One Solvent Swap Builds a More Durable Water-Splitting Catalyst
Water baths make cheap, fast metal-alloy electrodeposition possible — and they’re also the reason so many alloy catalysts come out with the wrong composition.
Water splitting only works as well as its slowest half-reaction, and that’s almost always the oxygen evolution reaction (OER) — the sluggish four-electron step that sets the efficiency ceiling for electrolyzers and photoelectrochemical cells alike. Nickel-iron alloys rank among the best non-precious-metal OER catalysts known, and electrodeposition is the cheapest, most scalable way to make them. The catch has always been the plating bath itself.
The Obstacle — Water Makes a Difficult Host for Alloy Plating
Standard electrodeposition uses water as the solvent, and water complicates alloy plating in two specific ways. Its narrow electrochemical window puts metals with very negative reduction potentials in direct competition with hydrogen evolution at the cathode, wasting current and degrading the deposit. Nickel and iron also co-deposit “anomalously” — the more easily reduced element suppresses deposition of the other, so the alloy that actually lands on the electrode rarely matches the ratio dissolved in the bath. The usual fix, complexing agents such as fluoroboric, glycolic, or citric acid, accumulates as reduction byproducts and degrades the deposit over repeated use.
The Innovation — Swap the Solvent, Not the Chemistry
In a study published in Dalton Transactions (Vo, Suarez Hidalgo & Chiang, 2019, DOI: 10.1039/c9dt03028j), researchers at National Taiwan University of Science and Technology addressed the problem at its source. Using a deep eutectic solvent (DES) — a low-cost mixture that behaves like an ionic liquid but is made from simple, inexpensive components — as the plating bath, the team electrodeposited nickel-iron films directly onto copper foil, no binder or conductive additive required.
Deep eutectic solvent replaces water as the plating bath, avoiding the anomalous co-deposition that skews alloy composition in aqueous electrodeposition.
The wider electrochemical window of the DES suppresses the side reactions that plague aqueous plating. Because the alloy ratio in the deposited film consistently tracked the bath’s ratio, composition became something the researchers could dial in on purpose, across a full series from pure nickel to pure iron, rather than something to correct for afterward.
Key Results — From Composition Control to a 10-Hour Stability Test
The team validated the approach in stages: confirming plated composition matched the bath, identifying the surface chemistry behind catalytic activity, then benchmarking the best composition’s OER performance and durability.
Composition Tracked the Bath, Across the Full Ni-Fe Series
Five compositions — pure Ni, Ni75Fe25, Ni50Fe50, Ni25Fe75, and pure Fe — were electrodeposited and composition-mapped (Table 1). Unlike aqueous baths, where anomalous co-deposition skews the ratio, the DES-plated films’ Ni:Fe content consistently matched the plating solution.
MRID Raman Confirms the Surface Oxide Phase on Every Composition
Raman spectra collected on ProTrusTech’s own MRID confocal Raman system (532 nm excitation) identified NiO (~497 cm-1) and Ni(OH)2 (450–472 cm-1) on nickel-rich films and iron-oxide phases on iron-rich films — confirming the thin, catalytically active oxide layer forms during use.
Ni75Fe25 Needed Just 316 mV Overpotential at 10 mA/cm2
Ni75Fe25 was the standout composition: a 316 mV overpotential to reach 10 mA/cm2, with a Tafel slope of 62 mV/dec — competitive with several precious-metal OER benchmarks.
The Catalyst Held Steady for Over 10 Hours
Chronopotentiometry at a fixed 10 mA/cm2 showed negligible activity loss over more than 10 hours of continuous operation — the durability data that separates a usable catalyst from a lab curiosity.
Note: This work was supported by Taiwan’s Ministry of Science and Technology, Grant MOST108-2636-E-011-001.
Why ProTrusTech Is Featuring This Research
This paper comes from the same NTUST lab and corresponding author, Chia-Ying Chiang, behind our feature on the anion-induced CuO co-catalyst work. Both studies drew on ProTrusTech’s MRID platform, though this 2019 study only needed the 532 nm Raman module — a metal alloy electrocatalyst has no photophysics to probe with photoluminescence. The 2020 CuO/BiVO4 paper additionally used MRID’s 405 nm PL/TRPL module to measure charge-carrier lifetime, a capability the NTUST team has used on the platform from early in our work together, across projects beyond this pair.
Watching a research group return to the same platform across projects with different measurement needs — reaching for 532 nm Raman here, 405 nm PL/TRPL there — is exactly the kind of sustained, varied use we look for before calling an instrument adopted by a lab rather than used once.
Engineering the Solution — Verification, Not Just Fabrication
The manufacturing insight in this paper isn’t a new coating process — it’s that switching the solvent, not the equipment, solved a decades-old plating problem. That’s a useful reminder for any team optimizing electrodeposited catalysts: composition drift is often a bath-chemistry problem before it’s a hardware problem. ProTrusTech’s MRID system fits into that workflow on the verification side — confirming, composition by composition, exactly which oxide or hydroxide phase actually formed on the electrode rather than assuming it from bath stoichiometry alone. That’s what let this team screen five Ni:Fe ratios and identify the OER-optimal one with confidence.
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Why This Matters
Beyond nickel-iron OER catalysts specifically, this is a proof point for a broader principle: many electrodeposition problems attributed to the metal system are actually solvent problems. A deep eutectic solvent is inexpensive, simple to prepare, and — unlike complexing-agent workarounds — doesn’t leave behind byproducts that degrade the final deposit. Other groups working on electrodeposited alloy catalysts, not just OER catalysts, can borrow this directly.
What’s Next
- Extending DES electrodeposition to other alloy systems, where aqueous-bath composition control is an even bigger obstacle than it is for Ni-Fe.
- Pairing dark electrocatalyst films like this one with light-driven photoelectrodes — the direction this same lab took a year later with its BiVO4/CuO co-catalyst work.
- Longer-duration stability testing beyond the 10-hour window shown here, to establish catalyst lifetime at industrial scale.
Evaluating a Raman-based verification step for your own catalyst development pipeline? Get in touch with our team.
Key Takeaways
- A deep eutectic solvent electrodeposition bath solves the composition-control problem that plagues aqueous nickel-iron alloy plating.
- Ni75Fe25 was the best oxygen evolution catalyst identified, needing just 316 mV overpotential at 10 mA/cm2 with a 62 mV/dec Tafel slope.
- The catalyst ran for over 10 hours at 10 mA/cm2 with negligible activity loss.
- Composition-dependent surface oxide and hydroxide phases were confirmed by Raman spectroscopy on ProTrusTech’s own MRID system.
- This is the same lab and the same MRID instrument behind ProTrusTech’s feature on anion-induced CuO co-catalysts for solar water splitting — applied here to a standalone dark electrocatalyst rather than a photoelectrode.
