Novel mixed heterovalent (Mo/Co)Ox-zerovalent Cu system as bi-functional electrocatalyst for overall water splitting

Abstract

A novel hybrid ternary metallic electrocatalyst of amorphous Mo/Co oxides and crystallized Cu metal was deposited over Ni foam using a one-pot, simple, and scalable solvothermal technique. The chemical structure of the prepared ternary electrocatalyst was systematically characterized and confirmed via XRD, FTIR, EDS, and XPS analysis techniques. FESEM images of (Mo/Co)O_x-Cu@NF display the formation of 3D hierarchical structure with a particle size range of 3-5 µm. The developed (Mo/Co)O_x-Cu@NF ternary electrocatalyst exhibits the maximum activity with 188 mV and 410 mV overpotentials at 50 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Electrochemical impedance spectroscopy (EIS) results for the (Mo/Co)O_x-Cu@NF sample demonstrate the minimum charge transfer resistance (R_ct) and maximum constant phase element (CPE) values. A two-electrode cell based on the ternary electrocatalyst just needs a voltage of about 1.86 V at 50 mA cm^-2 for overall water splitting (OWS). The electrocatalyst shows satisfactory durability during the OWS for 24 h at 10 mA cm^-2 with an increase of only 33 mV in the cell potential.

Figure 1

Illustration for the fabrication procedure of (Mo/Co)O_x–Cu@NF.

Figure 2

Surfaces of the blank uncoated-NF and the three coated-NF samples.

Figure 3

FE-SEM images (a–f) showing the texture and morphology of the fabricated (Mo/Co)O_x–Cu@NF at different magnifications.

Figure 4

(a) XRD pattern of (Mo/Co)O_x–Cu@NF and (b) EDS spectrum for the selected SEM area (c) and their corresponding elemental mapping (d–h).

Figure 5

ATR-FTIR for the fabricated (Mo/Co)O_x–Cu@NFand compared to that of the Fumaric acid, and β-cyclodextrin.

Figure 6

Detailed XPS spectra of (a) Mo3d, (b) Co2p, (c) Cu2p, and (d) O1s.

Figure 7

Cyclic voltammetry curves of different samples in 1 mol L⁻¹ KOH at scan rate of 50 mV s⁻¹ at 25 °C.

Figure 8

(a), (b), and (c) Cyclic voltammetric curves of different samples swept at different scan rates (10–100 mV s⁻¹) in 1 mol L⁻¹ KOH, at 25 °C and their corresponding (d) plot Δj/2 versus sweep rate.

Figure 9

(a) Linear sweep voltammetric curves and their corresponding (b) Tafel plots for the various prepared samples (c) Nyquist plots for different prepared samples measured at − 0.274 V (vs. RHE). (d) Graph showing R_ct and C_dl extracted from EIS for all samples. All measurements recorded in 1 mol L⁻¹ KOH, at 25 °C.

Figure 10

(a) Linear sweep voltammetric curves and their corresponding (b) Tafel plots for the various prepared samples (c) EIS Nyquist plots for different prepared samples measured at + 1.806 V (vs. RHE). (d) Graph showing R_ct and C_dlextracted from EIS for all samples. All measurements recorded in 1 mol L⁻¹ KOH, at 25 °C.

Figure 11

(a) overall water-splitting stability test of (Mo/Co)O_x–Cu@NF measured in a symmetrical two-electrode cell using chronopotentiometry at 10 mA cm⁻² for 24 h and (b) LSV for two (Mo/Co)O_x–Cu@NF electrode-cell measured at 5 mV s⁻¹ before and after the chronopotentiometric measurement for 24 h. All recorded in 1 mol L⁻¹ KOH at 25 °C.