Powder X-ray diffraction (PXRD):
The bulk TiMn2 alloy was purchased from Sigma-Aldrich, and this TiMn2 was converted to nanostructure via sonication in toluene. Powder X-ray diffraction (PXRD) analysis was performed to identify the crystal phases in the TiMn2 alloy and obtain structural information. The PXRD pattern confirmed the successful synthesis of the alloy and showed clear diffraction peaks corresponding to the crystallographic planes of the TiMn2 phase. In Fig. 1, the obtained diffraction pattern showed clear peaks at two theta values of 36.90 o, 39.92o, 43.56o, 71.08o, and 77.62o. The most prominent peaks were identified at two theta angles of 43.56o, 71.08o, and 77.62o, corresponding to crystallographic planes (112), (302), and (220). These results indicate that the TiMn2 alloy crystallizes in a hexagonal structure with a P6/mmc space group according to the reference JCPDS card No. 00-007-0146. The X-ray diffraction pattern of bimetallic alloy TiMn2 revealed the formation of a C-14 type hexagonal Laves phase structure, in which titanium atoms (Ti) occupy ordered positions resembling a hexagonal diamond lattice, while manganese atoms (Mn) occupy tetrahedral positions surrounding the titanium atoms [34]. The presence of sharp and intense peaks in the diffraction pattern suggests the formation of a well-defined TiMn2 alloy phase with minimal impurities.
Morphological study:
Field Emission-Scanning Electron Microscopy (FESEM):
In this study, we used field emission scanning electron microscopy (FE-SEM) to investigate the surface morphology of TiMn2 alloy. The images obtained using FE-SEM, as shown in Figs. 2(a)-(c), were carefully analyzed to understand the surface properties of the alloy. Our analysis revealed a relatively uniform and smooth surface morphology of the TiMn2 alloy. To further investigate the composition of the alloy, we used energy-dispersive X-ray spectroscopy (EDS). The EDS spectra shown in Fig. 2(g) confirm the presence of titanium (Ti) and manganese (Mn) in the sample and confirm the successful formation of the TiMn2 alloy. The detailed atomic and weight compositions in the table in Fig. 2(h) confirm the expected presence of Ti and Mn, which is consistent with the composition of the TiMn2 alloy. Additionally, we performed elemental mapping to visualize the spatial distribution of Ti and Mn within the alloy. The resulting mapping images in Figs. 2(d)-(f) illustrate an even distribution of both elements in the sample. This homogeneity of element distribution further supports the uniform composition of the alloy and confirms the absence of any significant phase segregation.
X-ray photoelectron spectroscopy (XPS):
X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical composition and valence states of the elements present on the surface of the Ti-Mn alloy. As shown in Fig. 3a, the scan spectrum of Ti-Mn alloy shows the presence of Ti and Mn elements. Before peak fitting, the binding energy of the components was calibrated using the carbon impurity peak at 284.6 eV in the C 1s region as a reference. The high-resolution Ti-2p spectra show 2p3/2 and 2p1/2 core levels at binding energies of 457.94 eV and 463.86 eV, respectively, confirming the existence of Ti in the + 4 oxidation state in TiMn2 [35]. The peak at 459.5 eV is ascribed to the Ti-Ti bond, and 463.86 eV was ascribed to the Ti-O bond. Similarly, the peaks at binding energies of 641.03 eV and 653.18 eV are attributed to the core levels Mn 2p3/2 and Mn 2p1/2, respectively, indicating the presence of Mn in the + 2 oxidation state [36].
BET analysis:
The surface properties of TiMn2 were analyzed through N2 physisorption measurements. Figure 4 (a) illustrates the N2 adsorption-desorption isotherms of TiMn2, revealing that the alloy follows a class IV isotherm. The synthesized TiMn2 was found to have a surface area of 6.951 m2/g. Additionally, the BJH technique was used to estimate the pore size distribution, as shown in Fig. 4 (b), with a recorded pore size of 3.82 nm. These results indicate that TiMn2 possesses a high surface area, which contributes to its superior activity as a catalyst for the HER.
Electrochemical Study:
To analyze the catalytic activity of TiMn2 and its hydrogen evolution reaction (HER), we used an electrochemical workstation equipped with a multi-channel potentiostat-galvanostat and Nova 2.1.4 software. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and impedance spectroscopy were performed using a three-electrode setup in a 0.5 M H2SO4 solution. The CV, LSV, and impedance spectroscopy scan rates were 100 mV/s, 50 mV/s, and 10 mV/s, respectively. The counter electrode consisted of a graphite rod, while the reference electrode consisted of silver/silver chloride (Ag/AgCl). The working electrode was fabricated by drop-dropping a TiMn2 alloy substrate ink onto the Glassy Carbon electrode (GC) with a diameter of 2 mm. The catalyst ink was formed by mixing 5 mg of the sample alloy material with 200 µL of ethanol, 250 µL of deionized water, and 50 µL of Nafion solution. The resulting ink was applied to the glassy carbon electrode as a 5 µL drop with a mass loading of 0.707 mg/cm2 and allowed to dry overnight at room temperature in a desiccator. All potentials were converted to RHE using the appropriate Nernst equation for HER calculations.
$$\:E\left(RHE\right)\:=\:E(Ag/AgCl)\:+\:0.059pH\:+\:ERef\:\:\:\:\:\:\:$$
1
……………
Electrochemical HER Performances:
The linear sweep voltammetry (LSV) curve of TiMn2 alloy shows remarkable performance in hydrogen evolution reaction (HER). Specifically, the overvoltage required to achieve a current density of 10 mA/cm2 is 139 mV. This is a significant achievement, although higher than the commercial Pt/C catalyst, which only requires 34 mV [37, 38]. Nevertheless, the TiMn2 alloy shows promising catalytic activity. Further insight into the HER performance of the TiMn2 alloy is provided by panel diagrams. These plots show that the TiMn2 alloy has a small Tafel slope of 68 mV/dec, indicating efficient catalytic kinetics. A lower Tafel slope generally indicates that the catalyst can facilitate HER with increasing current density with less overpotential, thereby improving the efficiency of the reaction.
We used linear sweep voltammetry (LSV), Tafel plot analysis, and electrochemical impedance spectroscopy (EIS) to investigate the charge transfer resistance (Rct) of TiMn2 alloy during the hydrogen evolution reaction (HER) process. The EIS analysis provided valuable insights into the dynamics of charge transfer at the interface. Our results showed that TiMn2 alloy had the lowest Rct value of 237.9 ohms, indicating smooth and efficient charge transfer during HER. In addition, the solution resistance (Rs) was measured at 2.15 ohms, further supporting the effectiveness of TiMn2 alloy in the HER process. We also performed a stability test for up to 24 hours, which showed a loss in current density at 10 mA/cm2 of approximately 13.2%, demonstrating excellent stability of the HER electrode (Fig. 5d).
To assess the electrochemically active surface area (ECSA) and double layer capacitance (Cdl) of the catalysts, we conducted cyclic voltammetry (CV) measurements at varying scan rates within the non-Faraday potential range. This approach allowed us to calculate both anodic (Ja) and cathodic (Jc) double-layer charging currents. By plotting the difference between Ja and Jc against the scan rate and determining the slope, we calculated the Cdl value, which was 0.282 mF/cm2.
ECSA = Cdl/Cs
where, Cdl denotes double-layer capacitance, Cs denotes specific capacitance of GC (40 µF/cm2) [39]. The ECSA value was then determined from the Cdl value using the known specific capacitance of a flat surface. The calculated ECSA value for the TiMn2 alloy electrocatalyst was 7.05 cm2. This relatively high ECSA value indicates a large electrochemically active surface area, which is advantageous for catalytic processes as it provides more active sites for the HER.
The mass activity and specific activity of the TiMn₂ alloy electrocatalyst were calculated to be 160.73 A/g and 0.2313 mA/cm², respectively. Additionally, the turnover frequency (TOF) value for the TiMn₂ alloy structure during the hydrogen evolution reaction (HER) experiment was determined to be 0.00545 s⁻¹ at an overpotential of 139 mV (-0.139 V vs. RHE).
HER mechanism:
The process of water splitting encompasses two primary half-cell reactions occurring at the cathode and anode, respectively. The oxygen evolution reaction is an anodic process, while the hydrogen evolution reaction (HER) is a critical cathodic process in water electrolysis for hydrogen production. The process is integral and involves the reduction of protons (H+) or water molecules to generate hydrogen gas (H2) on the surface of an electrocatalyst. This intricate reaction typically follows two main pathways: the Volmer-Tafel and Volmer-Heyrovsky mechanisms, which illustrate the complex interplay of reactions at the electrode-electrolyte interface.[40]
In the Volmer-Tafel (V-T) mechanism, the HER occurs in a two-step process. Initially, a transferred electron combines with a proton from the aqueous electrolyte and is adsorbed at the catalyst's active site, forming the adsorbed hydrogen atom (Hads*). This step is known as the Volmer or discharge reaction step. Subsequently, two Hads* species then undergo a recombination reaction, resulting in the production of hydrogen gas (H2). This second step is referred to as the Tafel step [41]. The V-T mechanism can be represented as follows, where the symbol * denotes the hydrogen adsorption site on the catalyst surface:
Volmer step: H+(aq) + e− + * → Hads*
Tafel step: Hads* + Hads* → H2(g) + 2*
In contrast to the Volmer-Tafel (V-T) mechanism, the Volmer-Heyrovsky (V-H) pathway involves a different sequence of steps following the Volmer reaction. After forming the adsorbed hydrogen intermediate (Hads*) in the Volmer step, it reacts with a proton (H+) from the electrolyte, and an additional electron is transferred from the external circuit. This results in the direct formation of hydrogen gas (H2) in the Heyrovsky step [42]. In these circumstances, the HER process follows an alternative pathway:
Volmer step: H+(aq) + e +* → Hads*
Heyrovsky step: Hads* +H+(aq) + e− → H2 (g) +*
Table 1
Electrochemical LSV and EIS data of as-synthesized TiMn2 alloy.
| Catalyst | Overpotential (mV) | Tafel Slope (mV/dec) | Rs (ohm) | Rct (ohm) |
| TiMn2 | 139 | 68 | 2.15 | 237.9 |
Table 2
Electrochemical cyclic voltammetry Cdl, ECSA, Mass activity, and Specific activity values of the as-synthesized catalyst.
| Catalyst | Cdl (mF/cm2) | ECSA (cm2) | Mass Activity (A/g) | Specific Activity (mA/cm2) |
| TiMn2 | 0.282 | 7.05 | 160.729 | 0.2313 |
Table 3
Comparison of HER performance with representative HER alloy electrocatalysts.
| Materials | Overpotential (mV vs RHE) | Tafel slope (mV/dec) | Reference |
| Co–Mn–Sn | 136 | 111 | [43] |
| Ni–Cu–Ti | 300 | 170.6 | [44] |
| MoCo alloy | 75 | 69.9 | [45] |
| NiCu@C | 164 | - | [46] |
| TiMn2 | 139 | 68 | Our Work |
