Ampere-level current density ammonia electrochemical synthesis using CuCo nanosheets simulating nitrite reductase bifunctional nature

Abstract

The development of electrocatalysts capable of efficient reduction of nitrate (NO₃^-) to ammonia (NH₃) is drawing increasing interest for the sake of low carbon emission and environmental protection. Herein, we present a CuCo bimetallic catalyst able to imitate the bifunctional nature of copper-type nitrite reductase, which could easily remove NO₂^- via the collaboration of two active centers. Indeed, Co acts as an electron/proton donating center, while Cu facilitates NO_x^- adsorption/association. The bio-inspired CuCo nanosheet electrocatalyst delivers a 100 ± 1% Faradaic efficiency at an ampere-level current density of 1035 mA cm^-2 at -0.2 V vs. Reversible Hydrogen Electrode. The NH₃ production rate reaches a high activity of 4.8 mmol cm^-2 h^-1 (960 mmol g_cat^-1 h^-1). A mechanistic study, using electrochemical in situ Fourier transform infrared spectroscopy and shell-isolated nanoparticle enhanced Raman spectroscopy, reveals a strong synergy between Cu and Co, with Co sites promoting the hydrogenation of NO₃^- to NH₃ via adsorbed *H species. The well-modulated coverage of adsorbed *H and *NO₃ led simultaneously to high NH₃ selectivity and yield.

Fig. 1. Preparation strategy and characterization of catalysts.

a Schematic diagram of CuCo alloy electrodeposition on nickel foam’s surface. b XRD spectra of Cu₅₀Co₅₀, Cu, and Co. HRTEM image (c), SEM image (d), linear topography profiles from AFM images (e), and EDS mapping analysis (f) of Cu₅₀Co₅₀. XPS peaks spectra of Cu 2p (g) and Co 2p (h) of Cu₅₀Co₅₀.

Fig. 2. Electrochemical responses of Cu₅₀Co₅₀, pure Cu, and pure Co catalysts.

a j-E curve (80% iR corrected) over Cu₅₀Co₅₀, pure Cu, and pure Co modified Ni foams (catalysts loading was 5 mg cm⁻²) in 1 M KOH solution containing 100 mM KNO₃ (solid lines) or in the absence of KNO₃ (dotted line) at a scan rate of 1 mV s⁻¹ (the red dash line presenting the j of 10 mA cm⁻², the shading S1 and S2 presenting the peak around 0.2 to 0.05 V and 0.05 to −0.15 V, respectively). b j-E curve (80% iR corrected) at 400 rpm and electron transfer numbers at different potentials calculated by the K–L equation for Cu₅₀Co₅₀ on RDE in 100 mM KNO₃ + 1 M KOH electrolyte at a scan rate of 10 mV s⁻¹ (catalysts loading was 0.25 mg cm⁻²). Tafel slopes in the potential range of peak S1 (c) S2 (d). e j-E curves over Cu₅₀Co₅₀ modified Ni foam in 1 M KOH solution containing 100 mM KNO₃ at different scan rates without agitation (solid line) and at a scan rate of 5 mV s⁻¹ with agitation (catalysts loading was 5 mg cm⁻²). f Time-dependent current density curves over Cu₅₀Co₅₀, Cu, Co modified Ni foam at −0.2 V with a magnetic stirring speed of 1000 rpm (catalysts loading was 5 mg cm⁻²).

Fig. 3. Electrochemical performance of catalysts.

${\text{FE}}_{{\text{NH}}_3}$and ${\text{FE}}_{{\text{NO}}_2^{-}}$ of NO₃⁻RR (a), bias-current density and products yield for NH₃ (b), and the ratio of NO₂⁻ to NH₃ generated (c) for different Cu/Co ratio at 0 V in 100 mM KNO₃ + 1 M KOH electrolyte (catalysts loading was 5 mg cm⁻²). ${\text{FE}}_{{\text{NH}}_3}$ (d) and NH₃ product yield (e) at different electrode potentials on Cu₅₀Co₅₀, pure Cu and pure Co catalysts modified Ni foam (catalysts loading was 5 mg cm⁻²). Comparison of the electrocatalytic NO₃^-RR performances of Cu₅₀Co₅₀ modified Ni foam with other extensively reported electrocatalysts (f). ${\text{FE}}_{{\text{NH}}_3}$ and ${\text{Yield}}_{\text{mass}-{\text{NH}}_3}$ on Cu₅₀Co₅₀/Ni foam under the applied potential of −0.2 V during 10 periods of 1 h electrocatalytic NO₃^-RR (g) (catalysts loading was 5 mg cm⁻²). The time-dependent concentration of NO₃⁻, NO₂⁻ and NH₃ and corresponding FE over Cu₅₀Co₅₀ modified Ni foam at −0.1 V (h) (catalysts loading was 5 mg cm⁻²). Error bars represent the standard deviations calculated from three independent measurements.

Fig. 4. Electrochemical in situ FTIR spectra.

Electrochemical thin-layer in situ FTIR spectra of NO₃⁻RR on Cu₅₀Co₅₀ (a), Cu (b), and Co (c) in 100 mM KNO₃ + 1 M KOH. d $\frac{{\mathrm{I}}_{{\mathrm{NO}}_2^{-}}}{{\mathrm{I}}_{{\mathrm{NH}}_2\mathrm{OH}}+{\mathrm{I}}_{{\mathrm{NO}}_2^{-}}}$ ratio at different electrode potentials. ATR-FTIR spectra on Cu₅₀Co₅₀ (e), Cu (f), and Co (g) in 1 M KOH.

Fig. 5. Electrochemical SHINERS spectra of NO₃⁻RR.

SHINERS spectra between 230–750 cm⁻¹ on Cu₅₀Co₅₀ (a), Cu (b), and Co (c). SHINERS spectra between 750–1700 cm⁻¹ on Cu₅₀Co₅₀ (d) in 100 mM KNO₃ + 10 mM KOH during cathodic polarization from 0.7 to −0.1 V.

Fig. 6. DFT calculations of NO₃⁻RR and HER on Cu(111), Co(111), and CuCo(111).

Reaction-free energies for different intermediates of NO₃^-RR (a) and HER (b) at −0.2 V vs. RHE on CuCo(111), pure Cu(111), and pure Co(111) surface, respectively.