Arun
Karmakar
ab,
Abhirami V.
Krishnan
c,
Rahul
Jayan
d,
Ragunath
Madhu
ab,
Md Mahbubul
Islam
*d and
Subrata
Kundu
*ab
aAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002, India
bElectrochemical Process Engineering (EPE) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630003, Tamil Nadu, India. E-mail: skundu@cecri.res.in; kundu.subrata@gmail.com; Fax: (+91) 4565-241487; Tel: (+91) 4565-241487
cDepartment of Chemistry, Wayne State University, Detroit-48201, USA
dDepartment of Mechanical Engineering, Wayne State University, Detroit-48201, USA. E-mail: gy5553@wayne.edu
First published on 24th April 2023
Transition metal-based layered double hydroxides (LDHs) have attracted significant attention in recent research in electrochemistry and hold promising capacity in a wide range of applications. Owing to their diverse chemical structure, improved mass- and charge-transfer kinetics, and fascinating 2D layered structure, LDHs have emerged to be promising electrocatalysts for water electrolysis. Yet, some limitations, including their low stability and poor conductivity, remain as major obstacles towards the application of LDHs in the field of electrocatalysis. As a result, various LDH-derived nanomaterials, like selenides, sulphides, phosphides, etc., have been employed as effective electrocatalysts towards water splitting. Here in this study, a solvothermal reaction strategy was employed to derive a selenide from NiFe-LDH and the successful selenization resulted in the formation of Fe-doped Ni3Se4 (Fe@Ni3Se4) with a 2D microporous structure. The Fe@Ni3Se4 showed excellent electrocatalytic performance for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline conditions, requiring just 185 and 33 mV overpotentials to drive 10 mA cm−2 current density in the OER and HER, respectively, which outperformed many recently reported benchmark electrocatalysts. Also, a 10-fold increase in turnover frequency (TOF) was noticed in Fe@Ni3Se4 compared to pristine NiFe-LDH, revealing an enormous improvement in specific activity as a result of the selenization. The presence of Fe with a lower valency provided an added advantage over the redox activity and stable structural outcomes, which was confirmed from operando impedance analysis and various other characterizations. First-principles DFT calculations revealed that doping low-valent Fe ions largely increased the 3d-2p repulsion in the Ni–O bonds, thereby facilitating the O2 desorption process as compared to pristine Ni3Se4, as a result of the electronic exchange. Moreover, the downward shifting of the d-band centre with respect to the Fermi level modulated the binding strengths of the Fe-doped substrate towards H* compared to the pristine substrates.
Water splitting by electrochemical means involves two half-cell reactions, namely the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode.8 Since the OER involves a complex four-electron and multi-step process, it can be a high bottleneck towards the overall water splitting reaction compared to the HER.9–13 Therefore, to remove this kinetic barrier, designing earth-abundant and stable electrocatalysts has gained a crucial importance owing to the high cost of the conventional noble metal (Ir, Ru, and Pt)-based electrocatalysts.14 Several, electrocatalysts based on earth-abundant 3d transition metals, like oxides, hydroxides, sulphides, selenides, phosphides, MOFs, etc., have been reported as potential candidates for the water splitting reaction.15–26
Among all the above-mentioned materials, layered double hydroxides (LDHs) with a brucite-like layered structure consisting of bi- and tri-valent metal ions as the metal-ion source along with charge balancing interlayer anions have attracted much attention.27,28 Significant studies on the utilization of LDHs for electrocatalytic water splitting have recently been performed. These studies have revealed that LDH-based materials can be an effective alternative to noble metal-based electrocatalysts. Among various LDHs, NiFe-LDH has recently been considered a benchmark catalyst for the OER. However, NiFe-LDH materials have a few limitations, like poor electrical conductance, restriction towards the number of active sites, and poor stability.29 Apart from these drawbacks, the improper active sites towards the cathodic counterpart of water splitting, limit their practical application. To address these issue, the derivatizations of LDHs to various counterparts, like sulphides, selenides, etc., have been investigated and has proven to be a very effective strategy. The various advantages of this strategy can be summarized as: (1) the transformation to a sulphide, selenide, or phosphide could alter the electronic structure of the parent materials; (2) an increase in conductivity; (3) a retention of a 2D sheet-like morphological structure could be achieved, and so on.30
So, by considering the various advantageous of this derivatization of LDHs, herein we report an effective selenization of the hierarchical flower-like NiFe-LDH material under solvothermal conditions. The selenization of NiFe-LDH led to the formation of an Fe-doped Ni3Se4 hierarchical nanostructure with a modified coordination environment on the active Ni sites. The transformation of the hydroxide moiety to a selenide entity resulted in an increase in the porosity of the material. As a result of such improvement in porosity, the Fe-doped Ni3Se4 possessed a better charge as well as mass transfer at the electrode–electrolyte interfaces. The detailed chemical analysis via X-ray photoelectron spectroscopy (XPS) showed that the doping of iron in the seleno-spinel structure in a reducing environment led to the transformation of Fe3+ ions to active Fe2+ ions. The high crystal-field-stabilization energy (CFSE) guided the Fe2+ ions to replace the Ni3+ in the octahedral (Oh) NiSe6 moiety.31 While to maintain the charge balance, the Fe3+ ions without any CFSE were been placed in the tetrahedral (Td) coordination site to form an FeSe4 unit. The formation of a Ni–Se–Fe linkage largely facilitated the electronic exchange in the electrocatalytic process. The hierarchical Fe-doped Ni3Se4 nanostructure demonstrated excellent OER and HER activity in alkaline conditions by requiring just 185 and 33 mV overpotentials to drive 10 mA cm−2 current density, respectively. First-principles DFT calculations revealed that doping low-valent Fe ions largely stabilized the high valent Ni–O intermediates by transferring their electron density to the active Ni sites. Moreover, the increased 3d-2p repulsion in Ni–O bonds largely facilitated the O2-desorption process compared to with pristine Ni3Se4 as a result of the electronic exchange.
Due to the large intensity of the substrate peaks, the other peaks corresponding to our desired materials were diminished. Therefore, XRD analysis of the powder materials obtained during the hydrothermal treatment was carried out and the obtained results are shown in Fig. S1b.† The diffraction pattern corresponding to NiFe-LDH showed different diffraction peaks corresponding to the (003), (006), (012), (110), and (113) planes of NiFe-LDH (ICDD card no: 00-051-0463). The XRD analysis of the selenized NiFe-LDH showed various diffraction peaks corresponding to the (−312), (−313), (−401), (310), and (−714) planes of the selenium counterpart of the spinel Ni, i.e. Ni3Se4.
The morphologies of the NiFe-LDH and Fe-doped Ni3Se4 (Fe@Ni3Se4) materials were analyzed via field emission-scanning electron microscopy (FE-SEM). From the FE-SEM images in Fig. 1b–d, it can be observed that the NiFe-LDH possessed a hierarchical micro-flower structure. The images show the flower-like morphological outcomes were made up by combining numerous uniform nano-flakes. The Fe@Ni3Se4 possessed a similar sheet-like morphology (Fig. 1e–g). It was interesting to note that after the selenization, the solid flakes of the flower-like structure created an internal porous structure. The formation of such an internal porous structure is beneficial for allowing the effective tunnelling of the electrolyte anions to favour the electrochemical process with less applied potential required.32 The highly porous nature of Fe@Ni3Se4 was further verified form the Brunauer–Emmett–Teller (BET) N2 adsorption–desorption isotherm analysis results given in Fig. S2a.† The calculated BET isotherm areas for NiFe-LDH and Fe@Ni3Se4 were 33.46 and 61.65 m2 g−1 respectively. Moreover, the pore diameters were obtained from the adsorption branch by using the Barrett–Joyner–Halenda (BJH) algorithm and the corresponding results are listed in Fig. S2b.† The NiFe-LDH and Fe@Ni3Se4 possessed characteristic BJH diameters of 18.7 and 13.32 nm, respectively. This high specific surface area and low pore diameter with distinct distribution is beneficial for allowing effective mass transfer. The quantitative analysis was carried out on the EDAX spectra given in Fig. S3a and b.† The EDAX spectra showed the presence of the expected elements, like Ni, Fe, C, O, and Se, without any impurity. It was interesting to note that the ratio of Ni to Fe in NiFe-LDH was almost about ‘1’, but after selenization, this ratio decreased substantially, which might be due to an additional selenization of the underlying Ni foam under the selenized conditions and as Fe has been placed as a doped entity in the Ni3Se4 lattice. The ratios of Ni to Fe in both NiFe-LDH and Fe@Ni3Se4 were further confirmed through inductive coupled plasma-mass spectroscopy (ICP-MS) analysis.
The microstructural outcomes of both the materials were analyzed via high-resolution transmission electron microscopy (HR-TEM). Fig. 2a and b show the low to high magnification TEM images of pristine NiFe-LDH, revealing the presence of the particles as a flower-like hierarchical nanostructure, which was in accordance with the FE-SEM results given in Fig. 1d. The selected area electron diffraction (SAED) pattern given in Fig. 2c revealed the polycrystalline nature of the particles and showed diffraction patterns from the (110) and (012) planes of the layered LDH lattice. The porous hierarchical nano-architecture of Fe@Ni3Se4 was further confirmed from the TEM images given in Fig. 2d. Further, the polycrystalline behaviour was confirmed from the SAED pattern given in Fig. 2e.
The high-resolution TEM image (Fig. S3†) displayed a distinct lattice fringes pattern with a d-spacing value of 0.342 nm corresponding to the (110) plane of the Ni3Se4 structure. Elemental confirmation and their distributions were analyzed from the high angle annular dark field (HAADF) colour mapping analysis. The selected HAADF area is displayed in Fig. 2f. The uniform distribution of all the elements, like Ni, Fe, and Se, was confirmed from the colour mapping results given in Fig. 2g–i, respectively.
The chemical nature of all the elements was analyzed by X-ray photoelectron spectroscopy (XPS). The corresponding survey spectra of NiFe-LDH and Fe@Ni3Se4 are given in Fig. S4 and S5,† respectively, which reveal the presence of all the expected elements, like Ni, Fe, Se, C, and O, without any other impurity. The high-resolution XPS spectrum of the Ni 2p orbital (Fig. 3a) of NiFe-LDH could be well-fitted with two spin–orbit-coupling-originated peaks corresponding to Ni 2p3/2 and Ni 2p1/2 orbitals along with two satellite peaks at 860.8 and 878.7 eV.33 The deconvoluted XPS spectrum showed the presence of Ni in both the +2 and +3 oxidation states. The Fe 2p XPS outcomes of NiFe-LDH are displayed in Fig. 3b, revealing the presence of two well-fitted maxima at 711.1 and 724.52 eV corresponding to Fe 2p3/2 and Fe 2p1/2 orbitals in a +3 oxidation state.
The other two peaks at higher binding energies of 715.74 and 733. 03 eV corresponded to Fe 2p3/2 and Fe 2p1/2 satellite peaks, respectively. The maximum for the 2p3/2 peak for Fe@Ni3Se4 was shifted towards a lower binding energy value of 0.62 eV, which indicated the existence or formation of lower valent iron as a result of the selenization under the reducing conditions. Moreover, the higher spin–orbit-coupling constant of Fe@Ni3Se4 (13.63 eV) compared to NiFe-LDH (13.11 eV) indicated the formation of a mixed valency, i.e. the co-existence of Fe2+ and Fe3+ ions. The O 1s XPS results given in Fig. 3c show the presence of three spin–orbit-coupling-originated peaks at binding energy values of 527.87, 529.75, and 530.29 eV, in accordance with the presence of oxygen as metal-hydroxide, metal-oxygen, and intercalated water molecule functionalities, respectively. The XPS spectrum of the Ni 2p orbital for Fe@Ni3Se4 (Fig. 3d) showed a similar XPS pattern as for NiFe-LDH, but maximum corresponding to the Ni 2p3/2 orbital was shifted towards a higher binding energy value by 0.65 eV, which might due to the oxidation of Ni2+ ions to maintain the charge in the seleno-spinel structure. It is interesting to look at the XPS spectrum of the Fe 2p orbital for the selenized counterpart given in Fig. 3e. A substantial amount of Fe3+ ions were reduced to Fe2+ under the reducing conditions, which could be identified from the XPS peaks at 710.9 eV.31 The presence of Fe2+ ions influenced the structural outcomes of the seleno-spinel structure, as confirmed through the DFT analysis discussed later. The Se 3d orbital XPS peak (Fig. 3f) showed two peaks at binding energy values of 53.64 and 54.94 eV, corresponding to the Se 3d5/2 and Se 3d3/2 orbitals and confirming the presence of selenium as Se2− ions. Another two higher energy peaks at 58.9 and 57.6 eV were also observed due to the presence of different surface oxides of Se.
According to the chemical nature of the spinel structure, Fe in the +3 and +2 oxidation states should occupy the Oh and Td sites, respectively, and would result in structure S-2. However, the presence of Fe3+ ions with the t2g3eg2 electronic configuration with a zero CFSE value and Fe2+ ions with the eg3t23 electronic configuration with a CFSE value of −0.26 Δo would destabilize the structure by 0.09Δo [CFSCFe2+(Fe@Ni3Se4) − CFSENi2+(Ni3Se4) = 0.09Δo]. Whereas, when considering the structure S-3, the Fe2+ ions with the electronic configuration of t2g4eg2 in Oh could bring additional stabilization to the spinel structure by 0.05Δo [CFSCFe(Oh)2+(Fe@Ni3Se4) − CFSENi(td)2+(Ni3Se4) = −0.05Δo]. The XPS analysis confirmed that Fe2+ and Fe3+ ions were present in equal proportions, and therefore, the even distribution of both metal ions could bring a structural stabilization to the Fe-doped Ni3Se4 structure. The presence of redox-active Fe2+ ions in the Oh coordination sites bought about added advantages over the redox activity of the selenide-based spinel structure with extra stability.
The specific activity of Fe@Ni3Se4 was further compared with NiFe-LDH from the ECSA-normalized polarization information given in Fig. 4e. The current density values acquired by Fe@Ni3Se4 and NiFe-LDH was calculated at a thermoneutral potential of 1.48 V vs. RHE. The inset of Fig. 4e displays the calculated current density, and shows that Fe@Ni3Se4 possessed a 5.23 times higher specific activity than the other catalyst. The intrinsic activity of all the catalyst were determined by calculating the turnover frequency (TOF), which is the number of O2 or H2 formed per second per active site.36 The TOF values of all the catalysts were determined from the redox surface area information given in Fig. S9a–c.†Fig. 4f shows the TOF values of Fe@Ni3Se4 (1.66 s−1), NiFe-LDH (0.96 s−1), and Ni3Se4 (0.5 s−1). Thus, by comparing the TOF result, it could be observed that Fe@Ni3Se4 delivered a high specific as well as high intrinsic activity compared to the other catalysts. Further the long-term stability of Fe@Ni3Se4 was verified by CV cycling for 1000 times with a high scan rate of 180 mV s−1. The resulting backwards polarization (to exclude the interference of the oxidation peak current in the forward CV cycles) information was compared with the polarization information before the cycling study (Fig. 5a). Fig. 5a portrays there was minimal degradation of the catalyst under the harsh cycling, inferring the excellent stability of the catalyst. A steady-state long-term stability study was performed via chronoamperometry (CA) with a fixed potential of 1.5 V vs. RHE. The resulting CA curve given in Fig. 5b shows the excellent stability of the catalyst over 50 h. The intrinsic activity of a catalyst demonstrates its site-specific or per-site activity information and thereby is related to the chemical and physical properties of the catalytic sites. The ECSA-normalized TOF, i.e. specific TOF value, is more acceptable than that of geometrically normalized polarization information (Fig. S10a and b†). To gain deeper knowledge about the site-specific intrinsic activity information, the specific TOF values were calculated and are portrayed in Fig. 5c.
It is interesting to notice that Fe@Ni3Se4 delivered a higher specific TOF value by 0.17 s−1, whereas for the pristine NiFe-LDH, the difference in TOF value was negligible. This is a quite interesting result and the possible reason behind this improvised specific activity is the presence of redox-active Fe2+ ions and the hierarchical porous structure in the selenide counterpart, as was evident from the BET results. The formation of a porous network allows mass transfer quite effectively, which enhances the charge transfer at the electrolyte interfaces. On the other hand, the presence of redox-active low-valent Fe entities could apparently enhance the site-specific active sites for OH* adsorption.31
To visualize the improved adsorption kinetics of OH* over Fe@Ni3Se4, operando electrochemical impedance (EIS) analysis was carried out. Operando EIS is an effective tool to investigate the adsorption and desorption kinetics for OER processes at the electrode–electrolyte interface.37–40 The corresponding Nyquist plots at different applied potentials are displayed in Fig. S11a and b,† which show the electrochemical behaviours of Fe@Ni3Se4 and NiFe-LDH. The Bode plots for both materials are shown in Fig. 6a and b, which could be differentiated into three different regions: the non-faradaic Cdl region, electro-activation or redox region (both at high frequency sites), and the OER region or the region of the concerned faradaic process (at low frequency sites). The localized and higher phase angle shift in Fe@Ni3Se4 in the Bode signal in the high-frequency region (Fig. 6b) revealed the higher charge-transfer kinetics at the electrode compared to pristine NiFe-LDH.
To gain an insight into the improved redox activity by Fe@Ni3Se4, the fitting resistance information (Fig. 6c–e) was analyzed at different applied potentials. An equivalent circuit diagram was noted with two successive redox reactions characterized with two resistance values, i.e. R1 (equivalent to redox transformation to reach the activated state) and R2 (related to the OER process). From the resistance information in Fig. 6c and d, a sharp drop in resistance could be seen at a certain potential (marked as the turnover potential), which indicated the transformation from a non-faradaic reaction to a faradaic process. Fe@Ni3Se4 delivered a turnover potential value of 1.27 V vs. RHE with a potential window of ∼0.1 V, whereas NiFe-LDH demonstrated a turnover potential of 1.32 V vs. RHE with a potential window of ∼0.2 V. Further, the calculated R1 value, i.e. the resistance towards the electrochemical activation steps of Fe@Ni3Se4 (0.16 Ω) and NiFe-LDH (0.83 Ω), indicated the faster redox transformation of the selenide counterpart as compared to NiFe-LDH.41 Therefore, the increase in porosity and redox-active metal ions as a result of the selenization in the sheet-like NiFe-LDH synergistically enhanced the electrocatalytic activity in the resulting Fe-doped Ni3Se4.
Further the potential application of Fe@Ni3Se4 towards the cathodic HER reaction was verified in 1 M KOH solution. As shown Fig. 7a, Fe@Ni3Se4 showed the highest HER activity and it required just 33 and 96 mV of overpotential to achieve 10 and 50 mA cm−2 current densities, respectively; whereas the other pristine materials, like NiFe-LDH and Ni3Se4, showed higher overpotentials of 113 and 143 mV (Fig. S12†) to drive the same 10 mA cm−2 current density. More importantly, from the given polarization information in Fig. 7a, it was interesting to observe that Fe@Ni3Se4 outperformed the commercial Pt/C (20%) catalyst, which required 59 mV overpotential at the benchmark current density.
The excellent electrocatalytic performance of Fe@Ni3Se4 compared to the other materials could further be verified from the bar diagram in Fig. 7b. The charge-transfer kinetics at the interface was verified from the Tafel slope analysis and the obtained results are illustrated in Fig. 7c. Fe@Ni3Se4 possessed a lower Tafel slope value of 76 mV dec−1, whereas pristine NiFe-LDH delivered a higher Tafel slope value of 126 mV dec−1. A lower Tafel slope value indicates faster kinetics, and a value of 76 mV dec−1 indicates that HER has proceeded by following the Volmer–Heyrovsky kinetics process. Faster electron transfer at the interface was further verified from the EIS analysis by fixing a constant over a potential value of −0.175 V vs. RHE (Fig. 7d). The lower Rct value (2.21 Ω) for Fe@Ni3Se4 compared to NiFe-LDH (6.62 Ω) illustrated the low internal resistance for the electron transfer at the interface towards the HER.42
The intrinsic activities of all the catalysts were verified by calculating their TOF values at 150 mV overpotential. Fe@Ni3Se4 delivered the highest TOF value of 3.07 s−1, whereas the TOF values for NiFe-LDH and Ni3Se4 were about 0.31 and 0.22 s−1, respectively. The high TOF value for Fe-doped Ni3Se4 illustrated its improved per-site activity as a result of the selenization and the low-valent Fe ions in the lattice. Moreover, the static stability of Fe@Ni3Se4 was investigated via chronoamperometry (CA) with an applied potential value of −0.125 V vs. RHE. The 50 h CA study revealed the excellent stability of the selenized counterpart.
In light of the exceptional activities and stabilities of Fe@Ni3Se4 towards the OER and HER, it is reasonable to suggest Fe@Ni3Se4 would be an auspicious bifunctional electrocatalyst for the overall water splitting reaction. Therefore, a two electrode set-up was constructed by taking two pieces of Fe@Ni3Se4 with an effective geometrical surface area of 1 × 1 cm2. The LSV polarization curve (Fig. 8a) for the total water splitting showed that the Fe@Ni3Se4‖Fe@Ni3Se4 electrolyser demanded just 1.64 V to drive 50 mA cm−2 current density. Also, the Fe@Ni3Se4‖Fe@Ni3Se4 electrolyser outperformed the state-of-art RuO2‖Pt/C electrolyser. More importantly, the Fe@Ni3Se4‖Fe@Ni3Se4 electrolyser displayed strong stability for total water splitting at a cell voltage of 1.63 V with a current density of 50 mA cm−2 for 33 h (Fig. 8b).
The Fe@Ni3Se4 porous nanostructured electrode exhibited remarkably low OER overpotentials of 201 and 185 mV at 50 and 10 mA cm−2, respectively, which make it among the best NiFe-based OER catalysts (Table S1†), and much more active than the state-of-the-art RuO2 catalyst. Moreover, Fe-doped Ni3Se4 possessed excellent alkaline HER activity too and it demanded just 33 mV overpotential to drive 10 mA cm−2 current density, which is a remarkable result for a non-platinum or non-noble-metal-based electrocatalyst in alkaline conditions, as can be seen from Table S2†.
Further, the structural and morphological stabilities of Fe@Ni3Se4 after long-term electrochemical application were assessed by different characterization techniques, such as XPS, TEM, and Raman analysis. From the XPS plots (Fig. S13a–d†), it was evident that for Ni and Fe, there was definite peak shifting of the XPS peaks towards higher binding energy values compared to the XPS maxima in the pre-OER materials. This peak shifting towards higher binding energy sites revealed the definite oxidation of the metallic elements under the harsh anodic environment. The deconvoluted XPS spectrum of the O 1s orbital displayed three spin–orbit-coupling-originated peaks corresponding to the M-O (oxyhydroxide), M-OH, and H2O molecules. The presence of selenide along with the oxide phase were observed from the Se 3d orbital XPS outcomes. Microstructural analysis of the Fe@Ni3Se4 material after the OER study was performed by TEM analysis, and the corresponding TEM results are portrayed in Fig. S14a–h.† The materials possessed similar 2D sheet-like morphologies after OER analysis, which were also like the catalyst before the OER analysis. The high-resolution TEM image showed different d-spacing patterns corresponding to the basic Ni3Se4 and electrochemically modified oxyhydroxide layer. This electrochemical modification would lead to the formation of a NiOOH@Ni3Se4 heterostructural entity. HADDF colour mapping analysis displayed a finite distribution of all the expected elements. The formation of an –OOH layer was further confirmed from the Raman analysis given in Fig. S15.†43
In addition, we investigated the free energy profile for H* adsorption for both the pristine Ni3Se4 and Fe@Ni3Se4 materials. It was demonstrated that the H* adsorption free energy is a critical predictor of the activity of HER catalysts, and that an efficient HER catalyst should have a small H* adsorption energy (Fig. S17a and b†).45,46 From Fig. 9f, the of the studied Fe-doped Ni3Se4 electrocatalyst was 0.094 eV, i.e. closer to the thermoneutral value compared to the pristine substrate (−0.573 eV), and moreover the studied electrocatalyst's HER activity was comparable to that of the state-of-the-art Pt. The d-band centre was determined using the position of the d-orbital, which regulates the adsorption strength of the examined electrocatalysts.47 By examining the energy of the antibonding states located above the Fermi level, the d-band centre can be used as a useful descriptor to understand the adsorption strengths of the investigated electrocatalysts. We noted that the d-band centre of Fe@Ni3Se4 was shifted further away from the Fermi level (−1.304 eV) compared to that of the pristine Ni3Se4 (−1.078 eV), as shown in Fig. 9d. The study of the d-band centres revealed that the binding strength of the Fe-doped substrate towards the HER was weaker than that of the pristine substrate, as evidenced by the shift in the Fermi level to a lower energy level with a greater number of occupied antibonding states (Fig. 9e). As a result, the mild adsorption behaviour of the Fe-doped Ni3Se4 substrate may facilitate the adsorption and desorption of H*, thereby enhancing its HER catalysis. The experimentally observed superior electrocatalytic activity of the Fe-doped Ni3Se4 substrate was in good agreement with the theoretical DFT calculations.
Footnote |
† Electronic supplementary information (ESI) available: Electrodes fabrications, material preparation for different characterizations, data for OER, and comparative electrocatalysis data are given. See DOI: https://doi.org/10.1039/d3ta00868a |
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