The electrochemical nitrogen reduction reaction (NRR) has emerged as a promising route for the production of green ammonia (NH3). In many studies, alloying strategies have been employed to enhance the performance of electrocatalysts in this process. This study assesses the NRR activity of a silver-tin (Ag-Sn) alloy, a mixture of metals, each of which has been previously reported for its high selectivity toward the NRR. Extended control experiments revealed that the detected NH3 stemmed from contamination rather than catalytic activity. Complementary density functional theory (DFT) calculations revealed that, although Ag3Sn exhibits a theoretically lower onset potential than pure Ag or Sn, the associated overpotentials remain excessively high. These findings highlight the critical need for stringent, long-duration control experiments to prevent erroneous attribution of NH3 formation to nitrogen (N2) reduction.

Metallic materials have been widely investigated for use in aluminum electrolysis cells due to their electrical conductivity, mechanical robustness, and ease of fabrication. Under Hall–Héroult conditions, however, their long-term stability is limited by chemical corrosion driven by interactions between alloying elements, oxide scales, and fluoride-based electrolytes. This review considers the chemical corrosion behavior of metallic systems exposed to molten fluorides relevant to aluminum electrolysis, with particular emphasis on Cu–Ni-based, Ni–Fe-based, high-entropy alloys, and Ni-based superalloys. Evidence from cryolite and cryolite-analog electrolytes is analyzed to identify dominant degradation mechanisms, including oxide dissolution, fluoridation reactions, selective elemental leaching, outward metal diffusion, and interfacial delamination. The roles of key alloying elements (Ni, Fe, Cu, Cr, Co, Mo, and dopants like Y) are discussed with respect to oxide-scale stability, interfacial chemistry, and contamination of the aluminum product. Rather than treating corrosion resistance as a bulk alloy property, the reviewed studies indicate that long-term stability is governed by the dynamic evolution of metal-oxide–electrolyte interfaces and by composition-dependent trade-offs between protective spinel formation and fluoride reactivity. On this basis, recurring limitations of conventional metallic systems are identified, along with unresolved gaps in early-stage oxide evolution, time-dependent degradation, and the transferability of results from alternative molten-salt systems.

The development of high-performance metal-nitrogen-carbon (M-N-C) catalysts for electrochemical CO2 reduction (CO2RR) requires precise control over atomic dispersion and coordination environments. Here, we report a mechanochemical ball-milling strategy to synthesize an iron-nitrogen-carbon catalyst (Fe-NC-BM) featuring uniformly dispersed Fe species within a nitrogen-doped carbon matrix. Ball-milling promotes homogeneous Fe site distribution, introduces abundant defects, and modulates the electronic structure. This catalyst achieves a CO Faradaic efficiency exceeding 99% across -0.5 to -1.2 V vs RHE, with a current density of 41.7 mA cm-2 at -1.1 V, more than twice that of the non-ball-milled counterpart (17.0 mA cm-2). Aberration-corrected STEM and XPS analyses confirm that ball-milling enhances Fe dispersion, prevents aggregation during pyrolysis, and fosters Fe-N4 site formation. The mechanical forces also induce an interconnected nanostructure, increasing active site exposure and enabling efficient charge and mass transport. Defect engineering further tunes the electronic structure and lowers the reaction energy barrier, as supported by DFT calculations. This work demonstrates that ball-milling is a solvent-free and effective pretreatment strategy for simultaneously enhancing the density and intrinsic activity of active sites, providing a promising pathway for the rational design and large-scale production of next-generation CO2RR electrocatalysts.

The rise in CO 2 levels presents a significant challenge to the climate, necessitating the development of advanced technologies to reduce emissions. In order to achieve the temperature rise limit set out in the Paris Agreement, which is 1.5 °C, it is necessary to implement innovative solutions such as electrochemical CO 2 reduction (ECR). ECR represents a promising method for the conversion of CO 2 into value-added products, such as formic acid, under mild conditions. Formic acid is of significant value in both the pharmaceutical and agricultural sectors, making it an ideal target for electrochemical CO 2 reduction (ECR). Tin (Sn) has emerged as a leading material for electrochemical CO 2 reduction (ECR), particularly for the production of formic acid, due to a number of favourable characteristics. These include high selectivity towards formate, low hydrogen evolution reaction (HER) activity, abundance and low cost, moderate overpotential, scalability for industrial applications and environmental friendliness. Nevertheless, stability remains a significant challenge, as industrial applications necessitate operational lifetimes of thousands of hours. In this study, the potential of Sn composites made of robust refractory metals, such as niobium (Nb) and tantalum (Ta), has been explored. Pulsed laser deposition (PLD) was employed to deposit Sn-Ta and Sn-Nb refractory metal composites. The composition of the composites was varied in order to optimise ECR performance. The initial experiments were conducted to investigate the activity of pure Nb and Ta for CO 2 reduction. The thin films of these metals, prepared via PLD, were analysed using Electrochemical Mass Spectroscopy (ECMS), which provides real-time insights into the reaction dynamics. The tests conducted on bare glassy carbon substrates demonstrated a limited capacity for CO₂ reduction, with hydrogen (H 2 ) evolution being the predominant phenomenon. In contrast, Nb films exhibited minimal catalytic activity with regard to both CO 2 reduction and H 2 evolution. The performance of PLD-prepared Sn-Nb and Sn-Ta composites will be presented in due course.

Carbon dioxide (CO₂) is a major contributor to climate change, driving global warming and associated environmental challenges. While CO₂ capture and storage technologies face critical barriers, including the risk of re-release, the use of captured CO₂ represents a direct and sustainable approach. By converting CO₂ into valuable fuels and chemicals, this strategy not only mitigates emissions but also reduces dependence on fossil fuels. Among the CO₂ utilisation methods, electrocatalysis holds great promise due to its ability to operate at ambient conditions and achieve high selectivity and efficiency. Lead (Pb) has attracted attention for the electrochemical production of formic acid/formate due to its low cost, chemical stability and high selectivity. Meanwhile, titanium dioxide (TiO₂) is well known for its applications in photodegradation and solar energy conversion. In order to improve the electrochemical performance of the porous Pb electrode for CO₂ reduction by incorporating a photocurrent-generating material, TiO₂ was deposited on the porous Pb surface using atomic layer deposition (ALD) and electrophoretic deposition (EPD). Afterwards, the prepared samples were characterized using a range of techniques, including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). Electrochemical CO₂ reduction tests showed that TiO₂-modified electrodes prepared by both techniques exhibited a reduced onset potential for formate formation, from -0.65 V for porous Pb to -0.45 V and -0.15 V vs RHE for EPD and ALD TiO 2 -modified Porous Pb (see Figure 1). This is accompanied by higher faradic efficiency at lower overpotential and improved energy efficiency in the absence of any light irradiation. Among the samples tested, the porous Pb electrode modified with 150 ALD cycles of TiO₂ showed exceptional performance, achieving a maximum faradaic efficiency of 100% and a maximum energy efficiency of 80% at an overpotential of 120 mV. These values exceed those of the unmodified electrode and are among the highest energy efficiencies reported in the literature. The enhanced performance is attributed to the altered interaction between water molecules and the catalyst surface induced by the presence of TiO₂ at the electrode surface. This study highlights the potential of TiO₂-modified Pb electrocatalysts as highly efficient and selective materials for CO₂ electrochemical conversion. The findings pave the way for the development of high-performance and energy-efficient electrocatalysts for carbon utilization. Figure 1

This study investigates the formation and electrochemical properties of Ni whiskers synthesized on a Ni substrate through heat treatment in an Al-rich environment, followed by leaching in potassium hydroxide (KOH). Scanning electron microscopy (SEM) revealed distinct layers (D1 and D2) formed during heat treatment, consisting of Ni₂Al₃ and Al₃Ni phases, respectively. Over time, Al₃Ni whiskers, oriented and embedded in an Al matrix, formed on the D2 layer. Leaching of Al from these whiskers revealed nanocrystalline Ni whiskers. The study showed that both Al mass and heat treatment time significantly affected the Electrochemical Active Surface Area (EASA). Results indicated that the Ni plate had an EASA of 3 mC cm⁻2, and adding Al linearly increased the EASA to 7100 mC cm⁻2 for 350 mg Al. Longer heat treatment times, from 1 to 10 min, resulted in coarser whiskers and a lower EASA, decreasing from 7125 mC cm⁻2 to 3800 mC cm⁻2. The Ni whiskers demonstrated promising oxygen evolution reaction (OER) activity, with lower overpotentials corresponding to increased EASA. The best sample required an overpotential of 240 mV at 10 mA cm⁻2 and 350 mV at 500 mA cm⁻2. Additionally, this sample showed no degradation at 500 mA cm⁻2 for 6 days (144 h).

The chemistry and localized structure of materials greatly affect their efficiency in gas evolution reactions leading to cell resistance loses. This work focuses on the role of morphological changes in the nature of the bubble distribution by a comparison of two different surface structures: polished Ni and various porous Ni surfaces fabricated by dynamic hydrogen bubble templating. This study introduces a refined method for counting bubbles in gas evolution reactions, focusing on high-density, overlapping bubble flows. By incorporating the Segment Anything Model (SAM), we significantly reduced the training cost of the You Only Look Once algorithm, maintaining accuracy while improving efficiency. Surface characteristics strongly influenced the bubble distribution, with superaerophobic surfaces exhibiting smaller Sauter Mean Diameter (D32) values than polished ones. A polynomial extrapolation indicated that D32s were roughly one third of the thickness of the bubble layer at higher current densities. Building on the established connection between bubble layer thickness and diffusion resistance through electrochemical impedance spectroscopy (EIS), this study establishes a direct relationship between D32 and diffusion resistance. We have also demonstrated that the characteristics of the bubble layer are independent of the electrochemical surface area (ECSA) beneath the surface. This combined approach uses EIS for broader current coverage and imaging for real-time data, providing a powerful framework for bubble management and performance monitoring in gas evolution applications.

Carbon dioxide reduction reaction (CO₂RR) offers significant potential for closing the carbon loop by converting CO₂ into value-added feedstocks for industries or renewable fuels. Among the diverse categories of CO₂RR electrocatalysts, single-atom catalysts (SACs), in particular, metal-nitrogen-doped carbon (M-N-C) catalysts are emerging as promising candidates, owing to their optimal atom utilization efficiency, well-defined active sites, relatively uniform active centers. Up to date, M-N-C catalysts have demonstrated excellent performance in electrochemical CO 2 -to-CO conversion, but only within very narrow potential windows. Such limited working potentials will negatively affect the real-world industrial applications with complex operation variations, which may introduce unwanted byproducts (i.g. H 2 ). Herein, we successfully synthesized a high-efficient Fe-N-C electrocatalysts for CO 2 RR, achieving near 100% CO conversion efficiency across a broaden potential range. In this work, zeolitic imidazolate frameworks (ZIF-8) were utilized as a template to create a porous structure, providing a high surface area and well-distributed pores, while the mechanical collisions from ball milling process can enhance the interaction of Fe source with the support materials. As a result, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the synthesized catalyst possesses atomically dispersed Fe single atom sites, which are uniformly distributed and anchored within a nitrogen-doped carbon matrix. The X-ray photoelectron spectroscopy analysis further corroborates this, showing characteristic peaks corresponding to Fe-N x coordination bonds, indicative of strong interactions between the Fe atoms and the surrounding nitrogen-doped carbon framework. The as-prepared Fe-N-C catalysts were implemented in CO 2 RR, achieving an exceptionally high faradaic efficiency (FE) for CO production, exceeding 99% across a wide potential range from -0.3 V to -1.2 V vs. RHE. In addition, the partial current density of CO at -1.2 V vs. RHE in a flow cell reaches up to 27.25 mA/cm 2 . Stability studies reveal that our catalysts exhibit consistent CO 2 RR performance in 5 hours, maintaining the CO selectivity with near 100%. XANES and theoretical studies will reveal the inherent electronic structure for great CO 2 RR performance with near-unity CO conversion. The details will be shown in the presentation. This work highlights the potential of Fe single-atom catalysts derived from ZIF-8 precursors as a scalable and robust solution for CO₂ electroreduction.

Due to their high thermal and chemical stability, ternary oxide coatings have been extensively investigated as potential solutions for high-temperature solid lubrication. In this study, the tribological behavior of the thermally sprayed Co0.75Ni0.25O and Co0.5Ni0.5O oxide coatings was investigated. The dry sliding reciprocating friction tests were performed against an Al2O3 counterface at room and elevated temperatures using a ball-on-flat tribometer. The results indicated that the friction and wear of Co0.75Ni0.25O oxide coatings did not change significantly at atmospheric and elevated temperature conditions, whereas the friction and wear of the Co0.5Ni0.5O increased at elevated temperatures. The correlations between the worn surface morphologies, the subsurface microstructure induced by wear, and the wear behavior of the coatings were discussed based on the characterizations, (i.e., microhardness, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI)).

High entropy alloys (HEAs) are currently the focus of particular attention, as some of them exhibit outstanding properties that far exceed those of conventional alloys (e.g. exceptional wear resistance, good structural stability, high resistance to corrosion and oxidation), making them of interest in many fields [1]. Their high-temperature stability further broadens their spectrum of applications. In addition, HEAs can be processed into coatings using existing equipment and technologies [2]. The control of the cryolithic bath temperature is crucial for the operation of aluminium electrolytic cells. At present, it is measured punctually, as the thermal probes (thermocouples protected by a 446 steel sheath) corrode rapidly in the cryolithic bath at 950°C, making them unsuitable for continuous operation. In this context, we have studied various HEAs with a view to their use as protective coatings for thermal probes used in aluminium electrolysis cells. Based on the thermodynamic modelling work of Senkov et al [3], we pre-selected different families of HEAs likely to be single-phase at 1000°C. In particular, we studied alloys based on Co-Cr-Fe-Ni. These alloys were synthesized by melting and characterized (XRD, SEM-EDX...) before and after oxidation in air at 1000°C and after immersion in cryolite at 1000°C (Fig. 1). We were thus able to establish their oxidation and corrosion mechanisms as a function of their composition and compare them with those of 446 steel. References [1] E.P. George et al ., High-entropy alloys , Nat. Rev. Mater. 4 (2019) 515. [2] A. Meghwal et al., Thermal spray high-entropy alloys coating: a review . J. Therm. Spray. Tech. 29 (2020) 857. [3] O.N. Senkov et al. Accelerated exploration of multi-principal element alloys with solid solution phases , Nature Comm. 6 (2015) 6529 Figure 1

To address the challenges of large-scale renewable hydrogen production, alkaline electrolyzers present a compelling solution due to their simple design and use of cost-effective materials. However, industrial alkaline water electrolysis often operates under harsh conditions, such as high current densities and concentrated alkaline solutions, which can induce structural degradation and reduce electrode longevity. In addition, the presence of ionic impurities in concentrated alkaline solutions could pause additional challenge in terms of catalyst de-activation during long-term operation. Optimizing electrode materials and electrodes is therefore essential to mitigate issues like high energy consumption, material costs, and limited durability in demanding electrolytic environments. In this context, high entropy alloys (HEAs) have emerged as promising candidates for electrode materials. Their unique elemental compositions could deliver synergistic effects and improving electrocatalytic performance, chemical stability, and structural resilience. In this study, FeCoNiCrMn, FeCoNiCrMo, and FeCoNiCrTi HEA electrocatalysts were fabricated using cold spray and warm spray solid-state deposition techniques. These scalable, industry-relevant methods enable the one-step production of high-performance electrodes while preserving the desirable properties of HEAs. Correlations between the spraying methods, spray parameters, electrode composition, coating characteristics, and performance for the hydrogen evolution reaction (HER) were investigated. Sprayed FeCoNiCrMn and FeCoNiCrMo coatings exhibited lower overpotential for the HER than both Pt and Ni electrodes at 100 mA/cm² in un-purified 10M KOH electrolyte. Comprehensive analyses were conducted on its microstructure, surface roughness, electrochemically active surface area (EASA), and electrocatalytic activity using a variety of techniques, including electrochemical impedance spectroscopy (EIS). This research underscores the potential of porous HEAs as cost-effective and durable materials for advancing sustainable hydrogen production and renewable energy technologies.

This study applies a dual bubble layer model and the Hydrogen Evolution Reaction (HER) test protocol originally developed by Kitajima et al., to both polished flat and 3D microporous Dynamic Hydrogen Bubble Template (DHBT) nickel surfaces. The results showed that bubble-induced diffusion resistance was present on both surface types, supporting the development of a universal, non-destructive protocol for assessing bubble management performance. By focusing on bubble diffusion impedance as a primary performance indicator, the study isolated bubble dynamics from electrochemically active surface area (ECSA) effects, which is often overlooked in traditional assessments. Time-domain transformations of impedance data helped identify the components of an equivalent circuit model, with porous DHBT surfaces exhibiting an additional charge transfer process compared to flat surfaces. Using photographic evidence and Nyquist plot observations, we define three distinct stages in the HER process: (1) Initial discrete bubble formation with charge transfer dominance, (2) emergence of hydrogen oxidation product (HOPD) and bubble accumulation [17], and (3) formation of a dense bubble layer. Circuit fitting revealed that bubble diffusion resistance was highly dependent on surface morphology, with polished surfaces exhibiting greater resistance than porous ones. Diffusion resistance correlated with bubble layer thickness and scales with the square root of applied current density.

The pledge to curb CO2 emissions with the help of clean energy is an ambitious undertaking, one that is tasked with the development of durable electrocatalysts for fuel cells and electrolyzers. Recent development of gold-palladium (Au-Pd) alloy nanoparticles outlines a contemporary strategy in combining the synergistic effects of a more electrochemically stable gold (Au) metal with catalytically active palladium (Pd). Unfortunately, not all compositions are optimal for an intended application, which requires further fine-tuning-a task that seems difficult with the prevalent wet-chemical synthetic methods. Here, we demonstrate a new approach for fine-tuning surface composition in Au-Pd nanoparticles via a procedure involving the selective diffusion of Pd atoms from the core to the surface induced by the adsorption of oxygen atoms at the surface of Au-Pd NPs. Going forward, a similar technique can be more generally applied toward development of 'self-healing' electrocatalysts that can alter properties suiting different external requirements with a simple switching of voltage, or a controlled pretreatment.

Recently, there has been a notable increase in interest in thin film technology within the fields of nanoscience and nanoengineering. Thin films offer significant potential for application in electronic, optoelectronic, and biomedical fields. This is due to the precise structure control and thereby, reproducibly tunable material properties that thin film deposition techniques can achieve. Among various such techniques, the pulsed laser deposition (PLD) technique has emerged as a prominent approach in the field of physical vapour deposition. It is renowned for its ability to produce thin films of high density and adjustable composition. In particular, PLD is a well-established method for the fabrication of multi-phase metal oxides over a range of substrates. Furthermore, it facilitates the investigation of the influence of particle size and stoichiometry on the physical, chemical, and mechanical attributes of the fabricated thin film. In the case of oxide, the stoichiometric adjustment of the elemental composition can be done by modifying the O 2 partial pressure of the deposition chamber and the substrate temperature. In the present study, we concentrated on the development of copper-based thin film electrocatalysts for biosensor applications. Copper has been identified as an optimal element for electrochemical biosensor applications due to its multiple oxidation states and chemical stability. Therefore, multi-phase copper oxide thin films were fabricated over a Si (100) substrate using the PLD technique. In the present study, we emphasize the fabrication of Cu thin films as a function of oxygen partial pressure (0, 10, 50, 100, 150 mTorr) at ambient temperature. The crystallinity and phase structure of the copper-based thin films were analyzed using grazing-incidence X-ray diffraction (GI-XRD). In addition, X-ray photoelectron spectroscopy analysis revealed the presence of multiple oxidation states of Cu and their chemical compositions. It was observed that changing the background oxygen partial pressure during the deposition influenced the oxygen content in the thin film, resulting in the formation of mixed oxide states Cu 2 O and CuO nanostructure. Among Cu-based electrodes prepared at different O 2 partial pressure (Cu-0, Cu-10, Cu-50, Cu-100, and Cu-150), Cu-0 exhibited an extraordinary sensitivity of 0.69 mA.cm -2 . mM -1 for glucose detection and the lowest detection limit of 1.76 μM in the linear range of 10 μM - 5 mM. Moreover, the Cu-0 sample showed the highest selectivity towards glucose detection among various organic, inorganic, and biological molecules. Thus, our study demonstrates an innovative method for nano-scale electrode fabrication and application in biological molecular detection. Figure 1

In this work, 3D Ni substrate with superaerophobic properties were catalyzed through the deposition of NiCo layered double hydroxide (NiCo-LDH). Deposition of NiCo-LDH was achieved using a mixed solution of Ni and Co nitrate salts as precursors. Precipitation of NiCo-LDH was achieved by the application of a negative current that caused a local raise of the solution pH. NiCo-LDHs were deposited in the form of vertically aligned nanosheets that infiltrate throughout the porous structure of the superaerophobic 3D Ni substrate. Variation of the deposition current changed the composition of the NiCo-LDH. The performance and the mechanisms responsible for the OER were investigated through a series of electrochemical experiments that include electrochemical impedance spectroscopy. As exemplified by a change of the Tafel slope from ca. 60 to 40 mV/decade, the OER mechanism is modified by the deposition the NiCo-LDH. In 1 M KOH at room temperature, the best electrode has overpotential as small as 269 and 340 mV at 10 and 250 mA cm−2, respectively. Longer-term electrolysis test during 56 h at 250 mA cm−2 showed a constant overpotential of 340 mV, without any sign of degradation.

A key component of green hydrogen production technologies is the fabrication of large-scale porous transport layer (PTL) for use in anion electrolyte membrane water electrolysers (AEMWEs). One strategy to achieve that goal is to manufacture Ni-based 3D electrode skeletons that can be further catalyzed to achieve high current densities at low overpotentials. In the present work, shock-wave induced spray (SWIS) and cold spray (CS) deposition techniques were used to prepare 20 cm(2) Ni-based electrode skeletons. In our experimental conditions, the porosity of coatings prepared using the SWIS deposition system and Ni powders with particle sizes D50 = 32 and 75 mu m never exceeded 28%. Higher porosity could only be achieved using the CS deposition system and a spheroidal Ni-Al powder, whose particles consist of an aluminum core encapsulated in a nickel shell. After Al leaching in an alkaline solution, the resulting electrodes showed good mechanical and structural integrity with up to 41% porosity. The electrochemical active surface area of the most porous electrodes is a factor of 2100 larger than a polished Ni plate, and it has superaerophobic properties with a captive air bubble contact angle of 151 degrees. During 1 h of electrolysis, the overpotential at 100 mA cm(-2) of the most active leached Ni-Al cold spray deposited electrode was 330 mV, compared to 380 mV for a Ni foam electrode.

Palladium (Pd) nanoparticles have been investigated for the hydrogen evolution reaction, the hydrogen oxidation reaction, the oxygen reduction reaction, and the methanol and ethanol oxidation reaction. The high methanol/ethanol tolerance of Pd is noteworthy. A powerful approach to further improving the catalytic properties of Pd is through alloying with suitable co-metals. In the case of the ethanol oxidation reaction in alkaline media, Pd alloyed with Au has demonstrated excellent activity exceeding even that of highly active Pt catalysts (1). A complete miscibility, small lattice mismatch, and contrasting properties of Pd and Au make Au-Pd a unique model system. Marked changes in the reactivity of Au-Pd catalysts have been reported when conditioning and operating conditions are altered, which should be related to physico-chemical changes in the nanoparticles. Indeed, dealloying/dissolution and segregation of metals at the surface of nanoparticles (NPs) takes place during conditioning and operation, which have pronounced effect on the activity and stability of NPs. The migration of metals from the bulk to the nanoparticles’ surface may take place during pre-catalytic conditioning and during operation, and can be triggered by the presence of oxidation and reducing agents (2), changes in temperature, and potential cycling (3). Thus, it is critical to investigate the effect of electrode potential on metals dissolution and segregation occurring in alloyed nanoparticles. In the present work, Au-Pd NPs prepared by pulsed laser ablation in liquids were used. Pulsed laser ablation is a simple method to synthesize alloy NPs with high purity out of different material systems and with different compositions. Typically, alloy-metal targets or pressed micro-powder mixture targets are ablated or fragmentated in liquid, respectively. Notably, the technique does not require any surfactant additives to achieve NPs stability in the electrolyte, thus allowing for NPs to remain free of associated contamination from the capping agents. We will show how one can fine-tuned the Au and Pd surface fraction of bimetallic Au-Pd nanoparticles using electrochemistry. The method involves electrochemical cycling in alkaline medium at a suitable upper vertex potential, resulting in a controllable diffusion of Pd atoms from the core to the shell. This preferential outward diffusion of Pd atoms occurs because of the electrochemical adsorption of oxygen atoms at the surface of NPs, which is driven by the application of a potential. This potential has to be positive enough for oxygen atoms to adsorb at the surface of the NPs, and therefore is dependent on the nature of the surface atoms. On AuPd NPs that are enriched with Au atoms, a potential of +1.5 V vs. RHE must be applied to form ca. 1 ML of adsorbed oxygen at the surface of gold and initiate the diffusion of Pd atoms from the core to the NP surface. This approach marks a new avenue in the development of compositionally-controlled electrocatalysts and can be tailored to suit different applications with a simple switch of voltage or a controlled pretreatment. References [1] J. B. Xu, et al, Int. J. Hydrogen Energy 35 (2010) 6490-6500. [2] H.L. Xin, et al, Nano Lett., 14 (2014) 3203-3207. [3] E. Pizzutilo, et al, ACS Catal. 2017, 7, 9, 5699–5705.

Catalytic activity and toxicity of mixed-metal nanoparticles have been shown to correlate and are known to be dependent on surface composition. The surface chemistry of the fully inorganic, ligand-free silver-gold alloy nanoparticle molar fraction series, is highly interesting for applications in heterogeneous catalysis, which is determined by active surface sites which are also relevant for understanding their dissolution behavior in biomedically-relevant ion-release scenarios. However, such information has never been systematically obtained for colloidal nanoparticles without organic surface ligands and has to date, not been analyzed in a surface-normalized manner to exclude density effects. For this, we used detailed electrochemical measurements based on cyclic voltammetry to systematically analyze the redox chemistry of particle-surface-normalized gold-silver alloy nanoparticles with varying gold molar fractions. The study addressed a broad range of gold molar fractions (Ag90Au10, Ag80Au20, Ag70Au30, Ag50Au50, Ag40Au60, and Ag20Au80) as well as monometallic Ag and Au nanoparticle controls. Oxygen reduction reaction (ORR) measurements in O-2 saturated 0.1 M KOH revealed a linear reduction of the overpotential with increasing gold content on the surface, probably attributed to the higher ORR activity of gold over silver, verified by monometallic Ag and Au controls. These findings were complemented by detailed XPS studies revealing an accumulation of the minor constituent of the alloy on the surface, e.g., silver surface enrichment in gold-rich particles. Furthermore, highly oxidized Ag surface site enrichment was detected after the ORR reaction, most pronounced in gold-rich alloys. Further, detailed CV studies at acidic pH, analyzing the position, onset potential, and peak integrals of silver oxidation and silver reduction peaks revealed particularly low reactivity and high chemical stability of the equimolar Au50Ag50 composition, a phenomenon attributed to the outstanding thermodynamic, entropically driven, stabilization arising at this composition.