X-ray Diffraction studies
Nature and the structure of the sample can be identified by X-ray diffraction pattern.. The XRD pattern for Ni2C electrocatalyst is displayed in Fig. 3. The Ni2C have different diffraction peaks at 2θ values 23.5°, 34.2°, 41.5°, 47.2°, 53.1°, 58.5°, and 69.3° were indexed to (100), (200), (101), (102), (001), (220), and (111) respectively. These planes are indicates the cubic phase crystalline structure of the synthesized Ni2C electrocatalyst and these results are well correlated with the JCPDS: 06-0693 patterns. These results confirmed the formation of Ni2C electrocatalyst with a cubic phase crystalline structure by hydrothermal synthesized method.
Electrocatalytic activity towards HER
CV studies
cyclic voltammetry technique is used to evaluate the catalytic activity in HER of the Ni2C electrocatalyst, was performed in 0.5M sulfuric acid solution at 2 mVs−1 scanning rate in room temperature. Fig. 4. shows the CV polarization curve for Ni2C electrocatalyst. The results of the polarization towards the HER show the peak with high current density at low over potential value. Ni2C electrocatalyst exhibits current density at 17mA/cm2 in 250 mV fixed potential and similarly, the over potential value is 203 mV at fixed current density 10 mA/cm2. Hence, these values’ signifying the prepared Ni2C has having the admirable HER activity.
Tafel studies
Tafel slope yields the electrocatalytic HER performance of Ni2C electrocatalyst. As shown in Fig. 5, the Ni2C electrocatalyst has achieved a small tafel slope at 93 mV/dec, It specify that the smaller Tafel slope of the Ni2C sample, the higher the HER activity. The Tafel plot (Fig. 5) is fitted by following Tafel equation is,
η = b x log j + a (1)
Where, η - over potential (mV),
j - Current density (mA·cm−2). ~93mV/dec is the calculated slope value of Tafel
The Tafel slope revealed that the HER catalyzed by Ni2C electrocatalyst occurs through the mixed Volmer mechanism. Tafel slope in Fig. 5 is due to the quicker proton discharge kinetic and shows superior HER activity in Ni2C electrocatalyst. In classic theory, HER at acidic aqueous media can be proceeds through two steps as shown below,
The first step is to electrochemical reduction (H+ reduction, Volmer-reaction)
H3O+ + e−→ Had + H2O (118 mV/dec) (2)
The second step (Hads desorption) is moreover the ion and atom reaction (Heyrovsky-reaction)
Had + H+ + e− → H2 (40 mV/dec) (3)
The atom combination reaction (Tafel-reaction)
Had + Had → H2 (30 mV/dec) (4)
The Tafel slope is insufficient to determine the precise mechanism, also the reduced slope in Ni2C sample confirms the promoted Volmer-step in hydrogen evolution reaction mechanism.
EIS analysis
The EIS measurements were performed to evaluate the electrocatalytic activity of the Ni2C electrocatalyst and also the corresponding Nyquist plot is displayed in Fig. 6. The charge transfer resistance was investigated carried throgh EIS analysis. The Ni2C catalyst exhibit a lower Rctvalue (54 Ω) indicate that the synthesized Ni2C electrocatalyst has a high electronic conductivity. EIS analysis shows the high electrochemical enactment in Ni2C sample. It may be seen that the EIS measurements are in sensible agreement with the results discovered from cyclic voltametry analysis.