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Mineral oil is the primary insulating medium in power transformers with DC electric field, yet the electrochemical processes governing its electrical response under DC stress remain poorly understood. This study combines chronoamperometry, cyclic voltammetry, and electrochemical impedance spectroscopy to identify the dominant current conduction mechanisms arising from trace water in transformer oil under DC stress. Chronoamperometry reveals that the current response contains both a capacitive component associated with electric double layer charging and a diffusion-limited Faradaic component arising from the redox reactions of water molecules at the electrode surface. Cyclic voltammetry confirms water redox reactions through characteristic current peaks, and analysis of spike features in the voltammograms reveals that water exists in the oil as micro- and nano-scale droplets, whose size distribution and abundance were quantified electrochemically. Repeated voltage sweep cycles lead to progressive water depletion, resulting in decreased current levels. Finally, electrochemical impedance spectroscopy resolves the frequency-dependent transition between ohmic and capacitive regimes and confirms the progressive depletion of water near the electrode surface through the evolution of charge transfer resistance across successive measurement cycles. These findings challenge the prevailing ion drift model and demonstrate that interfacial Faradaic reactions involving water play a central, previously overlooked role in the electrical response of transformer insulating oil.

Chromium-based coatings are of interest for applications in harsh environments due to their high hardness and excellent resistance to corrosion. However, coatings deposited by magnetron sputtering often exhibit columnar microstructure which can compromise their performance by promoting porosity and reducing mechanical integrity. In this work, we present an approach to overcome this limitation by alloying Cr with Nb and N, forming (Cr1-xNbx)N-y films. Increasing Nb content in Cr1-xNbx films (0.19 <= x <= 0.55) led to an X-ray amorphous microstructure, confirmed by X-ray diffractometry and high-resolution transmission electron microscopy, while hardness remains similar to 8 to 11 GPa regardless of Nb content. Adding N to Nb-containing films (0.19 < x < 0.55) produced a nanocrystalline, denser microstructure with reduced column width, resulting in a similar to 66 % higher hardness (similar to 15 GPa) than the metallic films. Electrochemical measurements show enhanced passivity of Nb-containing coatings above 1 V vs. Ag/AgCl, attributed to the suppression of soluble Cr6+ species. X-ray photoelectron spectroscopy results indicate that Nb2O5 could oxidize soluble species in the electrolyte forming NbO2 on the surface. These findings show how Nb and N additions can tailor the microstructure, mechanical properties, and corrosion resistance of (Cr1-xNbx)N-y films.

Lithium iron phosphate (LFP) is widely considered as a low-potential positive electrode material. Herein, the high-voltage stability and capacity retention of LFP composite electrodes are investigated at potentials up to 5.0 V (versus Li+/Li) using Li-metal containing half-cells and an electrolyte composed of 1.0 M LiPF6 dissolved in 1:1 ethylene carbonate (EC)/diethyl carbonate (DEC). The results indicate that LFP electrodes are stable at such high potentials and that cycling up to 5.0 V (versus Li+/Li) at a rate of 1 C yields a 15% higher capacity compared to cycling up to 4.0 V (versus Li+/Li). The results further indicate that the lithiation of delithiated LFP electrode is incomplete. This yields a diffusion-controlled capacity loss as some Li+ ions (and associated electrons) diffuse too far into the electrode to be accessible on the timescale of the subsequent delithiation. Analogue diffusion-controlled capacity losses are also demonstrated for LFP-graphite full-cells cycled up to 4.0 and 5.0 V. These insights, pave the way for new approaches to minimize capacity losses for lithium-ion batteries. The demonstrated high-voltage stability of LFP, also indicates that LFP can be used as a protective coating on high-voltage transition metal oxide positive electrodes.

Sodium metal is capable of unlocking a new sustainable pathway toward electrification thanks to its high energy density and abundant resources. However, uneven metal nucleation and growth during cycling remains a challenge to be fully understood and overcome. In this study, short high-overpotential pretreatment pulses are applied to enable 2D metal growth. This is achieved by separating the initial nucleation step from reversible sodium metal growth. Electrochemical measurements combined with scanning electron microscopy and image analysis prove that the reductive and oxidative pulses significantly decrease the initial overpotential during galvanostatic cycling, compared to cycling without pulses. The oxidative or reductive pulses activate the entire electrode surface by generating a dense population of single micrometer-sized pits or nuclei, respectively. This activation of the surface decreases the interfacial impedance and circumvents the generation of mossy sodium and forms a smooth hill-valley-like topography. Double pulse pretreatment further improves the 2D sodium growth. This improvement is also seen in full-cells with Prussian white as the cathode material as an increase in Coulombic efficiency. The results of this study demonstrate that separating the nucleation event from continuous growth and using the optimal conditions for each step is key to obtaining stable 2D sodium metal growth conditions.

Anodic dissolution of aluminum (commonly called aluminum corrosion) is a potential issue in sodium-ion batteries. Herein, it is demonstrated how different sodium-ion battery electrolyte solutions affect this phenomenon. The type of electrolyte was critical for the presence of anodic dissolution, while the solvent appeared to alter the dissolution process.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (¹H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF₆ in ethylene carboante mixed with diethylene carbonate electrolyte (i.e., 5 µAh h^−1/2_pause or 2.78 mAh g·h^−1/2_pause) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAh h^−1/2_pause or 0.72 mAh g·h^−1/2_pause).

A wide array of carbon materials finds extensive utility across various industrial applications today. Nonetheless, the production processes for these materials continue to entail elevated temperatures, necessitate the use of inert atmospheres, and often involve the handling of aggressive and toxic chemicals. The prevalent method for large-scale carbon material production, namely the pyrolysis of waste biomass and polymers, typically unfolds within the temperature range of 500–700 °C under a nitrogen (N2) atmosphere. Unfortunately, this approach suffers from significant energy inefficiency due to substantial heat loss over extended processing durations. In this work, we propose an interesting alternative: the carbonization of photothermal nanocellulose/polypyrrole composite films through CO2 laser irradiation in the presence of air. This innovative technique offers a swift and energy-efficient means of preparing carbon materials. The unique interaction between nanocellulose and polypyrrole imparts the film with sufficient stability to retain its structural integrity post-carbonization. This breakthrough opens up new avenues for producing binder-free electrodes using a rapid and straightforward approach. Furthermore, the irradiated film demonstrates specific and areal capacitances of 159 F g−1 and 62 μF cm−2, respectively, when immersed in a 2 M NaOH electrolyte. These values significantly surpass those achieved by current commercial activated carbons. Together, these attributes render CO2-laser carbonization an environmentally sustainable and ecologically friendly method for carbon material production.

Hard X-ray photoelectron spectroscopy (HAXPES) was used to perform a non-destructive depth profile of AlCrNbYZrN_x (x = 0 to ∼50 at.%) thin films. The outermost native oxide of the pristine thin films contained the highest coordination oxides of every metal. Substoichiometric oxides or oxynitrides were found underneath. After exposure to 1.0 M HCl, increases in the most highly coordinated oxides of Cr, Nb, and Al in films with up to 37 at.% N were observed, suggesting that the low coordination oxides and oxynitrides in the subsurface had been further oxidised and were intermediary compounds in the passivation process. Al and Y oxides were lost to the HCl electrolyte, in agreement with their respective Pourbaix diagrams. The film with 49 at.% N showed little to no change in the data due to its high porosity which led to the oxide being detected at all probed depths. The metal core level spectra revealed a preferential order in which nitrogen bonded with the different metals. Nitrogen interacted first with Y, then Zr, then Al and Nb, and lastly Cr as the nitrogen content was increased.

Memristive crossbar arrays (MCAs) offer parallel data storage and processing for energy-efficient neuromorphic computing. However, most wafer-scale MCAs that are compatible with complementary metal–oxide–semiconductor (CMOS) technology still suffer from substantially larger energy consumption than biological synapses, due to the slow kinetics of forming conductive paths inside the memristive units. Here we report wafer-scale Ag₂S-based MCAs realized using CMOS-compatible processes at temperatures below 160 oC. Ag₂S electrolytes supply highly mobile Ag+ ions, and provide the Ag/Ag₂S interface with low silver nucleation barrier to form silver filaments at low energy costs. By further enhancing Ag⁺ migration in Ag₂S electrolytes via microstructure modulation, the integrated memristors exhibit a record low threshold of approximately -0.1 V, and demonstrate ultra-low switching-energies reaching femtojoule values as observed in biological synapses. The low-temperature process also enables MCA integration on polyimide substrates for applications in flexible electronics. Moreover, the intrinsic nonidealities of the memristive units for deep learning can be compensated by employing an advanced training algorithm. An impressive accuracy of 92.6 % in image recognition simulations is demonstrated with the MCAs after the compensation. The demonstrated MCAs provide a promising device option for neuromorphic computing with ultra-high energy-efficiency.