Part I: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Synthesis Parameters on the Materials’ Physicochemical, Structural, and Morphological Properties

Abstract

The impact of process conditions on the synthesis of NiMoO4 nanostructures using a solution combustion synthesis (SCS) method, in which agar powder and Ni(NO3)2 were utilized as fuel and as the oxidant, respectively, was thoroughly studied. The results show that the calcination temperature had a significant implication on the specific surface area, phase composition, particle size, band gap, and crystallite size. The influence of calcination time on the resulting physicochemical/structural/morphological properties of NiMoO4 nanostructures was found to be a major effect during the first 20 min, beyond which these properties varied to a lesser extent. The increase in the Ni/Mo atomic ratio in the oxide impacted the combustion dynamics of the system, which led to the formation of higher surface area materials, with the prevalence of the β-phase in Ni-rich samples. Likewise, the change in the pH of the precursor solution showed that the combustion reaction is more intense in the high-pH region, entailing major implications on the physicochemical properties and phase composition of the samples. The change in the fuel content showed that the presence of agar is important, as it endows the sample with a fluffy, porous texture and is also vital for the preponderance of the β-phase.

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

“…At 300 • C, the electrocatalyst's performance in the HER saw a significant increase due to the crystallization and formation of NiMoO 4 as a result of the combustion reaction. As it was shown in Part I of this article [20], XRD and FTIR analyses of the material produced at 300 • C indicated the presence of a wide range of phases, including MoO 2 , MoO 3 , β-NiMoO 4 , α-NiMoO 4 , and NiO. Additionally, the BET (specific) surface area was not maximized at this temperature, and the band gap, which can be correlated with the electrical conductivity, was relatively small (2.88 eV).…”

“…Additionally, the BET (specific) surface area was not maximized at this temperature, and the band gap, which can be correlated with the electrical conductivity, was relatively small (2.88 eV). At 400 • C, the fraction of the β-NiMoO 4 phase in the synthesized material increased at the expense of MoO 2 and α-NiMoO 4 , in addition to a marked increase in the specific surface area (see Table 1 and Figure 2 in Part I of the manuscript [20]), which enhanced the HER performance, as seen in Figure 3a. As reported elsewhere, both MoO 2 and MoO 3 and are not active HER electrocatalysts [47].…”

“…We have previously identified NiMoO 4 as a promising HER electrocatalyst in the acidic medium [18] and reported on its synthesis as a nanostructured material [19]. In Part I of the current manuscript [20], we performed a detailed study on the production, optimization, and characterization of NiMoO 4 nanostructures using the solution combustion synthesis (SCS) method, with a comprehensive assessment of several synthesis-related parameters (the effects of calcination temperature, calcination time, Ni/Mo ratio, pH of the precursor solution, and fuel-to-oxidant ratio (ϕ)) that could influence the microstructure and physicochemical properties of the material. In the current (Part II) study, we evaluate the electrocatalytic activity of the nanostructured NiMoO 4 samples (synthesized and characterized in Part I [20]) toward HER in an acidic environment (resembling the one in a PEM electrolyzer) to deduce the influence of material production parameters on the HER activity.…”

“…After investigating the effect of non-synthesis parameters on the HER performance of the electrocatalyst layer, the effect of synthesis parameters used in the production of NiMoO 4 (effects of the calcination temperature, calcination time, Ni/Mo atomic ratio, pH of the precursor solution, and fuel-to-oxidant ratio (ϕ)) on the HER performance are discussed below. Those non-synthesis parameters were studied extensively in Part I of this article [20]. Unless being the parameter studied, the calcination temperature and time were 500 • C and 6 h, respectively, the Ni/Mo atomic ratio was 1, the pH of the precursor solution was 4.57, and the fuel-to-oxidant ratio (ϕ) was unity.…”

“…In Part I of the current manuscript [20], we performed a detailed study on the production, optimization, and characterization of NiMoO 4 nanostructures using the solution combustion synthesis (SCS) method, with a comprehensive assessment of several synthesis-related parameters (the effects of calcination temperature, calcination time, Ni/Mo ratio, pH of the precursor solution, and fuel-to-oxidant ratio (ϕ)) that could influence the microstructure and physicochemical properties of the material. In the current (Part II) study, we evaluate the electrocatalytic activity of the nanostructured NiMoO 4 samples (synthesized and characterized in Part I [20]) toward HER in an acidic environment (resembling the one in a PEM electrolyzer) to deduce the influence of material production parameters on the HER activity. Since HER cathodes in PEM electrolyzers are typically comprised of nanostructured metal electrocatalyst materials usually anchored on carbon black (CB) particles, and since the ionomer (Nafion) is a key part of the PEM configuration, we also elucidate here the effect of several other non-synthesis influential parameters related to the makeup of the electrocatalyst layer (Nafion loading, electrocatalyst loading, CB loading, and CB type) aiming to pinpoint the set of conditions that yield the best HER performance.…”