The Same Silver Catalyst Does Two Different Jobs Inside a Solid Oxide Cell

Inside a solid oxide cell, a speck of silver only a few nanometers wide switches which of its surfaces does the chemistry, depending on whether the cell is making electricity or splitting water into hydrogen.

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The team tested four metal catalysts, silver, cobalt, palladium, and platinum, and silver produced the strongest improvement in oxygen-reaction speed of any of them.

Why it matters

A solid oxide cell works in one of two modes: it can generate electricity by combining hydrogen and oxygen, called the oxygen reduction reaction, or it can run in reverse and split water into hydrogen, called the oxygen evolution reaction. Both modes depend on how fast oxygen atoms react at the air electrode, and adding a metal nanocatalyst like silver speeds that reaction up. Engineers building these electrodes have not known whether the catalyst's own surface, its edge where it touches the electrode, or both do the actual work, so they have largely built one electrode design to serve both jobs. Knowing that silver switches which surface it uses lets engineers instead shape the catalyst separately for each mode: more interface boundary for electricity generation, more exposed particle surface for hydrogen production, a design challenge with real echoes in fuel-cell catalysts, where researchers have separately tackled the tradeoff between platinum activity and durability.

What happened

Rather than testing a real, tangled commercial electrode, the team built a model version: a thin-film perovskite oxide electrode with silver nanoparticles deposited in controlled sizes and spacing, so they could isolate exactly where the reaction happened. They ran the electrode in oxygen reduction mode, generating electricity, and found that reaction rates rose as the boundary length between the silver particles and the electrode grew longer, pointing to that boundary as the main reaction site. Then they ran the same electrode in oxygen evolution mode, producing hydrogen, and the pattern flipped: reaction rates instead tracked the surface area of the silver particles themselves.

Synchrotron-based photoelectron spectroscopy, which let the team watch the electrode surface while it operated, combined with atomic-scale calculations, showed why: in electricity-generating mode, the silver mainly transfers electrons to oxygen at that boundary; in hydrogen-producing mode, it instead helps oxygen atoms combine into molecules and release from its exposed surface. The findings, led by professors WooChul Jung and Jeong Woo Han of Seoul National University with KAIST and the Korea Basic Science Institute, were published in Energy & Environmental Science and featured on the journal's back cover.

What happens next

Jung's team describes the finding as a design principle rather than a finished product: an electrode built for a solid oxide fuel cell could now be engineered to maximize interface boundary, while one built for a solid oxide electrolysis cell could instead maximize exposed catalyst surface. The same model-electrode platform, the researchers say, could apply beyond solid oxide cells to other electrochemical devices where a catalyst's reaction site was previously assumed rather than measured directly. Dr. Jinwook Kim, who led the experimental work, plans to continue this line of research at the University of Seoul, extending it toward reversible cells that switch between generating power and producing hydrogen in the same device, a materials-design problem similar in spirit to the machine-learning-guided search for better sodium-battery electrolytes.

Takeaway

The two reaction sites, boundary versus surface, are not two theories competing for the same explanation; they are two separate jobs the same silver particle performs depending on which direction the cell is running. Whether that distinction survives the jump from a controlled lab electrode to a working power plant or hydrogen plant is the open question the next round of testing has to answer.