Do the Evaluation Parameters Reflect Intrinsic Activity of

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lectrochemical water splitting is one of the hot areas of energy research in which electrical energy is stored as chemical fuels.1 Surplus energy derived from intermittent sources can be used as input and stored as hydrogen. In water splitting electrocatalysis, there are two important halfcell reactions, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).2,3 Both of them are given equal importance due to various associated challenges, which forbid the successful commercialization of water electrolysis til now.4−6 The notable advancements made in this field are use of nanostructured noble metal oxides and metals and use of nonprecious electrocatalysts.6−8 An enormous amount of work has been done in this field in the past 5 years, and there are several good reviews on the developments of these electrocatalysts.6−8 Unfortunately, the sudden elevation that occurred in this field led researchers to have many unnoticed errors in the use of evaluation parameters.3 There had been significant efforts made by the global research community in order to ensure correct and appropriate use of evaluation parameters in electrocatalysis of energy conversion reactions as flawed use of evaluation parameters will make uncorrectable scientific records.9−11 Hence, it is important to come up with a report such as this Viewpoint that could guide the use of appropriate evaluation parameters reflecting intrinsic activity of an electrocatalyst. In electrochemical water splitting, more than five evaluation parameters such as the overpotential at 10 mA cm−2geo (η@10 mA cm−2geo), Tafel slope, exchange current density, mass activity, specific activity, turnover frequency (TOF), endurance studies, and Faradaic efficiency are used.9−11 However, a common error that is being made by many researchers is the overrated and prime use of the overpotential at 10 mA cm−2geo (η@10 mA cm−2geo) and Tafel slope (derived from iRcorrected LSV) without knowing that they can be influenced to a greater extent by increasing the mass of the catalyst (Figure 1). Even though η@10 mA cm−2geo is accepted in the engineering perspective of the overall electrode performance, it cannot reflect the intrinsic activity of an electrocatalyst. In this Viewpoint, we intended to show such a dependence of η@10 mA cm−2geo and Tafel slope on the mass of the catalyst. In addition, we also show that the other rarely adapted activity parameters, like the specific activity and TOF, are relatively better in reflecting the intrinsic activity, taking the wellcharacterized NiO catalyst for OER in an alkaline medium. Five NiO (Sigma-Aldrich)/CC electrodes of periodically increasing catalyst loading as 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 were prepared by the conventional drop casting method. The prepared electrodes were then subjected to a typical OER study in 1 M KOH (see the Supporting Information (SI) for details). Before studying OER activities, © 2019 American Chemical Society

Figure 1. Plot of loading against η@10 mA cm−2geo and the Tafel slope (derived from iR-corrected LSV), showing their dependence on the catalyst’s mass.

the electrodes were activated by a consecutive potential sweeping for 10 cycles. Having known the OER region for NiO, electrochemical impedance spectroscopy (EIS) analysis was done first to know the charge transfer nature of each electrode at a fixed potential of 1.6 V vs RHE, where OER is the only charge transfer process. The resultant Nyquist plots are shown in Figure S1a. All five NiO/CC electrodes showed almost the same uncompensated resistance (Ru) of 1.5 ohm, whereas significant differences were witnessed in the charge transfer resistances (Rct). The measured Rct of NiO/CC electrodes with periodically increasing loading of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 were 13.8, 15.2, 16.7, 19.5, and 21.1 ohms, respectively. This observation implied that the increasing catalyst loading did not affect the Ru, whereas it had significantly increased the Rct. This can be attributed to the increasing thickness of the catalyst layer over the substrate electrode. After studying the electrochemical charge transfer nature, the OER activity was first studied by linear sweep voltammetry (LSV). The iR uncompensated LSVs of the NiO/ CC electrode with catalyst loadings of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 are shown in Figure S1b. As expected, the overall current density of NiO increased with increasing catalyst loading, which implied that the potential at any fixed current densities will be loading-dependent. This proved our claim that η@10 mA cm−2geo is not an intrinsic activity parameter rather than an engineering perspective. The actual origin of η@10 mA cm−2geo was from the solar to fuel (STF) energy conversion devices where the cell potential @10 mA cm−2geo was used to measure the efficiency of the STF device. Hence, it is not completely appropriate to use the same for an electrocatalyst that just catalyzes a half-cell reaction in Received: March 30, 2019 Accepted: May 3, 2019 Published: May 10, 2019 1260

DOI: 10.1021/acsenergylett.9b00686 ACS Energy Lett. 2019, 4, 1260−1264

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Cite This: ACS Energy Lett. 2019, 4, 1260−1264

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ACS Energy Letters water electrolysis. Though it is just an engineering perspective to use η@10 mA cm−2geo, people have mistakenly been using it for a very long time as the primary evaluation parameter. Moreover, we have also shown here that overpotentials at any fixed current density are relative and will vary with varying loading. To further prove the same, we have corrected (for 100%) all LSVs for iR drop and provided the iR drop compensated LSVs as Figure S1c. The first observation that could be made with Figure S1c is that the same activity trend is retained here as that of Figure S1b. This again indicates that overpotentials are catalyst mass-dependent and will vary with varying loading. The second widely used evaluation parameter in electrochemical water splitting is the Tafel slope derived from iR-corrected LSV. However, it is not noted by many that it will also vary with varying catalyst loading, particularly when it is extracted from their respective iR-corrected polarization curves. The Tafel plots of NiO/CC with loadings of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 are given as Figure S1d, from which it was witnessed that the Tafel slopes began lowering from 118 to 90 mV dec−1 while increasing the catalyst loading. By this, it is once again proven that η@10 mA cm−2geo is not the only mass-dependent evaluation parameter but also the Tafel slope, particularly when they are extracted from iRcorrected LSVs. Hence, the primary use of these two evaluation parameters is proven to be no longer reflecting the intrinsic activity of an electrocatalyst other than just being an engineering perspective that measures the overall electrode performance. This study was repeated five consecutive times, and the catalyst loading-dependent overpotentials and Tafel slope were studied. The obtained data with mean deviation are plotted as loading vs η@10 mA cm−2geo and loading vs Tafel slope. The same are provided here as Figure 2a,b. From Figure 2a,b, it is evidenced that both η@10 mA cm−2geo and the Tafel slope directly depend on the catalyst loading. Hence, these two essentially used evaluation parameters cannot reflect the intrinsic activity of a given electrocatalyst. Therefore, it has become mandatory to find out relatively more reliable evaluation parameters. For this purpose, we propose the use of specific activity, which is obtained by normalizing the current with the actual electrochemical surface area (ECSA) and TOF. The ECSA-normalized specific activity and the corresponding overpotential at any fixed current density will remain constant irrespective of loading of catalyst provided that increasing the loading does not alter the charge transfer nature. However, the main challenge in this method is to determine the exact ECSA of a given catalyst. There are several methods to determine the ECSA of a catalyst. However, it varies from material to material. Some known methods are the double-layer capacitance (Cdl) method, redox peak method, hydrogen underpotential deposition (H-UPD), and CO stripping.3 The specific activity works better in reflecting the intrinsic activity of an electrocatalyst only when all of the nanoparticles are of the same size and until the electrocatalyst coverage (θ) reaches 100%. Beyond 100% coverage, self-masking of catalytically active sites will result in lowered ECSA if measured. Among said methods of ECSA determination, the redox peak method is the most accurate one when it comes to OER, and the material used is having a distinct redox couple just before the OER onset. Examples are OER catalysts that have Fe, Co, Ni, Ru, and Ir in it. From the mechanism of OER catalyzed by a 3d transition metal oxide/hydroxide in alkali, it

Figure 2. (a,b) Plots of loading vs overpotential at 10 mA cm−2geo and loading vs Tafel slope (derived from 100% iR-corrected LSVs).

is discovered that the metal site will undergo continuous oxidation and reduction in each cycle of water oxidation in which M−OOH is a key intermediate. M2 +−OH + OH−(aq) → M3 +(O)−OH + H+(aq) + e− (1)

M3 +(O)−OH + OH−(aq) → M2 +−OH + H+(aq) + O2 (g)

(2)

Hence, it is acceptable to say that the number of M(II) ions that get converted into M(III) ions as M−OOH is equal to the number of metal sites that actually catalyze the OER. Therefore, to calculate the ECSA of a 3d transition metal oxide/hydroxide catalyst like NiO, the redox peak method is the most accurate of all. On the other hand, the H-UPD and CO stripping analyses are accurate in determining the ECSA of noble metals like Pt and Pd.9 Figure 3a shows the NiOOH formation peaks in the LSVs of NiO/CC electrodes of loading 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2. From Figure 3a, it can be seen that with increasing catalyst loading the current density associated with NiOOH formation is also increasing. This is because increasing loading allowed the electrolyte to access more Ni sites. From this, the actual number of Ni sites that catalyze the OER can be calculated by integrating the area under the peaks. Figure 3b shows the area-integrated NiOOH formation peaks 1261

DOI: 10.1021/acsenergylett.9b00686 ACS Energy Lett. 2019, 4, 1260−1264

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ACS Energy Letters

Figure 3. (a,b) LSVs showing the NiOOH formation peak used for area integration. (c) ECSA-normalized LSVs. (d) Plot of loading vs TOF.

0.310, 0.410, 0.615, and 0.820 mg cm−2 are 1.10, 1.21, 1.81, 3.28 cm2, respectively. For a comprehensive understanding, a plot of loading vs relative ECSA was derived and is provided as Figure S3 in the SI, from which a similar trend to that of other parameters discussed earlier is noted. The relative ECSA is also increasing with increased loading of the catalyst. We have so far been highlighting the significance of loading in HER and OER and the inappropriateness of η@10 mA cm−2geo and the Tafel slope (derived from iR-corrected LSV) that are used as essential evaluation parameters despite just being of engineering perspective for overall electrode performance. From now on, we would like to show how the specific activity, associated overpotential, and TOF reflect the intrinsic activity regardless of loading of the catalyst. To have overpotentials at 10 mA cm−2ECSA (η@10 mA cm−2ECSA) for all five studied NiO interfaces with loading of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2, plots of relative ECSAnormalized LSVs are pictured together in Figure 3c. From Figure 3c, the calculated η@10 mA cm−2ECSA for loadings of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 are 423 ± 3, 422 ± 5, 420 ± 6, 457 ± 5, and 504 ± 6 mV, respectively. The first thing to be noted in these overpotential values is the closeness among the loadings of 0.205, 0.310, and 0.410 mg cm−2. Despite having different loadings, their η@10 mA cm−2ECSA is almost the same. This indicates that the overpotential determined from the ECSA-normalized current density (specific activity) reflects the intrinsic activity of the catalyst, and the same is unaffected by the loading, whereas the geometrical area normalized current density showed a strong

from which the calculated areas for loadings of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 are 4.465 × 10−5, 4.912 × 10−5, 5.418 × 10−5, 8.088 × 10−5, and 1.463 × 10−4 A V, respectively. An exponential increase can be noted from these calculated areas, which implies that the ECSA or electrochemically accessible Ni sites also increase with increasing loading. Using these integrated areas under NiOOH formation peaks, the corresponding charge associated with NiOOH formation and the number of accessible Ni sites was calculated (see the SI for a sample calculation of 0.205 mg cm−2 loading). Calculated charges of NiOOH formation with 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 loadings of NiO are found to be 8.93 × 10−3, 9.82 × 10−3, 1.08 × 10−2, 1.62 × 10−2, and 2.93 × 10−2 C, respectively, and the number of active Ni atoms calculated for loadings of 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 of NiO are found to be 5.57 × 1016, 6.13 × 1016, 6.76 × 1016, 1.01 × 1017, and 1.83 × 1017, respectively. Taking these, plots of loading vs charge associated with NiOOH formation and loading vs number of involved Ni sites are drawn and given as Figure S2a,b in the SI. Both had shown exponential increase with increasing loading. This once again implies that increasing the loading is directly proportional to ECSA and the number of active sites. Hence, the mass of the catalyst should not be neglected when reporting the catalytic activities. Using the number of active sites, the relative ECSA was calculated by assuming that the number of Ni sites that are involved in OER with a loading of 0.205 mg cm−2 exactly covers a geometrical area of 1 cm2. Hence, the relative ECSA of other loadings of 1262

DOI: 10.1021/acsenergylett.9b00686 ACS Energy Lett. 2019, 4, 1260−1264

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ACS Energy Letters

Tafel slopes are highly dependent on the mass of the catalyst, and they will vary with the varying loading. As a consequence of the same, it is concluded that neither η@10 mA cm−2geo nor Tafel slopes can reflect the intrinsic activity of the catalyst, and they can lead to false interpretations. To overcome this issue, we propose and recommend the use of η@10 mA cm−2ECSA and TOF, which are proven to reflect the intrinsic activity of the studied catalyst regardless of the loaded amount of catalyst. However, to ensure flawless scientific data in this particular field, other precise and reliable evaluation parameters are still to be found.

dependence on catalyst loading. Hence, we suggest here that η@10 mA cm−2ECSA should be preferred over η@10 mA cm−2geo for assessing a catalyst in HER and OER electrocatalysis. However, the last two loadings (0.615 and 0.820 mg cm−2) had shown significantly larger overpotentials even from the relative ECSA-normalized current density. This could be related to the increased Rct with increasing catalyst loading, which in turn was aroused due to self-masking of catalytically active sites at higher loadings where catalyst coverage (θ) is more than 100%. This could have forbidden the catalyst to achieve the same specific activities as that of the ones with the lower loadings. Hence, we suggest here that optimum loading must be found by different trials by varying the loading levels to get a loading-independent η@10 mA cm−2ECSA. TOF is another intrinsic activity parameter that could be derived from that current density at a fixed potential and the surface concentration or number of actually involved metal sites. Like the specific activity, the TOF is also independent of loading of the catalyst and can reflect the intrinsic activity. However, it should also be emphasized here that, like the specific activity, the TOF is also highly coverage-dependent and shows a linear relationship only when the coverage is below 100%. Moreover, getting an accurate TOF is always a difficult task unless the catalyst under study is a single-crystal electrocatalyst of well-faceted planes or a molecular electrocatalyst as several assumptions have to be made. Because the electrocatalyst that we have taken is a commercial product of similar particle size, getting TOFs with closer precision is quite possible at least up to 100% catalyst coverage (i.e., until the catalyst loading reaches 0.410 mg cm−2). To prove this, we have calculated (see the SI for a sample calculation of 0.205 mg cm−2 loading) the TOFs for all five studied catalytic interfaces using the following equation3 TOF = j ·NA /n·F ·Γ

Sengeni Anantharaj* Subrata Kundu*

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Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201 002, Uttar Pradhesh, India Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630006, Tamil Nadu, India

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00686.

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Experimental section, plots of loading vs NiOOH formation charge, loading vs number of active Ni, and loading vs relative ECSA (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] and anantharaj1988@ outlook.com. *E-mail: [email protected] and [email protected].

(3)

where j is the current density, NA is the Avogadro number, n is the number of electron transferred for the evolution of a single O2 molecule, F is the Faraday constant, and Γ is the surface concentration or the number of active Ni sites. Calculated TOFs for 0.205, 0.310, 0.410, 0.615, and 0.820 mg cm−2 loadings of NiO are 8.99 × 10−1, 8.96 × 10−1, 8.93 × 10−1, 6.85 × 10−1, and 4.43 × 10−1s−1, respectively. A clear closeness in TOF values among the NiO loadings of 0.205, 0.310, and 0.410 mg cm−2 can be witnessed. To have a comprehensive understanding, a plot of loading vs TOF was derived and is given in Figure 3d, from which the same closeness in lower loadings (i.e., until the coverage of catalyst reaches 100%) is witnessed again. This testifies that, like η@10 mA cm−2ECSA, the TOF is also another reliable intrinsic activity parameter that can be preferably used in electrochemical water splitting. The results of this study are tabulated as Table S1 in the SI. From the overall study, we recommend the use of η@10 mA cm−2ECSA and TOF as essential evaluation parameters in electrochemical water splitting to reflect the intrinsic activity of a given electrocatalyst. To avoid errors that occur with the Tafel slope, we also suggest the use of polarization curves that are normalized with ECSA rather than the geometrical surface area. In summary, we have discussed and showed the limitations and inappropriateness of η@10 mA cm−2geo and the Tafel slope (derived from iR-corrected LSV) in assessing a given electrocatalyst for electrochemical water splitting in detail. In addition, we have also proven that both η@10 mA cm−2geo and

ORCID

Sengeni Anantharaj: 0000-0002-3265-2455 Subrata Kundu: 0000-0002-1992-9659 Notes

Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.A. thanks CSIR, New Delhi for financial support through the SRF award. S.A. and S.K. are thankful to Dr. Vijayamohanan K. Pillai, (Former Director) Outstanding Scientist, Dr. N. Kalaiselvi (Present Director) and Dr. B. Subramanian, Principal Scientist of CSIR - CECRI for the extended support.

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DOI: 10.1021/acsenergylett.9b00686 ACS Energy Lett. 2019, 4, 1260−1264