Insights on Tafel Constant in the Analysis of Hydrogen Evolution

Oct 1, 2018 - Tafel equation based electrochemical analysis is widely employed in hydrogen evolution reaction to evaluate and characterize electrocata...
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Cite This: J. Phys. Chem. C 2018, 122, 23943−23949

Insights on Tafel Constant in the Analysis of Hydrogen Evolution Reaction Arun Prasad Murthy,*,† Jayaraman Theerthagiri,‡ and Jagannathan Madhavan*,† †

Solar Energy Laboratory, Department of Chemistry, Thiruvalluvar University, Vellore 632 115, Tamilnadu, India Centre of Excellence for Energy Research, Sathyabama Institute of Science and Technology, Chennai 600 119, India

‡

J. Phys. Chem. C 2018.122:23943-23949. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/26/18. For personal use only.

S Supporting Information *

ABSTRACT: Tafel-equation-based electrochemical analysis is widely employed in hydrogen evolution reaction (HER) to evaluate and characterize electrocatalysts. Tafel slope and exchange current density are the only two parameters that are invariably obtained and discussed with respect to Tafel equation in the literature. Herein, insights on Tafel constant in the analysis of HER are discussed, and its practical advantage is indicated. It is proposed that Tafel constant can be considered as the onset potential of HER. Tafel constant becomes the defining parameter between two electrocatalysts when other parameters such as Tafel slope or exchange current density become same. The significance of the same Tafel constants is illustrated using CoSe2 and NiSe2. Variation of the Tafel constant within a series of Co(1−x)NixSe2 is examined. The occurrence of the same Tafel constants between two electrocatalysts leads to one electrocatalyst becoming the better catalyst in the lower current regime, whereas the other exhibits higher activity in the higher current regime. Furthermore, concepts developed here are applied to several literature examples, and the significance of Tafel constant in the analysis of HER is established. log(j) versus overpotential (η) plot.12 The linear portion of the curve is fitted to the Tafel equation, and the Tafel slope (b) is extracted from the fitting. More often than not, exchange current density (j0) is another parameter that is usually obtained from the Tafel plot through extrapolation method. The Tafel slope and the exchange current density form important electrochemical parameters of an electrocatalyst derived from the Tafel equation. The Tafel equation is one of the well-known and highly verified expressions which was experimentally derived by Swiss chemist Julius Tafel in 1900s.13 The Tafel equation is as famous as the Arrhenius equation in chemistry, and they have remarkably similar forms.14 The Tafel equation is applicable to a broad range of reactions and processes such as corrosion, metabolism, photosynthesis, and so on Furthermore, the reactions that do not form surface intermediates also exhibit Tafel relationship.15 Khan and Bockris16 noted the applicability of the Tafel equation to a wide range of reactions including HER. The activity and the mechanistic aspects of HER are generally discussed based on the Tafel analysis in the literature. In this context, Tafel analysis in HER is invariably confined to obtaining b and j0 from the experimental data. In the present work, physical insights on the Tafel constant (a) in the analysis of HER are described, and its practical advantage pointed out.

1. INTRODUCTION Energy crisis is one of the most important and pressing topic of the 21st century. The fast dwindling fossil fuel reserve will not sustain the energy demands of the ever-growing world population. Furthermore, the combustion of fossil fuel results in the emission of carbon oxides, sulfur compounds, and so on, which are highly inimical to the environment. Several countries have started abandoning fossil-fuel-based energy sources in the near future.1 The above issues mandate developing alternate green fuels that can be obtained from renewable sources with zero carbon emission. Solar energy is renewable and available in abundance; however, its intermittent nature necessitates its storage in high-energy-density sources. In this context, hydrogen is an attractive source in which solar energy can be stored as the chemical energy. Hydrogen is rightfully touted as the energy currency,2 the energy carrier,3 and the energy economy of the future.4 Hydrogen can be produced by electrocatalytic/photoelectrocatalytic water splitting in which hydrogen evolution reaction (HER) forms the cathodic reaction of the water splitting.5 During the past decade, there has been renewed interest in HER, and many low-cost alternatives to noble-metal-based electrocatalysts have been developed.6−11 Voltammetric methods are widely and routinely used to evaluate and compare the HER activities of the electrocatalysts. In Tafel analysis, linear sweep voltammetry at relatively lower scan rates is employed, and the current density (j) versus potential (E) data of the voltammogram is transformed into © 2018 American Chemical Society

Received: August 9, 2018 Revised: September 29, 2018 Published: October 1, 2018 23943

DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION 3.1. Tafel Constant in the Analysis of HER. The potential dependency of the current density j can be related to the interfacial electrocatalytic reaction rate v as j = nFv, where n is the number of electrons involved and F is the Faraday constant. It follows that because j is potentially dependent, v also is potentially dependent and is composed of many elementary steps, as shown below Initial discharge or Volmer step

It is shown that the Tafel constant becomes the defining parameter between two electrocatalysts when the other parameter, for example, the Tafel slope becomes the same. As mA cm−2 is the commonly used unit of current density, the Tafel constant is proposed as the onset potential of HER. From the electrochemical point of view, the Tafel constant becomes complementary to the Tafel slope. The evaluation of electrocatalysts wherein the Tafel constants become the same is illustrated using CoSe2 and NiSe2. Furthermore, the variation of the Tafel constant with a Tafel slope and composition is discussed in a series of Co(1−x)NixSe2, and its practical significance is discussed. The concepts developed are illustrated with several literature examples as well.

H3O+ + e− → Hads + H 2O

Atom + ion or Heyrovsky step Hads + H3O+ + e− → H 2 + H 2O

2. EXPERIMENTAL SECTION Nickel(II) chloride hexahydrate (NiCl2·6H2O), cobalt(II) chloride hexahydrate (CoCl2·6H2O), cetyl trimethylammounium bromide (CTAB), selenium powder, and hydrazine hydrate (N2H4·H2O) were purchased from SDFCL, India. Co(1−x)NixSe2 (0 ≤ x ≤ 1) was synthesized using the reported one-step hydrothermal method.17 Briefly, NiCl2·6H2O and/or CoCl2·6H2O with a total concentration of 1 M along with 0.23 g of CTAB were added to 30 mL of deionized water. Se powder (2 mmol) was added to the above mixture and stirred for 15 min. N2H4·H2O (20 mL) was added dropwise to the above homogeneous mixture under stirring for another 30 min. The reaction mixture was then transferred to a Teflon-lined stainless steel autoclave of 150 mL. The autoclave was then sealed and placed in a hot air oven, and the hydrothermal reaction was carried out at 120 °C for 12 h. After cooling down the autoclave to the room temperature naturally, the contents were filtered and washed several times with deionized water and absolute ethanol to remove impurities. Finally, the black precipitate was dried in a vacuum oven at 80 °C overnight. Electrochemical experiments were conducted using a CH Instruments potentiostat in a conventional single-compartment three-electrode cell configuration. Silver/silver chloride (3 M KCl) electrode was employed as a reference electrode, whereas a graphite rod served as a counter electrode. The working electrode potentials were referenced with respect to a reversible hydrogen electrode using the equation18 E RHE = EAg/AgCl + 0.0591pH + 0.1976

(2)

(3)

Atom + atom or Tafel step Hads + Hads → H 2

(4)

The above elementary steps lead to two mechanisms, namely, Volmer−Heyrovsky and Volmer−Tafel. Three ratedetermining steps (RDS)Volmer, Heyrovsky, and Tafel are possible for the above two mechanisms. In the above two HER mechanisms, the catalytic current increases exponentially with the overpotential; the rate of increase, however, differs for different RDSs presenting an easier way of identifying RDS and possible mechanism operating at the catalyst surface. The Tafel slope assumes a specific value related to one of the three RDSs mentioned above and is independent of the magnitude of the catalytic current. When the Volmer step, which is common to both of the mechanisms, becomes the RDS, a Tafel slope of 120 mV dec−1 ensues. Alternatively, when either Heyrovsky or Tafel step becomes the RDS, a slope value of 40 or 30 mV dec−1, respectively, results. The Tafel equation is an important diagnostic tool delineating RDSs and possible mechanism operating in an electrocatalyst. In voltammetry, the electric potential beyond the equilibrium potential is applied to the working electrode at a desired rate, and the current is measured as the response. The experimental potential dependency of the current density is given by the Tafel equation20 η = b log(j) + a

(5)

The Tafel plot is obtained by plotting the overpotential against the logarithmic current density. η in the above equation should be greater than RT/F below which the current− potential relationship becomes Ohmic, that is, linear. In this work, we follow the convention that η is positive, and the Tafel equation has the form given in eq 5.21−29 It can be seen in the above equation that when log(j) becomes zero, then η becomes a, hence a undertakes the unit of potential. Furthermore, a assumes the value of overpotential at 1 mA cm−2 (if j is given in the unit of mA cm−2). If the potential required to reach 1 mA cm−2 is defined as the onset potential of HER,30 then the Tafel constant directly indicates the onset potential of HER. Because mA cm−2 is widely used as the unit of current density in the literature, the Tafel constant can be realized as the onset potential of HER. This reasoning leads to the inference that the lower the value of a (the onset potential), the better is the catalyst for HER. Butler−Volmer equation describes the electrochemical redox reactions, and the Tafel equation can be conjugated with the Butler−Volmer equation. The Tafel equation can be derived from Butler− Volmer kinetics assuming the HER rate significantly larger

(1)

Before coating with a catalyst layer, the glassy carbon working electrode (CH Instruments; 3 mm diameter) was polished to a mirrorlike finish using 0.05 μm alumina media (Buehler). The electrocatalyst ink was prepared by dispersing 4 mg of electrocatalyst in 1 mL of the mixture of deionized water and ethanol in a 4:1 ratio (v/v). Nafion solution of 80 μL (Electrochem Inc., obtained as 5 wt % solution) was then added; subsequently, the mixture was sonicated for 30 min to form a homogeneous ink. The catalyst ink of 5 μL was dropcasted onto the glassy carbon electrode and dried in air at room temperature. The mass loading of the catalyst was 0.28 mg cm−2, a commonly employed mass loading in the literature.19 HER activities of various catalysts were evaluated using linear scan voltammetry at a scan rate of 2 mV s−1 in 0.5 M H2SO4 at room temperature (298 K). The iR compensation was applied during voltammetric experiments using the in-built iR compensation function in the electrochemical workstation. 23944

DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949

Article

The Journal of Physical Chemistry C

(200), (210), (211), (220), (221), (311), (222), (230), (321), and (400) planes (JCPDS no. 41-1495). Co0.5Ni0.5Se2, on the other hand, exhibits the diffraction peaks of both CoSe2, and NiSe2. HER activities of Co(1−x)NixSe2 electrocatalysts were evaluated using voltammetry, and the current−potential data were transformed into Tafel plots. For an accurate Tafel analysis, the following experimental conditions were followed;31 (1) voltammetric data should be free of Ohmic distortions. (2) j−η data need to be measured over 2 orders of magnitude of the current, and at least one decade of linearity in the Tafel curve is needed to derive the correct slope. (3) The potential regime investigated should be in such a way that it is not under diffusion control. At high overpotentials, the electrochemical reaction may be under diffusion control. Very low overpotential region also ought to be avoided wherein the current−potential relationship becomes Ohmic (linear). (4) The catalytic current should be under steady-state or quasi-steady-state condition. (5) j−η data should be free of background currents. Voltammograms showing the HER activities of NiSe2 and CoSe2 are shown in Figure 2a, and the corresponding Tafel

than that of the backward reaction. Accordingly, the Tafel constant can be given by rearranging eq 5 when η = 0 a = −b log(j0 )

(6)

Because a is the product of b and log(j0), ideally three cases arise when comparing two electrocatalysts; (1) two catalysts may have the same Tafel slopes but different exchange current densities. (2) Catalysts may have the same exchange current densities but different Tafel slopes. (3) The catalysts may have different Tafel slopes and exchange current densities but same Tafel constants. The above scenarios arising between two electrocatalysts are illustrated in Figure 1a. In case one, where

Figure 1. (a) Tafel plots illustrating three cases; the first pair exhibits same Tafel slopes, the second pair shows same exchange current densities, and the third pair displays same Tafel constants. (b) Powder XRD patterns of CoSe2, Co0.5Ni0.5Se2, and NiSe2.

the two catalysts possess same Tafel slopes, the better catalyst would be the one with lower Tafel constant in line with the inference we obtained above. Similarly in case two, where both of the catalysts have the same exchange current densities, the better catalyst between the two would be the one with lower Tafel constant. The third case, where the two catalysts have the same Tafel constants (Figure 1a) but different Tafel slopes and exchange current densities is interesting wherein one catalyst is better in the lower current regime and the other becomes a better catalyst in the higher current regime. This case is illustrated using a series of Co(1−x)NixSe2 (x = 0, 0.25, 0.50, 0.75, and 1.00) electrocatalysts in the study of HER in 0.5 M H2SO4. 3.2. Analysis of Tafel Constants in Co(1−x)NixSe2. Figure 1b shows powder X-ray diffraction (XRD) patterns of CoSe2, NiSe2, and Co0.5Ni0.5Se2. In CoSe2, the diffraction peaks at 2θ = 27.6°, 29.0°, 30.8°, 34.4°, 36.0°, 35.0°, 40.3°, 43.9°, 47.8°, 50.2°, 53.4° 55.3°, 56.9°, 59.3°, and 63.2° can be indexed, respectively, to (110), (011), (101), (111), (120), (200), (210), (121), (211), (002), (031), (221), (131), (310), and (122) planes of orthorhombic CoSe2 (JCPDS no. 53-0449). Similarly, in the case of NiSe2, the peaks appearing at 2θ = 30.0°, 33.6°, 36.9°, 42.4°, 45.1°, 50.7°, 53.3°, 55.5°, 58.0°, and 62.4° are in good agreement with the cubic NiSe2 indexed to

Figure 2. (a) Voltammograms showing HER activities of CoSe2 and NiSe2. (b) Corresponding Tafel plots.

plots are given in Figure 2b. The voltammograms and the Tafel plots illustrate the third case discussed in the previous section. CoSe2 and NiSe2 exhibited Tafel slopes of 46 ± 2 and 60 ± 2 mV dec−1 and exchange current densities of 7.5 ± 0.8 × 10−5 and 6.5 ± 0.5 × 10−4 mA cm−2, respectively. It may be noted that similar Tafel slopes and exchange current densities were obtained by Kong et al. for CoSe2 and NiSe2.32 The Tafel constants obtained for CoSe2 and NiSe2 were 0.190 ± 0.003 and 0.191 ± 0.002 V, respectively. The same Tafel constants exhibited by CoSe2 and NiSe2 led to the crossing of voltammetric and Tafel curves at about 1 mA cm−2 and zero log(j), respectively (arrows shown in Figure 2a,b). The implication of the above phenomenon, that is, the occurrence of the same Tafel constants between two catalysts, is that NiSe2 would be the better catalyst for applications that operate below 1 mA cm−2, whereas CoSe2 would be the better catalyst for 23945

DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949

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The Journal of Physical Chemistry C applications that require a current density above 1 mA cm−2. CoSe2 and NiSe2 exhibited different Tafel slopes and exchange current densities indicating that HER likely occurs through different mechanisms and their inherent HER activities are also different. However, the occurrence of the same Tafel constants defines their practical applications. In other words, the electrocatalysts are complementary to each other with respect to HER activity, namely, one is better at lower j, whereas the other is better at higher j. We further investigated the variation in Tafel constants in ternary Co(1−x)NixSe2 with x = 0.25, 0.5, and 0.75. It would be interesting to learn whether the Tafel constants remain the same or vary with composition in the series. Figure 3a shows

below 14 mA cm−2. Thus, the manipulation of Tafel constants allows for tailoring the catalysts for desired applications. 3.3. Evaluation of Electrocatalysts Based on Tafel Constants. In this section, we shall illustrate the significance of Tafel constants in the analysis of HER with several instances from the literature. Tafel constant values were obtained either using eq 6 or from the Tafel plots of the references sited. First, we consider the electrocatalysts with same Tafel slopes but different Tafel constants. Using chemical vapor deposition method Behranginia et al.34 fabricated crystalline 3D MoS2 on graphene film (MoS2/graphene film) with bare Mo edge atoms as the HER electrocatalyst. Kong et al.35 fabricated CoSe2 nanoparticles on the carbon fiber paper (CoSe2 NP/CP) as the HER electrocatalyst in the acid electrolyte. Pham et al.36 deposited amorphous molybdenum sulfide on high-surfacearea conductive graphene−carbon nanotube hybrids (MoSx− GCNT) and used them as highly active HER electrocatalysts. The three HER electrocatalysts mentioned above, namely, MoS2/graphene film, CoSe2 NP/CP, and MoSx−GCNT, exhibited similar Tafel slopes of 41, 42, and 41 mV dec−1, respectively. However, their Tafel constants were different, namely, 0.071, 0.097, and 0.101 V, respectively, as shown in the first three entries of Table S1. On the basis of Tafel constant values, it can be deduced that MoS2/graphene film with the lowest Tafel constant of 0.071 V would be the best electrocatalyst among the three. Indeed, we see that MoS2/ graphene film exhibited η10 of 0.100 V, followed by 0.137 V for CoSe2 NP/CP and 0.141 V for MoSx−GCNT, as shown in Table S1. Another example comes from molybdenum carbidebased electrocatalysts. Chen et al.28 synthesized porous onedimensional Mo2C/amorphous carbon using MoO3/polyaniline nanorod precursor in a solid-state reaction. Mo2C supported on the carbon nanotube−graphene hybrid (Mo2C/CNT−GR) was fabricated by Lee et al.37 through a modified urea-glass method. Mo2C/amorphous carbon and Mo2C/CNT−GR exhibited very similar Tafel slopes of 57.5 and 58 mV dec−1, respectively, and different Tafel constants of 0.057 and 0.070 V, respectively. It may be inferred from the Tafel constant values that Mo2C/amorphous carbon with a lower Tafel constant would be the better catalyst. We find that Mo2C/amorphous carbon has a better η10 (0.115 V) compared to that of Mo2C/CNT−GR (0.130 V), as shown in Table S1. A series of three electrocatalysts; self-standing MoP2 nanosheet on carbon cloth (MoP2 NS/CC),38 MoSoy (βMo2C and the γ-Mo2N) anchored on reduced graphene sheets (MoSoy/RGO),39 and molybdenum carbide nanoparticledecorated graphitic carbon sheets (Mo2C/GCS)40 exhibited similar Tafel slopes of 63.6, 62.7, and 62.6 mV dec−1, respectively, but different Tafel constants, as shown in entries 6−8 of Table S1. MoP2 NS/CC showed the lowest Tafel constant of 0.005 V, whereas Mo2C/GCS showed the highest Tafel constant of 0.119 V. We may conclude that MoP2 NS/ CC would be the best HER electrocatalyst in the series, and this conclusion is justified by the respective η10 values, as shown in Table S1. It is interesting to note that the difference in η10 values between MoP2 NS/CC (0.058 V) and Mo2C/ GCS (0.200 V) is large similar to the large difference in Tafel constants between these two electrocatalysts. The example of the same catalyst supported on different supports is worth considering. Finn and Macdonald41 hydrothermally fabricated petaled molybdenum disulfide on Mo and Au supports. MoS2/ Mo and MoS2/Au exhibited same Tafel slopes of 68 mV dec−1 because the active phases of the electrocatalysts were same

Figure 3. (a) Variation of Tafel constants with Tafel slopes and compositions in Co(1−x)NixSe2. (b) Voltammograms showing HER activities of CoSe2 and Co0.5Ni0.5Se2.

Tafel constants of Co (1−x) Ni x Se 2 plotted against the composition and the Tafel slope. It can be observed that Tafel constants do vary with composition as well as the Tafel slope. The Tafel constants of Co(1−x)NixSe2, where x = 0.25, 0.50, and 0.75, are lower than those of CoSe2 and NiSe2 with Co0.5Ni0.5Se2 showing the lowest value (0.182 ± 0.002 V). Nevertheless, the difference between the Tafel constants of CoSe2 and NiSe2 and that of Co0.5Ni0.5Se2 is not large. This allows significant improvement from the application point of view. For example, the Tafel slope of Co0.5Ni0.5Se2 (53 ± 2 mV dec−1) is still higher than that of CoSe2 (46 ± 2 mV dec−1); however, its Tafel constant is lower than that of CoSe2, which results in Co0.5Ni0.5Se2 being the better catalyst in the lower current regime. Figure 3b shows voltammograms of CoSe2 and Co0.5Ni0.5Se2 wherein the HER current density of Co0.5Ni0.5Se2 is higher until 14 mA cm−2, whereas CoSe2 becomes a better catalyst above 14 mA cm−2. It may be noted that the figure of 10 mA cm−2 is relevant in solar hydrogen production; hence, Co0.5Ni0.5Se2 would be a suitable catalyst for devices that operate at 12.3% solar to hydrogen efficiency.30,33 CoSe2 would be the better catalyst for the applications that operate above 14 mA cm−2, whereas Co0.5Ni0.5Se2 would be the better catalyst for the applications that require current densities 23946

DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949

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The Journal of Physical Chemistry C

potential of HER. When either Tafel slope or exchange current density becomes same, then the Tafel constant becomes the defining criterion. In the case where same Tafel constants occur, the performance of the electrocatalyst depends on the current regime it is applied. CoSe2 and NiSe2 exhibited different Tafel slopes and exchange current densities but same Tafel constants. CoSe2 is the better catalyst in the higher current regime, whereas NiSe2 becomes the better catalyst below 1 mA cm−2 and Co0.5Ni0.5Se2 becomes the better catalyst below 14 mA cm−2 in the series of Co(1−x)NixSe2. In other words, electrocatalysts may be related by Tafel constant values in which the mechanism through which HER occurs or the inherent activity of electrocatalysts may be different. However, when they are related by the Tafel constant, the suitability of an electrocatalyst for a specific application can be predicted. Furthermore, several literature examples were considered to establish the concepts developed on the Tafel constant in the analysis of HER.

(MoS2). However, they did exhibit different Tafel constants, as shown in Table S1; hence, we may expect different HER activities for these electrocatalysts. The electrocatalyst with lower Tafel constant (MoS2/Mo; 0.173 V) performed better (η10 = 0.242 V) compared to the other one, as indicated in Table S1. We may consider an example for the second case wherein the electrocatalysts exhibit similar exchange current densities but different Tafel constants. MoSoy/RGO39 and Mo2N/ CNT−GR37 exhibited similar j0 values of 3.70 × 10−2 and 3.94 × 10−2 mA cm−2, respectively. However, MoSoy/RGO showed a lower Tafel constant of 0.090 V, whereas Mo2N/CNT−GR exhibited a higher Tafel constant of 0.101 V. Accordingly, MoSoy/RGO performed better than Mo2N/CNT−GR, as implied by their η10 values (Table S2). It may be remarked that even though MoSoy/RGO exhibited marginally lower j0, its Tafel constant was better than Mo2N/CNT−GR. Hence, a significant enhancement in the HER activity was observed in MoSoy/RGO. We now consider more interesting cases of electrocatalysts with same Tafel constants but different Tafel slopes and exchange current densities. Cui et al.42 described a growth process of MoSe2 layers vertically aligned on the substrates such as nanowires and microfibers. MoSe2 film on the carbon fiber paper (MoSe2/CFP) was shown as an efficient electrocatalyst for HER and exhibited a Tafel constant of 0.205 V. The hydrothermally fabricated petaled MoS2/Au41 exhibited a similar Tafel constant of 0.206 V. However, Tafel slopes and exchange current densities of these electrocatalysts were different. Because Tafel constants of MoSe2/CFP and MoS2/Au were similar (Table S3), their voltammetric curves cross near 1 mA cm−2 and their Tafel plots cross near zero at abscissa, as shown in Figure S1a,b. It may be shown that MoSe2/CFP has a better Tafel slope, as shown in Table S3, and would be the better electrocatalyst in the higher current regime. Indeed, MoSe2/CFP is the better electrocatalyst in the higher current regime and MoS2/Au becomes the better electrocatalyst in the lower current regime, as shown in Figure S1a. Another example of occurrence of similar Tafel constants can be found in Mo2C supported on carbon nanotube− graphene hybrid (Mo2C/CNT−GR)37 and MoS2 on graphene film (MoS2/graphene film).34 Mo2C/CNT−GR and MoS2/ graphene film exhibited Tafel constants of 0.070 and 0.071 V, respectively. Tafel slopes of these electrocatalysts were different, as shown in Table S3. Voltammetric curves of these electrocatalysts cross near 1 mA cm−2 and their Tafel plots cross near zero at abscissa, as shown in Figure S2a,b. MoS2/graphene film showed better Tafel slopes and hence becomes the better electrocatalyst in the higher current regime (above 1 mA cm−2), and Mo2C/CNT−GR becomes the better electrocatalyst in the lower current regime, as shown in Figure S2a. The above discussion illustrates the significance of the Tafel constant in the analysis of HER, and several literature instances are available43−45 to further establish the role of the Tafel constant in tailoring the electrocatalysts for desired applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07763.

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Electrochemical data from the literature presented in figures and tables (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +918248049246 (A.P.M.). *E-mail: [email protected]. Phone: +919585692101 (J.M.). ORCID

Arun Prasad Murthy: 0000-0003-4293-6280 Jayaraman Theerthagiri: 0000-0002-5746-5541 Jagannathan Madhavan: 0000-0003-4005-4604 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors A.P.M. and J.M. are grateful to the authorities of Thiruvalluvar University for the support. REFERENCES

(1) https://www.electrochem.org/redcat-blog/france-end-salefossil-fuel-powered-vehicles-2040/ (accessed July 15, 2018). (2) Murthy, A. P.; Theerthagiri, J.; Madhavan, J.; Murugan, K. Enhancement of Hydrogen Evolution Activities of Low-Cost Transition Metal Electrocatalysts in Near-Neutral Strongly Buffered Aerobic Media. Electrochem. Commun. 2017, 83, 6−10. (3) Chen, Y.; Yang, K.; Jiang, B.; Li, J.; Zeng, M.; Fu, L. Emerging Two-Dimensional Nanomaterials for Electrochemical Hydrogen Evolution. J. Mater. Chem. A 2017, 5, 8187−8208. (4) Jiang, C.; Moniz, S. J. A.; Wang, A.; Zhang, T.; Tang, J. Photoelectrochemical Devices for Solar Water Splitting − Materials and Challenges. Chem. Soc. Rev. 2017, 46, 4645−4660. (5) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Earth-abundant Hydrogen Evolution Electrocatalysts. Chem. Sci. 2014, 5, 865−878. (6) Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalyst for Artificial Photosynthesis and Enviranmental Remediation: Are We a Step

4. CONCLUSIONS The Tafel analysis in HER is invariably confined to obtaining the Tafel slope and the exchange current density. The significance of the Tafel constant in the analysis of HER was demonstrated using representative electrocatalysts. Because mA cm−2 is the commonly used unit of current density in the literature, the Tafel constant was proposed as the onset 23947

DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949

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DOI: 10.1021/acs.jpcc.8b07763 J. Phys. Chem. C 2018, 122, 23943−23949