New insights into the hydrogen evolution reaction using Ni-ZIF8/67-derived electrocatalysts

One of the present great challenges is finding nonprecious materials characterized by efficient electrocatalytic behavior in order to substitute the expensive platinum-based materials for the purpose of hydrogen evolution reactions (HERs). In this study, ZIF-67 and ZIF-67 were used as precursors in order to fabricate metallic-doped N-enriched carbon successfully through a simple process of pyrolysis for applying the hydrogen evolution reaction. In addition, nickel was added to these structures in the course of the synthesis procedure. While under high-temperature treatment, Nickel doped ZIF-67 was transformed into metallic NiCo doped N enriched carbon (NiCo/NC), under high-temperature treatments, Ni-doped ZIF-8 changed into metallic NiZn doped N enriched carbon (NiZn/NC). By combining metallic precursors, the following five structures were synthesized: NiCo/NC, Co/NC, NiZn/NC, NiCoZn/NC, as well as CoZn/NC. It is noteworthy that the produced Co/NC shows optimum hydrogen evolution reaction activity along with superior overpotential of 97 mV and the minimum Tafel slope of 60 mV/dec at 10 mA cm. In addition, the superb behavior of hydrogen evolution reaction can be attributable to the numerous active sites, the superior electrical conductivity of carbon, and the firm structure. As a result, the present paper suggests a novel strategy in order to produce nonprecious materials characterized by superb HER efficiency for future scholars.

www.nature.com/scientificreports/ It is noteworthy that metal-organic frameworks (MOFs) are characterized by controllable metal centers, large specific surface area, significant chemical and physical stability, and modifiable pore sizes and are commonly used in drug delivery 20,21 , gas adsorption 22,23 , catalysis 24,25 , sensing 26,27 , etc. Metal-organic frameworks are fabricated by organic ligands and metal ions (II) 28 . Additionally, one can use them as precursors in order to prepare C-based metal ions doped substances through the procedure of pyrolysis. As a result, they are of potential applicability in the HER process.
The present study used a facile pyrolysis procedure in order to synthesize five structures (NiCo/NC, Co/NC, NiZn/NC, NiCoZn/NC, as well as CoZn/NC) from ZIF-67 and ZIF-8 and employed them as electrocatalysts for the HER purpose. The as-synthesized Co/NC showed superb hydrogen evolution reaction performance along with a mild Tafel slope of 60 mVdec as well as a low overpotential of 97 mV. In this paper, we present a novel strategy in order to produce nonprecious materials to serve as substitutes for platinum-based materials in hydrogen production reactions.

Fundamentals and theories
Hydrogen evolution reactions. One can present the overall form of the water-splitting reaction as below 29,30 : The hydrogen evolution reactions are deemed as the cathodic half-reactions of the overall H 2 O splitting reactions, the mechanisms of which are comprised of two major phases. The first phase, called the Volmer phase (discharge reaction), is the adsorption reaction of the electrochemical hydrogen ions. In the same stage, the reaction of a water molecule or proton with an electron within an alkaline/acidic medium generates an atom of hydrogen (H*) adsorbed on the surface of the electrode (refer to reactions 2 and 3).
Alkaline medium: Acidic medium: The second stage forms molecular hydrogen. This may take place via electrochemical desorption reactions (Heyrovsky reactions) or chemical desorption reactions (Tafel reactions). In Heyrovsky reactions, a water molecule or a hydrogen ion (H + ) within the electrolyte is combined with an electron and an adsorbed hydrogen (H*) onto the surfaces of the electrode in order to generate a molecule of hydrogen.
Alkaline medium Acidic medium: The two adsorbed hydrogens (H*) are combined in order to generate molecular hydrogens in Tafel reactions.
As a result, the hydrogen evolution reaction is composed of the mechanisms of Volmer-Tafel as well as Volmer-Heyrovsky. Classifying the reactions on the basis of the alkaline and acidic mediums reveals the dominant reaction; however, it does not mean that only the cited reaction proceeds within specific mediums. In addition, the production of hydrogen in an electrolyte progresses via both mechanisms of Volmer-Tafel, and Volmer-Heyrovsky, not just a single mechanism. One can employ the Tafel curve in order to decide the governing mechanism and also the rate-determining stage explained in "Tafel curve".

Evaluation criteria of the electrocatalysts used in HER.
Overpotential. The net current density in the equilibrium state is zero, and the energy applied to the system must be higher in comparison with the equilibrium electromotive force in order to carry out electrolytic reactions at current densities exceeding zero 31 . This additional energy is employed to eliminate a number of resistances and energy barriers, including mass transfer resistance and electron transfer resistance. One can define the overpotential (η) is as the difference between the equilibrium and the applied electromotive forces.
One of the major parameters used to assess electrocatalysts is overpotential. The lower overpotential of an electrocatalyst to present a particular current density is rendered as a lower consumed amount of energy, and thereby, the better performance of the electrocatalyst. In general, the evaluation and comparison of the electrocatalysts are conducted on the basis of the overpotentials required at current densities of 100, 10, and 1 mA cm −2 . Also, at 1 mA cm −2 , the overpotential is frequently called the onset potential, which indicates the intrinsic activities of electrocatalysts in order to trigger electrochemical reactions and affects the total performance remarkably. The current density on which photovoltaic cells are generally active is the current density of 10 mA cm −2 . The current density of 100 mA cm −2 is chosen as a decisive criterion in order to reflect the efficiency www.nature.com/scientificreports/ of the electrocatalysts on industrial scales. Also, such values are determined via LSV. It is noteworthy that, in general, the ohmic losses caused by the electrolyte resistance (R s ) are compensated in order to scrutinize only the electrocatalysts' performance (E LSV = E measured − iR s ).
In the field of hydrogen evolution reactions, the electric potential is frequently presented as compared to the reversible hydrogen electrodes (RHEs). Nonetheless, reference electrodes, e.g., saturated calomel (SCE) and also Ag/AgCl electrodes, are practically utilized in general. One can use the equations below in order to transform the potential calculated using the two reference electrodes with saturated electrolyte at a temperature of 25 °C: When these two reference electrodes are employed at a variety of concentrations and temperatures, one must correct the above equations proportionally.
Tafel curve. As previous section demonstrated, the overpotential can be regarded as a function of the current density 31 . The association between the current density and overpotential, in which electron transfers (generally) control the electrochemical reactions, is explainable via the Butler-Volmer relation as presented in the following: in which i, η, α, n, F, i 0 , T, and R respectively stand for the current density, overpotential, transfer coefficient, number of electrons exchanged, Faraday constant, exchange current density, temperature, and gas constant. One can simplify Eq. (10) on the basis of the anodic/cathodic nature of the reactions as well as the overpotential range. As a cathodic reaction, at high overpotentials in the hydrogen evolution reaction, one can simplify Eq. (10) as below: By extracting η and also transforming the logarithm base from e to 10, the below relationship is obtained: It is clear that one can plot the overpotential versus the logarithm of current density as a straight line characterized by the slope of b = − 2.3RT αnF and intercept of a = 2.3RT αnF log|i 0 | . Slope (b) and such a linear equation are known as the Tafel slope and the Tafel equation, respectively. These concepts were first coined by Julius Tafel in 1905.
In general, a and b are determined in accordance with the linear sweep voltammetry. If the overpotential is plotted versus the logarithm of current density, a plot is obtained, which is characterized by a linear section. One can interpolate it via a line equation; b and a are respectively acquired from the slope and intercept of the same line. One can determine the exchange current density a in accordance with its definition.
Tafel slope is measured in mV dec −1 that shows the extent of the overpotential required to be exerted in order to increment the current density by tenfold. As a result, lower Tafel slope values for an electrocatalyst lead to its improved efficiency. Also, the exchange current density is defined as the current density measured at the state of equilibrium (zero overpotential), which reflects the intrinsic activity of electrocatalysts. As a result, a higher value of the exchange current density for electrocatalysts leads to their improved electrocatalytic efficiency.
In addition to the use of the Tafel slope for the purpose of evaluating an electrocatalyst, the Tafel slope can also be applied to decide the governing mechanisms of the hydrogen evolution reactions and their rate-determining steps (RDSs). The Tafel slopes of the Heyrovsky, Volmer, and Tafel reactions by assuming α = 0.5 at 25 °C are determined as 39, 118, and 30 mV dec −1 , respectively. The above values indicate that the rate-determining step of the hydrogen evolution reaction can be decided. For instance, in the case that the Tafel slope of an electrocatalyst exceeds the value of 118 mV dec −1 , the Volmer reactions (electrochemical adsorption) will govern the hydrogen evolution reaction, or in the case, the Tafel slope ranges within 39-118 mV dec −1 , then, the Volmer-Heyrovsky will be the mechanism of the hydrogen evolution reactions, which will proceed via a controlling Heyrovsky reaction as well as comparatively fast Volmer reactions.
Charge transfer conductivity/resistance. Charge transfer resistance or conductivity is among the basic specifications of an electrocatalyst 31 . In particular, the higher conductivities of electrocatalysts result in lower energy losses in the course of the charge transfers. Electrochemical impedance spectroscopy (EIS) is one of the most advanced and the most precise methods used to measure and compare the charge transfer resistance of electrocatalysts (one can consider the impedance in systems of alternating current (AC) the same as the resistance in systems of direct current (DC)). By employing an alternating current voltage featuring a constant frequency (typically ranging within 100 kHz to 100 MHz) and amplitude, the same technique measures the impedance of the electrocatalysts. The utilization of alternating currents paves the way for the frequency adjustment in order to eliminate or consider the impacts of resistances resulting from a redox reaction. The Nyquist plot is among the outputs of the electrochemical impedance spectroscopy, which illustrates the imaginary elements of impedances versus the real ones. The Nyquist plot of electrocatalysts, which exhibit both resistive and capacitive behaviors in general, has a roughly semi-circle shape. The primary part of the curve (i.e., the semi-circle) at www.nature.com/scientificreports/ high frequencies indicates the electrolytic resistance, which is attributable to the fact that redox reactions are not capable of proceeding at high frequencies as a result of the quick changes of the cathode and anode. In addition, only the ionic conductivity and movement are monitored. The final part of the curve (i.e., the semi-circle) at low frequencies (in the vicinity of zero), where the current is direct to some extent, represents all system resistances, e.g., the charge transfer resistance pertaining to the electrocatalyst and electrolyte. As a result, the diameter of the semi-circle represents the charge transfer resistance of the electrocatalyst (it should be noted that, typically, fitting methods should be utilized in order to extract the values of resistance). In order to make a comparison between a variety of electrocatalysts, it should be noted that the lower size of the diameter of the electrochemical impedance spectroscopy semi-circle of an electrocatalyst will indicate smaller charge transfer resistances, which is rendered as the best electrocatalyst with regard to its conductivity. It should be mentioned that electrochemical impedance spectroscopy and Nyquist plots are only practically addressed in this paper, and their fundamentals require a detailed explanation. As a result, interested readers are advised to study electrochemical impedance spectroscopy in more detail in the associated references 32,33 .
Adsorption of hydrogen. It is noteworthy that one of the most critical dimensions of the electrocatalysis of hydrogen evolution reactions is the hydrogen adsorption 31 . The materials featuring moderate (neither too high nor too low) capability of adsorption of hydrogen exhibit improved performance in the field of hydrogen evolution reactions compared to the materials featuring too high or too low capability of hydrogen adsorption. All in all, in accordance with the Sabatier principle, a better performance of the hydrogen evolution reaction is conceivable for those values of the Gibbs free energy of hydrogen adsorption (ΔG H ) that are closer to zero. This is attributable to the fact that even though the materials featuring high capability of hydrogen adsorption (ΔG H < 0) are capable of adsorbing hydrogen species, e.g., H + or H 2 O, perfectly, they are not capable of releasing hydrogen in the desorption stages appropriately (Tafel or Heyrovsky reactions), which limits the reaction rates. Alternatively, the substances featuring a low adsorption capability for hydrogen (ΔG H > 0) will encounter the dilemma associated with the first stage (Volmer reaction). This is because they are not capable of adsorbing the HER reactants appropriately. As a result, the best electrocatalysts with regard to the adsorption of hydrogen are the ones with a ΔG H closer to zero. Besides being considered as a criterion reflecting the capability of hydrogen adsorption, ΔG H is also applicable as combined with the exchange current density in order to present an appropriate viewpoint for the comparison of the HER performance of a variety of materials. To this end, the exchange current densities of materials will be represented against their ΔG H . This provides a semi-theoreticalsemi-empirical volcano-type plot in which the best substances, i.e., platinum-related metals, are situated at the top of the plot. As a result, the electrocatalysts situated near the top of the above-said volcano plot are better. Electrochemical measurements. By employing the three-electrode technique, the entire characteristics of the hydrogen evolution reactions were assessed on an electrochemical workstation (CHI660E). Also, we used Ag/AgCl as the reference electrode. 1.0 M KOH solution and carbon rods served as the working electrolyte and counter electrodes, respectively. It is noteworthy that in order to fabricate the working electorate, we used the glassy carbon electrode (GC, diameter = 5.0 mm). The glassy carbon electrodes were rinsed using distilled water three times, which was then dried up at room temperature before the preparation of the working electorate.

Materials and methods
All the samples were deposited on the abovementioned polished GC electrode with loading of 0.32 mg/cm 2 and to avoid detachment of the powders from the GC electrode, Nafion ionomer (5 wt -% dispersion in ethanol and water) was applied as a binder.
Then, in the course of the HER, N 2 (g) bubbled constantly bubbled into the 1.0 M KOH aqueous solution. Subsequently, at a sweep rate of 10 mVs, the entire potentials were recorded, which were then transformed into a reversible hydrogen electrode (RHE). We obtained the whole linear sweep voltammetry (LSV) plots within the range of −500 to 0 mV at a 1600 rpm rotation rate. Also, the measurement of the long-time stability was carried out for 10 h at a voltage of 100 mV.
Characterizing the developed catalysts. An S-4700 microscope (made by Hitachi, Krefeld, Germany) featuring field emission scanning electron microscopy has been utilized in order to scrutinize the materials morphologically. In a powder diffractometer (manufactured by Rigaku TTRAX III, Japan) operating at 20 mA and 30 kV, Cu-K radiation (λ = 0.15418 nm) was applied for the X-ray diffraction. The employed pace was 4/ min in order to collect data on the X-ray diffraction patterns collected within the 5-90 range in two modes. By employing the technique of the thermal stability experiment called thermogravimetric analysis (TGA), the flowing N (60 cm 3 /min) underwent heating at a 10 °C/min rate. Also, we employed a thermogravimetric analyzer (manufactured by TA Instrument 5100, Dynamic TGA Q500) for the purpose of the quantification of all changes www.nature.com/scientificreports/ in temperature-dependent weight observed in the specimen. Using the ASAP 2020 accelerated surface area and porosity technology, N 2 adsorption-desorption isotherms were evaluated at −196 °C, and the surface specifications of the samples were assessed (Micro metrics, Norcross, Georgia, USA). Using the Brunauer-Emmett-Teller approach for P/P o = 0.05-0.3, the specific surface areas (SSAs) were calculated. In addition, ICP Mass Spectrometry or Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is employed in order to decide the weight percentage of hydrocarbon and metal within the synthesized specimens.

Results and discussion
Results of characterization. As reported in the earlier investigation 34 , the entire samples indicated a pattern much similar to that of ZIF-8. ZnNi-ZIF, ZnCoNi-ZIF, and ZnCo-ZIF exhibited an insignificant decline in their peak intensities, which is attributable to the lower growth observed in the ZIF crystals. It should be mentioned that no indication of NiO, Ni, or amorphous peaks was evident, which indicates that Ni was embedded within the ZIF structures. In addition, following the completion of the carbonization process, the XRD peaks declined significantly in comparison with the primary structures of ZIF. Fig. S1 and Fig. S1 present the XRD figures after and before the process of carbonization. In Fig. S2, the prominent peak near the 42° assign to the reflection of the (111) crystalline plan of cubic-phased Co. In addition, low broad peak centered at 25° assigned to amorphous carbon. Additionally, as the previous investigation mentioned, the N 2 desorption/adsorption isotherms of the entire specimens are between the types IV and I, which indicates the mesoporous and microstructures. The generated hysteresis loops will be of the H4 type. This might reflect the existence of narrow slit-like micropores within the material. In addition, following the carbonization process, the N 2 desorption/adsorption isotherms of specimens showed that the hysteresis was extended. Figures S3 and S4 present N 2 desorption/adsorption isotherms of the entire samples after and before the process of carbonization. In addition, Tables S1 and S2 presents a summarization of the Brunauer-Emmett-Teller surface area, total pore volume of the synthesized catalysts, and the average pore diameters before and after the process of carbonization, respectively.
It is evident that ZIF-67 features the maximum surface area characterized by the minimum diameter of pore. By incorporating the second/third metals into the ZIF structures, the diameters of pores increase, and the surface areas decrease as a result of the pore blocking. Following a carbonization stage, the N 2 desorption/adsorption isotherms of the whole specimens changed into the type IV. This indicates that the entire samples have turned into mesoporous forms as a result of the destructed ZIF structure and formation of the N-doped carbon. The generated hysteresis loops will be of the H3 type. This reveals the fact that the mesopores predominantly include interparticle pores. However, the Brunauer-Emmett-Teller surface area for the entire samples is still more extensive in comparison with the regular catalyst supports, such as Ɣ-Al 2 O 3 (~ 100 m 2 /g).
According to the earlier investigation, Fig. S5 showed the thermal decomposition point of the as-prepared ZIFs, where the evaporation of the solvent from the structure (20 wt.%) could be witnessed within the approximate primary range of 250 °C. For the entire samples, the initiation of the thermal decomposition occurred at 450 °C approximately. Figure 1 shows the TEM images of NiCoZn-ZIF in order to assess its bulk-scale morphologies.
These figures indicate that the entire samples show the presence of nano-sized crystals with approximately rhombic dodecahedron shapes. Additionally, the average particle size, according to these figures, is 100 nm.
Additionally, the results of ICP-OES presented in Table S3 indicate that in single metal ZIFs, such as ZIF-67 and ZIF-8, the respective total contents of metal are 34% and 33%,. In polymetallic specimens with Ni, the contents of Nickel were lower in comparison with those of Zn and Co; however, in the process of synthesis, these elements were utilized in equal amounts. This is attributable to the weaker bones formed between 2-MeIM and Ni 2+ in comparison with the Zn 2+ and Co 2+ on the basis of the theory of hard and soft acids and bases (HSAB) 35 . After conducting the pyrolysis, a significant decrease was observed in the metal content of the entire samples.
Evaluation of the HER activity. By employing the three-electrode technique, the performance of the hydrogen evolution reactions was assessed on the electrochemical workstation for the entire as-synthesized samples. The measurement of the hydrogen evolution reactions was carried out at a scan rate of 10 mV/s as well as a rotation rate of 1600 rpm.
As demonstrated in Fig. 2, the Co/NC catalyst shows a higher hydrogen evolution reaction activity characterized by overpotential of 97 mV at 10 mA/cm. Table 1 presents the overpotential of other electrocatalysts at 10 mA/cm. As can be seen, activity of NiCo/NC electrocatalyst was close to Co/NC due to simultaneous presence of Ni and Co metals in its structure.
Also, Fig. 3 illustrates the Tafel curves corresponding to the linear sweep voltammetry plots. As the figure shows, Co/NC exhibits a Tafel slope of 60 mV dec. As described in previous sections, as the Tafel slope decreases, the activity of electrocatalysts for HER process increase. Table 2 shows the Tafel slope and exchange current density of all electrocatalysts. As described, the exchange current density is defined as the current density measured at the state of equilibrium (zero overpotential), which reflects the intrinsic activity of electrocatalysts. As a result, a higher value of the exchange current density for electrocatalysts leads to their improved electrocatalytic efficiency. Accordingly, the Co/NC was indicated better performance among other electrocatalysts.
The kinetic governing mechanisms of the catalyst in the procedure of the hydrogen evolution reaction are manifested as the Tafel slope. This is in agreement with the fact that Co/NC presents the best hydrogen evolution reaction activity.
A comparison has been presented in Table 3 to indicate HER performance of the current synthesized electrocatalysts and other previously synthesized electrocatalysts by researchers in 1 M KOH medium. As can be www.nature.com/scientificreports/ seen, the activity parameters of current synthesized electrocatalysts are good compare to the most of previously synthesized electrocatalysts, especially noble metal free electrocatalysts. In order to investigate catalyst stability, chronoamperometry technique has been used at constant overpotential of 100 mV for 10 h. As can be seen in Fig. 4, the stability of Co/NC electrocatalyst was great and it has constant performance during 10 h.  www.nature.com/scientificreports/

Conclusions
In the present study, ZIF-67 and ZIF-67 were used as precursors in order to fabricate metallic-doped N-enriched carbon successfully through a simple process of pyrolysis for applying the hydrogen evolution reaction. In addition, nickel was added to these structures in the course of the synthesis procedure. The synthesized catalysts before carbonization process were matched with ZIF-8 and ZIF-67 structures owning to similar XRD patterns. Moreover, after carbonization process, the micro structures converted to mesostructures and their specific areas were reduced.
According to electrochemical measurements, the Co/NC catalyst shows a higher hydrogen evolution reaction activity characterized by overpotential of 97 mV at 10 mA/cm, while the activity of other electrocatalysts is NiCo/NC > CoZn/NC > NiCoZn/NC > NiZn/NC.
In addition the Tafel slope and exchange current density of Co/NC electrocatalyst were 60.14 and 225.66, respectively. It is worth mentioning that the performance of NiCo/NC was relatively similar to Co/NC electrocatalyst as proved by electrochemical analyses.   www.nature.com/scientificreports/

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Material
Tafel slope (mV dec −1 ) Overpotential @10 mA cm −2 /mV Reference   www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.