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Nickelo–Sulfurization of DNA Leads to an Efficient Alkaline Water Oxidation Electrocatalyst with Low Ni Quantity Kannimuthu Karthick, Sengeni Anantharaj, and Subrata Kundu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00633 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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ACS Sustainable Chemistry & Engineering

Nickelo–Sulfurization of DNA Leads to an Efficient Alkaline Water Oxidation Electrocatalyst with Low Ni Quantity Kannimuthu Karthick‡†, Sengeni Anantharaj‡†and Subrata Kundu‡†*

‡

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute (CSIR-CECRI) Campus, New Delhi, India.

†

Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India.

* To whom correspondence should be addressed, E-mail: [email protected], [email protected], Phone: (+ 91) 4565-241486 and (+ 91) 4565-241487.

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ABSTRACT Non-precious metals based electrocatalysts are highly anticipated in electrocatalytic water splitting as the increasing energy demand can be handled by large scale H2 production with minimum expenses. Herein, a facile and faster nickelo-sulfurization of DNA in ambient conditions has been developed that resulted in NiS anchored wire-like assemblies of DNA. Effect of DNA concentration on material stability and electrocatalytic activity was studied and found that with the DNA to Ni2+ ratios of 0.048 M and 0.072 M, the NiS anchored DNA colloidal solution were stable. In addition, it was found that theNiS(0.048 M) with a relatively lower DNA concentration showed better oxygen evolution reaction (OER) activity than the NiS(0.072 M).Overpotentials of 352 mV and 401 mV were required by NiS(0.048) and NiS(0.072) to deliver the current density of 10 mA cm-2 even with an ultra-low quantity of NiS(0.0123 mgcm-2) in both.The same trend was reflected in the Tafel slopes of NiS(0.048 M) and NiS(0.072 M) which showed 58.6 and 112.4 mVdec-1 indicating that the optimum ratio for better OER activity is 0.048. In this study, DNA plays a versatile role such as acting as a stabilizer, scaffold and a microstructural stage for NiS in solution. Moreover, DNA also acts as an efficient binder and as a conductor of both ions and electrons in its OER activity trend. The proposed method can be used for preparing the stable colloids of other metal sulfide based nanoelectrocatalysts and can directly be employed for water oxidation in alkaline condition. Keywords: nickel sulfide, DNA, self-assemblies, oxygen evolution reaction, voltammetry, Tafel analysis Introduction Increasing use of energy per head around the globe is massively accelerating the depletion rate of fossil fuels which are the major energy resources in thecurrent energy scenario. Such an abnormal and elevated consumption of fossil fuels and other non-renewable energy sourceshave begun polluting the environment in the highest rate like it had never been before.1–3 Recent aggressive changes in climate and the increased melting of ice reservoirs in polar region are the clear evidences of the same.These environmental harms had recently alarmed the research community to trigger the research on non-conventional energy conversion and storage systems that do not affect or have least negative effects on the environment.1,3,4 Following which, the 2 ACS Paragon Plus Environment

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recent evolution of direct conversion of seasonal energies like wind, tidal and solar into electricity gained intense attention in recent years. However, when it comes to energy storage in large scale,a big void exists there when compared to the developments made in the field of energy conversion.1,2 Unfortunately, conventional electrical energy storage systems such as batteries and supercapacitors cannot be used for storing large electrical energy derived from seasonal sources for the on demand use as it would otherwise require huge space and cost high. Though the redox flow batteries can offer a relatively effective energy storage mean, the environmental hazards associated with it are more dangerous than consuming fossils.5 Due to all above stated reasons, electrochemical water splitting has recently been revisited again to make it as an efficient way to store the electrical energy derived from seasonal sources as chemical fuels such as H2 and O2 which on demand can be combined back via fuel cells to generate electricity.1– 4

Water electrolysis has other advantages like high purity and can be produced at ambient

conditions as far as the H2 production alone considered.6 However, there are issues with the halfcell reactions of water electrolysis which are thermodynamically highly un-favored which means that they are to be catalyzed with appropriate electrocatalyst.In earlier days, IrO2,7 RuO28 and Pt9–11 were used as electrocatalysts for water splitting. However, their preciousness and associated cost made the overall process very expensive. Luckily, the recent evolution of non-precious metals based water splitting electrocatalysts made the water electrolysis for hydrogen production easier and cost-effective than ever before.1,3,4 Electrocatalytic water splitting involves OER at anode and hydrogen evolution reaction (HER) at cathode. The kinetics in case of OER is more sluggish compared to HER which is attributed to the more kinetic steps involved in it to release O2 which causes huge energy loss.2 Hence, naturally there has been an immense effort to bring down the energy loss caused by the sluggish OER. For better OER electrocatalysis, layered double hydroxides of 3d iron group metals are the state-of the-art.4 However, the pnictogenides and chalcogenides of the same are also being reported as efficient pre-catalysts for OER and HER with improved activity.1,3,12–14 Other than this, alloys and oxidesof 3d iron group metals are also being reported frequently which are said to be highly active for both water splitting and supercapacitor applications.15,16 In case of sulphides of iron group metals, there are several reports for both HER and OER. However, compared to the sulfides of Fe and Co, sulfides of Ni are less explored systems when it comes to OER in alkaline conditions.3 It is observed that the nickel sulfide was 3 ACS Paragon Plus Environment


reported with different stoichiometry such as NiS,17 Ni3S2.18–20 As far as OER with nickel sulfide is considered, the less reported one is the NiS.21–24 Hence, it is still highly desired to work on such a less explored nickel sulfides systems for OER. Besides, to make use of the versatility of DNA in material synthesis and in electrocatalytic water splitting studies, assembling of as formed NiS nanoparticles (NPs) over DNA was performed and the resultant colloidal solution of NiS anchored DNA self-assemblies were found to be stable enough under the applied potentials of water oxidation. The supramolecular chemistry of various types of DNA makes it as the attractive support materials for stable colloidal nanomaterials synthesis. Structures of the DNA self-assemblies are sensitive to their metal to DNA molar ratio that makes different morphological outcomes of staged NPs. Therefore, tuning the same, it is possible to obtain structurally exciting nanostructures. This versatile nature of DNA was utilized by our group earlier to make various nanostructures out of metal/metal compounds anchored assemblies of DNA that includes Pt, CoS and IrO2 for OER and HER.7,9,12 Moreover, use of DNA in electrocatalytic studies has several advantages such as its ability to act as both ionic and electronic conductor and the ability to get strongly adhered to all kinds of substrate electrodes.With this view, we herein report the facile synthesis of NiS anchored over DNA through a wet chemical pathway for the first time. The as formed NiS@DNA solutions were highly stable. The same was then subsequently characterized and screened for electrocatalytic water oxidation in alkaline conditions.

EXPERIMENTAL SECTION Nickelo-Sulfurization of DNA To prepare NiS staged DNA colloidal solution, 40 ml of DNA (0.12 M)was taken with 60 mL of DI water to which nickel acetate (1 mmol) was added that corresponds to the molar ratio of 0.048 and the solution was stirred. The negatively polarized base pairs present in DNA and phosphate moieties will get attracted towards the positively charged Ni (II) ions which will be assembled over the DNA chain to form concentration dependent superstructures in solution. Following this, Na2S (1 mmol) is swiftly added to the above solution during which Ni(II) ions assembled over the DNA chain reacted with S2- ions that could form the self-assembled superstructures of NiS NPs staged DNA and the stirring continued up to 30 min to ensure complete assembling of NiS NPs over DNA.The same protocol was followed to prepare another 4 ACS Paragon Plus Environment

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set of NiS staged DNA colloidal solution of molar ratio 0.072, where 60 mL of DNA (0.12 M) was taken with 40 mL of DI water. In both the cases, stable solution of NiS nanoparticles staged over DNAand was stable up to 36 h when kept in refrigerator and after that it settles down slowly which can be regained by sonication for just 10 min. When the molar ratio of DNA was reduced to lower concentrations like 0.024 M and 0.036 M, the NiS@DNA settles down within4 to 5 h. Therefore, the optimum molar ratios of DNA required to form stable colloidal solutions of DNA are 0.048 M and 0.072 M. The as formed colloidal solutions were stable enough which allowed the direct utilization of the same for the fabrication of working electrodes to be used in electrocatalytic water oxidation. The briefed information on the synthetic protocol for both sets about the reagents used, color observed and reaction time are tabulated as Table S1in SI and the role of DNA in synthesis is also schematically shown as Scheme 1. The materials used for the synthesis and for electrochemical characterizations are given in supporting information (SI).

Scheme 1: Role of DNA in staging the Nickelo-Sulfurization for electrocatalytic studies. Electrochemical characterizations The as formed colloidal solution of NiS staged self-assemblies of DNA (NiS@DNA) were subjected to electrochemical studies in 1M KOH at ambient conditions to screen its ability



to catalyze OER electrochemically. As DNA itself has good adhesion properties to all kinds of substrate electrodes and also has the tendency to act as an ionic and electronic conductor. The as prepared NiS@DNA solution was directly drop casted over acid treated carbon cloth (CC) of dimensions of 4 × 0.5 cm. After drop casting NiS@DNA over CC, it was dried at room temperature and then used for electrochemical characterizations. Here, as DNA itself acted as a binder, no foreign additive as binder was added which might cause additional problems like active site masking and swelling of interface upon prolonged exposure.All electrochemical studies were done in a three electrode cell with Pt counter electrode, Hg/HgO reference electrode and NiS@DNA modified CC as working electrode. Linear sweep voltammetric (LSV) curves were acquired at a scan rate of 5 mV s-1 and are manually corrected for iR drop. The cyclic voltammetry were carried out at a scan rate of 200 mV s-1 to check the stability of the interface at accelerated conditions and chronoamperometric study was done at overpotentials that could drive 10 mA cm-2 without iR correction. Electrochemical impedance spectroscopy (EIS) analysis was carried at a frequency range between 1 Hz to 100 KHz with amplitude of 0.05V at the onset overpotentials for OER on each interface. The potential scales all polarization curves were converted into reversible hydrogen electrode(RHE) scale following literature reports for ease of evaluation and comparison of the activity of our catalyst systems.25–28

RESULTS AND DISCUSSION Material Characterization The formation of NiS@DNA was confirmed through various advanced characterization techniques and is elaborated in detail below. Initially, to know the specific interactions in between DNA and nickel sulphide, UV-Visible study was carried out.The obtained UV-Visible spectral features are given in Figure 1. In Figure 1, curve a is for nickel acetate which has shown peaks near 395 nm as a broader one and near 204 nm which are due to LMCT and hydrated Ni2+ ions respectively.29–31 Curve b, for DNA which has peaks near 259 nm is due to the presence of aromatic base pair groups with π−π∗ transitions.32–34


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Once the nickel acetate is added to the DNA solution, the negatively polarized base pairs and phosphate moieties will get attracted to Ni2+ ions through electrostatic interactions and thereby causing a shift in the UV-Visible spectral features of both. This can clearly be seen in curve c that the intensity of the peak of Ni2+ ions is decreased with DNA in higher wavelength regions and increased in lower wavelength region which leaves a clue that Ni2+ ions are definitely interacting with DNA. Curves d and e are the UV-Vis absorption curves of NiS@DNA synthesized with increasing DNA concentrations from which we can see that upon increasing concentration of DNA the intensity of the characteristic π−π∗ transitions peaks of base pairs in DNA is also getting increased. From this information, it is postulated that there are certain interaction between Ni2+ ions/NiS and DNA that resulted in a stable colloidal solution of NiS@DNA. Similar observations were made in our earlier studies of anchoring and staging various metal, metal oxide, mixed metal oxide and metal sulfide nanostructures for diverse 7 ACS Paragon Plus Environment


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applications.12,32,33,35 These are also in good agreement with the earlier reports where other similar materials were stabilized over DNA in aqueous solution.

Figure 2: (a-b) are the low and high magnified TEM images of NiS@DNA (0.048) and (c-d) are NiS@DNA (0.072) respectively. The corresponding SAED patterns are given in the inset of a and c. (e) is the HR-TEM micrograph of NiS (0.072)@DNA. Having found and postulated the possible interactions between Ni2+ ions/NiS and DNA, microstructural analysis was done with transmission electron microscopy (TEM) to know the specific morphology of the self-assemblies formed in the as prepared NiS@DNA colloidal solution and the obtained micrographs have been given as Figure 2, A-D. The micrographs of NiS (0.048) and NiS (0.072) of lower and higher magnifications are provided as Figure 2, A-D respectively. At first sight itself it is very clear that the NiS is staged over DNA and formed two different self-assemblies depending on the concentration of the same. With NiS@DNA(0.048), sheet-like self-assembled structures were seen over whichpoly-dispersed NPs of NiS is also seen. In sharp contrast, with NiS@DNA(0.072), wire-like superstructures of NiS@DNA self-


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assemblies were witnessed. Interestingly, the particle size of NiS staged over DNA in case of NiS@DNA(0.072) was very small which was in the range of 4 to 5 nm. This particular observation infers that the higher concentration of DNA restricted the growth of NiS and resulted in particles of sub-nano level. The inset of Figure 2 A and Figure 2C show the corresponding selected area electron diffraction (SAED)patterns which have once again confirmed that the particles are polycrystalline in nature. Observed ring patterns were indexed to corresponding miller indices in accordance with the JCPDS card number 00-001-1286as per earlier reports. Figure 2E is the high resolution TEM (HRTEM) micrograph of NiS@DNA(0.048) in which fine arrangements of atomic planes legible and the measured distance between two lattice planes were 0.388 nm and 0.456 nm which corresponds to the miller indices (410) and (220) respectively. From the above discussions, it is witnessed that the NiS NPs are assembled over DNA and just by varying concentrations of DNA the morphological outcome of NiS@DNA is changed that resulted in different morphologies and the same was observed in our earlier studies also.9 The size of NiS (0.048) NPs are ~4 to 5 nm with the chain length of ~0.22 µm which were seen with significant agglomerations as the concentration of DNA was not adequate to restrict the agglomeration of NiS NPs.Similarly sized particles were observed with NiS (0.072) also with the chain length of ~1.46 µm. From combined microstructural analysis it is confirmed that the concentration of DNA affects the morphological outcome of the DNA derived catalysts. The X-ray diffraction (XRD) pattern of NiS@DNA(0.048) has been given as Figure S1 in SI. From this figure, we can see that there are no legible diffractionpeaks which indicate that the as prepared NiS NPs are very small to diffract the applied X-rays and therefore resulted in a pattern with no peaks of considerable intensities.36,37 This is in accordance with the results of TEM and SAED analysis which suggested the same information. The other set i.e., NiS@DNA(0.072) had also resulted in similar XRD pattern (not shown as they were almost identical) implying the same information. To further ascertain the presence of all expected elements from NiS staged DNA, elemental color mapping in high-angle annular dark field (HAADF) was done and the corresponding EDS spectrum and the smart maps are shown as Figure S2 and Figure S3, A-F in SI. Figure S3A shows the HAADF micrograph of NiS@DNA(0.048) which is similar to the observed TEM micrograph (Figure 2A). Figure S3, BG are the elemental color maps of C, N, O, P, Ni and S of their K shells respectively. From these maps and the EDS spectrum, we can see that the composition of S is six fold higher than Ni. 9 ACS Paragon Plus Environment


Presence of N, P, C and O along with Ni and S also gives us important and much needed information that the as formed NiS is grown exclusively over the DNA chains and forms selfassembled superstructures. Since, both NiS@DNA(0.048) and NiS@DNA(0.072) are chemically equivalent in nature, the EDS elemental color mapping was performed only with NiS@DNA(0.048). The high intensity of C is from the grid used in HRTEM analysis.The copresence of DNA along with NiS is confirmed by both EDS spectrum and the elemental color mapping analyses.

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Wavenumber (cm-1) Figure 3: FT-IR transmittance spectral features of only DNA with the nickel sulfide anchored selfassembly of DNA. Further, in order to know the exact binding sites where NiS NPs are staged, Fourier transform infra-red (FT-IR) spectroscopic studies were done in addition to the UV-Visible studies. The FT-IR spectra of NiS@DNA in comparison with DNA alone are shown as Figure 3. In this Figure, Spectral curve a is for pristine DNA and spectral curve b is for NiS@DNA 10 ACS Paragon Plus Environment

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(0.048). For the same fact that both NiS@DNA (0.048) and NiS@DNA (0.072) are chemically equivalent, only NiS@DNA (0.048) has been studied here. From these curves we can see that the interaction of DNA and NiS altered the intensity and positions of various bands corresponding to different functional groups. In DNA, at lower wavenumber region, the bands observed at 603, 749 and 1123 cm-1 are of stretching vibrations of phosphate groups in DNA. These peaks became diminished significantly in intensities and deviated from their parent band positions which suggested that the phosphate moieties in DNA are one of the major binding and interacting sites of NiS NPs. The band located near 1637 cm-1 is of characteristic amide functional group in DNA which is shifted to 1690 cm-1 in NiS@DNA(0.048) indicating that NiS is also interacting with DNA via the amide functionalities that present in the base pairs of the same.The bands observed near 593 and 620 cm-1 are of sulphide functionalities that are bonded to metal centers which resonate well with earlier literature reports.12,32,34,38 The broad band near 3416 cm-1 is for hydroxyl group present in DNA which became less intensified and deviated to 3601 cm-1 leaving a strong indication that NiS NPs are staged over DNA by interacting with hydroxyl groups of sugar moieties present in DNA too. Similar observation was also made in our earlier studies.12,32,34,38 Hence, from the UV-Visible studies and FT-IR spectral studies, it was confirmed that the as formed NiS NPs were staged over hydroxyl, phosphate and amide functional groups of DNA and forming a self-assemblies of varying morphologies as revealed by TEM and HRTEM analyses. Being confirmed the interaction between NiS NPs and self-assembled DNA superstructures, chemical nature of each element in the prepared sample was analyzed through X-ray photoelectron spectroscopic (XPS) analysis. As the chemical nature of both NiS@DNA (0.048) and NiS@DNA (0.072) are the same, XPS analysis of NiS@DNA (0.048) was only done. Figure 4, A-D are the high resolution spectra of Ni 2p, S 2p, C 1s and O 1s states of respective elements of NiS@DNA (0.048). Figure 4A is of Ni 2p state in which two peaks were observed at 853.5 and 871.2 eV which are characteristic to Ni 2p3/2 and Ni 2p1/2 sub-states which arouse due to spin-orbit coupling respectively.The small humps observed at slightly higher binding energies to these sub-states are their characteristic satellite peaks in Ni 2p which is matching well with the previously reported results of NiS.21–24



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Figure 4: (a-d) are the high resolution spectra of Ni 2p, S 2p, C1s and O1s respectively. Figure 4B shows the high resolution spectrum of S 2p state which also has two peaks at 162.5 and 164.3eV that correspond to the sulfidic S as S 2p3/2and S 2p1/2 sub-states and along with these there are peaks in the region of 167-172 eV which are responsible for the surface oxidized sulfites and sulfates that present along with the sulfides in the as-prepared colloidal NiS@DNA solution.21–24 Similar surface oxidation was observed in many earlier studies of metal sulfides and selenides.12,13 Figure 4C shows the high resolution spectrum of C 1s state where we can see different kinds of peaks with respect to various chemically distinguishable carbon in DNA.A peak located at 284.6eV is for C=C, 285.1 eV is for C-C and 288.3 eV is for C(O)-NH from 12 ACS Paragon Plus Environment

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DNA. Figure 4D shows high resolution spectrum of O 1s state which shows different peaks at 528.8 eV is for –P-O bond, 530.12 eV is for –C-O-C bond, 530.9 eV is for –Ni-OH bond and 531.6 eV is for C(O)-NH bond. These results are in accordance with the previous reports and further confirm that Ni2+ reacted with S2- to form NiS NPs with some surface oxidation of S2and also suggested a possible formation of Ni(OH)2 formation which is irresistible in aqueous medium.9,32,39 Similar observation was made earlier for surface oxidation of anion and metal hydroxide formation by the metal cation in the synthesis of metal sulfides and selenides in aqueous medium. The detailed material characterization studies have confirmed that the NiS was successfully formed and staged over DNA self-assemblies in solution. After confirming the formation of NiS, we went on subjecting the same to electrocatalytic OER studies in alkaline conditions as discussed in detail below. Electrocatalytic OER studies of NiS@DNA As a primary activity parameter, LSVs were acquired for both NiS@DNA(0.048) and NiS@DNA(0.072) modified CC electrodes in comparison with the RuO2/CC. Figure 5A shows iR corrected LSVs obtained at a scan rate of 5 mV s-1. Polarization curves for both CC and DNA@CC did not show any appreciable current density in the OER region within the applied potential window whereas NiS@DNA (0.048)/CC, NiS@DNA (0.072)/CC and RuO2/CC have shown significant activities. The overpotential required to reach the benchmarking current density of 10 mA cm-2for NiS@DNA (0.048)/CC, NiS@DNA (0.072)/CC and RuO2/CC were 352, 401 and 348 mV respectively. From the polarization curves it is also observed that with the higher concentration of DNA i.e., for NiS@DNA (0.072), the overpotential required for driving 10 mA cm-2 was significantly increased. The observed OER activity for RuO2/CC was with the huge catalyst loading of 0.205 mg cm-2 compared to which the loading of NiS is around 20 times lesser (0.0123 mg cm-2).



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Figure 5:(a) Polarization curves of NiS@DNA in comparison with RuO2/CC, bare CC and DNA modified CC acquired at a scan rate of 5 mVs-1 in 1 M KOH. (b) the plot of j vs. η of NiS@DNA. (c) the plot of η vs. Mass activity of NiS@DNA. (d) Tafel plots of NiS@DNA in comparison with bare CC and DNA modified CC.

Even with lower loading, the observed activity is still comparable which is attributed to the synergistically enhancing properties of DNA which facilitates the charge transfer even with lower concentration of NiS. With the ability to stabilize the sub-nano level particles, DNA also provides maximum sites for electrochemical accessibilities. This observation was made earlier by our group for IrO2@DNA,7 Pt@DNA9 and [email protected] The error bars on the reproducibility of the polarization studies and mass activities of all three catalytic interfaces that were screened in this study with minimum uncertainties further confirm the consistency of the NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC as shown in Figure 5B and Figure 5C. The 14 ACS Paragon Plus Environment

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nature of kinetics of OER on CC, DNA@CC, NiS@DNA(0.048)/CC, NiS@DNA(0.072)/CC and RuO2/CC was analyzed via their corresponding Tafel plots as given in Figure 5D. The measured Tafel slope values for CC is 209.9 mV/dec and for DNA/CC is 319.5 mV/dec which indicates that the intrinsic resistance of DNA/CC is higher compared to bareCC. The lower Tafel slope values of NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC are 58.6 and 112.4 mV/dec respectively which are lower than RuO2/CC which has the Tafel slope value of 123.3 mV/dec implying that the charge transfer kinetics is facile in case of NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC compared to RuO2@CC under OER conditions in 1 M KOH. These results are in good agreement with the activity trend observed in polarization studies.The accelerated degradation test was carried at a scan rate of 200 mV s-1 for 500 cycles to ensure the endurance of the NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC and the same is given in Figure 6. The overpotential at 10 mA cm-2was increased from 352 mV to 541 mV and from 401 mV to 619 mV for NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC respectively after accelerated degradation. The significant degradation in activity after 500 cycles with a high scan rate of 200 mV s-1 implies that NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC interfaces had moderate stability under applied potential region which can be tuned by varying the loading and drying conditions such as time, atmosphere and temperature upon interest.

30 NiS@DNA(0.048) NiS@DNA(0.048) AD NiS@DNA(0.072) NiS@DNA(0.072) AD

20

j (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


401 mV

619 mV 541 mV

352 mV

10

0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

E/V Vs RHE Figure 6: LSVs of NiS@DNA before and after the accelerated degradation test at a scan rate of 200 mV s-1.



However, it is not mandatory to carry out those optimization studies here as this degradation does not have anything to deal with the intrinsic catalytic activity of the catalysts as there won’t be any such high scan rate cycling in real water electrolysers.40 The higher change in overpotential is attributed to the surface oxidation of sulphides to sulphates and also to the destabilizing action of alkali on the thin film formed out of DNA self-assemblies upon cycling.

a 160

14

Before AD After AD

140

12 10 8

120

6

-Z" / Ω

100

4 2

80

0

0

2

4

6

8

10

12

14

60 40 20 0 0

20

40

60

80

100 120

140

160

Z' / Ω

b Before AD After AD

250

9.0 7.5 6.0

200

4.5 3.0

-Z" / Ω

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

1.5

150

0.0 0.0

1.5

3.0

4.5

6.0

7.5

9.0

100 50 0 0

50

100

150

200

250

Z' / Ω

Figure 7:a and b are the Nyquist plots of NiS@DNA (0.048) and NiS@DNA (0.072) before and after cycling tests. 16 ACS Paragon Plus Environment

Page 17 of 26

The electrochemical nature of these interfaces is analyzed through EIS before and after the accelerated degradation study and is given in Figure 7A and 7B. The change in Rct value after endurance test is145.5 and 221.2Ω for NiS@DNA (0.048)/CC and NiS@DNA (0.072)/CC respectively. More meaningful stability study on NiS@DNA (0.048)/CC interface was carried out under continuous chronoamperometric conditions without iR compensation at overpotential of 410 mV for 10 h and the resultant chronoamperometric curve is shown as Figure 8. The degradation in activity was very less which is indicating that though NiS@DNA(0.048)/CC interface was not fairly stable under accelerated degradation conditions still it is stable enough under potentiostatic conditions which will be the real conditions in water electrolysers.

25 NiS@DNA(0.048)

20 15

j (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 5 0 -5 -10 0

1

2

3

4

5

6

7

8

9

Time (h) Figure 8:Chronoamperometric analysis of NiS @DNA (0.048) at the specified overpotentials without iR compensation.

In addition to this, we have also calculated the relative electrochemical surface area (ECSA) through a well-known double layer capacitance method. The plot between ∆j(ja-jc) vs. scan rate is shown in Figure S4 in SI section. From the Figure S4 it is seen that the 2Cdl value is 0.16µF for NiS@DNA(0.048)/CC and 0.12µF for NiS@DNA(0.072)/CC which clearly imply that the exposed surface area with lower concentration of DNA relatively higher than the one with higher concentration of DNA. This trend is well matching with the results of polarization 17 ACS Paragon Plus Environment


Page 18 of 26

studies. We have also calculated TOF values for both NiS@DNA(0.048)/CC and NiS@DNA(0.072)/CC by integrating the area under the oxidation peak observed justbefore the onset overpotential. Here, it is assumed that the Ni2+ sites whichever get oxidized to oxyhydroxide are the only sites that participated in the electrocatalysis of OER (See Figure S5, a-b in SI and related calculation). Table 1: Results of electrocatalytic studies

Loading (mgcm2 )

Overpotential @ 10 mAcm2 (mV)

Tafel Slope (mVdec1 )

Mass activity @ η = 398 mV (Ag-1)

TOF at η = 350 mV (s-1)

Change in Overpotential @ 10 mAcm-2 after AD test (mV)

NiS@DNA (0.048)/CC

0.0123

352

58.6

2387

0.7791

189

NiS@DNA (0.072)/CC

0.0123

401

112.4

731.45

0.3615

218

0.205

348

123.3

82.92

-(a)

-(b)

Catalyst

RuO2/CC

Note: (a) TOF value was not calculated for RuO2/CC. (b) AD test was not carried out over RuO2/CC interface.

Table 2: Benchmarking NiS@DNA with other related electrocatalysts in terms of overpotential and Tafel slope with respect to mass loading. Catalysts

Ni3S2/Ni NiS NiS/SLS NiS/Ni h-NiSx Ni–Ni3S2/NF

Overpotential at 10 mA cm-2

Tafel slope mV/decade

Loading mg cm-2

References

187 320 297 335 (50 mA cm-2) 180 310

159.3 59 47 89 96 63

37 0.7±0.2 1 43 3.3

14 19 16 17 18 15


Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thus the calculated TOF values at 350 mV are 0.7791 s-1 and 0.3615 s-1in NiS@DNA(0.048) and NiS@DNA(0.072) respectively which are comparable to earlier reports. The overall results of the electrocatalytic study are tabulated as Table 1 and the detailed comparison has also been made with the earlier reports18–24 of similar catalysts as Table 2. By comparing our results with some of other closely related reports, we have proven here that utilizing the method we opted for the formation of NiS@DNA, we can harvest better and comparable activity in OER electrocatalysis even with 20 times lower loading. In addition, from Table 1, Table 2 and the above detailed discussions, it has been made very clear that the use of DNA in forming the self-assembly of NiS was quite advantageous to form a stable colloidal solution and also in synergistically enhancing the OER activity. As DNA itself acted as a good binder,it avoided the use of an external binder anyhow. With all these advantages, it is declared that the prepared colloidal solutions of NiS NPs staged DNA self-assemblies can be preferred over the precious and costly anode materials like IrO2 and RuO2in future water electrolysers. Conclusion Successful in situ formation and staging of NiS on the self-assembled superstructures of DNA in aqueous solution had first time been achieved within a relatively lower time of reaction by employing various optimization studies. The as prepared colloidal solutions of NiS@DNA with increasing DNA concentration were directly casted over acid treated CC electrode and screened for OER electrocatalytic activity in 1 M KOH in comparison with the state-of-the-art RuO2. Even with 20 times lesser loading (0.0123 mgcm-2), the NiS@DNA had shown significant activity which was comparable to RuO2/CCwith a catalyst loading of 0.205 mg cm-2. To reach a current density of 10 mA cm-2, NiS@DNA(0.048) and NiS@DNA(0.072) required just 352 and 401 mV respectively. As DNA possessed a fair adhesive ability, no external binder was used during stability studies and found good stability upon prolonged potentiostatic electrolysis. The proposed protocol to form NiS staged colloidal solution of DNA could be extended for other metal sulfides also and thereby the same could be opted for other electrochemical applications like HER, oxygen reduction reaction (ORR), alcohol and polyol oxidation in near future. ASSOCIATED CONTENT Supporting Information Available: Information on the final concentration of the precursors used in the synthesis as Table S1, materials and characterization techniques calculation of TOF 19 ACS Paragon Plus Environment


Page 20 of 26

and Figures of XRD pattern, color mapping and EDS spectra and Figures related to electrochemical studies are available in supplementary section. ACKNOWLEDGEMENTS We wish to acknowledge Dr. Vijayamohanan K. Pillai, Director, CSIR-CECRI for their continuous support and encouragement. K. Karthick wishes to acknowledge UGC for JRF award. S. Anantharaj wishes to acknowledge CSIR for SRF award. Support from all the faculties of CIF-CECRI, Karaikudi, India is thankfully acknowledged.

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Nickelo-sulfurization of DNA has been proposed to design a low-cost, highly efficient water oxidation electrocatalyst with ultra-low Ni quantity.


Page 26 of 26

Nickelo-Sulfurization of DNA Leads to an Efficient ... - ACS Publications

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