Electrochemically dealloyed nanoporous Fe40Ni20Co20P15C5 metallic glass for efficient and stable electrocatalytic hydrogen and oxygen generation

Abstract

The anion exchange membrane (AEM) in fuel cells requires new, stable, and improved electrocatalysts for large scale commercial production of hydrogen fuel for efficient energy conversion. Fe₄₀Ni₂₀Co₂₀P₁₅C₅, a novel metallic glass ribbon, was prepared by arc melting and melt spinning method. The metallic glass was evaluated as an efficient electrocatalyst in water-splitting reactions, namely hydrogen evolution reaction under acidic and alkaline conditions. In addition, oxygen evolution reaction in alkaline medium was also evaluated. In 0.5 M H₂SO₄, the metallic glass ribbons, after electrochemical dealloying, needed an overpotential of 128 mV for hydrogen evolution reaction, while in 1 M KOH they needed an overpotential of 236 mV for hydrogen evolution. For the oxygen evolution reaction, the overpotential was 278 mV. The electrochemical dealloying procedure significantly reduced the overpotential, and the overpotential remained constant over 20 hours of test conditions under acidic and alkaline conditions. The improved electrocatalytic activity was explained based on the metastable nature of metallic glass and the synergistic effect of metal hydroxo species and phosphates. Based on the excellent properties and free-standing nature of these metallic glass ribbons in electrolyte medium, we propose the current metallic glass for commercial, industrial electrocatalytic applications.

Fig. 1. (a) Optical image of the as-prepared MG ribbons, (b) ribbon sample twisted, cut, and wrapped around a tube, (c) evolution of H₂/O₂ gas bubbles at the MG working electrode toward HER/OER reactions.

Fig. 2. XRD spectrum (a) of the MG ribbon in the as-prepared form. (b–d) after 20 h chronoamperometric tests. (e) The crystallized ribbon is composed mainly of crystalline phase FeCo, FeNi₂P, Fe₃P, Ni₂P, and Co₂P phases, which are confirmed from the PDF#65-4131, PDF#51-1367, PDF#19-0617, PDF#03-0953, PDF#89-3030, respectively. (f) TEM image of the as-prepared MG sample; scale bars: 100 nm and 5 nm⁻¹ (inset).

Fig. 3. HER and OER performance of Fe₄₀Co₂₀Ni₂₀P₁₅C₅ MG. (a–c) Linear polarization curves of pristine glassy, after 2000 CV, electrochemically dealloyed and crystalline samples. (d–f) Tafel plots of their corresponding polarization curves.

Fig. 4. (a and b) SEM image of electrochemically dealloyed MG ribbon in 0.5 M H₂SO₄ (c and d) MG ribbon electrochemically dealloyed in 1 M HCl. (e) EDX point-shoot spectrum for the porous structure after dealloying in 1 M HCl. Inset shows XRD spectra for electrochemically dealloyed in 1 M HCl; scale bars: (a) 40 μm, (b) 10 μm, (c) 5 μm, and (d) 1 μm.

Fig. 5. TEM image of electrochemically dealloyed MG ribbon in 1 M HCl; scale bars: (a) 20 nm and (b) 5 nm.

Fig. 6. (a–e) HAADF-STEM image and elemental mapping of Fe₄₀Ni₂₀Co₂₀P₁₅C₅. (f–j) MG ribbon electrochemically dealloyed in 1 M HCl; (b and g) Fe, (c and h) Co, (d and i) Ni, and (e and j) P.

Fig. 7. Chronoamperometric durability tests for 20 h (a) HER @ 0.5 M H₂SO₄ at overpotential 214 mV, (b) HER @ 1 M KOH at overpotential 319 mV, (c) OER @ 1 M KOH at overpotential 330 mV. (d–f) LSV curves before and after chronoamperometric tests.

Fig. 8. (a–c) Impedance Cole–Cole plots for the Fe₄₀Co₂₀Ni₂₀P₁₅C₅ MG during HER in acidic medium and alkaline medium, and OER in alkaline medium.

Fig. 9. XPS spectra for as prepared and chronoamperometric tested MG samples. (a–d) Fe 2p, (e–h) Co 2p, (i–l) Ni 2p, and (m–p) O 1s.