[MoS4]2â-Intercalated NiCo-Layered Double Hydroxide Nanospikes

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

[MoS4]2−-Intercalated NiCo-Layered Double Hydroxide Nanospikes: An Efficiently Synergized Material for Urine To Direct H2 Generation Ayasha Nadeema,†,‡ Varchaswal Kashyap,†,‡ Rakshitha Gururaj,†,§ and Sreekumar Kurungot*,†,‡ †

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India Academy of Scientific and Innovative Research (AcSIR), New Delhi 110001, India § Christ University, Bengaluru 560029, India ‡

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S Supporting Information *

ABSTRACT: Substituting the energy-uphill water oxidation half-cell with readily oxidizable urea-rich urine, a ground-breaking bridge is constructed, combining the energy-efficient hydrogen generation and environmental protection. Hence, designing a robust multifunctional electrocatalyst is desirable for widespread implementation of this waste to fuel technology. In this context, here, we report a simple tuning of the electrocatalytically favorable characteristics of NiCo-layered double hydroxide by introducing [MoS4]2− in its interlayer space. The [MoS4]2− insertion as well as its effect on the electronic structure tuning is thoroughly studied via X-ray photoelectron spectroscopy in combination with electrochemical analysis. This insertion induces overall electronic structure tuning of the hydroxide layer in such a way that the designed catalyst exhibited favorable kinetics toward all the required reactions of hydrogen generation. This is why our homemade catalyst, when utilized both as a cathode and anode to fabricate a urea electrolyzer, required a mere ∼1.37 V cell potential to generate sufficient H2 by reaching the benchmark 10 mA cm−2 in 1 M KOH/0.33 M urea along with long-lasting catalytic efficiency. Other indispensable reason of selecting [MoS4]2− is its high-valent nature making the catalyst highly selective and insensitive to common catalyst-poisoning toxins of urine. This is experimentally supported by performing the real urine electrolysis, where the nanospike-covered Ni foam-based catalyst showed a performance similar to that of synthetic urea, offering its industrial value. Other intuition of selecting [MoS4]2− was to provide a ligand-based mechanism for hydrogen evolution half-cell [hydrogen evolution reaction (HER)] to preclude the HER-competing oxygen reduction. Another crucial point of our work is its potential to avoid the mixing of two explosive product gases, that is, H2 and O2. KEYWORDS: urine/urea electrolysis, sewage denitrification, urea oxidation reaction, hydrogen evolution reaction, layered double hydroxide, nickel oxyhydroxide

1. INTRODUCTION

pollutants such as ammonia and nitrates, creating adverse health issues and water reservoir eutrophication.13,14 In addition, urea electrolysis provides one of the ways to deal with the competing oxygen reduction reaction during hydrogen generation to improve the energy efficiency of the process.8,9 It also offers one of the potential ways to avoid the mixing of two explosive product gases of water electrolysis, that is, H2 and O2.15 Most importantly, urea electrolysis in alkaline medium thermodynamically shifts the potential from 1.23 V to a mere 0.37 V, theoretically generating 70% cheaper hydrogen.14 Hence, urea can be a promising hydrogen carrier in the development of sustainable hydrogen economy. In spite of a number of factors in favor of replacing WOR with UOR, challenges lie in the complexity of the six-electron transferred urea oxidation, undergoing sluggish kinetics and

A switch from today’s hydrocarbon economy to hydrogen economy, having highest gravimetric energy density (∼140 M J/kg), has gained sufficient momentum in recent years to mitigate the issues associated with current energy scenario.1,2 This transition, however, depends upon our efforts of providing cheaper and renewable source of hydrogen and the best solution is water.3−5 However, poor energy efficiency of this process because of the sluggish water oxidation reaction (WOR) kinetics4,6,7 as well as its poisoning effect on the H2generating half-cell8,9 is hampering its widespread application. Among various efforts, replacing the energy-uphill water oxidation half-cell with readily oxidizable molecules, for example, alcohols,10,11 hydrazine,12 urea,13,14 and so forth., has emerged as one of the potential substitutes to make the H2 production energy-efficient. Among which, the urea oxidation reaction (UOR) not only makes the H2 production energyefficient but also allows the denitrification of urea-rich wastewater, which otherwise naturally hydrolyzes into © XXXX American Chemical Society

Received: April 15, 2019 Accepted: June 27, 2019 Published: June 27, 2019 A

DOI: 10.1021/acsami.9b06545 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Representation of the Steps Involved in the Synthesis of MoS4-LDH/NF and the Schematic Illustration of Our Waste to Fuel Generation Strategy

restricting its widespread implementation.16−18 On the basis of available literature studies, in the alkaline medium, urea electrolysis takes place as follows14,16

catalytic efficiency even in the presence of common catalyst inhibitors, for example, CO, S2−/HS of natural water bodies.9 Considering this, we thought of incorporating the [MoS4]2− anion with WOR/UOR-active materials to design a multifunctional robust catalyst for H2 generation directly from real urine. Among various WOR/UOR active materials, layered double hydroxides (LDHs) are found to best fulfill the requirement because of their tunable structure,26,27 chemical environment, and electronic properties.28−30 For instance, α-Ni(OH)2 and its oxidized form, that is, γ-NiOOH, are the building blocks of Ni-based LDHs27 and have shown an outstanding role toward catalyzing the two half-cell reactions of water electrolysis.31−33 Moreover, Subbaraman et al.34 corroborates the essential OHacceptor role of Ni(OH)2 moieties over Pt sites toward improving the hydrogen evolution reaction (HER) kinetics of Pt in alkaline medium. In addition, the WOR-promoting roles of γ-NiOOH6 and other (oxy)hydroxides (e.g., NiFe (oxy)hydroxides,35 CoFe(oxy)hydroxides36 etc.) are well documented. In spite of that, the essential role of Ni(OH)2/ NiOOH toward urea/urine oxidation is thoroughly investigated by Boggs et al.14 Thus, our strategy for the [MoS4]2− intercalation in the NiCo-LDH interslab may fulfill the requirement of an ideal catalyst to boost up the overall urea/urine electrolysis via an efficient catalytic interface generated between the interlayer [MoS4]2− moieties and LDH. The obtained catalyst showed efficient performance toward all required reactions of hydrogen generation in the alkaline medium. The fabricated whole-cell urea electrolysis device required a mere ∼1.37 V to generate sufficient amount of H2 by achieving the benchmark 10 mA cm−2 current density. Interestingly, our catalyst works just as well with real urine, representing the proficiency of the catalyst system toward the utilization of waste for generating useful fuel.

Anode: CO(NH 2)2 + 8OH− → N2 + CO32 − + 6H 2O + 6e−,

E = − 0.46 V vs SHE

(1)

Cathode: 6H 2O + 6e− → 3H 2 + 6OH−, E = −0.83 V vs SHE

(2)

Overall: CO(NH 2)2 + 2OH− → N2 + CO32 − + 3H 2 , E = 0.37 V vs SHE

(3)

Although UOR can be efficiently catalyzed by Pt- and Rhbased catalysts, their high cost and scarcity makes the process impractical.14 Considering this, the designing of an ideal catalyst, composed of least-toxic, earth-abundant elements, economical to manufacture, and electrochemically stable, is crucial. In this context, a number of efforts have been directed, such as NiCo compounds, 18 2D MnO2 crystals, 19 Ni(OH)2 nanostructures,17,20 and NiMo-based nanostructures.21,22 Moreover, in recent works, Zeng et al.23 and Hu et al.24 also demonstrated improved UOR performance by designing NiCo-LDH-NO3 and Ni3N/NF-based electrocatalysts, respectively. Through all these works, urea oxidation is experienced as one of the feasible approach to replace WOR for energyefficient H2 generation. However, none of these catalysts was demonstrated to utilize real urine-based urea directly, the ultimate mandate to commercialize this waste to fuel technology. The probable reason for this limitation is their poor selectivity as well as instability in the presence of common catalyst inhibitors of urine, for example, CO, proteins, creatinine, phosphates, and so forth.25 Hence, it is of paramount to design a catalyst providing good selectivity as well as reasonable activity toward urea/urine electrolysis along with remarkable durability. In this context, Dey’s group, in one of their works, demonstrated the unprecedented role of ammonium tetrathiomolybdate (ATTM) toward catalyzing highly selective H+ reduction while maintaining the long-term

2. EXPERIMENTAL SECTION 2.1. Synthesis of NO3-Intercalated NiCo-LDH/NF. The NO3intercalated NiCo-LDH nanospike arrays were directly grown over the nickel foam (abbreviated as NF) by a single-step hydrothermal method in the presence of excess NaNO3 (Scheme 1). The NF was first cleaned with 3 M HCl solution, water, and ethanol. The synthesis B


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Figure 1. Comparative XRD (a) and FTIR (b) patterns recorded for different samples. The FE-SEM images of NO3-LDH/NF are presented in (c,d) and that of MoS4-LDH/NF are displayed in (e,f); the inset in image (c) represents the SEM image of the bare Ni foam.

successful NO3− intercalation in NiCo-LDH interslab.26,37 In case of NF-supported NO3-LDH, although the intercalationdeciding (003) plane showed diminished intensity, the position of (006) plane matched well with that of unsupported system, affirming the successful growth of NO3-LDH over NF (Figure S1b). After confirming the NO3-LDH formation, both unsupported and NF-supported sample were subjected to the ethanol-assisted [MoS4]2− anion exchange process in the degassed ethanol/water mixture (1:1, volume ratio) with constant 48 h stirring at ambient atmosphere. Figure 1a also comprises the XRD pattern of powdered MoS4-LDH, depicting a clear increment of the (003) basal spacing from 0.88 to 1.06 nm, because of the expansion of NiCo-LDH unit cell along the c-axis.37−39 The XRD analysis, when extended on MoS4-LDH/NF, showed poorly defined (00l) planes, due to the increased amorphousness after the anion exchange process (Figure S1b). In fact, the (003) plane gets totally unresolved. Nevertheless, the lower 2θ value shift of the other Bragg reflections, that is, (006), (101), and (015), clearly indicates the introduction of larger [MoS4]2− in the LDH interslab.38 Figure 1a also comprises the XRD pattern of carbonate-intercalated LDHs having 0.74 nm d-spacing. It is synthesized using chloride metal salts with no NaNO 3 (abbreviated as CO3-LDH).40 The intention to show the comparative XRD analysis of NO3-LDHs and CO3-LDHs is to support our efforts of minimizing the number of synthesis steps by synthesizing direct NO3−-intercalated LDHs, which otherwise needs an extra step via NO3− anion exchange from CO3-LDH.26,38 To verify the types of anion existing in the LDH interlayer, IR spectroscopy analysis was performed on powdered samples. For NO3-LDH, the strong band appearing at 1384 cm−1 corresponds to the interlayer NO3− ions (Figure 1b).39 To further support the major intercalated species as NO3− anions, Fourier-transform infrared spectroscopy (FTIR) analysis was extended on CO3-LDHs (Figure S1c). In case of CO3-LDHs, the broad band starting near 1365 cm−1 unlike the sharper NO3− band affirms the success of our synthesis strategy.40 After the [MoS4]2− anion exchange process, the NO3− band is found to get merged with ATTM absorption bands, suggesting almost-complete exchange (Figure 1b). Moreover, a weak band positioned at ∼984 cm−1 is encountered in both ATTM and MoS4-LDH, corresponding to the characteristic S−O absorption band, probably resulting from the slow oxidation at

procedure involves the dissolution of 1 mmol of Ni(NO3)2·6H2O and 2 mmol of Co(NO3)2·6H2O in 100 mL of DI-water; subsequently, urea was added in a molar ratio of 1:5 of the metal ions to urea. Afterward, NaNO3 was added to the above solution in the ratio of 1:2 of the metal ions to NaNO3 and the solution mixture was transferred into a Teflon lined stainless steel autoclave. Six pieces of cleaned NFs (1 × 2 cm2) were immersed into the reaction solution and heated at 170 °C for 12 h. After the 12 h reaction, the autoclave was allowed to cool down naturally and the as-modified NFs were washed via bath sonication for 10 min using an ethanol/water mixture. Finally, the sample was dried at 60 °C for 8 h and abbreviated as NO3-LDH/NF. The mass loading of NO3-LDH-nanospikes over NF was found to be ∼1 mg cm−2 as determined by weighing the NF before and after the hydrothermal treatment. The powdered NO3-LDH was synthesized by adopting the same above-mentioned procedure without adding any NF. 2.2. Conversion of NO3− to [MoS4]2−-Intercalated NiCoLDH/NF. The NO3− to [MoS4]2− anion exchange was performed by adopting an ethanol-assisted approach to effectively minimize the carbonate contamination from the atmosphere and promote the anion exchange. Typically, all 6 pieces of the as-grown NO3-LDH/NF were immersed in the degassed ethanol/water binary solution (1:1 v/v) containing 100 mg of ATTM. After purging with N2, the vessel was tightly sealed and kept for stirring for 48 h at room temperature. Afterward, the material was washed thoroughly with ethanol/water mixture via 10 min bath sonication and dried at 60 °C for 8 h (the sample is abbreviated as MoS4-LDH/NF). Lastly, any change in the mass loading of NO3-LDH/NF after anion exchange was determined by weighing the MoS4-LDH/NF and found on an average a little increment in the mass loading from ∼1 to ∼1.2 mg cm2. The change in color of NF after NO3-LDHs growth as well as after the anion exchange process is presented via optical image in Figure S1a of Supporting Information. Materials and some supporting synthesis procedures are also given in Section S1 of Supporting Information.

3. RESULTS AND DISCUSSION One-dimensional solid nanospike networks of [MoS4]2−intercalated NiCo-LDHs were directly grown over conducting nickel foam following the two-step synthesis process (Scheme 1). First of all, the LDH formation as well as NO32− insertion was confirmed through the X-ray diffraction (XRD) analysis. Because of the strong XRD diffraction peaks from the NF substrate, the (00l) planes of LDH may get suppressed and indistinguishable. This is why the XRD pattern was first analyzed on powdered NO3-LDH, showing well-defined (003) and (006) LDH planes (Figure 1a). The center of the (003) plane illustrates a d-spacing value of 0.88 nm, confirming the C


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Figure 2. TEM images of (a) NO3-LDH and (b) MoS4-LDH nanospikes; (c) higher-magnification TEM image of NO3-LDH nanospike representing the lattice fringes where the inset shows the d-spacing profile; (d) HRTEM image of MoS4-LDH nanospike showing the lattice fringes and the inset shows the d-spacing profile; image (e) represents the SAED pattern on a single MoS4-LDH nanospike; and panels (f−j) represent the HAADF−STEM image and the EDS mappings corresponding to different elements.

the sulfur centers.41 Furthermore, the increased broadness of the −OH band after the anion exchange supports the XRD finding of the increased interslab space, incorporating excess amount of interlayer water molecules. The adopted hydrothermal growth of NO3-LDH over the Ni foam resulted into the formation of dense, vertically grown nanospike-like structures with quite uniformity in size and diameter. Such a growth pattern is first examined through fieldemission scanning electron microscopy (FE-SEM). Figure 1c displays the vertically grown NO3-LDH-nanospikes over NF which can be clearly differentiated from the bare NF (inset of Figure 1c). The higher magnification image further clarifies the vertical growth of nanospikes (Figure 1d) as compared to the smooth surface of bare NF (Figure S1d). Figure 1e shows that the anion-exchange process does not alter the morphology much, except little increased amorphousness and hence thickness of the nanospikes. Figure 1f is the highermagnification SEM image of MoS4-LDH/NF, representing the loose nanospikes-texture and open spaces between neighboring nanospikes. Such kind of designed catalyst texture can effectively facilitate the electrolyte-infiltrations for the maximum utilization of engraved active sites to boost-up the overall electrochemical performance. Figures S2 and S3 represent the SEM−energy-dispersive X-ray spectroscopy (EDS) analyses of MoS4-LDH (powdered sample) and ATTM, respectively, depicting that (1:4) ratio of Mo/S is maintained in MoS4-LDH. Moreover, Figure S2 identifies the presence of all the elements in the MoS4-LDH, having 1:2 ratio of Ni/Co, supporting the initial precursor ratio. The transmission electron microscopy (TEM) analysis relocates the one-dimensional nanospikes-like morphology in both NO3-LDH (Figure 2a) and MoS4-LDH (Figure 2b) with the diameter ranging from 50 to 70 nm. The higher magnification TEM image of NO3-LDH (Figure 2c) identifies a d-spacing of 0.88 nm. However, in case of MoS4-LDH, because of the highly amorphous and hydrated surface texture, the burning of sample from the high energy electron beam is encountered. This is why high-resolution TEM (HRTEM) analysis was employed to identify the d-spacing. It shows the expansion of the lattice fringes to ∼1.05 nm, hence supporting the XRD findings (Figure 1a). The selected area electron

diffraction (SAED) pattern of MoS4-LDH, captured on a single nanospike, exhibits a spot electron diffraction pattern, illustrating the single crystalline nature of the MoS4-LDH (Figure 2e). Figure 2f−j represents the high-angle annular dark-field (HAADF)−scanning TEM (STEM) image and the EDS mappings for all the elements. The EDS mappings depict the uniform distribution of molybdenum and sulfur along with Ni and Co all over the MoS4-LDH nanospikes, suggesting the successful anion-exchange process. To further identify the interlayer composition of all the synthesized samples as well as to support the FTIR and XRD findings for the incorporated anions, X-ray photoelectron spectroscopy (XPS) analysis is performed. First of all, adventitious carbon signals of all the controlled samples were comparatively analyzed. Figure S4a illustrates that as compared to CO3-LDH, there is no additional higher binding energy carbonate (292.3 eV) signal42 both in case of NO3-LDH and MoS 4 -LDH. This indicates the success of our anion intercalation process, which is further elaborated by analyzing XPS spectra in N 1s region (Figure S4b). The N 1s spectrum of NO3-LDH clearly identifies the characteristic NO3− peak centered at 408.2 eV as the interlayer species30 (Figure S4b). In contrast, the deconvoluted N 1s core level of MoS4-LDH shows a broad peak, resulting from the merging of Mo 3p3/2 peak and N 1s signal of NH4+ centered at 395.5 and 399.2 eV along with a small peak at ∼404.3 eV attributable to remnant nitrite9 (Figure S4b). The N 1s spectrum of ATTM also depicts a similar feature, but having two distinct peaks for Mo 3p3/2 and N 1s of NH4+ unlike merged broader peak in case of MoS4-LDH (Figure S4c). Recently, the effect of intercalated anions on the water oxidation activity is investigated by Zhou et al.29 and Hunter et al.30 via detailed electrochemical and XPS characterizations combined with density functional theory calculations. In these studies, they emphasized that interlayer anions play key roles toward tuning the electrocatalytic performance of LDH. In addition, Zeng et al. also studied the role of interlayer anion toward improving the UOR kinetics.23 In line with this, we employed a detailed XPS analysis to probe the change in the electronic structure and coordination environment of metal ions of the LDH with the [MoS4]2− introduction as well as its D


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further supporting our claim of electronic structure modification via the [MoS4]2− anions intercalation. In addition, the XPS analysis of MoS4-LDH in the S 2p region depicts broader peak in the 161−165 eV range, along with the SO42− peak (169.6 eV) arising from the moisture sensitivity of the [MoS4]2− moieties (Figure 3c).39,43 The broader peak when deconvoluted shows that, along with S2− of [MoS4]2− anions, partial oxidation at S2− centers to S− and S0 is encountered.43−45 Such kind of oxidation can be attributed to the extension of sulfur coordination environment toward the metal hydroxide layer, further supporting our claim of partial charge transfer. To analyze the stability of [MoS4]2− moieties in the LDH gallery, XPS analysis was examined in the Mo 3d region as well. Figure 3d depicts a doublet at 232.6 eV (Mo 3d3/2) and 229.1 eV (Mo 3d5/2), originating from the spin− orbit coupling of the Mo6+ 3d orbital.39 Another peak at 235.8 eV corresponds to the octahedral configuration of Mo6+ ions.46 In addition, the weak peak at 226.5 eV is suggestive to be the signature of S 2s energy of [MoS4]2− moieties.39 Hence, the XPS study in the Mo 3d core region demonstrates a typical [MoS4]2− anion feature, suggesting the protective role of LDH toward moisture-sensitive [MoS4]2− moieties. A slight shift in binding energy is consistent with the mutual interactions between metal hydroxide layers and the [MoS4]2− moieties. Thus, these XPS data support the intercalation as well as stability of [MoS4]2− anions in the NiCo-LDH interslab space along with its electronic coupling with the metal hydroxide layers probably via M(HO)2−S−Mo linkage (M = Ni, Co). The as-proven electronic coupling induces stabilization of metal hydroxide sites to a lower valence-state. Such kind of hydroxide sites have shown significant contribution to improve the HER kinetics by facilitating the rate-determining water dissociation step as justified by Liang et al.47 In addition, such kind of synergy is proven to stabilize the high-valence M states when charged,29 that is, to MOOH (M = Ni, Co), active sites of WOR/UOR. To support the XPS findings of electronic structure modification and its effect on the electrochemical performance, cyclic voltammetry analysis was performed in the

role toward the tuning of electrochemical performance. Figure 3a,b provides the comparative Ni 2p and Co 2p spectra of

Figure 3. XPS analysis: (a) and (b) are the comparative Ni 2p and Co 2p spectra, respectively; (c,d) are the S 2p and Mo 3d core level spectra of MoS4-LDH, respectively.

NO3-LDH and MoS4-LDH, respectively (powdered sample is examined to avoid any electronic contribution from NF). The lower Ni 2p binding energy shift, as well as the sharpness of the two spin−orbit doublets after the anion exchange, can be concluded as the partial charge transfer from the intercalated [MoS4]2− leading to a lower Ni valence state throughout the catalyst. The Co sites show a trend similar to the Ni ones,

Figure 4. Comparative: (a) UOR (solid lines) and WOR (dashed lines) polarization curves where the inset shows the polarization curve in the onset potential region; (b) polarization curves in 1 M KOH/0.33 M urea showing better kinetics of the main sample at higher current densities; (c) UOR polarization plots with controlled samples; (d) UOR Tafel plots; (e) UOR and OER chronopotentiometric stability test of MoS4-LDH/NF; and (f) comparative ring currents at different disk currents for MoS4-LDH in the presence of urea. E


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boosting characteristics of MoS4-LDH/NF was also analyzed by varying the urea concentration from 0.1 to 0.5 M while keeping the KOH concentration constant (1 M KOH). Figure S6b represents that there is a sufficient performance improvement from 0.10 to 0.33 M urea concentration. However, going from 0.33 to 0.50 M urea, our catalyst exhibited unexpected behavior, depicting decreased UOR performance. To find out the underlying reasons that govern the observed anomalous behavior of our catalyst with varied urea concentration, CVs are analyzed comparatively. Figure S6c shows the cathodic peak portion of the CV curves in varied KOH concentration having a constant urea concentration of 0.33 M. The comparative CV curves reveal that the cathodic redox peak potential and the current are KOH concentration-dependent. As the concentration of OH− ions increases, the cathodic redox peak potential shifts negatively, illustrating the facilitated redox transition and hence improved urea oxidation. However, in case of increasing urea concentration with constant 1 M KOH, the oxyhydroxide formation is limited by the OH − concentration, restricting the complete oxidation of excess urea. Because of the partial oxidation, the excess urea, as proposed by Daramola et al.49 and Guo et al.,16 may generate plenty of catalyst poisoning adsorbed cyanate and carbonate intermediates. This is the reason, why the UOR performance decreased when the urea concentration is increased from 0.33 to 0.5 M in 1 M KOH (Figure S6b). This anomalous behavior is also supported by comparative CV study, showing a positive cathodic peak potential shift along with decreased area under the curve as the urea concentration increased from 0.33 to 0.5 M (Figure S6d). In this way, the comparative CV analyses corroborate 0.33 M urea as the ultimate concentration to achieve maximum UOR performance. Next to the activity, mechanistic insights into the catalytic process were accessed through the Tafel analysis from the polarization curves (eq S1). Comparative Tafel analyses of the UOR process revealed remarkably faster reaction kinetics on MoS4-LDH/NF and NO3-LDH/NF as compared to the commercial catalyst (Figure 4d). One important reason of this finding is the in situ growth of active material over conductive NF substrate. This is in comparison to the ex situ coated 20% RuO2/C over NF, experiencing increased potential drop. Interestingly, MoS4-LDH/NF shows a Tafel slope value of only 29 mV dec−1 rise in current, which is substantially lower than the reported ones,21,50 further illustrating the importance of electronic property tuning strategy via [MoS4]2− anion exchange. Next fundamental performance evaluation step for practical application is the long-term catalyzing stability of a material. Figure 4e represents the comparative chronopotentiometric stability test for UOR and oxygen evolution reaction (OER) at 10 mA cm−2. In both the cases, our catalyst exhibited outstanding stability even after 24 h of the continuous catalysis. Because of a slow increment in the UOR overpotential after 12 h of analysis, the stability test was continued for 100 h to find out the robustness of the catalyst. Figure S7a represents the chronopotentiometric stability test for the UOR at 10 mA cm−2, revealing excellent long-term catalyzing capability of the designed material. The designed catalyst showed negligible performance degradation even after 100 h of continuous catalysis, placing our catalyst at the top of yet reported catalysts in terms of durability.19,21,22,48 To support such a high UOR stability of the material, rate capability of the designed catalyst is analyzed via performing

redox region. Figure S5a represents the comparative cyclic voltammogram (CV) of NO3-LDH/NF and MoS4-LDH/NF in the redox potential region. The comparative CVs exhibit a negative cathodic redox peak potential shift after [MoS4]2− anion exchange, suggesting the role of intercalated anions toward facilitating the active oxyhydroxide species formation. In addition, the increased area under the cathodic curve is directly associated with the increased interlayer space. It facilitates excess electrolyte infiltrations and hence involvement of more number of active M sites. Altogether, the detailed XPS and CV analyses rationalize that the designed catalyst offers a valuable strategy to efficiently catalyzing the electrochemical reactions involved in the energy-efficient hydrogen generation. To test its proficiency, the designed catalyst is first examined toward half-cell reactions, that is, UOR, WOR, and HER. Afterward, the whole-cell water, urea, and urine electrolysis was performed by fabricating the two-electrode device, as described in the upcoming sections. 3.1. Electrocatalytic Performance toward the UOR and WOR. We first evaluated the electrocatalytic UOR and WOR performance in 1 M KOH with and without 0.33 M urea, respectively. Figure 4a represents the comparative UOR (solid line curves) and WOR (dashed line curves) linear sweep voltammograms (LSVs) for NO3-LDH/NF and MoS4-LDH/ NF. The comparative WOR polarization curves show that the anion exchange process induces negative anodic redox (Ni(OH)2 → NiOOH) peak shift from 1.45 to 1.42 V versus RHE. It facilitates the Ni(OH)2 → NiOOH transition, leading to improved WOR performance. Of note, the Ni(OH)2 → NiOOH oxidation onset potential and the UOR onset potential overlap (inset of Figure 4a). This observation emphasizes that UOR onset and the Ni2+ to Ni3+ redox transition onset share the same potential. This gives a strong evidence of the Ni3+ to be the active centers for urea oxidation. After the anion exchange, the UOR is found to improve by 20 mV and required only ∼1.34 V versus RHE to reach the benchmark 10 mA cm−2. The activity tuning of [MoS4]2− intercalation is found to be more pronounced at higher current densities. Therefore, MoS4-LDH/NF required only 1.41 and 1.48 V versus RHE to reach 100 and 200 mA cm−2, respectively, as compared to 1.48 and 1.56 V versus RHE required for NO3-LDH/NF to reach the same current density (Figure 4b). It is important to note that NO3-LDH/NF itself showed comparative performance to the best yet reported literature on UOR.19,21,22,48 Also, our unique anion-exchange strategy further surpasses the UOR performance of most of the noble and non-noble metal-based catalysts (as listed in Table S1, Supporting Information). For comparison, UOR is also catalyzed using bare NF, ATTM, and commercial 20% RuO2/ C-NF (1.0 mg cm−2). The commercial catalyst showed far inferior activity, displaying 100 mV positive potential shift (1.458 V) from MoS4-LDH/NF to reach 30 mA cm−2 (Figure 4c). ATTM is found to get leached out in KOH solution and showed negligible UOR current as depicted in Figure S5b. We next turn to interrogate the outstanding urea oxidizing property of MoS4-LDH/NF by varying the KOH concentration from 0.5 to 5 M KOH with a constant urea concentration of 0.33 M. Figure S6a represents the comparative UOR LSV plots with varied KOH concentration, illustrating direct dependence of UOR on the OH − concentration. In fact, with 5 M KOH, our catalyst showed surprisingly enhanced performance and required only 1.287 V versus RHE to generate benchmark 10 mA cm−2. The UOR F


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ACS Applied Materials & Interfaces scan rate-dependent CV in the range of 30−110 mV s−1 in 1 M KOH/0.33 M KOH (Figure S7b). The scan rate independency of the UOR current and potential even at higher values (i.e., 90 and 110 mV s−1) revealed the good rate capability of the designed catalyst. This is attributable to the excellent electronic conductivity of the catalyst substrate (NF) along with open nanospike-like morphology. Certainly, sufficient inter-hydroxide layer space also facilitated active center exposure, faster reactant, and product transport as well as electrolyte infiltrations. In addition, the plot of cathodic redox peak current density (icp) variation with the square root of scan rate (ν1/2) represented a linear dependency (Figure S7c). It suggests the in situ (oxy)hydroxide formation to be a reversible diffusion controlled process, imparting high stability to the catalyst.14 Moreover, the continuous increment in the cathodic peak current as a function of scan rate, with no apparent change in UOR current and potential, indicates that the rate of electrooxidation of Ni species is faster than the urea electrooxidation. This finding supports the reaction between NiOOH and urea absorbed on the surface to be the ratelimiting step.14 Next to the activity and stability demonstrations, another important parameter to evaluate the proficiency of a catalyst is its Faraday efficiency.51 In fact, high Faradaic efficiency toward urea oxidation is desired, because the catalytic active intermediates, for example, active oxygen species, during WOR may bring deleterious side reactions.23 Hence, we have carried out the rotating ring disk electrode (RRDE) analysis to determine the Faraday efficiency for UOR-competing WOR. By comparatively analyzing the Faraday efficiency for WOR in the presence and absence of urea, the UOR selectivity of the electrocatalyst can be determined.23 While performing the RRDE measurement, the disk electrode was subjected to 30 s current steps varying from 0 to 10 mA cm−2 and the ring electrode was kept at a constant potential of 0.5 V versus RHE to detect the product O2. The O2 molecules evolved by water oxidation at the disk electrode will be reduced by 2e− on the surrounding ring electrode and the corresponding ring-disk current signatures will be used to measure the Faradaic efficiency using eq S3.6 Figures 4f and S8a depict the plots of the ring current against time in the presence and absence of 0.33 M urea, respectively. At zero disk current, both plots represent near zero ring current, because there is no product formation. However, in the absence of urea, at 1 and 5 mA cm−2 applied disk current, our catalyst showed 95.86 and 49.89% Faraday efficiency toward WOR, respectively (Figure S8a). Interestingly, after urea addition, in all the disk current range, that is, 0−10 mA cm−2, negligible ring current is detected (Figure 4f). Following this, the Faraday efficiency was calculated at higher current densities, that is, 5 and 10 mA cm−2, illustrating only 4.89 and 2.65% WOR contributions, respectively. Hence, our comparative RRDE analysis for WOR concludes 95.11 and 97.37% Faraday efficiency for UOR at 5 and 10 mA cm−2, respectively, showing no apparent WOR even at higher current densities. 3.2. Electrocatalytic Performance toward the HER. To overall improve the energy efficiency of the hydrogen generation process, it is also essential to boost up the other half-cell reaction of electrolyzer, that is, HER, by employing a suitable catalyst. Here, the meaning of a suitable catalyst is not only limited to populate active centers but also to protect the active centers as well as to save the H+ reducing electrons from

the parallel competing oxygen reduction reaction.9 Intriguingly, our final catalyst, that is, MoS4-LDH/NF, is designed in such a way that it contains the HER-selective [MoS4]2− anions to improve the energy-efficiency of the H2 generation process even in urine and other toxic environments.9 Figure 5a

Figure 5. Comparative (a) HER polarization curves, (b) HER Tafel plots, and (c) HER polarization curves in the presence and absence of 0.33 M urea. (d) HER chronopotentiometric stability test of MoS4LDH/NF where the inset shows the LSV curves before and after the stability test.

represents the comparative HER LSV curves of our designed materials with commercial 20% Pt−C/NF (1.0 mg cm−2) in 1 M KOH. The comparative plots reveal that MoS4-LDH/NF and NO3-LDH/NF exhibit a mere 91 and 140 mV negative potential shifts, respectively, with respect to 20% Pt−C/NF to reach benchmark 10 mA cm−2. This potential gap further reduces at higher current densities (i.e., above 20 mA cm−2), illustrating the electronic structure tuning of the designed catalyst. The electronic structure tuning was further examined by employing Tafel analysis. Also, MoS4-LDH/NF showed a Tafel slope value of 125 mV dec−1 as compared to 115 mV dec−1 for 20% Pt/C (Figure 5b). Of note, a slope value of >120 mV dec−1 is generally associated with a ligand-based mechanism rather than metal hydride-based mechanism to catalyze HER.9,52,53 Also, the importance of a ligand-based mechanism is to make the catalyst effective both in a toxic environment as well as in the presence of foreign species, for example, urea. The reason why MoS4-LDH/NF follows a ligand-based mechanism is the strong H+ accepting power of [MoS4]2− via thio groups, providing reasonable activity as well as stability even in the presence of catalyst poisons. In support of this claim, HER is performed with 0.33 M urea/1 M KOH. As shown in Figure 5c, MoS4-LDH/NF does not show any negative impact of urea on HER, further guaranteeing its attracting H2 generating property in the urea electrolyzer. Next, to support catalyst robustness, long-term catalytic stability of MoS4-LDH/NF is performed through time-dependent potential curve, depicting a steady-state performance even after 24 h continuous catalysis (Figure 5d). To further support the robustness of the designed catalyst, post-analysis characterizations are performed through XRD G


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Figure 6. Whole-cell electrolysis: (a) schematic representation of the benefits of urea electrolysis over water electrolysis in alkaline medium, (b) comparative polarization curves of urea electrolysis with various electrodes employed in this study; (c) comparative LSV curves for water, urea, and urine electrolysis of MoS4-LDH/NF; chronoamperometric stability test recorded for (d) water electrolysis, (e) urea electrolysis, and (f) urine electrolysis. In all the urea electrolysis cases, 0.33 M urea was added. Urine electrolysis was performed in 1 M KOH.

Now, based on all the above-mentioned half-cell electrochemical studies as well as in accordance with the reported reaction pathways, a plausible reaction mechanism is proposed. As we know, among the two-half cell reaction, cathodic HER is found to follow the ligand-based mechanism. It starts via the reduction of [MoS4]2− anions in the aqueous environment to [MoS3SH]2−, because of the strong H+ accepting tendency of its thio groups.9 It is then followed by the stabilization of the other water dissociation product, that is, OH− over OHacceptor Ni(OH)2.34 Subsequently, by evolving H2, the [MoS3SH]2− regenerates [MoS4]2− and Ni(OH)2 gets oxidize to NiOOH.15 The NiOOH moieties, by catalyzing the other half-cell, that is, WOR/UOR, undergo cathodic reduction back to Ni(OH)2.15 This is the possible pathway through which all the H2-electrolyzer reactions get boosted up synergistically via the catalytically tuned MoS4-LDH/NF. The as-hypothesized catalytic mechanism as well as catalyst regeneration is illustrated as follows: Cathodic reaction:

and XPS. Figure S8b represents the comparative XRD pattern after UOR and HER stability test in the presence of 1 M KOH/0.33 M Urea. The UOR-catalyzed electrode does not show any clear peak, probably because of the coverage of amorphous reaction intermediates. However, the HERcatalyzed electrode revealed prominent (006) diffraction plane of [MoS4]2−-intercalated LDH. Thus, the XRD analysis suggests stronger interlayer stability of [MoS4]2−, even during the carbonate-generating urea electrolysis. Next, to support the XRD findings of the interlayer [MoS4]2− stability, XPS analyses are performed in C 1s, O 1s, Mo 3d, and Ni 2p core level regions of post-UOR and post-HER stability samples. The comparative C 1s core level examination of fresh MoS4-LDH with the post-HER sample exhibited only adventitious carbon features, ruling out any interlayer carbonate exchange (Figure S9a). Nevertheless, weak carbonate and bicarbonate peaks at 293.1 and 295.7 eV, respectively, can be observed in the postUOR sample. However, the C 1s spectrum of post-UOR sample does not match well with that of CO3-LDH (Figure S9b). It strongly corroborates that the observed carbonate feature is certainly originating from the surface-adsorbed urea degradation products. Comparative O 1s spectra of post-analyzed samples are also in accordance with that of C 1s findings, further revealing the interlayer [MoS4]2− stability (Figure S9c,d). To support the asclaimed UOR-generated carbonate adsorption over LDH, instead of its intercalation, XPS analysis is extended in Mo 3d and Ni 2p core regions. Figure S10a,b comprises the deconvoluted Mo 3d XPS spectra of the post-UOR and post-HER samples, respectively. Because of the increased amorphousness, both the spectra lack a discrete [MoS4]2− peak pattern; however, the deconvoluted spectra identify the intact [MoS4]2− phase. In fact, the prominent S 2s peak suggests that the major bonding interaction is still Mo−S not the Mo−O. In addition, comparative Ni 2p spectra of as-synthesized MoS4LDH/NF with post-UOR sample illustrate no apparent change in the binding energy position of Ni 2p core level before and after the UOR stability test, supporting the intact interlayer chemistry. The above XPS findings, hence, suggest the stronger interaction of [MoS4]2− with the hydroxide layer and hence the robustness of the catalyst even in harsh carbonate conditions.

Volmer step: [MoS4 ]2 − + Ni(OH)2 + H 2O + e− → [MoS3SH]2 − + Ni(OH)2 −OH−ad

(4)

Heyrovsky step: [MoS3SH]2 − + H 2O + e− → [MoS4 ]2 − + H 2 + OH−

(5)

Anodic reaction: Ni(OH)2 −OH−ad → NiOOH + H 2O + e−

(6)

UOR: 6NiOOH + CO(NH 2)2 + H 2O → 6Ni(OH)2 + CO2 + N2

(7)

The above-mentioned reaction pathways can also be supported experimentally by examining the post-analyzed samples in Ni 2p region via XPS. The Ni 2p core level spectrum of post-HER sample displays the emergence of an additional higher binding energy peak at 860.6 eV, corresponding to the Ni3+ species6 (Figure S10c). This additional peak emergence supports our as-proposed HER H


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ACS Applied Materials & Interfaces mechanism, where the catalyst undergoes Ni(OH)2 → NiOOH transformation. In contrast, the Ni 2p core level of post-UOR sample shows a binding energy similar to that of fresh MoS4-LDH, suggesting the back NiOOH → Ni(OH)2 transition during UOR (Figure S10c). In this way, the Ni 2p core level XPS analysis of post-HER and post-UOR samples revealed quite good reversibility and hence robustness of the catalyst system. Thus, the postmortem characterization data further highlight the potential of designed catalyst to place it among one of the robust catalysts for its direct implementation to sewage water utilization. 3.3. Whole-Cell Alkaline Urea Electrolysis. Taking the above mentioned superiorities into account, here, we have employed MoS4-LDH/NF both as cathode and anode to construct a two-electrode electrolyzer. Figure 6a illustrates the importance of urea electrolysis over water electrolysis schematically, which is proven experimentally in our work. Figure 6b represents the comparative LSV curves of whole-cell urea electrolysis tested in 1 M KOH/0.33 M KOH. As expected, the MoS4-LDH/NF-based couple (MoS4-LDH/ NF∥MoS4-LDH/NF) outperformed the-state-of-art Pt−C/ NF∥RuO2−C/NF electrode pairs with a cell voltage of only ∼1.37 V as compared to 1.485 V required for the commercial catalyst pair to deliver benchmark 10 mA cm−2. (NO3-LDH/ NF∥NO3-LDH/NF) and (bare NF∥bare NF)-based electrode pairs were also tested for urea electrolysis, requiring 1.52 and 1.73 V to deliver 10 mA cm−2. These comparative results again suggest the remarkable role of [MoS4]2− intercalation into NiCo-LDH interslab, leading to superb performance. Interestingly, when we switched from synthetic urea to direct human urine, our catalyst did not show much change in the performance and required ∼1.44 V cell voltage to deliver 10 mA cm−2 (Figure 6c). At higher current densities, our catalyst delivered a performance similar to that of synthetic urea, supporting the suitability of our designed strategy for direct urine-based urea utilization toward useful fuel. The proficiency of the designed catalyst is further proven through the long-term catalyzing capability with and without urea. Figure 6d,e represents the chronoamperometric durability test for MoS4-LDH/NF without and with 0.33 M urea. In both the cases, the catalyst maintained the current density above 10 mA cm−2 even after 24 h of continuous catalytic process with no apparent performance degradation. To experimentally verify the catalyst robustness, we have examined the used-electrolyte after the urea electrolysis stability test to detect any catalyst component leaching through SEM−EDS. Figure S11 represents negligible metal leaching even after 24 h continuous stability, further illustrating the outstanding robustness of the designed catalyst. To prove the viability of the designed catalyst in sewage water, the long-term catalytic efficiency was analyzed with the direct-urine electrolysis. Figure 6f depicts that while delivering a-steady-state current density of 10 mA cm−2 for continuous 24 h of operation, no obvious degradation and catalyst deactivation is encountered from the proteins and other catalyst poisons present. Most importantly, during all the mentioned durability tests, even after the consumption of urea in between the operation, no performance degradation is encountered, offering industrial interest in our catalyst. In this way, our designed catalyst gives a great promise of its implementation for the direct-sewage water utilization toward sustainable hydrogen generation and simultaneous sewage denitrification.

4. CONCLUSIONS To sum up, here, we have successfully grown the [MoS4]2− anion-intercalated NiCo-LDH on the nickel foam via an energy-saving two-step process. XPS and electrochemical analyses revealed the potential role of [MoS4]2− anion intercalation toward tuning the electronic structure of LDH and hence the electrochemical performance. The electrocatalytic activity of the designed heterostructures is attributed to several superiorities including: (i) the synergistic effect between the intercalated [MoS4]2− and the positively charged NiCo-hydroxide layers reducing the internal potential drop, (ii) the abundant, uniformly distributed and exposed active sites of LDH, (iii) the plentiful hydrated LDH channels for the reaction species transportation, and (iv) 3D nanospike texture over the nickel foam further facilitating the electrolyte infiltrations and product gas transport. Here, we discovered that the designed catalyst works equally efficiently with the real urine and delivered the constant performance for more than 24 h with no apparent deactivation. Hence, our designed catalyst illustrates the feasibility of the as-claimed inspiration for anodic water oxidation substitution as well as waste water management. Lastly, the above-mentioned superiorities of our designed strategy place the obtained catalyst at the top of yet reported works toward its direct industrial implementation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06545.

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Practical demonstration of H2 generation from direct urine electrolysis employing the MoS4-LDH/NF-based electrolyzer via LSV study in the potential window of 1.1−1.7 V (AVI) Materials and synthesis procedures, structural characterization techniques, electrochemical procedures employed, calculation methods, additional material characterizations, electrochemical studies, proposed reaction pathways, and comparison tables (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 20-25902566. Fax: +91 20-25902636. ORCID

Sreekumar Kurungot: 0000-0001-5446-7923 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was conducted within the framework of the project no. TLP003526. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.N. and V.K. acknowledge UGC, New Delhi, India, for the NET-SRF. A.N. acknowledges Sachin Kumar for his help during scheme designing. S.K. acknowledges CSIR, New Delhi, for the project funding (TLP003526). I


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