Enabling Unprecedented Ultra‐Efficient Practical Direct Seawater Splitting by Finely‐Tuned Catalyst Environment via Thermo‐Hydrodynamic Modulation

Direct sea water splitting as asource of clean renewable energy is indeed a holy grail and necessitates the invention of unprecedented avenues. Toward this goal, for the first time, the effect of thermo‐hydrodynamic processes modulation (electrolyte flow and heating) on water splitting reactions, through the controlling of the nanocatalyst surface environment, is studied thoroughly. A catenated sulphur type‐nickel polysulphide‐based single crystalline, high surface area 3D electrocatalyst (NiS2pSxsurface), with surface‐enriched oxygen evolution reaction (OER, Ni3+) and hydrogen evolution reaction (HER, pSn2−) catalyzing species, is prepared by a single‐step process. Thermo‐hydrodynamic processes‐induced electrochemical analysis demonstrates a dramatic improvement in the electrocatalytic performance of the catalyst, by both flow and temperature modulation. Decoupling contributions from the electrolyte and electrodes heating demonstrate an intrinsic electrode property influence on the overall temperature‐dependent electrochemical performance. Furthermore, a chlorine‐phobic behavior of the NiS2pSxsurface catalyst is observed, even at 80 °C, for direct seawater oxidation, confirming the electrocatalyst potential for direct seawater splitting. Notably, a cell voltage of 1.39 V (at 10 mA cm−2), reaching industrially practical large‐scale of >500 mA cm−2 is observed for additive‐free direct seawater splitting, which is the lowest reported cell voltage to date, even for alkaline additive‐based electrolysers. Consequently, an alternative approach for direct seawater splitting is realized and can be universally extended to any present‐day electrocatalyst platform.


Introduction
Sustainable development of mankind is implausible without next-generation alternative renewable sources of energy production.3] Electrocatalytic splitting of water has emerged as one of the major sources of hydrogen and can produce pure hydrogen and oxygen gas at the cathode and anode electrolyzer's components, respectively. [4,5]Considering the limited supply of pure water and its possible scarcity as feedstock for water splitting in the long run, direct seawater electrolysis is one of the most appreciated alternative for the uninterrupted supply of next generation fuels, i.e., hydrogen.However, the practical electrocatalytic direct seawater splitting process faces intrinsically severe issues of extremely high overpotentials for hydrogen as well as oxygen production.[8][9] Hence, as an alternative to direct seawater splitting, and considering the extremely sluggish four-electron oxygen evolution reaction in general, various reports on alkaline seawater splitting were recently published. [10,11]However, the requirement of highly corrosive alkaline electrolytic conditions, along with the use of expensive separators, limit the economic scalability of the seawater splitting electrolyzer, while also imposing various arduous maintenance and operating-life challenges. [12][23][24][25][26][27] However and unfortunately, neutral water splitting still necessitates the use of buffered electrolytic conditions, even for very moderate impractical electrocatalytic performances. [28]These buffered electrolytic conditions are required as the electrocatalytic HER and OER reactions under neutral conditions lead to a concomitant rapid and dramatic local increase of basicity and acidity at the HER and OER electrodes vicinity, respectively, which are known to be detrimental for corresponding gas-forming reactions, and thus for the observed overall water splitting. [28,29]Buffered seawater splitting additionally requires major maintenance issues, related to precipitation, catalyst selectivity, and stability, and still the electrocatalytic performance is far from practicality.
Direct seawater splitting also suffers a major challenge of concomitant toxic chlorine species evolution at the anode, because of the similar overpotential for oxygen and chlorine oxidation reactions. [30]In this context, different ingenious approaches were recently explored in the literature for the selective oxygen evolution in chloride-containing concentrated solutions.A common approach involves the use of highly alkaline electrolytes (pH:14) for decreasing the oxygen evolution overpotential, thus selectively evolving oxygen. [31]A similar approach involves the use of chemical additives that exhibit oxidation potentials much lower than oxygen and chloride oxidation, thus preventing chlorine evolution.Hydrazine, [32] alcohols. [33,34]and urea [35] were majorly explored recently for inhibiting the evolution of toxic chlorine species in the chloride-containing concentrated electrolytes.Apart from this, the use of chlorine-resistant over-layers on the active electrocatalyst has been also explored for the selective oxygen evolution.For example, Vos et al. explored MnO x as chlorineresistant overlayer on IrO x catalyst, reporting a 93% oxygen evolution yield. [36]In a recent study, Guo et al. explored Lewisacid modified electrodes (CrO 3 -CoO x ) for generating surface local alkalinity to suppress chlorine evolution, reporting a remarkable 98% selectivity toward oxygen evolution in direct seawater electrolysis. [37]In this benchmarking report, although great electrocatalytic performance was achieved, along with low cell potential and practically high current densities, the drawbacks of the electrodes material (4 mg cm −2 ) demand the use of chemical binders and the requirement of a specialized Nafion membrane as separator, challenges the overall scalability of the electrolyzer.In a recent study, an intrinsic chlorine-phobic behavior is reported using a catenated sulphur-based nickel polysulphide catalyst, demonstrating an unprecedented 100% oxygen evolution selectivity for direct seawater splitting. [38] thorough analysis of the state-of-the-art literature clearly indicates a current huge gap in this research field, where a universal approach toward the highly desired additive-free direct seawater splitting for pure hydrogen and oxygen evolution is still unachieved.Thus, in this work, a universally extendable approach to tackle all the current practical challenges of direct seawater splitting is addressed by optimizing the thermo-hydro dynamic conditions (mainly flow and temperature) in an electrocatalyst surface environment, leading to the highly efficient practical additive-free direct seawater splitting.Electrolyte flow can act as a dynamically-buffering system that can inhibit the formation of strong acidic or basic environments at the anode and cathode surface, respectively, by providing a fresh renewing electrolyte environment at a rate higher than the H 2 O consumption rate, at each electrode.Furthermore, the flowing electrolyte can mechanically remove the reaction-formed bubbles, or inhibit their formation completely, which is a well-known source of catalytic active surface area loss and electrical resistance dur-ing electrolysis. [39,40]Thus, a thermo-hydrodynamic approach is successfully explored herein, in synergy with an intrinsicallyselective OER electrocatalyst, NiS 2 pS x surface , for the efficient largescale practical additive-free direct seawater splitting, exhibiting the lowest cell-voltage ever reported in the literature, as well as displaying high current densities.Overall, a simple and economically scalable, maintenance-free, uninterrupted hydrogen energy source is here realized using naturally occurring direct seawater as abundant electrolyte based on this novel thermohydrodynamic water splitting approach.Primarily this work focuses on a novel avenue to direct sea water splitting field by demonstrating an enhancement in the overall performance by tuning the thermo-hydrodynamic processes in the electrolysis system.It is important to mention that the tuning of the thermohydrodynamic processes can be universally applied to any electrocatalyst for enhanced performances.In this work, NiS 2 pS x surface , Ni and Pt||IrO 2 are explored as few examples of different electrocatalyst for enhancing the overall cell performance.

Results and Discussion
Efficient water splitting for pure hydrogen production has always been realized by the use of OER or HER favorable basic or acid electrolytic conditions, respectively, for practically considerable performances.However, these electrolytic conditions (highly acidic or basic), require the use of corrosion-resistant catalysts and electrochemical setups for extremely harsh electrolytic conditions.Electrochemical splitting of neutral water is however highly sluggish due to the rapid detrimental pH changes around the electrodes during electrolysis (decrease in pH around anode and increase in pH around cathode) (Figure 1a).Thus, buffering electrolytic conditions are still required to resist any pH changes at the vicinity of the electrodes, which otherwise is considered practically impossible for large-scale electrocatalytic water splitting.The use of buffers, however, requires various cost-related and maintenance issues, thus causing scale-up limitations in the practical deployment of electrolysis systems.In this work, the use of moderately high temperatures and flow electrolytic conditions are realized for the first time to completely circumvent the use of additive-based corrosive or buffered electrolyte environments for the practically scalable, almost maintenance-free water splitting in natural seawater.The choice of the electrolysis prototype set-up applied in this work is based on the hypothesis that a flow-system can continuously supply the catalytic electrodes with a fresh electrolyte environment, thus providing the required buffering action at the catalytic electrode intimate environment, while the moderately high applied temperatures can increase the ionic conductivity and reaction kinetics of the electrolysis system (Figure 1b).Flow systems possess an added advantage of continuous removal, or complete inhibition, of gas bubbles formation, which is well-known to minimize the catalytic active surface area and induce nonconducting barriers (gas bubbles) between the electrolyte and the electrode surface.Furthermore, using the abundantly available seawater resource as an electrolyte can further reduce the system maintenance by the continuous supply of this naturally occurring ionically-conducting electrolyte.Thus, the proposed flowand temperature-based electrochemical system can potentially revolutionize the hydrogen production industry by providing a (i) non-corrosive, (ii) bubble-free, (iii) intrinsic mechanicallybuffered, (iv) additive-free and, (v) ionically-conducting electrochemical setup for the realization of ultrahighly-efficient direct seawater splitting (Figure 2a).
The choice of the electrocatalyst was based on previous literature reports, where earth-abundant nickel-based materials were successfully demonstrated as effective bifunctional catalysts for overall water splitting. [21,41]Thus, Ni foil substrates were transformed into electrocatalytic, binder-free electrodes via a welloptimized single-step transformation process, as detailed in the experimental section.The NiS 2 pS x surface catalyst is prepared by the direct in situ transformation of Ni foil under elemental sulfur environments in the ratio (2:1).A 1.5 times increase in the overall weight is observed upon conversion of Ni foil to NiS 2 pS x surface .Xray diffraction analysis reveals the successful transformation of the Ni foil into the catalytic NiS 2 substrate (Figure 2b).The presence of NiS traces in the XRD pattern can be explained due to the data acquisition from bulk (as XRD is not a surface-sensitive analysis technique that also includes a signal from the bulk) of the electrodes, where limited diffusion of sulfur leads to the formation of NiS in the bulk (cross-sectional EDX analysis is shown in Figure S2a, Supporting Information).The crosssectional SEM analysis shows the full transformation of the Ni foil to sulphide phase which can overall act as the electrochemical active catalyst, however, the electrocatalysis is only limited to the immediate surface of the catalyst.The elemental composition at the cross-section of the electrocatalyst shows an enrichment of sulfur at the surface in comparison to Ni, and the sulfur content decreases at a few microns from the surface with negligible sulfur content at the center of the electrocatalyst (Figure S2a, Supporting Information).Further, the porosity and cracks at the middle of the cross-section are because, the sulfur diffuses from the surfaces (top and bottom) and reacts to Ni from both sides, causing a volume change upon conversion to sulphide phase which ends at the middle of NiS 2 pS x (surface) cross-section.Scanning electron microscopy images show the formation of a highly porous 3D morphology of the transformed electrodes, which can act as high surface area electrodes for electrocatalysis applications (Figure 2c).Electron dispersive X-ray spectroscopy analysis displays a uniform distribution of Ni and S elements at the catalyst surface (EDX mapping shown in Figure 2d, EDX spectrum shown in Figure S2b, Supporting Information), with an S: Ni ratio of 2.3, indicating toward the enrichment of sulfur at the electrode surface.The TEM images of NiS 2 pS x surface at different magnifications are shown in Figure S3 (Supporting Information).In conjugation with the SEM analysis, a porous morphology was observed in the low-resolution TEM images (Figure S3a,b, Supporting Information).Crystallites of different shapes with sizes majorly in the range ≈50-60 nm were observed (Figure S3c,d, Supporting Information).The crystallite size observed in the TEM analysis matches well with that calculated using Scherrer equation from XRD peaks (≈51 nm).High-resolution TEM images display well-defined crystals with easily observable lattice fringes (Figure S3e,f, Supporting Information).A highly crystalline crystal lattice of NiS 2 was observed, with the lattice fringes having the crystal planes corresponding to 200 and 111 planes (Figure 2e).The SAED pattern (inset, Figure 2e) along the zone axis [110] reveals single crystalline nature, with the pattern matching well with that of single crystalline NiS 2 (ICSD: 646 342).Eventually, X-ray photoelectron spectroscopy analysis was performed to analyze the exact chemical state of the moieties present at the electrode surface (Figure 2f,g).XPS analysis reveals an enrichment of S in comparison to Ni, with a S:Ni ratio of 3.8.The highresolution Ni 2p XPS spectrum of nickel sulphide reveals typical Ni 2p 3/2 and Ni 2p 1/2 peaks along with their corresponding satellite peaks (Figure 2f).As a result of the non-local screening of core holes, Ni 2p 3/2 and Ni 2p 1/2 are individually deconvoluted to two peaks. [42,43]Based on the area-under-the-curve calculation of Ni 2p 3/2 and satellite 1 , a relative ratio of Ni 3+ : Ni 2+ is calculated.It is observed that the prepared nickel sulphide catalyst has a high Ni 3+ : Ni 2+ ratio of 1.33, indicating the presence of the OER active Ni 3+ centers at the surface. [44,45]Recently, it is suggested via in situ characterization techniques that Ni 4+ is the active OER site that forms during electrolysis under voltage bias via oxidation of lower oxidation state Ni moieties. [46,47] higher Ni 3+ : Ni 2+ can favor the formation of active Ni 4+ during electrolysis as Ni 3+ would require less activation energy to form Ni 4+ in comparison to that required for Ni 2+ conversion.Thus, a high Ni 3+ : Ni 2+ for NiS 2 pS x surface catalyst prepared in this work can be directly related to enhanced OER performance.Furthermore, the high-resolution S 2p spectrum displays the signature peaks of terminal sulphides S 2− , usually observed in metal sulphide species (Figure 2g).A significant amount of bridged polysulphide species (pS n 2− ) [48] are also observed, which are wellestablished as catalytically active sites for HER, as they facilitate easy adsorption and desorption of H* species during HER (adsorption energy (H*) for pS n 2-is closer to zero). [49,50]pS n 2-species at the catalyst surface can also be associated with the enriched percentage of sulfur observed in the elemental composition analysis.Thus, NiS 2 pS x surface is characterized as a single-crystalline 3D binder-free catalyst, with OER and HER catalyzing species at the surface, and can act as an exceptional electrocatalyst for overall water splitting.Since the preparation method is a facile single-step heating and does not require any complicated synthesis route, it was easily scalable to form larger electrodes.As an example, Ni foil of ≈35 × 1.5 cm 2 was successfully transformed to NiS 2 pS x surface (Figure S4, Supporting Information) justifying the easy scalability of the synthesis process.The NiS 2 pS x surface catalyst with an electrochemically-active surface area (ECSA) of ≈178 cm 2 displays exceptional electrochemical performance under neutral conditions (Figures S5-S7, Supporting Information).NiS 2 pS x surface demonstrates OER and HER overpotentials of 170 mV (Tafel slope: 209.5 mV dec −1 ) and 250 mV (Tafel slope: 97.4 mV dec −1 ) at 10 mA cm −2 respectively, and a high turnover frequency (TOF) of 73.3 sec −1 (OER) and 62.2 sec −1 (HER) at an overpotential of 300 mV (Figures S5 and S6, Supporting Information).NiS 2 pS x surface demonstrates remarkable overpotentials in 0.5 m PBS buffer, however the Tafel slope values are higher than that reported for standard catalyst in the literature. [51,52]e would like to point out that the Tafel plots in this work are reported without any iR corrections in contrary to those reported for NiBi or CoPi in the literature.For a better comparison, the iR corrected Tafel slopes for OER and HER were analyzed (Figures S5c and S6c, Supporting Information).As expected, the iR corrected Tafel slopes for both OER (74.19 mV dec −1 ) and HER (77.22 mV dec −1 ) display comparable values as that reported for standard catalyst in the literature.As a result, a NiS 2 pS x surface || NiS 2 pS x surface -based electrolysis full cell exhibits a low cell voltage of 1.59 at 10 mA cm −2 (Figure S7, Supporting Information).The Faradic efficiency of the NiS 2 pS x surface as a bifunctional catalyst was calculated by the gas displacement technique using a H-cell in a homemade airtight setup [53,54] using 0.5 m PBS buffer at a fixed current density of 10 mA cm −2 (electrochemical data shown in Figure S8, Supporting Information).The theoretical values were calculated assuming 100% current efficiency according to Faradays law which states that 96 845.3 C of charge leads to one equivalent of reaction. [26]The NiS 2 pS x surface surface, as bifunctional electrocatalyst, exhibits a faradic efficiency of >99%, with a hydrogen to oxygen evolution ratio of 2:1 (Figure S9, Supporting Information).Thus, an efficient bifunctional electrocatalyst is developed exhibiting a unique combination of high surface area, 3D nanostructural morphology along with electrocatalytic HER and OER centers at the surface, here investigated further for the thermo-hydrodynamic influence on its direct seawater splitting performance.
Water splitting under neutral electrolytic conditions is one of the key pre-requisites for achieving overall water splitting in natural seawater conditions, which still is extremely  Cell Voltage (V) sluggish.In this regard, different precious and non-precious catalysts have been explored exhaustively in the literature, however, the present neutral water splitting efficiency is far from realworld practicality.Hence, a novel yet unexplored alternative approach based on temperature-and flow-enabled enhancement of the catalyst electrolytic performance is herein explored, using the experimental setup shown in Figure 3a (digital photograph of the setup is shown in Figure S1, Supporting Information).The water splitting performance of NiS 2 pS x surface ||NiS 2 pS x surface cell in un-buffered neutral conditions (0.25 m Na 2 SO 4 ) was tested at different temperatures under static and flow conditions (6 L min −1 ).As a control experiment, electrolyzers composed of Ni||Ni and the commercially available benchmarking electrocatalyst Pt||IrO 2 were also tested.The voltage polarization curves for Ni||Ni demonstrate poor electrocatalytic performance with a huge cell voltage of 2.76 (5 mA cm −2 ) at 25 °C (Figure 3b).A systematic increase in the current density, and a corresponding decrease in the overpotential is observed upon increasing the temperature in the range of 25-80 °C, still displaying a huge cell voltage of 2.58 (@ 5 mA cm −2 ) at 80 °C.A decrease in the cell voltage is expected due to an increase in the ionic conductivity of the electrolyte and a decrease in the reaction activation energy with increasing temperatures.Surprisingly, an extra enhancement in the electrochemical performance of Ni||Ni is observed under flowing electrolyte conditions, flow rate of 6 L min −1 exhibiting cell voltages of 2.75 V and 2.38 V at 25 °C and 80 °C, respectively (Figure 3c).This enhancement can be related to the flow buffering action, due to the continuous availability of fresh electrolyte and bubble removal and formation inhibition under flowing electrolytic conditions.As expected Pt||IrO 2 exhibits enhanced performance in comparison to the Ni||Ni cell in terms of reduced overpotentials and higher current densities.Pt||IrO 2 demonstrates a cell voltage of 2.4 mV (@ 5 mA cm −2 ) at 25 °C, under static conditions, which decreases to 2.22 at 80 °C, demonstrating the influence of temperature as observed for the Ni||Ni electrolyzer (Figure 3d).It is to be noted that the electrochemical performance for Pt||IrO 2 could not be performed under flow conditions due to the non-binder free nature of the Pt and IrO 2 electrodes (described in the experimental section) which leads to peeling-off of the active material under flow conditions.Finally, the NiS 2 pS x surface ||NiS 2 pS x surface cell was tested under different temperatures under static and flow conditions.Interestingly, voltage polarization curves of the NiS 2 pS x surface ||NiS 2 pS x surface cell demonstrate an exceptionally enhanced performance in comparison to Ni||Ni and even Pt||IrO 2 under all the tested conditions.Specifically, NiS 2 pS x surface ||NiS 2 pS x surface exhibits a cell voltage of 1.97 V at 25 °C, further decreasing to 1.65 V at 80 °C under static flow-free conditions.Notably, the measured cell voltage further decreases to 1.46 V under flow conditions.Figure 3g compares the polarization curves for different electrodes at 80 °C under static and flow conditions, clearly displaying enhanced performance of the NiS 2 pS x surface catalyst.All the tested cells display a linear decrease in the cell voltage with increasing temperature, both under static and flow conditions, with a higher slope for the corresponding decrease in cell voltage under flow conditions.Obviously, a clear distinction in the cell voltage is observed, with NiS 2 pS x surface ||NiS 2 pS x surface , exhibiting the lowest cell voltage followed by Pt||IrO 2 , and the highest cell potentials observed for Ni||Ni.It is very important to notice that the slope of change in cell voltage with respect to temperature is higher for NiS 2 pS x surface ||NiS 2 pS x surface in comparison to Pt||IrO 2 and Ni||Ni, indicating toward a significant role of the intrinsic catalytic nature of the catalyst (such as morphology, catalytic action, catalytic reaction intermediates) on the thermo-hydrodynamic influence on the water splitting performance observed.Thus, a direct influence of temperature and flow conditions is observed on the electrocatalytic properties, which is much more pronounced in the NiS 2 pS x surface ||NiS 2 pS x surface cell, thus deserving further in depth investigation on the effect of flow and temperature on this dramatically enhanced splitting performance.
The preliminary temperature-dependent voltage polarization curves demonstrate a linear dependence of the electrocatalytic performance on the electrolyte temperature, however, the rate of change in cell potential (at specific current density) varies with different electrocatalysts.Thus, it can be understood that the intrinsic properties of the electrocatalysts play a significant role in determining the overall electrochemical properties, in addition to the usual electrolyte heating effect that can be observed for any electrocatalyst.Thus, it is important to perform further investigations on the electrocatalytic performance in such a way that the sole contribution of electrode heating is analyzed, by decoupling the effect of electrode heating from overall electrolyte heating.For this purpose, a dedicated experimental setup was designed with custom made temperature controlled heaters that can specifically heat the electrodes (Figure 4a, photograph shown in Figure S10, Supporting Information).The temperature of the electrolyte was fixed, using external thermostat to avoid electrolyte heating (upon heating the electrodes), and thus decoupling the effect of electrode heating from that of heating the whole electrochemical setup.Both Ni||Ni and NiS 2 pS x surface ||NiS 2 pS x surface cells display an enhancement in their electrochemical performance with increasing electrode temperature, which is expected due to improved reaction kinetics by decreasing the activation energy upon increasing the electrode temperature (Figure 4b,c).Interestingly, NiS 2 pS x surface ||NiS 2 pS x surface cell exhibits a higher change in cell voltage (ΔV) upon a corresponding increase in the electrode temperature (Figure 4d).At 75 °C electrode temperature, NiS 2 pS x surface ||NiS 2 pS x surface cell demonstrates a threefold decrease (ΔV : 130 mV) in the cell voltage, in comparison to the Ni||Ni cell with a ΔV of 42 mV.Thus, a significant role of the electrocatalyst's intrinsic property can be observed at moderately high temperatures.Temperature-dependent XPS analysis was performed on NiS 2 pS x surface electrodes to analyze the chemical states at the surface of the catalyst at different temperatures outside the cell (Figure S11, Supporting Information).Interestingly, a systematic increase in the Ni:S elemental composition ratio was observed upon increasing the electrodes temperature (Figure 5e).There was an insignificant change in peak shape and position in Ni2p and S2p spectra (Figure S11a,b, Supporting Information).Further, no change in the O1s peak position or shape was observed (Figure S11c, Supporting Information) with increasing temperature with non-emergence of any new NiO/Ni-OH/Ni-OOH peaks which is characterized at lower binding energy of ≈530 eV. [55]Thus, the observed increase in Ni content can lead to concomitant increase in the density of OER active Ni 3+ along with Ni 2+ centers at the electrode surface with increasing temperature, hence leading to enhanced performance.For further investigating this phenomenon, only the cathode (for HER) or only the anode (for OER) was heated at different temperatures during the electrochemical analysis, as shown in Figure 4f.It can be observed that upon heating, the anode displays a considerable decrease in the cell voltage, which otherwise does not decrease much upon heating the cathode.Thus, heating the electrodes has significantly higher impact on the oxygen evolution reaction, conjugating well with the XPS analysis, where a systematic increase in OER catalyzing Ni 3+ centers surface density is observed with increasing electrolysis temperatures.It can be observed that upon heating, the anode displays a considerable decrease in the cell voltage, which otherwise does not decrease much upon heating the cathode.The heating effect of only the anode and only the cathode does not sum up to the heating effect for simultaneous heating.This could be understood due to the added contribution of electrolyte heating when both electrodes are heated, which in turn enhances the overall performance.Thus, despite a dominating role of enhanced kinetics electrolytic properties, intrinsic morphological electrode properties play a significant role in dictating the overall performance of the electrocatalyst at higher temperatures, and thus can be finely tuned for optimum performance.
Figure 3 clearly demonstrates the influence of temperature and flow on the electrocatalytic performance of hydrogen gas production.The effect of flowing the electrolyte on the electrocatalytic performance is significant and requires further investigation for better understanding the hydrodynamic influence on electrocatalytic water splitting performance of the catalyst.Based on our hypothesis, the hydrodynamics of electrolyte flowing is expected to substitute the currently applied "buffering action" approach, which is obligatory even for moderate water splitting performances under neutral electrolytic conditions.Furthermore, water flowing can also eliminate the detrimental bubble formation, which inhibits the optimum performance of electrocatalysts in general, specially under practical high current density operating conditions.Thus, the influence of flowing water was further analyzed by performing the electrochemical measurements at varying flow rates (0-90 L min −1 ) in the electrochemical setup, as shown in Figure 5a.It is to be noted that the turn-over frequency for electrolyte replacement over the electrodes (during water flow) was measured in a wide range, from 0-1500 volume replacements per second.As a result of this high electrolyte replacement rate, water molecules in the order of 10 5 were replenished every microsecond per Å 2 of the surface of the electrode.This number is important, as the electrocatalytic water splitting reactions take place in the microsecond time scale, and it can be realized that the flow rates tested in this work are much higher than the water molecules that can get consumed to form acidic and basic moieties at the anode and cathode, respectively.Voltage polarization curves for NiS 2 pS x surface ||NiS 2 pS x surface with varying flow rates, at 25 °C and 80 °C, demonstrate a systematic enhancement in the electrochemical performance in terms of increased current density and decreased cell potentials (Figure 5b,c).A sudden decrease in the cell potential is observed from static to flow conditions, and then a systematic decrease is observed in the entire flow rate range (Figure 5d).At 25 °C NiS 2 pS x surface ||NiS 2 pS x surface demonstrates a cell voltage drop of 240 mV, exhibiting a cell voltage of 1.81 V at a flow rate of 90 L min −1 .Remarkably, a cell voltage drop of 342 mV is observed at 80 °C with NiS 2 pS x surface ||NiS 2 pS x surface , exhibiting a cell voltage of slightly less than 1.4 V, which is the lowest reported cell voltage ever reported for neutral water splitting, or any additivebased electrolyzer systems as well.The trend in the cell voltage with varying flow rates was further analyzed in the low and high flow rate regimes, at 25 °C and 80 °C (Figure 5e,f respectively).In the low flow rate regime (1-6 L min −1 ), a linear decrease in the cell voltage is observed, however, an exponential drop in the cell voltage is observed at 80 °C (Figure 5e).Concomitantly, in the high flow regime (20-90 L min −1 ) a linear decrease in the cell voltage was observed, with a higher rate of change (slope of voltage with respect to flow rate: 1.95 mV per Lmin −1 ) at 80 °C.At higher current density of 50 mA cm −2 , the influence of flow saturates (slope: 0.88 mV per Lmin −1 ) at 80 °C (Figure S12, Supporting Information).Contrary at 25 °C, the influence of flow rate is similar to that observed at 10 mA cm −2 with a slope of 1.66 mV per Lmin −1 .Based on these observations, it can be concluded that flowing electrolyte has a significant effect on the electrocatalytic performance, and the effect of flow is more dominant at higher temperatures.Furthermore, at higher temperatures, even moderate flow rates are sufficient for optimum performance, which otherwise does not saturate even under 90 L min −1 at temperatures of 25 °C.
Inspired by the unprecedented thermo-hydrodynamicallyenabled performance of NiS 2 pS x surface ||NiS 2 pS x surface cells under un-buffered neutral electrolytic conditions, it is of prime importance to analyze its electrocatalytic properties under the naturally occurring abundantly available electrolyte; seawater.However, seawater splitting suffers a major limiting drawback of chlo-ride oxidation at the anode, instead of oxygen formation, due to their similar electrocatalytic overpotentials.Thus, apart from the efficient electrocatalytic activity, the selectivity of the NiS 2 pS x surface anode toward oxygen evolution is a prerequisite.In a recent work, a catenated sulphur-type nickel polysulphide catalyst was shown to intrinsically induce completely selective oxygen evolution, even at practically high current densities. [38]The chlorinephobic behavior is due to the limited exposure of sterically and electrostatically hindered OER centers (Ni metal centers) to large Cl − ions.However, under the present working conditions, it is important to analyze the selective oxygen evolution behavior at varying temperatures as well.For this purpose, a three-electrode OER electrochemical analysis was performed for NiS 2 pS x surface as the working electrode using seawater as the electrolyte.As expected, OER voltage polarization curves at different temperatures demonstrate a systematic enhancement in the electrolysis performance in terms of decrease in OER overpotential and an increase in current density (Figure 6a).Upon increasing the temperature to 80 °C, a decrease of 230 mV in OER overpotential (@10 mA cm −2 ) is observed, displaying an overpotential value of 197 mV and 406 at 10 mA cm −2 and 50 mA cm −2 , respectively.The gaseous and liquid electrolysis products were analyzed using residual gas analyzer mass spectrometer and iodometric titration, respectively (detailed in the experimental section).Remarkably, our NiS 2 pS x surface -based electrocatalyst demonstrates 100% selectivity toward oxygen evolution, even at moderately high temperatures of 80 °C (Figure 6b).In contrast,     [37,[56][57][58] f) Chronoamperometric stability test for NiS 2 pS x surface ||NiS 2 pS x surface (foam) cell at 10 mA cm −2 .(Note: all the electrochemical data is without iR compensation except Figure 6d).standard commercially available IrO 2 and Pt electrodes demonstrate significant evolution of gaseous chlorine and liquid hypochlorous acid even at 25 °C.Thus, the intrinsic chlorinephobic behavior of NiS 2 pS x surface electrocatalyst pertains even at 80 °C and can be explained due the temperature-independent steric and electrostatic hindrance of Cl − ions to reach OER active sites.Eventually, the NiS 2 pS x surface ||NiS 2 pS x surface cell was tested for the direct overall splitting of seawater under optimum conditions (Figure 6c).A major influence of flow and heating is observed in the electrocatalytic water splitting performance, even while using seawater as the electrolyte.The effect of water flow is considerably more pronounced at 80 °C in comparison to that observed at 25 °C, which is in direct correlation with the performance observed when using Na 2 SO 4 as an electrolyte.A cell potential of 1.44 V is observed, which is the lowest ever reported cell voltage for direct natural seawater splitting.The specific catalytic activity of NiS 2 pS x surface for hydrogen production was calculated at 1.5 V by evaluating the moles of hydrogen produced per hour per unit area under static and thermo-hydrodynamic conditions.A steady state Tafel analysis was also performed to calculate the Tafel slope for low flow at 25 °C and high flow at 80 °C for overall sea water splitting (Figure S13, Supporting Information).A lower Tafel slope of 310 mV dec −1 was observed under thermo-hydrodynamic condition of high flow at 80 °C revealing higher activity in comparison to low flow at 25 °C (Figure S13, Supporting Information).Further, a threeorders of magnitude increase in the catalytic activity was ob-served under electrolytic flow at 80 °C (1938.3μMcm −2 h −1 ) in comparison to that observed at 25 °C no flow conditions (3.26 μMcm −2 h −1 ).These results clearly demonstrate the dramatic amplification effect thermo-hydrodynamic conditions can cause on the catalyst water splitting activity.The applicability of the synthetic process on different substrates was tested by successfully extending the synthesis process on 3D large-active area nickel foam substrates as precursor material.A thorough characterization of the Ni foam transformed to NiS 2 pS x surface (foam) has now been performed (Figure S14, Supporting Information).The XRD pattern of NiS 2 pS x surface (foam) shows the formation of NiS 2 (Figure S14a, Supporting Information) similar to as observed for NiS 2 pS x surface .SEM imaging and EDX mapping demonstrates the formation of 3D porous morphology with a uniform distribution of Ni and S at the surface (Figure S14b-d, Supporting Information).Further, the XPS analysis shows the formation of nickel sulphide with peak position and shape matching exactly to that observe for NiS 2 pS x surface (Figure S14e,f, Supporting Information).Thus the single-step sulphidation process was successfully explored for transformation of Ni foam substrates to NiS 2 pS x surface (foam) and test for electrochemical analysis.As expected, NiS 2 pS x surface electrodes (foam) demonstrate remarkable electrochemical performance under flow conditions at 80 °C, with an exceptionally low cell voltage of 1.39 V and 2.2 V at 10 mA cm −2 and 500 mA cm −2 , respectively (Figure 6d).Additivefree direct sea water splitting under neutral conditions is considered unachievable, because of the numerous challenges yet to be addressed and overcome, and thus there are only few recent articles dealing with this challenging topic (Figure 6e).Remarkably, this work reports the lowest reported cell voltage ever reported in the literature using an economically-scalable bifunctional electrocatalyst (Figure 6e).Further, the long-term stability of the electrocatalyst was tested under chronoamperometric conditions, and found to be stable for long continuous operations (Figure 6f).An extensive characterization of the OER and HER electrode after 24 h of chronoamperometry at 10 mA cm −2 under high flow and heating operation is performed to know how the electrodes change.The post operation OER and HER electrodes were thoroughly cleaned with DI water and dried overnight for characterization.SEM images demonstrate morphological changes in the OER and HER electrodes post cell operation which is expected after long hour cell operation in flow and heating conditions (inset Figure S15b,c, Supporting Information).To further analyze these changes in the structure, TEM analysis of the post characterized samples was performed (Figure S15a-f, Supporting Information).The OER electrode demonstrates insignificant change in the crystallite size and crystallinity of the samples (Figure S15b, Supporting Information).Contrary, the HER electrode displays a decrease in the crystallite size probably due to structural reformation over time (Figure S15c, Supporting Information).However, even after structural reformation, the crystallinity of the samples was maintained with clear lattice fringes observed in the high resolution TEM images (Figure S15d-f, Supporting Information).Further, the XPS analysis was performed to analyze changes in the chemical states of the NiS 2 pS x surface post electrolysis (Figure S15g-i, Supporting Information).As expected, a shift in the Ni 2p peak toward higher binding energy was observed due to the formation of NiO/Ni-OH/ Ni-OOH type species at the surface post electrolysis (Figure S15g, Supporting Information).The O1s spectra demonstrates a shift in the peak position with non-symmetric peaks due to the emergence of nickel oxygen peak post electrolysis (Figure S15h, Supporting Information).The S 2p XPS spectra (Figure S15i, Supporting Information) of post-OER electrode demonstrates no significant change, however the post HER electrode demonstrates an increase in the sulphate groups at the surface along with peak broadening possibly due to the structural rearrangements and breaking of the crystallites at the surface as observe in the TEM analysis.Overall, thermo-hydrodynamic tuning of the catalyst intimate environment (flow and moderating heating of electrolyte), along with the intrinsic chloro-phobic properties of our exceptional electrocatalyst NiS 2 pS x surface , allowed here to synergistically achieve unprecedented electrocatalytic direct natural seawater splitting performances with real world practical qualities.
A concern about the overall energy required, mainly for heating the system and the flowing fluid.For this reason, the heating of only the electrodes (which requires much less energy) is also explored in this work as an alternative, which can significantly enhance the overall system performance (Figure 4).Furthermore, working on the specific electrode temperatures, electrode distance, and the specific cell geometry to reduce the effective electrolyte resistance can be explored in the future which can totally eliminate the use of heating the entire electrolyte bath.In spite, the state-of-the-art electrolyzers use similar electrolyte temperatures as presented in our work, and thus the present thermohydrodynamic approach does not bring any significant increase in applied energy in comparison to the state-of-the-art water splitting electrolyzers.However, it shall be realized that upon application of direct seawater splitting, the cost of water purification units (which is even required for pure water splitting), chloride removal osmosis system, cost of additives, etc. will be totally eliminated.The topic of direct seawater splitting is represented as debatable in the literature, especially with reports demonstrating a small contribution in the overall cost reduction upon direct seawater splitting in comparison to a two-step scenario with the current technologies (water desalination and then current water splitting). [60,61]However, it is very important to realize that the state-of-the-art cost estimations for seawater splitting are based on conventional alkaline electrolyzer where high-cost precious electrocatalysts, highly alkaline condition stable electrolyser materials, and expensive specialized alkali stable membranes are required.Apart from this, the cost of reverse osmosis system for chloride removal, a purification unit for ultra-pure water, and the cost of an alkaline additive is also included.However, this work cannot be generalized based on these assumptions as it paves the path for unconventional direct seawater splitting where a nonprecious abundantly available electrocatalyst is explored for direct seawater splitting under neutral conditions.Apart from the low cost of the electrocatalyst, non-requirement of chlorine removal/ water purification, low-cost membrane for neutral water electrolysis in comparison to the alkaline stable membrane and a cheap alternative for neutral water electrolyser setup.Thus, this work proposes almost no maintenance direct seawater splitting and thus outdates the previous price analysis performed where pre-processing cost of sea water, cost of additives, cost of precious electrodes, specialized membranes, and various other maintenance costs questioned the economics of direct sea water splitting.Furthermore, direct sea water splitting can lead to the realization of decentralized hydrogen production (with minimal setup and maintenance cost), which will reduce the hydrogen transportation costs and safety risks significantly.Apart from this, the toxicity of the alkaline additives in the electrolytes, maintenance/waste management of water treatment plants, chemical abatement, etc. can be totally eliminated as well.

Conclusions
In conclusion, tuning of thermo-hydrodynamic conditions of the catalyst near environment, flow, and temperature control, has been here thoroughly studied for the first time and applied to additive-free direct seawater splitting.The prepared binder-free 3D electrocatalyst, NiS 2 pS x surface , with surface-enriched Ni 3+ and pS n 2− moieties demonstrates exceptional electrochemical performance in neutral seawater electrolysis.Temperature-and flowassisted electrocatalytic measurements demonstrate a remarkable improvement in this performance.Furthermore, the electrocatalytic performance of the catalysts was decoupled for electrolytic and electrode-related contributions, revealing a role of the intrinsic catalytic material nature on the observed overall temperature dependence.The overall enhancement in the electrochemical performance can be related to flow-induced mechanical buffering and bubble removal/inhibition, along with heatinginduced increase in ionic conductivity and decrease in reactions activation energy.Furthermore, the electrocatalyst prepared in this work demonstrates selectivity toward oxygen evolution in chlorine-concentrated solutions (e.g., seawater).As a result, this work demonstrates the lowest cell voltage ever reported of 1.39 V for additive-free, or additive-based, direct seawater splitting, reaching practical current densities of >500 mA cm −2 .Overall, a three order increase in the catalytic activity for hydrogen production was observed under thermo-hydrodynamic influence on the overall electrocatalytic performance.Thus, this work opens novel avenues for real world seawater splitting applications based on the straightforward thermo-hydrodynamic processes modulation of the catalyst environment and can be universally extended to any existing electrocatalyst system, thus bringing to light the potential of novel approaches for the development of improved practical water splitting systems, besides the present dominating focus on the synthetic development of new electrocatalysts materials.

Experimental Section
Synthesis of Nickel Polysulphide-Based Electrodes: Pre-cleaned nickel foil (99.99%) and nickel foam (99.99%) samples were annealed in a preevacuated furnace at 450 °C with elemental sulphur (99.99% Sigma), at a temperature of 450 °C.The temperature was increased at rate of 10 °C min −1 , and maintained for 24 h.Upon completion, the furnace is cooled under ambient conditions.A sulphur-to-nickel weight ratio is optimized to 2:1.The sea water was obtained from Tel Baruch beach, Tel Aviv, Israel in the month of February 2023 and used as it is without any pre-treatment.The sea water was visually transparent and clean with a pH of ≈7.
Characterization: X-ray diffraction (XRD) studies are performed using a Bruker D8 Discover diffractometer.The surface morphology and Electron Dispersive X-ray measurements were performed using a highresolution Scanning Electron Microscope (ZEISS Gemini SEM 360).Transmission Electron Microscopy (Fei Themis Z G3) was performed by first cutting a lamella sample from the surface of the catalytic material using a ThermoFisher Helios 5 UC focused ion beam system (FIB), further applied for the analysis of the morphological and crystallographic structure.The surface chemical composition was analyzed using X-ray photoelectron spectroscopy system (XPS), and measurements conducted on a ESCALAB QXi X-ray Photoelectron Spectrometer Microprobe.The XPS data was fitted using the Fityk software.
Electrochemical Measurements: All the electrochemical measurements were conducted by a Metrohm Autolab Potentiostat.All the electrochemical measurements are performed at pH 7 using 0.25 m sodium sulphate (Na 2 SO 4 ) and natural seawater as electrolytes.The temperature-and the flow-dependent measurements were performed in a custom-made electrochemical setup especially designed for this purpose.The electrochemical measurements under flow conditions were performed in a custom made setup (photographs shown in Figure S1, Supporting Information) where a high power water pump (Pedrollo©) was connected to a 50 L electrolyte tank.The pump was connected to an external pump flow controller which can regulate the flow rates in the range 0-90 L min −1 .The pump created a steady, uniform, incompressible, turbulent flow.The pipes through the pump were connected to the electrochemical cell (inner diameter 3.2 cm) with external electrical connections and a cathode to anode distance of 1 cm.The temperature in the electrolyte tank was controlled using a Huber © thermostat and the electrolyte temperature was measured using an external thermocouple.The electrolysis is performed under high flow conditions which provides the self-cleaning conditions for the electrodes.All the potentials are converted to the reference hydrogen electrode (RHE), and are presented without performing any iR corrections, unless specified.A working area of 0.5 cm 2 was fixed for the working electrode.The cyclic voltammetry measurements were conducted at a scan rate of 5 mV s −1 , and the reverse scan was considered for calculating the overpotential values.The TOF of NiS 2 pS x surface is calculated at an overpotential of 300 mV using the formula TOF = 1 2nF [1] where, I is current density at a spe-cific overpotential, F is Faraday constant, and n is number of active sites; n = Q/2F, calculated by performing CV in the non-faradic region.Further, a different comparison parameter of specific catalytic activity considering the geometric area of the electrocatalyst was calculated in neutral electrolytic conditions.The specific catalytic activity of NiS 2 pS x surface was calculated at 1.5 V cell voltage for evaluating the moles of hydrogen produced per hour per unit geometric area under static and thermo-hydrodyamic conditions.The double layer capacitance (C dl ) as an evaluation of electrochemically active surface area (ECSA) of the NiS 2 pS x surface was evaluated by performing cyclic voltammetry in the non-faradic region.NIS-450 exhibits a significantly high double layer capacitance (C dl ) of 1.6 mF cm −2 .Finally, the ESCA was calculated using the formula, ECSA = C dl C s cm 2 ESCA ; where, C s is the specific capacitance of flat surface (40 μF cm −2 ). [26]The activity of the NipS x surface electrocatalysts is compared to the commercial IrO 2 (99% Angene Chemicals Pvt.Ltd.) and Pt electrodes (Wuhan Corrtest Instruments Corp., Ltd.).For electrode preparation, 1 mL dispersion of active material (15 mg) was prepared via sonication in a water-ethanol (4:1) mixture with 50 μL Nafion (of 5%) in ethanol.The electrodes were prepared by drop-casting the dispersion of the active material on a Ni foam with a mass loading of ≈1.5 mg cm −2 .For maximum utilization of the substrate surface area, IrO 2 was loaded on 3D Ni foam in spite of planar Ni foil with a significantly low surface area to avoid the surface area induced overestimation of the performance of NiS 2 pS x surface in comparison to IrO 2 catalyst.
Quantitative Analysis of Electrolysis Products: For analysis of the liquid and gaseous products, the OER performance was analyzed in a sealed electrochemical setup.Three-electrode OER measurements were conducted using platinum as counter electrode and Ag/AgCl as reference electrode.The gaseous products were analyzed using the residual gas analyzer mass spectrometer (MKS Instruments) during the chronoamperometric test, at 10 mA cm −2 .The liquid products were analyzed for the formation of hypochloric acid using the idometric titration. [59]Freshly prepared 0.5 potassium iodide (KI) and starch were added to the solution to form a dark red complex indicating the presence of hypochlorides.The hence formed solution was titrated using 0.01 m sodium thiosulphate (Na 2 S 2 O 3 ) until it forms a colorless solution.

Figure 1 .
Figure 1.Schematic of electrocatalytic overall water splitting in nonbuffered conditions under a) static and b) thermo-hydrodynamic conditions.

Figure 2 .
Figure 2. a) Schematic demonstration of the advantages of efficient additive-free direct seawater splitting using flow-and temperature-controlled conditions as an alternative to corrosive or toxic additives-based electrolyzers.b) X-ray diffraction pattern (* represents the NiS 102 plane), c) Scanning electron micrograph d) Electron dispersive X-ray of Ni and S mapping e) High-resolution TEM micrograph (inset in (e) shows the selected area diffraction pattern and inverse FFT patterns), high-resolution f) Ni 2p and g) S2p X-ray photoelectron spectra of NiS 2 pS x surface electrodes.

Figure 3 .
Figure 3. a) Schematics of the experimental setup for performing electrochemical measurements under variable flow and temperature conditions.b) Voltage polarization curves at different temperatures under b,d,e) no flow, c,f) flow and g) comparative curves at 80 °C for Ni||Ni, Pt||IrO 2 and NiS 2 pS x surface ||NiS 2 pS x surface electrochemical cells in 0.25 m Na 2 SO 4 electrolyte solution.h) comparative cell voltage at 5 mA cm −2 at different temperatures for Ni||Ni, Pt||IrO 2 , and NiS 2 pS x surface ||NiS 2 pS x surface under no flow and flow conditions (Note: all the electrochemical data is without iR compensation).

Figure 4 .
Figure 4. a) Schematics of the experimental setup for performing electrochemical measurements at different electrode temperature and fixed electrolyte temperature.b) Voltage polarization curves at varying electrode temperatures for b) Ni||Ni, and c) NiS 2 pS x surface ||NiS 2 pS x surface .d) comparative representation of cell voltage (@10 mA cm −2 ) and change in cell voltage (ΔV) upon increasing the temperature from 25 °C.e) Bar-graph representation of Ni:S elemental composition ratio analyzed via temperature dependent XPS analysis.f) Chronoamperometric test performed at 10 mA cm −2 while heating just anode, just cathode, and both anode and cathode simultaneously.(Note: all the electrochemical data is displayed without iR compensation).

Figure 5 .
Figure 5. a) Schematics of the experimental setup for performing electrochemical measurements at different flow rates at fixed temperatures.b) Voltage polarization curves for varying flow rates at an electrolyte temperature of b) 25 °C and c) 80 °C using NiS 2 pS x surface ||NiS 2 pS x surface cell.d) Comparative representation of cell voltage (@10 mA cm −2 at different flow rate at 25 °C and 25 °C.Cell voltage dependence on flow rates at e) low and f) high flow rate regimes.(Note: all the electrochemical data is displayed without iR compensation).

Figure 6 .
Figure 6.a) Voltage polarization curves for OER in three electrode geometry at different temperatures using sea water as electrolyte.b) percentage of electrolysis products formed at the anode at 10 mA cm −2 .c) voltage polarization curves for NiS 2 pS x surface ||NiS 2 pS x surface directly in sea water under static and flow conditions at 25 °C and 80 °C.d) Voltage polarization curve NiS 2 pS x surface ||NiS 2 pS x surface (foam) under flow condition at 80 °C.e) Literature comparison of cell voltage for direct sea water splitting.[37,[56][57][58]f) Chronoamperometric stability test for NiS 2 pS x surface ||NiS 2 pS x surface (foam) cell at 10 mA cm −2 .(Note: all the electrochemical data is without iR compensation except Figure6d).