# Engineering Hierarchically Nano‐Structured Cu Foams: Dynamic Hydrogen Bubble Templated Binder‐Free Freestanding Electrodes for Energy Applications

## TL;DR

This paper introduces a new method to create copper foam structures using hydrogen bubbles, which can be used in energy applications like CO2 reduction.

## Contribution

The novel DHBT method uses hydrogen bubbles as templates to fabricate hierarchical copper foams with tunable properties for energy applications.

## Key findings

- Copper foam gas diffusion electrodes achieved 50% Faradaic efficiency for C2+ products at -1.1 V versus RHE.
- DHBT parameters can systematically control foam morphology, including pore size, ECSA, and surface features.
- Operational stability of the electrodes was demonstrated for 12 hours under CO2 reduction conditions.

## Abstract

The intelligent design of hierarchical metallic structures with optimized performance for targeted applications, such as energy devices, sensors, and catalysis, remains a significant challenge. In this study, electrochemically generated hydrogen bubbles are employed as dynamic negative templates for copper electrodeposition. This so‐called dynamic hydrogen bubble templating approach (DHBT) yields highly porous hierarchical copper foams adorned with surface nano‐structures. A comprehensive investigation of DHBT synthesis parameters is provided, organized into four categories: (1) deposition current density and time; (2) current modes, namely direct, pulsed, reversed, and alternating regimes; (3) physical conditions, including stirring and temperature; and (4) bath composition. The results demonstrate that morphological descriptors, such as pore size and density, foam thickness, electrochemically active surface area (ECSA), and nanoscale surface features, can be systematically and reproducibly tuned by varying these DHBT parameters. As a proof of concept, a simple three‐step protocol for the fabrication of copper foam gas diffusion electrodes (GDEs) is presented. The resulting GDEs show promising CO2 reduction performance, achieving C2+ products Faradaic efficiencies of approximately 50% at ‐1.1 V versus reversible hydrogen electrode (RHE) and partial current densities of up to 104 mA cm−2 at ‐2.5 V versus RHE, with good operational stability tested for 12 h.

The competition between H2 evolution (HER) and Cu electrodeposition during the dynamic hydrogen bubble templating (DHBT) crucially influences the synthesized foam morphology. Generally, when HER is dominating, numerous small H2 bubbles form, resulting in compact Cu foams. Conversely, when Cu electrodeposition outcompetes, fewer and larger H2 templates form, building up open Cu foam morphologies.