One-pot synthesis of crystalline structure: Nickel-iron phosphide and selenide for hydrogen production in alkaline water splitting

Electrocatalytically active nanocomposites play a vital role in energy generation, conversion


Introduction
Developed nations have made a commitment to switching over immediately to environmentally friendly electrochemical energy. Renewable energy sources like wind and solar energy are crucial in order to reduce the use of fossil fuels and environmental degradation. Electrochemical water splitting (EWS) is one of the most promising strategies that can be used to address energy issues due to its purity, safety, and ease of use [1,2]. EWS includes two half-cell reactions, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Both the anodic and cathodic reactions result in the creation of hydrogen and oxygen, respectively [3,4]. In addition, several studies have focused not only on the production of new catalysts but also on the discovery of new affordable fuels. In this regard, particular attention has recently been paid to hydrogen fuel, which has been suggested as the secondary energy source for the future [5][6][7].
Among noble metals, platinum-based catalysts are the most active for HER. However, the high price and resource shortage of noble metal electrocatalysts limit their application in water splitting application. To address this problem, scientists began to explore earth-rich transition metals (TMs) to replace precious metal-based catalysts. Transition metal phosphide and selenide (TMP/TMSe) have got a lot of interest due to their flexibility in a variety of applications, such as catalysts, batteries, and supercapacitors [8][9][10]. There have been several efforts to produce a less expensive composite material that can replace noble metals for hydrogen production. According to previous studies, TMP/TMSe have been synthesized through one-pot processes resulting in highly active catalysts [11][12][13][14]. Nickel (Ni) and iron (Fe) TM alloys were also considered efficient materials owing to their exceptional properties, such as hardness and conductivity. Therefore, preparing Ni and Fe composites with P and Se could be a valuable approach for hydrogen production. For example, Pei et al. [15] developed a series of highly active and inexpensive Co-Ni-P films by a one-step constant current density electrodeposition method. These films were demonstrated to be efficient bifunctional catalysts for both H2 and O2 evolution (HER and OER), while deposition time was deemed the crucial factor governing electrochemical performance. In particular, it requires −103 mV overpotential for HER and 340 mV for OER to achieve the current density of 10 mA cm -2 , corresponding Tafel slopes of 33 and 67 mV dec -1 , respectively. Xu et al. [16] fabricated the Fe-Co-P multi-heterostructure arrays consisting of CoP, Co2P and FeP, built for water splitting by a selfsacrificial template method. Such multi-heterostructures adjust the local electronic structure and then improve the adsorption of reaction intermediates on active sites, which was further proved through density functional theory (DFT) calculations. Xing et al. [17] fabricated a unique 2D FeSe2/CoSe nanosheet structure synthesized by hydrothermal and selenization processes. In an alkaline solution, FeSe2/CoSe nanosheet electrode exhibited a low overpotential of 73 mV at 10 mA cm -2 for the HER. Kwak et al. [18] reported the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for the simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibited superior electrocatalytic efficiency in OER, with a Tafel slope of 38 mV dec -1 (1 M KOH) and HER with 44 mV dec -1 (0.5 M H2SO4). One-pot method is considered advantageous over traditional multi-step synthesis methods, as it reduces the number of reaction steps and the amount of waste generated. It also has the potential for scalability and lower cost [19,20]. This novel one-pot method is offering inherent advantages such as step economy, operational simplicity, and synthetic efficiency.
Herein, Ni and Fe-based P and Se composites such as Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se were prepared by using the one-pot synthesis method and Ni-foam (NF) was used as a substrate. The asprepared composites were firstly characterized physically by different techniques, including XRD, SEM, EDS, and HRTEM, for their crystallographic analysis, surface features, elemental presence and deeper morphological aspects, respectively. After that, the as-prepared materials were analysed for HER response in an alkaline medium. The obtained physicochemical and electrochemical outcomes demonstrated that the Ni-Fe-P-Se could be considered as a prominent electrocatalyst for HER in the alkaline medium.

Chemicals
The experimental procedure involved the use of Ni foam (NF), which was supplied by Sigma-Aldrich. The Ni foam used had a high purity of 99.9 % and a specific surface area ranging from 0.5 to 3 m 2 g -1 . Additionally, the foam had a pore size of around 100-200 micrometres, commonly used in electrochemical applications. The 2-mercaptoethanol with a purity of 93.4 % was provided by Tianjin Guangfu Fine Chemical Research Institute. Selenium (Se) powder with a purity of 97 % was purchased from Sinopharm chemical reagents Co. LTD. Sodium dihydrogen phosphate (NaH2PO4) was purchased from Tianjin Yaohua chemical reagents Co. LTD, and nickel(II) nitrate Ni(NO3)26H2O was obtained from Sinopharm chemical reagents Co. LTD. All chemicals used in the synthesis process were of analytical grade. The chemicals were added to the reaction without further treatment, ensuring that the experimental conditions were consistent and reliable.

Preparation of Ni-P-Se, Fe-P-Se and Ni-Fe-P-Se materials
In a typical experimental procedure, commercially purchased chemicals were taken into a mixture with Ni(NO3)2·6H2O (1.45 g), Fe(NO3)3 (1.40 g), NaH2PO2 (2.00 g), and selenium (Se) (1.2 g) were mixed to prepare different solutions. Ni-P-Se, Fe-P-Se and Ni-Fe-P-Se were prepared, dissolved separately in 35 mL distilled water (DW), and stirred for one hour to form clear solutions. Later, the obtained solutions were centrifuged and washed several times with DW and ethanol for 10 minutes. The centrifuged products, such as Ni-P-Se, Fe-P-Se and Ni-Fe-P-Se, were collected and dried in a vacuum oven overnight at 70 °C. Finally, the dried products were placed in a furnace for annealing under a nitrogen atmosphere. The furnace was heated at 400 °C for 3 hours with a heating rate of 5°C min -1 . After cooling down the samples (Ni-P-Se, Fe-P-Se and Ni-Fe-P-Se) at room temperature, the asobtained powder samples were collected and used for further characterizations and electrochemical analysis. The schematic procedure of samples preparation is depicted in Figure 1. The X-ray diffraction (XRD) patterns were observed by using X-Ray diffractometer (XRD, Rigaku TTR-III) operated at 40 kV/150 mA with high-intensity Cu Ka radiation (λ = 0.15406 nm). Transmission electron microscopy (TEM, Tecnai-G220S-Twin, FEITEM) was used to check the samples' topography and interplanar analysis at 120 kV an accelerating voltage. The corresponding d-spacing value has been calculated through Image-J software (via HRTEM images). The morphology and structure of the samples were characterized by scanning electron microscopy (SEM, JSM-6480A, JOEL) run at 20 kV an accelerating voltage and used to determine the shape and structure of the deposited materials on Ni-foam. For deposition, a dispersed solution of binder and the as-prepared catalyst was dispersed on NF via drop casting. EDS (energy dispersive x-ray spectroscopy) mapping attached with SEM was used for the confirmation of present elements. This information helped us to optimize their synthesis methods and improve the properties of the resulting materials.

Electrocatalytic measurements
Electrocatalytic measurements of HER were conducted by using the traditional three-electrode workstation (CHI760E) bought from Shanghai Chenhua Instrument Co. Ltd. A platinum rod electrode was used as the counter electrode, while a silver silver-chloride (Ag/AgCl) filled with 3.0 M KCl served as the reference electrode. The Pt rod electrode is often used as a counter electrode because it is inert and does not participate in the chemical reactions taking place in the electrochemical cell. As-prepared samples were deposited separately on NF (1×1cm) pieces and used as working electrodes. In 1.0 M KOH, all potentials were calibrated to the reversible hydrogen electrode (RHE) using the Nernst equation and all electrochemical experiments were performed at room temperature. First, 5 mg of each catalyst solution dissolved in 1 mL of DW and 50 mL of 5 % Nafion solution (as a binder) were individually added for catalyst ink preparation. Then, the catalyst ink was properly dispersed using an ultrasonic bath for 20 minutes. Later, 10 mL of dispersed solution was coated by drop casting on NF foam and dried at room temperature. The loaded mass of each catalyst was about 0.2 mg cm -2 . The electrocatalytic activity for HER was investigated in 1.0 M KOH through the linear sweep voltammetry (LSV) at 5 mV s -1 scan rate. KOH is a strong base and can provide a high-conductivity solution, making it a good choice for many electrochemical applications, including HER [21,22]. All potentials measured by Ag/AgCl reference electrode (EAg/AgCl) were converted to the potential of reversible hydrogen electrode (ERHE) using equation (1): where E°Ag/AgCl is equal to 0.21 V. The electrode overpotential (ƞ) in the HER region is defined as the difference between ERHE and standard equilibrium potential for H + /H2 (0.00 V), resulting in ƞ = ERHE.
The Tafel slope, on the other hand, was computed using the Tafel equation (2).
where the overpotential is ƞ, the Tafel slope is b, and the current density is j. The current-time curve is required for assessing the performance of electrocatalyst for HER because it gives essential information on the catalyst efficiency, kinetics, and long-term stability. Based on a particular current level, a current-time curve displays the interrupting time of an overcurrent device, all tests were carried out in an electrocatalysis cell. Preliminary studies revealed that a little adjustment in V had no discernible effect on the current-time curves in the cell. In other words, the small adjustments did not affect the current-time curves in the cell. If significant modifications are made, the current-time curves are likely to be altered [23]. The proposed mechanism of HER on Ni-Fe-P-Se is presented in Figure 2.

Physicochemical analysis
The X-ray diffraction has been applied to analyse the crystallographic characteristics of various materials, including Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se, and results are shown in Figure 3. crystallographic planes and strongly justify the existence of Ni within the as-prepared composite material [25,26]. Fe-P-Se diffraction patterns can be seen in Figure 3b, revealing the presence of Se with additional peaks for iron (Fe) at 44.3 and 65.1° that are attributed to the (110) and (200) crystal planes, having a JCPDS card no. 03-065-4899, which is entirely supported by existing research [27,28]. X-ray patterns of Ni-Fe-P-Se shown in Figure 3c resemble the existence of Ni, Fe, and Se elements. Such findings through XRD analysis confirmed the successful preparation of Ni-Fe-P-Se composition, and this combination of three different elements was already shown to promote the synergistic effect toward HER performance [29,30].

2 / °
HRTEM is one of the most advanced analytical imaging measurement techniques to distinguish the size and shape of as-prepared samples [31]. In Figure 4a-4c, the images perfectly reflect the detailed morphology of the as-prepared material with the scale bars of 500, 200 and 10 nm, respecttively. The HRTEM image for the synthesized material Ni-Fe-P-Se (Figure 4) reveals an uneven surface and different domains resulting from the tight aggregation of nanoparticles with a wellresolved lattice fringes space assigned to the plane of Ni-Fe-P-Se [32]. Figures 4a and 4b illustrate the different magnifications of the samples. The lattice fringe, along with their planner distance measurement, is depicted in Figure 4c, and its fast Fourier transform (FFT) conversion can be found on the right side of the image. The d-spacing value for as-prepared Ni-Fe-P-Se is calculated as 0.221 nm, which is well-matched with recently reported research [33].

Figure 4. (a-b) HRTEM images of Ni-Fe-P-Se at different magnifications; (c) lattice fringes and corresponding FFT images for d-spacing calculation
Distance, nm Figure 5 presents the elemental mapping result of Ni-Fe-P-Se to demonstrate the uniform distribution of Ni, Fe, P, Se, C, N, and O, which confirms the uniform growth of the Ni-Fe-P-Se composite.

Figure 5. Elemental mapping of as-synthesized material (Ni-Fe-P-Se)
Different morphology of electrocatalytic materials, as well as chemical composition, were studied via SEM. Average morphologies of the as-obtained Ni-P-Se, Fe-P-Se and Ni-Fe-P-Se were observed at different magnifications, as shown in Figure 6. It is observed that the Ni-P-Se shows the morphology on NF with its desired quantity, as depicted in Figure 6a. Figure 6b reveals the morphological aspects of Fe-P-Se, which is grown on NF as well. It can be seen that Fe-P-Se has an agglomerated lumpy structure. In Figure 6c, the morphology of Ni-Fe-P-Se can be seen, which shows a higher amount of prepared material on NF, with varied surface features than both previous samples of Ni-P-Se and Fe-P-Se, respectively. Through the SEM images, we have observed agglomerated surface features of various elements within the as-prepared Ni-Fe-P-Se structure as a potential composite for electrochemical water splitting application [34,35].

Electrochemical analysis
The electrocatalytic HER performance on Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se was initially investigated in an alkaline electrolyte (1.0 M KOH) using LSV. Figure 7a exhibits the LSV curves of three proposed electrocatalysts with reference to a Pt-based electrocatalyst. It can be seen that the constructed Ni-Fe-P-Se has an activity for HER with a lower overpotential value of 316 mV at 10 mA cm -2 as compared to other electrocatalysts (Ni-P-Se and Fe-P-Se) with overpotential values of 385 and 458 mV, respectively. Tafel slope has always been a crucial analysis tool in HER reaction kinetics for the determination of the rate-determining step [36][37][38]. The Tafel slope value of Ni-Fe-P-Se was calculated as 89 mV dec -1 , which is significantly lower than for the other two prepared electrocatalysts, computed as 162 and 138 mV dec -1 for Ni-P-Se and Fe-P-Se, respectively. Based on the Tafel slope values, it should be discussed what HER mechanism is in question and which reaction step is ratedetermining in each case (Tafel, Volmer or Heyrovsky). The HER mechanism for Tafel slopes for TMs involves the electrochemical reduction of protons to hydrogen gas on the surface of a transition metal catalyst. There are two categories of the HER mechanism: Volmer-Tafel and Heyrovsky mechanisms. Tafel slope has been expressed as the logarithmic derivative of the current density with respect to the applied potential. Figure 7 b shows the computed Tafel slope values for Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se, respectively.
Moreover, the stable and durable nature of an electrocatalyst is essential toward its practical usage [39]. Therefore, the stability of the as-prepared electrocatalyst (Ni-Fe-P-Se) was analysed through chronopotentiometry technique for 24 hours at the current density of 10 mA cm -2 , as shown in Figure 8a. It is observed that the Ni-Fe-P-Se has a strong, stable nature in terms of potential value, which showed a negligible change with time. Furthermore, durability is another important factor for the electrocatalyst, especially in H2 production through electrocatalysis [40]. The stability of Ni-Fe-P-Se is tested through chronopotentiometry at 10 mA cm -2 (Figure 8b), and durability before and after chronopotentiometry test, as demonstrated in Figure 8c. The material (Ni-Fe-P-Se) showed a very durable behaviour for electrochemical HER in an alkaline medium without any major change in LSV curves. Such stable and durable behaviour of Ni-Fe-P-Se material supports this composite as a potential candidate for active HER response. The corresponding overpotential values at 10 mA cm -2 for every electrocatalyst material are shown in the form of histogram, as depicted in Figure 8a. The lower overpotential value, lower Tafel slope, and stable nature of Ni-Fe-P-Se have proven this composite as a comparable transition metal-based electrocatalyst appropriate for electrochemical applications in alkaline water splitting [41,42]. Figure 9 demonstrates the functioning of a current-time curve. Chemical reaction kinetics, diffusion processes, and adsorption are all studied using current-time analysis. This approach entails applying a quick shift in electric potential to the electrode and then observing the resulting current as a function of time [43,44]. It is shown that the greater the current, the straight the time, and its activity may preserve the efficiency of the device [44,45]. The corresponding current-time values of Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se can be seen in Figures 9a-9c). Using the parameters listed in Table  1, the current-time curves were created. After 600 to 1200 s of the water-splitting reaction, no corrosion on the electrodes could be seen in the plots.

Conclusion
Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se solutions were effectively synthesized via a facile one-pot method. After the corresponding treatment, the prepared materials were deposited on the nickel foam by drop casting method. The XRD analysis of samples confirmed the crystallinity of different materials, including Ni-P-Se, Fe-P-Se, and Ni-Fe-P-Se. SEM showed the surface structure of these materials with some modifications between them, while EDS confirmed the presence of various elements within as-prepared materials. According to the electrochemical analysis, Ni-Fe-P-Se exhibited much better catalytic activity for HER as compared with Ni-P-Se and Fe-P-Se, showing the overpotential value of 316 mV at 10 mA cm -2 , and 89 mV dec -1 value of Tafel slope, respectively. The prepared electrocatalyst is stable for 24 hours at a current density value of 10 mA cm -2 . Such electrochemical features, including lower overpotential value, lower Tafel slope and high stability, suggest that this composed material based on Ni, Fe, P and Se is a robust electrocatalyst for H2 production in alkaline media and energy conversion applications.