in the RuO2 stability. The heterogeneity of corrosion will be discussed in the context of the catalyst longevity along with possible mitigation strategies.

(Invited) Molecular Imaging of the Local Solvation, Nucleation and Growth Processes at Electrode-Electrolyte Interfaces

Electrochemical energy storage is a complex process that requires the synergistic participation of electrodes, electrolytes, and separators in the system. However, the key energy storage functions usually depend critically on the electrode-electrolyte interfaces, which include both the electrode surface and the electrical double layers (EDLs). Here I will discuss our efforts on developing and using electrochemical 3D atomic force microscopy (EC-3D-AFM) to simultaneously image the electrode and EDLs under operando conditions, with atomic-scale resolution. We study the interface between graphite electrode and various types of electrolytes. We observe rich EDL reconfiguration effects at different electrode potentials, which we find to be directly responsible for the capacitive charge storage functions. As the electrode potential goes beyond the electrochemical stability window of the electrolyte, we observe nucleation and growth processes where the EDLs and the deposited solid clusters exhibit intertwined structures. Such initial stage of electrode processes is likely connected to the long-term cycling behaviors of batteries.

Electrochemical Scanning Probe Microscopy Evaluation of Li-Ion Battery Cathode Degradation through in Situ Oxygen Evolution Measurement and Interfacial Property Characterization

Degradation processes involving oxygen evolution significantly limit the performance, reversibility, and capacity of high energy density Li-ion battery (LiB) cathodes. In addition, oxygen evolution processes create hazardous conditions for LiB operation. Despite the importance of these processes, characterization of evolved O2 with sufficient spatial and temporal versatility is hampered by the complex setups required to perform electrochemical mass spectrometry, the gold standard for evaluating this process. Recently, we introduced a scanning electrochemical microscopy (SECM) approach that enables in-situ, real-time (i.e., few ms temporal resolution), chemically selective detection of O2.[1] Measurements were accomplished by positioning an SECM probe (i.e., an Au ultramicroelectrode) near operating cathode surfaces for detecting O2 via the oxygen reduction reaction (ORR). The superior signal-to-noise of this method enabled the observation of two peaks corresponding to a steady-state evolution at high voltage and a transient O2 evolution at low voltage on materials such as lithium cobalt oxide (LCO) and NMC electrodes.Here, we expand the capabilities of our technique by incorporating correlated measurements aimed at understanding how oxygen evolution profiles are related to surface electrochemical and mechanical properties. Specifically, we will focus on the use of feedback-mode SECM and the use of reactive mediators to evaluate the changes in the rates of electron transfer measured on materials undergoing O2 and after structural degradation. Furthermore, the use of electrochemical AFM allows us to measure the mechanical properties of these interfaces, in an attempt to tie the structural changes and the evolution of oxidizing species with the formation of cathode-electrolyte interfaces. Altogether, these multimodal SECM and AFM methods elucidate the manifold chemical, structural, and performance changes underpinning LiB cathode operation.[1] Mishra, A.; Sarbapalli, D.; Hossain, M.S.; Gossage, Z.T.; Li, Z.; Urban, A.; Rodríguez-López, J. Highly Sensitive Detection and Mapping of Incipient and Steady-State Oxygen Evolution from Operating Li-Ion Battery Cathodes via Scanning Electrochemical Microscopy. J. Electrochem. Soc., 2022, 169, 086501. Figure 1

Synergistic inhibition of molybdate and phytate on chloride-induced corrosion of carbon steel in simulated concrete pore solutions

Improving the corrosion resistance of reinforcing steel is particularly important to prolong the service life of reinforced concrete (RC) structures. This study proposes a novel mixed corrosion inhibitor of sodium molybdate and sodium phytate to enhance the corrosion resistance of Q235 carbon steel in concrete exposed to aggressive environments. Based on the synergistic inhibition of the mixed corrosion inhibitor with an optimal concentration ratio, the passivation and chloride-induced corrosion of steel in simulated concrete pore solutions were investigated using electrochemical measurements, XPS, AFM, SEM/EDS and ATR-FTIR. The results indicated that the mixed corrosion inhibitor could promote the passivation process of steel by increasing the passive film thickness, despite no significantly improved passivation ability was highlighted for steel. Furthermore, due to the formation of a protective and thick passive film with a specific three-layer structure, the mixed corrosion inhibitor markedly improved the pitting corrosion resistance of steel and exhibited a high inhibition efficiency after 30 days of chloride exposure. Accordingly, the corresponding synergistic inhibition mechanism of the mixed corrosion inhibitor was proposed.

Influence of Fe and Ho additions on Sn-3.0Ag-0.5Cu solder alloy: Microstructure, electrochemical and mechanical properties

In this work, the microstructure, electrochemical and mechanical properties of Sn-3.0Ag-0.5Cu solder alloy with Fe and Ho additions were studied. The results demonstrated that adding of Fe and Ho improved the microstructure of solder alloy and formed FeSn2 phase and Ho5Sn3 phase. Besides, the addition of Fe weakened the microhardness and tensile properties of the solder alloy, which was ascribed to a weak interface between FeSn2 and β-Sn matrix. The microhardness and tensile properties of solder alloy were enhanced by adding Ho owing to the improvement of microstructure and the strengthening of the second phase. Electrochemical tests and atomic force microscopy (AFM) results showed that comparing to the base solder alloy, higher corrosion current densities and rougher surfaces after corrosion were found in the solder alloys with the Fe and Ho additions.