The structural properties of the electrocatalysts were characterized by FE-SEM images (Fig. 2a-f). According to Fig. 2a, the bare Ti sheet has a relatively smooth surface, presumably due to the surface substrate pretreatment. However, the surface of the Ni/Ti electrocatalyst (Fig. 2b) shows cracks with exposed nickel crystals with intense internal agglomerated links and the introduction of manganese has resulted in a relatively smooth surface without any crystalline particle for the NiMn/Ti electrocatalyst (Fig. 2c). This observation signifies that the addition of manganese might treat the cracks and somewhat flatten the surface. The high- magnification SEM image of the NiMn/Ti (Fig. 2d) depicts more surface details by showing an interconnected, rough, and porous structure. Such a surface could afford a high surface area and more exposed active sites, facilitating charge and mass transfer. Adding cobalt to NiMn decreased the porosity and surface area of the ternary composite (Fig. 2e). Also, the FE-SEM image of CoMn (Fig. 2f) shows an agglomerated non-porous structure which signifies the low surface area of the electrocatalyst.
The chemical composition of the electrocatalysts was identified by energy-dispersive X-ray spectroscopy (EDS). The EDS spectrum of NiMn/Ti (Fig. 2g) shows that NiMn/Ti contains manganese and nickel with atomic percentages of 9.15 and 9.74, respectively. These close atomic percentages are consistent with the equal concentration of the Ni and Mn precursors in the electrodeposition bath, confirming the synthesis procedure's homogeneity and precision. Oxygen atoms are present because of exposure to air and surface oxidation during preparation, drying, and electrode measurements. The elemental mapping of NiMn/Ti (Fig. 2h) confirms the uniform distribution of nickel and manganese on the substrate.
The XRD pattern of the as-synthesized electrocatalysts has been presented in Fig. 2i. In all patterns, four peaks assigned at 2θ of \({36}^{0}\), \({40}^{0}\), \({60}^{0}\) and \({72}^{0}\), attributed to the Ti substrate. The appearance of the Ti substrate characteristic peaks, in line with the EDS spectrum, is ascribed to the low mass loading of electrocatalysts and X-ray penetration into the Ti substrate. In the XRD pattern of the samples with nickel content (Ni, NiMn, and NiMnCo), there is a peak around 2θ of \({22}^{0}\) which is indexed to the (004) plane of NiO (JCPDS no.78–0423). In the case of samples containing manganese (NiMn, CoMn, and NiMnCo), a diffraction peak corresponding to the (220) plane of MnO2 appears (JCPDS no. 44–0141). The XRD patterns of the cobalt-containing electrocatalysts (CoMn and NiMnCo) show three weak peaks around 2θ of \({38}^{0}\), \({43}^{0}\) and \({74}^{0}\) that belong to the (111), (200), and (311) planes of CoO, respectively (JCPDS no. 48-1719). The detection of metal oxide compounds is in good agreement with the EDS results, which indicated the presence of oxygen in the electrocatalysts' composition. It is worth mentioning that the broad diffraction peaks indicate that Ni, NiMn, CoMn, and NiMnCo with crystalline sizes of 12.65, 14.37,10.96, and 9.84 nm, have small crystallinity, which is due to the low synthesis temperature.
LSV evaluated the electrocatalytic activity of the electrocatalysts toward HER with a scan rate of 5 mV.s− 1 under alkaline media (1.0 M KOH solution). Figure 3a shows the LSV curve of the electrocatalysts. As can be seen, NiMn/Ti, with the lowest onset potential of 83 mV, which is 151 and 144 mV smaller than that of Ni/Ti and Mn/Ti respectively is the most active electrocatalyst but is still inferior to Pt/C catalyst. NiMn/Ti needs an overpotential of 210 mV for delivering a current density of 10 mA.cm− 2, which is much lower than that of Ni/Ti (390 mV) and Mn/Ti (502 mV). Such activity suggests that the high catalytic activity of NiMn/Ti arises from the synergistic effects of the Ni-Mn composite [16]. While the composite of Mn and Ni improved the activity and NiMn/Ti shows a better electrocatalytic performance than Ni/Ti and Mn/Ti, the addition of cobalt to the structure of NiMn composite (NiCoMn/Ti) results in no significant effect on the electrocatalytic activity of the NiMn/Ti. This implies that the synergy between Mn, Co, and Ni in the ternary NiCoMn composite is not as great as the bimetallic NiMn composite. This observation could also be related to the negative effect of adding cobalt to the NiMn structure, as seen in their FE-SEM images.
The Tafel pathway was studied to evaluate the electrocatalytic activity further and characterize HER kinetics. According to Fig. 3b, while NiMn/Ti displays the lowest Tafel slope of 103 mV.dec− 1, the other developed electrocatalysts show Tafel slopes greater than 118 mV.dec− 1. Such Tafel slope values suggest that the electrochemical hydrogen adsorption is the rate-determining step and controls the reaction except for NiMn/Ti, for which HER proceeds through the Volmer-Heyrovsky mechanism with relatively fast electrochemical hydrogen adsorption and controlling electrochemical desorption [17, 18]. While the Volmer reaction (discharge step) needs filled d orbitals, the Heyrovsky reaction needs half-filled d orbitals for properly proceeding [19]. Hence, combining a metal with half-filled d orbitals (Mn) with a metal with filled d orbitals (Ni) facilitates the reaction progress. Thus, the better performance and the accelerated kinetics of NiMn could be partly related to this phenomenon.
Besides the Tafel slope, the exchange current density of electrocatalysts was obtained from the Tafel plots. The exchange current density of Ni, Mn, and NiMn were 0.033, 0.743 and 0.760 \(\frac{mA}{{cm}^{2}}\). Exchange current density, which is the current density at an overpotential of 0 V, somehow demonstrates the intrinsic activity of electrocatalysts [20]. As can be seen, nickel solely has low exchange current density and so poor intrinsic activity toward HER; however, when combined with manganese, which historically is known as an active metal for HER, its intrinsic activity was improved. Thus, it could be asserted that a part of the higher activity of NiMn is due to its higher intrinsic activity. Though, the exchange current density of NiMn2 was 0.428\(\frac{mA}{{cm}^{2}}\), which indicates that excess Mn did not resulted in the improvement of HER activity, and the optimum amounts of Mn and Ni have been used to prepare the NiMn electrocatalyst.
In addition to the Tafel slope and exchange current density, the charge transfer coefficient of electrocatalysts was also extracted from the Tafel plots. Mn, Ni, and NiMn electrocatalysts' charge transfer coefficients were obtained at 0.134, 0.155, and 0.290, respectively. The charge transfer coefficient signifies the fraction of the consumed energy that contributes to decreasing the free energy barrier of an electrochemical reaction [21]. Based on the charge transfer coefficient values, Ni has a low ability to decrease the activation energy of HER. However, when Mn was combined with Ni, the charge transfer coefficient was increased considerably more than 2 times, which indicates NiMn devotes a higher fraction of applied energy to decreasing activation energy. Thus, it could be proposed that NiMn has higher energy efficiency than Ni and Mn, and part of its better performance arises from its higher energy efficiency. This observation also was seen in the combination of Mn and Co, where the charge transfer coefficient of Co and CoMn was obtained at 0.132 and 0.171 respectively. While Mn and Co solely have relatively low charge transfer coefficients, their combination gave rise to a higher charge transfer coefficient and so higher energy efficiency. It is worth mentioning that further addition of Mn to NiMn might not favor the increase of charge transfer coefficient and bring it closer to manganese since NiMn2, with a charge transfer coefficient of 0.139, has lower energy efficiency than NiMn. This could be considered a reason behind the worse performance of NiMn2 compared with NiMn.
The charge transfer behavior of the electrocatalysts was also investigated by performing electrochemical impedance spectroscopy (EIS). Figure 3c shows the Nyquist plot of the different electrocatalysts at η = 300 mV. An equivalent circuit as depicted in the inset of Fig. 3c was considered to study the EIS plots. Table 1 presents the value of elements for different electrocatalysts. The NiMn/Ti with Rct value of 13.18 Ω has the smallest charge transfer resistance and the superior HER activity of the NiMn/Ti could be related somewhat to its lower charge transfer resistance [22].
Table 1
Electrochemical parameter estimated from EIS data
Electrocatalyst | Rs (Ω) | Rct (Ω) | C(µF) | n |
Co | 1.94 | 27.32 | 478.8 | 0.99 |
CoMn | 2.01 | 32.48 | 421.1 | 0.96 |
Ni | 2.47 | 233.39 | 414.8 | 0.92 |
NiCoMn | 1.79 | 22.1 | 477.8 | 0.99 |
NiMn | 2.07 | 13.18 | 362.9 | 0.94 |
Ti | 1.80 | 7779.72 | 465.8 | 0.93 |
In addition to the electrochemical activity, the stability and durability of electrocatalysts are also important for real applications. Therefore, the durability of the NiMn/Ti as the best electrocatalyst of this study was evaluated by performing 1000 continuous cyclic voltammetry (CV) tests. According to Fig. 3d, after 1000 cycles, a minor loss in the activity of NiMn/Ti is seen, which proves its good durability. Moreover, the stability of NiMn/Ti was investigated by chronopotentiometry test under a static overpotential of − 70 mV (inset of Fig. 3d). As can be seen, NiMn/Ti showed a stable performance after 10 h of continuous polarization.