Two Metals, One Plating Order — and Nitrate Becomes Harmless Nitrogen Gas

Reduce nitrate electrically and you get a fork in the road: harmless nitrogen gas, or ammonia you now have to deal with. This electrode takes the clean fork over 90% of the time — and the trick is which metal you plate last.

Nitrate (NO₃^–) is everywhere clean water isn’t — leaching from fertiliser, pouring out of electroplating lines — and it is regulated hard (the US EPA caps drinking water at 10 mg-N/L) because excess nitrate causes methemoglobinemia (“blue-baby syndrome”). Electrochemical nitrate reduction (NO₃-RR) is attractive because the only reagent is electrons. But there is a catch: the reaction can end at N₂ gas (harmless, escapes) or at ammonia (still a pollutant). For water treatment, you want N₂ — and steering it there reliably has been the hard part. A 2025 study from Yu-Jen Shih’s group at National Sun Yat-sen University does it by engineering a two-metal electrode with a job for each metal.

The Obstacle

Nitrate reduction is a multi-step relay: NO₃^– → NO₂^– → … → N₂ or NH₃. The first step (deoxygenation, stripping oxygen off nitrate) and the later steps (hydrogenation, adding hydrogen) want different surface chemistries. A single metal is rarely good at both, so it either stalls at nitrite or over-hydrogenates to ammonia. You need one surface that does deoxygenation and another that supplies adsorbed hydrogen — and you need them arranged so the hydrogen arrives at the right moment to push the intermediates to N₂ rather than NH₃.

The Innovation

The electrode is a layered bimetallic coating on nickel foam: a transition metal M (copper or zinc) paired with tin (Sn), electroplated one layer at a time so the order and ratio can be tuned. The design gives each metal a role:

• Cu or Zn = the “oxygen metal” — its M(I)/(II)⇌M⁰ phase transition mediates the first two-electron step, NO₃^– → NO₂^– (Cu is the stronger nitrate adsorber).
• Sn = the “hydrogen metal” — it doesn’t reduce nitrate directly, but it enriches adsorbed hydrogen that hydrogenates the intermediates.

The key insight: which metal you plate on top decides the outcome. Put Sn on top of the Cu/Zn base and the H-rich surface drives intermediates all the way to N₂; put Cu or Zn on top and you get more ammonia or nitrite. (Wu, Shih, Chen, He & Huang; Chemical Engineering Journal 511, 2025, 161956; DOI 10.1016/j.cej.2025.161956.)

Key Results

Three results carry the story: dividing the labour beats either metal alone, the plating order is the decisive lever, and operando Raman shows why each combination behaves as it does.

Splitting the Job Beats Either Metal Alone

Monometallic Cu or Zn managed nitrate removal and faradaic efficiency below 40%. The bimetallic electrodes leapt past that: Cu₁Sn₁ at −1.0 V reached 68% removal, 75% faradaic efficiency and 84% N₂ selectivity; Zn₁Sn₁ at −1.2 V reached 76% N₂ selectivity.

Plating Tin on Top Is the Winning Move

Reversing the stack so Sn sits on the surface pushed both further: Sn₁Cu₁ hit 82% removal at 90% N₂ selectivity, and Sn₁Zn₁ hit 88% removal at 92% N₂ selectivity — together exceeding 90% at −1.0 V. On real electroplating effluent (~500 mg-N/L), Sn₁Cu₁ achieved >95% nitrate removal at ~85% N₂, under 0.25 kWh/m³ per order removed.

Operando Raman Explains the Difference

Recording spectra on the MRID (785 nm) during electrolysis, the team watched the surface change with potential. On Cu₁Sn₁, a cuprous film and SnO formed below −1.0 V alongside a ρ(NH₂) rocking mode at 960 cm^-1 — the fingerprint of an ammonia intermediate. On Zn₁Sn₁, ZnO bands (460, 639 cm^-1) and an adsorbed-NO_x band at 1030 cm^-1 appeared, tracking a pathway that favours nitrite/N₂. DFT reproduced the picture: Sn-on-top keeps the surface hydrogen-rich and lowers the barrier to N₂.

Why ProTrusTech Is Featuring This Research

This is as direct as attribution gets: the paper’s methods name “an operando Raman spectrometer (MRID model, ProTrusTech, Taiwan), λ = 785 nm.” Only by taking Raman spectra inside the running electrolysis cell, potential by potential, could the team catch the metal-oxide phase transitions and adsorbed nitrogen intermediates (ρNH₂ at 960 cm^-1, NO_x at 1030 cm^-1) that decide whether nitrate ends up as ammonia or nitrogen gas. See the Tech Insights hub for how operando EC-Raman works.

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Why This Matters

Denitrification is the endpoint most nitrate treatment actually wants — turn the pollutant into harmless gas and walk away — and doing it electrochemically, reagent-free, at low energy cost, on real electroplating wastewater is a genuinely deployable result. It also shows how much leverage sits in fabrication order: same two metals, different stack, different product. This is the remediation chapter alongside the same group’s pharmaceutical detection and molybdate recovery, all read by the same operando-Raman method.

What’s Next

• Long-term electrode durability against the tin passivation that competes with the reaction.
• Selectivity in high-chloride/brackish matrices, where competing chemistry intrudes.
• Coupling recovered N₂ (or valorising the ammonia branch) into a closed nitrogen loop.

Working on electrocatalysis, nitrate reduction, or bimetallic electrode design — and need operando Raman to see the surface phase and intermediates under potential? Get in touch with our team.

Key Takeaways

• A bimetallic Cu-Sn / Zn-Sn electrode reduces nitrate to harmless N₂ gas, splitting the job: Cu/Zn deoxygenates, Sn supplies hydrogen.
• Plating Sn on top is decisive — Sn₁Cu₁ (90%) and Sn₁Zn₁ (92%) N₂ selectivity, versus <40% for either metal alone.
• On real electroplating wastewater: >95% nitrate removal, ~85% N₂, under 0.25 kWh/m³ per order.
• Operando Raman on the MRID (785 nm) — named explicitly in the paper — revealed the metal phases and N-intermediates (ρNH₂ 960, NO_x 1030 cm^-1) behind the selectivity.

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