In Situ Measurements of Interfacial Electric Fields at Graphene-Modified Electrode Surfaces During High Current Density Operation

Measurements of Interfacial Electric Fields at Graphene-Modified Electrode Surfaces During High Current Density Operation

Understanding electric fields at electrochemical interfaces is essential for optimizing electrocatalytic processes. However, their measurement under high current densities is limited by gas evolution, which obstructs access to the electrode surface. To address this, we developed a custom flow cell featuring an internal nozzle that directs fluid across the electrode surface. This high-velocity flow suppresses bubble nucleation by limiting local hydrogen supersaturation and thereby preserves optical access to the electrode. This design enables in situ Raman spectroscopy and direct quantification of interfacial electric fields under current densities relevant to electrolysis. High-speed optical imaging shows that bubble coverage decreases by 85–88% at 100 mL min⁻¹ relative to stagnant conditions. With this improvement, stable Raman measurements are possible at current densities up to 25 mA cm⁻², which is more than two orders of magnitude higher than in a non-flowing cell. Under these conditions, graphene-encapsulated Pt and Au electrodes exhibit Stark shifts in the graphene G-band. These shifts allow extraction of charge-carrier densities and local interfacial fields up to 3.8 × 10⁶ V cm⁻¹. Differences between graphene/Pt and graphene/Au highlight the substrate-dependent nature of the doping behavior. We propose a framework in which metal work function, proton adsorption, and electrostatic gating collectively determine the potential-dependent graphene doping and the resulting interfacial electric field.

Interfacial engineering of high-curvature NiFe nanocone arrays for accelerated oxygen evolution via tip-induced electric field and rapid bubble detachment.

In gas-evolving electrocatalysis, the trade-off between mass transfer polarization and intrinsic activity remains a fundamental bottleneck. This work reports the interfacial engineering of high-curvature NiFe bimetallic nanocone arrays (NiFe-HC@NF) via a facile cathodic electrodeposition strategy. Finite element method (FEM) simulations reveal that the high-curvature tips trigger a strong localized electric field, which drives the directional migration and enrichment of OH- ions, thereby suppressing concentration polarization at the reaction interface. This physical enhancement is effectively integrated with the intrinsic Ni-Fe electronic synergistic effects, as verified by XPS, which optimize the surface electronic states for superior intrinsic kinetics. Furthermore, the unique 3D array configuration constructs a superhydrophilic and superaerophobic interface, ensuring ultrafast bubble detachment and maintaining effective active site exposure under vigorous gas-evolving conditions. As a result, the NiFe-HC@NF electrode delivers a low overpotential of 269.8 mV at 500 mA cm-2 and maintains remarkable structural and catalytic integrity for over 400 h at 100 mA cm-2. This study provides a compelling curvature-engineering strategy to break the mass-transport limitations in high-performance alkaline water electrolysis.

Experimental study on hydrogen bubble growth and detachment on surface-modified electrodes

Potential of Zero Charge at Electrochemical Interfaces: Concepts, Measurement Strategies, and Emerging Applications

The potential of zero charge (PZC) is a key reference potential for understanding the electrochemical double layer at electrode/electrolyte interfaces. It describes the potential at which the excess charge on the electrode side becomes zero and provides a basis for discussing interfacial electric fields, ion distributions, solvent orientation, adsorption, and charge transfer reactions. Although the PZC can be defined clearly for liquid-metal electrodes that behave close to ideally polarizable interfaces, its assignment becomes more complex for solid electrodes because surface structure, oxide formation, hydrogen and oxygenated species adsorption, and specific adsorption can contribute to the measured response. This mini-review discusses classical and modern strategies for PZC determination, including electrocapillarity, differential capacitance, immersion measurements, CO displacement, piezoelectrochemical spectroscopy, scanning electrochemical cell microscopy, phase-sensitive in situ second harmonic generation, and atomic force microscopy approaches. By comparing their principles, applicable interfaces, and limitations, this mini-review emphasizes that PZC values should be interpreted together with the electrode material, electrolyte composition, surface state, local electronic structure, spatial scale, and measurement method.

( Invited ) Mass-Transfer Resistance across Bubble Layers on Wire- and Mesh-Type Electrodes during Alkaline Water Electrolysis

A major limitation of the practical implementation of alkaline water electrolysis (AWE) is the substantial increase in overvoltage caused by gas bubbles adhering to, and accumulating on the electrode surface under high current-density operation. Such bubble accumulation inhibits ion transport and blocks active electrochemical sites, resulting in lowered energy efficiency. Because the extent of bubble coverage depends on both the applied current density and the geometric properties of the electrode, the bubble-induced contribution to total overvoltage can vary significantly from system to system. Therefore, when evaluating catalyst or electrode performance, it is essential to consider not only intrinsic catalytic activity, but also to carefully identify and quantitatively separate the bubble-related mass-transfer overvoltage from the overall cell polarization. Our group previously developed the mass-transfer models in bubble layer based on electrochemical impedance spectroscopy (EIS) using simple Ni wire electrodes, combined with equivalent-circuit analysis for the anode/OER and cathode/HER, respectively [1, 2]. These studies enabled the isolation of the bubble contribution. For OER, the measured i-V curve deviates from the expected Tafel line at current densities above approximately 0.2 A cm ‒2 , indicating the onset of a mass-transfer limitation associated with bubble accumulation. A comparable deviation is observed for HER at current densities exceeding roughly 0.3 A cm ‒2 . In both cases, the mass‐transfer resistance was found to consist of two key components: diffusion resistance and ohmic migration resistance within the bubble layer. The migration resistance 𝑅 sb was successfully interpreted in terms of the thickness and void fraction of the bubble layer, the conductivity of the electrolyte, and a tortuosity model characterizing the ion-migration pathways through the inter-bubble gaps. Diffusion transport differs between OER and HER: for OER, the diffusion of hydroxide ions was modeled by a finite-length Warburg impedance element, reflecting the restricted diffusive boundary created by the bubble layer. For HER, two distinct inductive elements were identified at high and low frequency, suggesting that gas evolution dynamically enhances the reaction rate in a frequency-dependent manner. The charge-transfer resistance (𝑅 ct ) at the electrode surface was proven to scale inversely with current density, indicating that 𝑅 ct becomes relatively insensitive to bubble coverage and that nearly all accessible electrode surface remains electrochemically active even beyond 𝑖 geo = 0.5 A cm −2 . These two mass-transfer models were subsequently extended to a cell-type AWE configuration [3], operated in 2 M KOH at 30 °C. Both CV and EIS measurements were performed under control of anodic potential vs. RHE, allowing simultaneous acquisition of three i-V curves and three Cole-Cole plots corresponding to anodic potential, cathodic potential, and whole-cell voltage. From the three i-V curves, the cell voltage could be decomposed into seven distinct contributions: the theoretical decomposition voltage, the reaction overpotential and mass-transfer overpotential for both anode and cathode, the diaphragm resistance, and the mass-transfer overpotential arising near the diaphragm. Subsequent analysis of the Cole-Cole plots allowed the mass-transfer overpotential to be further separated into diffusion- and ohmic-type components. Through these procedures, we were able to quantitatively demonstrate that bubble-derived mass-transfer overpotential accounts for approximately 30–50% of the total cell overvoltage under high-current density conditions, depending strongly on electrode geometry and gas-release behavior. This finding highlights that bubble management—whether by electrode structuring, surface modification, hydrodynamic control, or cell design—can have an impact comparable to improving intrinsic catalyst performance. Accordingly, the methodology presented here provides a rational framework for diagnosing limiting mechanisms in practical AWE devices and may serve as a useful benchmark when screening electrode materials for high-current industrial hydrogen production. Acknowledgments A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO, JPNP25002). References [1] H. Ikeda, R.Misumi, Y. Nishiki, Y. Kuroda, S. Mitsushima, Electrochimica Acta , 430, 141053 2022 [2] D. Kitajima, R. Misumi, Y. Kuroda, S. Mithushima, Electrochimica Acta , 502, 144772, 2024 [3] Y. Nishio, R. Misumi, S. Mithushima, PRiME 2024 , I01B-2773, 2024

Monolithic ion-electron coupling interfaces enable low-impedance moisture-electric generators.

Moisture-electric generators (MEGs) offer a promising route for harvesting ubiquitous atmospheric energy, yet their performance is fundamentally constrained by inefficient ion-electron coupling at the electrode-electrolyte interface. Existing strategies primarily focus on optimizing hygroscopic materials or ionic transport, while overlooking the critical role of interfacial impedance arising from structurally discontinuous contacts. Here we show a monolithic silicon nanowires (SiNWs)/ionic hydrogel architecture that establishes continuous ion-electron coupling pathways through conformal integration of the electrode and active layer. The monolithic interface reduces the charge transfer resistance by over one order of magnitude compared to conventional stacked configurations. The resulting device delivers an open-circuit voltage of 1.35 V and a short-circuit current density of 27.5 µA cm-2 at 25  °C and 60% relative humidity, outperforming benchmark devices by ∼60%. Impedance spectroscopy combined with distribution of relaxation time analysis reveals suppressed interfacial polarization and accelerated ion-electron conversion kinetics. This work establishes a general interface engineering strategy for low-impedance hydrovoltaic systems and highlights the central role of ion-electron coupling in moisture-enabled energy conversion.