Examination of Hydrogen Evolution Bubble Trapping in Ordered Porous 3D Printed Metal and Metal Oxide-Coated Microlattice Electrodes

Determining the nature of surface roughness and electrode pore structure on H2 bubble evolution rate and quantity, and bubble trapping under electrolytic conditions is important for quantifying useful gas production during total water splitting and hydrogen evolution reactions. Controlled electrode systems involving the design of geometry, surface area, and porosity provides options to understand trapped/redissolved gas bubble evolution and improve overall efficiency. In this study, we use vat polymerization (Vat-P) 3D-printing to create ordered microlattice electrode structures from metal and metal-oxide coated photopolymerized methacrylate-based resins. These micro-lattice structures are designed with various geometries to influence bubble traffic from gas nucleation and evolution during electrochemical HER processes. Using cyclic and linear sweep voltammetry, and chronopotentiometry, this work analyzes the response of metallized (NiO/Ni(OH)2 and Au) microlattice HER electrodes as a function of geometric structure, to gauge influence of material activity, small scale surface roughness, and the larger substrate pore network on the traffic or larger bubbles formed during HER. This work also uses broadband acoustic resonance dissolution spectroscopy (BARDS) to quantify bubble evolution and reabsorption in the electrolyte during electrolysis. The results show that coated 3D printed electrodes are robust HER electrodes, allow efficient transport of small bubbles, but significant limitations are found for larger bubble transport through ordered porous microlattice shown through model simulations and experimental measurements.

💡 Research Summary

The paper investigates how surface roughness and pore architecture of 3‑D‑printed microlattice electrodes influence hydrogen‑evolution‑reaction (HER) performance, specifically focusing on bubble nucleation, growth, transport, and trapping. Using a vat‑polymerization (Vat‑P) printer (Form‑2, 25 µm layer resolution), the authors fabricated two types of ordered microlattice structures—octet‑truss and octahedral—each measuring roughly 1 cm³. After printing, the lattices were cleaned in isopropanol, UV‑cured, and then coated with a thin (≈50 nm) layer of either nickel (Ni) or gold (Au) by sputtering from six directions to ensure complete coverage. The Ni‑coated samples were further oxidized to generate a mixed NiO/Ni(OH)₂ surface, providing a well‑studied HER catalyst in neutral pH.

Electrochemical characterization was performed in a three‑electrode cell containing 1 M K₂HPO₄/KH₂PO₄ (pH ≈ 7). Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronopotentiometry (CP) were used to obtain overpotential curves, Tafel slopes, and stability data. To monitor bubble dynamics in real time, the authors employed broadband acoustic resonance dissolution spectroscopy (BARDS). Because dissolved or entrained gas changes the compressibility of the electrolyte, the resonant frequency of a standing acoustic wave shifts proportionally to the fractional volume occupied by bubbles. By recording this frequency continuously, BARDS provides a non‑invasive measure of bubble volume, escape rate, and re‑absorption.

Experimental BARDS data showed that both Ni‑ and Au‑coated microlattices efficiently release small bubbles (≤ 100 µm), resulting in modest overpotentials. However, when bubbles grew larger than the local pore‑throat diameter (≈ 200 µm, as determined from the lattice geometry), they became trapped within the lattice network. Trapped bubbles increased the effective coverage of catalytic surface, raised the local ohmic resistance, and caused a noticeable rise in overpotential during prolonged CP runs. Au electrodes, despite their higher intrinsic catalytic activity, suffered more pronounced bubble trapping because the higher current density accelerated bubble nucleation, leading to a larger fraction of oversized bubbles.

To rationalize these observations, the authors built a computational model directly from the STL files of the printed lattices. A ray‑casting algorithm mapped the pore‑throat diameter field throughout the structure. Spherical bubbles were seeded at the bottom of the lattice and continuously injected to mimic sustained HER. Each bubble’s motion obeyed a simple kinematic update: a deterministic upward drift (≈ 100 mm s⁻¹) plus a stochastic dispersion term. When a bubble’s diameter exceeded the local throat diameter, its velocity was set to zero, representing “trapping.” Coalescence was deliberately disabled to isolate geometric confinement effects. The model tracked individual trajectories, allowing calculation of mean‑squared displacement, residence‑time distributions, and frame‑resolved counts of free, trapped, and escaped bubbles. Simulated bubble‑volume fractions and escape rates matched the BARDS measurements, confirming that the dominant limitation is geometric rather than chemical.

The study concludes that ordered microlattice electrodes, while offering high surface area and low mass, can suffer from performance loss if the pore network is not sized to accommodate the largest bubbles generated under operating conditions. Design recommendations include (i) enlarging pore‑throat diameters beyond the expected bubble size, (ii) introducing hydrophilic surface treatments or micro‑texturing to promote bubble detachment, and (iii) optimizing current density to limit bubble growth. By integrating additive manufacturing, surface engineering, operando acoustic diagnostics, and physics‑based modeling, the work provides a comprehensive framework for engineering next‑generation HER electrodes where catalyst activity, architecture, and bubble dynamics are co‑optimized.