Part II: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Material Synthesis and Electrode-Fabrication Parameters on the Electrocatalytic Activity in the Hydrogen Evolution Reaction

Abstract

Earth-abundant NiMo-oxide nanostructures were investigated as efficient electrocatalytic materials for the hydrogen evolution reaction (HER) in acidic media. Synthesis and non-synthesis parameters were thoroughly studied. For the non-synthesis parameters, the variation in Nafion loading resulted in a volcano-like trend, while the change in the electrocatalyst loading showed that the marginal benefit of high loadings attenuates due to mass-transfer limitations. The addition of carbon black to the electrocatalyst layer improved the HER performance at low loadings. Different carbon black grades showed a varying influence on the HER performance. Regarding the synthesis parameters, a calcination temperature of 500 °C, a calcination time between 20 and 720 min, a stoichiometric composition (Ni/Mo = 1), an acidic precursor solution, and a fuel-lean system were conditions that yielded the highest HER activity. The in-house NiMoO₄/CB/Nafion electrocatalyst layer was found to offer a better long-term performance than the commercial Pt/C.

Keywords: hydrogen evolution reaction; nanostructures; nickel molybdate; parametric study; solution combustion synthesis; water electrolysis.

Figure 1

Dependence of the HER current density on the (a) Nafion loading, (c) electrocatalyst (NiMoO₄) loading, and (e) carbon black loading in the electrocatalyst layer consisting of NiMoO₄, carbon black, and Nafion. The current density values were recorded under potentiostatic conditions at an overpotential of −0.58 V after reaching a quasi-steady-state HER rate (1000 s). This was followed by LSV curves at a scan rate of 1 mV s⁻¹ and corrected for ohmic (iR) drop, shown in Figure S1a,c,e. The variation in Tafel slopes and exchange current density for the (b) effect of Nafion loading, (d) electrocatalyst (NiMoO₄) loading, and (f) carbon black loading. Those parameters were obtained from the Tafel plots shown in Figure S1b,d,f. In Nafion testing: the loading of NiMO₄ was 0.141 mg cm⁻², while the amount of CB was 33 wt.% relative to the total weight of the NiMoO₄/CB mixture. In the electrocatalyst loading tests, the Nafion amount was 23 wt.%, and the amount of CB was 33 wt.%. In the CB loading test, the Nafion amount was 23 wt.%, while the catalyst loading was 0.283 mg cm⁻².

Figure 2

(a) Dependence of the HER current density on the carbon black type used in the electrocatalyst layer consisting of NiMoO₄, carbon black, and Nafion. The corresponding NiMoO₄ and Nafion loadings were 0.283 mg cm⁻² and 23 wt.% (dry weight), respectively. The current density values were obtained from a potentiostatic test conducted at −0.58 V in 0.5 M H₂SO_4, after reaching a quasi-steady state HER rate (1000 s). (b) XRD. (c) Raman spectra of the pristine CB types.

Figure 3

Dependence of the HER current density on the (a) calcination temperature, (c) calcination duration, and (e) Ni/Mo atomic ratio in Ni_xMo_1−x-oxide (0 ≤ x ≤ 1). For (a,c), the electrocatalyst layer consisted of NiMoO₄, carbon black, and Nafion while for (e) the electrocatalyst layer consisted of Ni_xMo_1−x-oxide (0 ≤ x ≤ 1), carbon black, and Nafion. The current density values were recorded under potentiostatic conditions at an overpotential of −0.58 V after reaching a quasi-steady-state HER rate (1000 s). This was followed by LSV curves at a scan rate of 1 mV s⁻¹ and corrected for ohmic (iR) drop, shown in Figure S1g,i,k. The variation in the Tafel slope and the exchange current density for the (b) effect of calcination temperature, (d) calcination time, and (f) Ni/Mo atomic ratio in Ni_xMo_1−x-oxide; these kinetic parameters were obtained from the Tafel plots presented in Figure S1h,j,l, respectively.

Figure 4

Dependence of the HER current density on the (a) pH of the precursor solution and (c) Figure 3. ratio (φ). The electrocatalyst layer consisted of NiMoO₄, carbon black, and Nafion. The current density values were recorded under potentiostatic conditions at an overpotential of −0.58 V after reaching a quasi-steady-state HER rate (1000 s). This was followed by LSV curves at a scan rate of 1 mV s⁻¹ and corrected for ohmic (iR) drop, shown in Figure S1m,o. The variation in Tafel slope and exchange current density as a function of the (b) pH of the precursor solution and (d) fuel-to-oxidant ratio (φ); these kinetic parameters were obtained from the Tafel plots presented in Figure S1n,p.

Figure 5

(a) Polarization curves of the best-performing NiMo-oxide/CB/Nafion electrocatalyst layer (denoted by NiMoO₄ and exhibited the following specifications; Nafion (dry weight) loading: 23%, electrocatalyst loading: 0.354 mg cm⁻², CB loading: 38%, CB type: XC-72R, calcination temperature: 500 °C, calcination time: 6 h, Ni/Mo: 1, precursor solution’s pH: 4.57, φ: 1) and other benchmarks, recorded in 0.5 M H₂SO₄ at a scan rate of 1 mV s⁻¹ and corrected for ohmic (iR) drop (b) Tafel regions of the LSV plots in (a). (c) Long-term stability test of the NiMo-oxide/CB/Nafion electrocatalyst layer (denoted by NiMoO₄ with the same specifications as those mentioned in (a)) and 20 wt.% Pt/C conducted at −0.58 V and −0.33 V, respectively. (d) ICP-MS analysis of the blank H₂SO₄ electrolyte, and the electrolyte collected after the long-term stability test presented in (c) using a graphite rod and a Pt coiled wire as counter electrodes (CE). (e) LSV of the electrocatalyst layer before and after the long-term test presented in (c). (f) Nyquist plot of the electrocatalyst layer before and after the long-term test presented in (c), conducted at an overpotential of −0.23 V.

Figure 6

Selected SEM images showing the NiMoO₄/CB/Nafion electrocatalyst layer (same specifications as those indicated in the caption of Figure 5) before (a,c) and after electrolysis (b,d) at different magnifications. The label “Before HER” refers to the sample not subjected to electrolysis, while “After HER” denotes the sample after 24 h of electrolysis.

Figure 7

The wettability of the NiMoO4/CB/Nafion electrocatalyst layer (same specifications as Table 5 in Part I). Measured by the contact angle method for different electrolysis durations; (a) pre-electrolysis, (b) 15 min, (c) 1 h, and (d) 24 h after electrolysis.

Figure 8

XPS analysis of the NiMoO₄/CB/Nafion electrocatalyst layer (same specifications as those indicated in the caption of Figure 5) conducted before and after 24 h of electrolysis, performed at an overpotential of −0.58 V, portraying the elemental scans of the electrocatalytic constituents: (a) Ni 2p, (b) Mo 3d, (c) O 1s, (d) C 1s, (e) F 1s, and (f) S 2p. The label “Before HER” refers to the XPS spectrum of the sample before electrolysis, while “After HER” denotes the XPS spectrum of the sample after the 24-h electrolysis test. The points represent the experimental data, while the lines represent the deconvoluted spectra.