Kuniaki Murase

In neutral and mildly alkaline water electrolysis, electrolytes are primarily viewed as media controlling macroscopic solution properties such as pH and ionic conductivity. Beyond this conventional role, electrolyte species can interact with metal ions and dynamically reconstruct electrode surfaces through dissolution and reformation. Although such reconstruction is widely recognized, the extent to which operating surfaces retain the initial oxidation history or are governed by the electrolyte environment remains unclear. Here, a high-throughput electrochemical platform evaluated cell voltages under 108 conditions using 304 stainless steel anodes, independently varying anodic pretreatment, electrolyte pH (9.5, 10.5, and 11.5), phosphate concentration (1 and 3 M), and electrolysis regime. Analysis of variance showed that pH accounted for 70–81% of cell-voltage variation, whereas anodic pretreatment accounted for only 6–7%, demonstrating the dominant influence of the electrolyte environment on the electrochemical response. Surface analyses revealed widespread nickel depletion and heterogeneous chromium-rich films under lower-pH, high-phosphate conditions. Iron behavior was consistent with the equilibrium solubility trend, whereas nickel depletion did not follow the equilibrium solubility trend, indicating the involvement of processes beyond equilibrium thermodynamics. Together, these results reveal that multicomponent electrode surfaces do not reconstruct through a uniform pathway. Instead, the operating surface emerges from distinct element-specific dissolution and retention processes. The surface formed during operation should therefore be treated as a materials-design target rather than considering the as-prepared electrode and electrolyte independently.

We report the depth-resolved characterization of silver incorporated into electrodeposited cobalt–copper oxyhydroxide (CoCuOOH) films and its correlation with alkaline oxygen evolution reaction (OER) performance. The OER enhancement is consistent with improved charge transport associated with a metallic-like Ag distribution rather than direct electronic modification of the Co or Cu active sites. Extended profiling to ~86 nm nominal depth reveals a constant Ag concentration of ~5–6 at.% for the CoCuOOH-2.4Ag film. Cross-sectional EDX supports an approximately uniform Ag distribution across the analyzed film when normalized to the metallic components, with Ag constituting ~14 at.% of the metals. The silver-modified films show a reproducible improvement in OER activity in 1 M KOH, with the CoCuOOH-2.4Ag composition reaching an overpotential of ~344 mV at 100 mA cm−2 and a Tafel slope of 66 mV dec−1. Electrochemical impedance spectroscopy reveals a decrease in charge-transfer resistance from 5.9 to 1.9 Ω with increasing Ag content. A 48 h chronopotentiometric test at 100 mA cm−2 shows a potential drift of only −0.37 mV h−1 for the Ag-containing film versus +1.17 mV h−1 for the Ag-free counterpart. Post-OER sputter-depth XPS indicates that the subsurface metallic-like Ag signature and the silver distribution persist after prolonged operation.

The present study investigated the electrodeposition of nickel from hydrate melts comprising highly concentrated aqueous solutions of CaCl ₂ . The objective of this work was to elucidate the manner in which the extreme chloride activity of such melts affected both electrocrystallization kinetics and hydrogen co-deposition. Visible spectra demonstrated that the high chloride concentrations in these solutions transformed symmetric Ni(II) aquo ions into lower-symmetry chloride complexes. Consequently, the kinetics of the Ni electrodeposition process were enhanced, as evidenced by decreased charge-transfer resistance compared with that associated with a conventional Watts bath. The Ni electrodeposited from these hydrate melts exhibited a random crystallographic orientation whereas a {100} preferred orientation was obtained from the Watts bath. Thermal desorption spectroscopy indicated the substantial co-deposition, or occlusion, of atomic hydrogen in the form of vacancy-hydrogen clusters in the resulting Ni films. This co-deposition occurred despite a current efficiency of nearly 100%. Interestingly, enhanced hydrogen co-deposition together with the suppression of gaseous H ₂ evolution were previously observed during the electrodeposition of other metals and appear to be characteristic of electrodeposition from chloride-based hydrate melts.

The discharge process at the positive electrode in lithium–oxygen batteries (LOBs) involves coupled O2 transport, electrochemical generation of LiO2 intermediates, and subsequent Li2O2 deposition that progressively restricts O2 transport. In practical porous electrodes, geometric complexity obscures this interplay and hinders mechanistic understanding. Here, these processes were spatially separated and analyzed by employing an anodic aluminum oxide (AAO) positive electrode with one-dimensional nanochannels as a model reaction field. The results reveal that the deposition location of Li2O2 reflects the travel distance of dissolved LiO2 intermediates from the electrode. Furthermore, even partial accumulation of discharge products on the order of tens of nanometers can significantly restrict O2 transport and lead to early attainment of the discharge cutoff. This study provides a framework to reinterpret complex electrode reactions using a simplified AAO reaction field and offers guidelines for achieving high capacity in LOBs.

As an alternative to hexavalent chromium plating, chloride-based hydrate melts enable crystalline chromium electrodeposition from trivalent chromium without the use of organic complexing agents. Cr films deposited from these hydrate melt-based baths uniquely contain a metastable δ -Cr phase absent in conventional baths. Achieving hard coatings with adequate mechanical properties requires a systematic understanding of the crystallographic features including phase formation. In this study, the deposition of δ -Cr and its subsequent phase transformation to α -Cr were systematically investigated from the perspectives of crystal growth mode and hydrogen incorporation. The results indicate that δ -Cr preferentially forms during the initial nucleation and coalescence stage, where isotropic fine grains are generated. As electrodeposition proceeds, the crystal growth mode transitions to columnar growth, in which α -Cr becomes the dominant phase. The obtained δ -Cr was confirmed to transform into α -Cr at relatively low temperatures ranging from room temperature to 250 °C. The thermal desorption spectrum of hydrogen and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) experiments using deuterium as a stable isotope label suggest that the formation and stability of the electrodeposited δ -Cr phase may well be attributed to incorporated hydrogen. These findings provide a fundamental basis for microstructural control of electrodeposited Cr from chloride-based hydrate melts.

In lithium−oxygen batteries (Li−O2 batteries, LOBs), the solution pathway involving the disproportionation of solvated lithium superoxide (LiO2s) is recognized as a major discharge reaction route. In this pathway, the deposition of lithium peroxide (Li2O2), the main discharge product, also occurs beyond the conductive substrate. The accumulation of such deposits, which lack electronic contact with the electrode, can mitigate pore clogging of the positive electrode and enable higher discharge capacities, while it deteriorates charge−discharge reversibility. Therefore, establishing guidelines for controlling the spatial distribution of Li2O2 deposition according to the desired battery characteristics, such as high capacity or reversibility, is of critical importance. In this study, we investigated the effects of solvent and lithium salt selection on the fraction of Li2O2 deposited outside the conductive substrate and elucidated the governing factors. A newly designed hybrid nonwoven positive electrode with spatially adjacent conductive and non-conductive regions was employed, and quantitative colorimetric measurements were combined with finite element method simulations. The results revealed that both the LiO2s diffusivity and the disproportionation rate govern the deposition location, along with the solvent donor number. Specifically, solvents with relatively low disproportionation rate constants and high donor numbers, such as N,N-dimethylacetamide, promote long-range LiO2s migration, resulting in enhanced Li2O2 deposition outside the conductive substrate. In contrast, the solubility of dissolved oxygen and its diffusivity have only a limited influence on the fraction of Li2O2 deposited outside the conductive substrate. These findings provide a rational design framework for controlling the deposition location of discharge products through electrolyte design in LOBs.

Phosphate buffers selectively dissolve nickel to a depth of approximately 4–6 monolayers on 304 stainless steel anodes, depleting nickel redox-active sites and irreversibly suppressing OER, unlike KOH, which forms a nickel-rich active layer.

Chromium (Cr) electroplating is an industrially important surface finishing technology. However, conventional Cr plating processes suffer from a pronounced hydrogen evolution reaction (HER), resulting in extremely low current efficiencies of 10–20%. Because the HER leads to energy loss and can cause embrittlement of the deposited films, controlling the HER is crucial for establishing sustainable electroplating processes. In this study, we achieved a markedly high current efficiency of approximately 90% for crystalline Cr electrodeposition by selectively suppressing the HER using deuterated Cr(III) electrolytes based on a highly concentrated CaCl₂ system. This selective suppression of the HER originates from the lower acidity of the Cr(III) electrolytes as a result of the weaker dissociation of D₂O, alongside inhibition of typical proton/deuteron hopping diffusion as a result of the addition of excess CaCl₂. Notably, we succeeded in tailoring the water dissociation around Cr(III) ions without relying on organic additives, thereby enabling the efficient crystalline Cr electrodeposition. This study provides a design principle for aqueous electroplating systems that exploits the isotope effect of water.

The bottom-up synthesis of three-dimensional chiral metallic nanostructures without molecular chiral ligands remains a fundamental challenge. Here we report a template-confined, ligand-free strategy producing intrinsically morphochiral gold nanoparticles (GNPs) and nanohelices (GNHs) by AuCl₄⁻ reduction within rigid silica nanohelical nanocavities. Reductant choice selects the operative kinetic regime — charge-transfer-limited or diffusion-limited — yielding discrete GNPs or continuous GNHs, respectively. Under diffusion-limited conditions, crystallographic twinning redirects growth along the cavity, encoding chirality into the metal lattice. Pitch and diameter are template-fixed; helix length is kinetically controlled, decoupling these parameters and making helix length the sole tunable variable. The dissymmetry factor (g) scales sigmoidally with aspect ratio, programmable from ~10⁻³ to ~0.2 across two orders of magnitude. Electron tomography and simulations confirm morphology-intrinsic chiroptical activity persisting after template removal. This work establishes kinetic regime selection under nanoscale confinement as a general synthetic principle for programming three-dimensional chirality in metals.

Room-temperature electrodeposition of Al−Si alloys was achieved using phosphonium chloroaluminate ionic liquids (ILs) with saturated SiCl ₄ (2.5 mol dm ⁻³ ) as Si ⁴⁺ precursor. Solvated SiCl ₄ in the ILs interacted with [P _6,6,6,14 ] ⁺ via hydrogen bonding. This interaction enabled the reduction of SiCl ₄ along with chloroaluminate species at a more positive potential than that required for pure Si electrodeposition in other well-known baths. Co-deposition yielded Al−Si alloys with 6.1−9.8 wt% Si, dependent on overpotentials. These Al−Si alloys exhibited a homogeneous microstructure without elemental Si precipitation, resulting in a supersaturated solid solution. This study provides an efficient electrodeposition approach for Al−Si alloy coatings.