Alkaline Seawater Research Articles

Oxalate anions-intercalated NiFe layered double hydroxide as a highly active and stable electrocatalyst for alkaline seawater oxidation

Seawater electrolysis is gaining recognition as a promising method for hydrogen production. However, severe anode corrosion caused by the high concentration of chloride ions (Cl–) poses a challenge for the long-term oxygen evolution reaction. Herein, an anti-corrosion strategy of oxalate anions intercalation in NiFe layered double hydroxide on nickel foam (NiFe-C2O42– LDH/NF) is proposed. The intercalation of these highly negatively charged C2O42– serves to establish electrostatic repulsion and impede Cl– adsorption. In alkaline seawater, NiFe-C2O42– LDH/NF requires an overpotential of 337 mV to gain the large current density of 1000 mA cm−2 and operates continuously for 500 h. The intercalation of C2O42– is demonstrated to significantly boost the activity and stability of NiFe LDH-based materials during alkaline seawater oxidation.

Hetero MOF‐On‐MOF of Ni‐BDC/NH2‐MIL‐88B(Fe) Enables Efficient Electrochemical Seawater Oxidation

AbstractSeawater electrolysis is a sustainable technology for producing hydrogen that would neither cause global freshwater shortages nor create carbon emissions. However, this technology is severely hampered by the insufficient stability and the competition from the chlorine evolution reaction (ClER) in actual application. Herein, a metal–organic framework (MOF)‐on‐MOF heterojunction (Ni‐BDC/NH2‐MIL‐88B(Fe)) denoted as (Ni‐BDC/NM88B(Fe)) is synthesized as an effective oxygen evolution reaction (OER) electrocatalyst for high‐performance seawater electrolysis, which exhibits a long stability of 200 h and low overpotentials of 232 and 299 mV at 100 mA cm−2 in alkaline freshwater and seawater solution, respectively. The exceptional performance is attributed to the rapid self‐reconstruction of Ni‐BDC/NM88B(Fe) to produce NiFeOOH protective layer, thereby avoiding ClER‐induced dissolution. Moreover, the interface interaction between Ni‐BDC and NM88B(Fe) could form the Ni─O─Fe bonds in Ni‐BDC/NM88B(Fe) to promote the electron transfer and lower the energy barrier of the rate‐determining step, thereby accelerating the OER. These electrochemical properties make it intriguing candidate as an efficient electrocatalyst for practical alkaline seawater electrolysis.

Fe/P dual-doping NiMoO4 with hollow structure for efficient hydrazine oxidation-assisted hydrogen generation in alkaline seawater

Electrosynthesis of hydrogen straightforwardly from seawater represents a potential solution towards carbon-neutral economy. However, with the sluggish oxygen evolution reaction (OER) at anode, the sustainable and cost-effective application is greatly hindered by extra energy consumption and serious chlorine chemistry in seawater. Herein, based on the advanced hydrazine-assisted electrolysis strategy, we reported a trifunctional Fe, P dual-doping NiMoO4 nanorods with a hollow structure for highly active hydrogen evolution reaction (HER) (23 mV @ 10 mA cm−2) and OER (213 mV @ 10 mA cm−2) activity, which also significantly decreased the cell voltage (activity variation for ∼1.40 V) in two-electrode system by replacing OER with thermodynamically beneficial hydrazine oxidation reaction (HzOR). Notably, a record low electricity expense of 2.3 kW h m−3 was obtained among commercial reactor in alkaline seawater/0.5 M N2H4. Meanwhile, the corrosion resistance of catalyst allows stable catalytic performance in seawater without any ClO- generation. Density functional theory calculations showed that Fe/P co-doping effectively endowed optimized electronic structure and modulated d-band centre for intermediates adsorption/desorption.

Bifunctional Amorphous Transition-Metal Phospho-Boride Electrocatalysts for Selective Alkaline Seawater Splitting at a Current Density of 2Acm-2.

Hydrogen production by direct seawater electrolysis is an alternative technology to conventional freshwater electrolysis, mainly owing to the vast abundance of seawater reserves on earth. However, the lack of robust, active, and selective electrocatalysts that can withstand the harsh and corrosive saline conditions of seawater greatly hinders its industrial viability. Herein, a series of amorphous transition-metal phospho-borides, namely Co-P-B, Ni-P-B, and Fe-P-B are prepared by simple chemical reduction method and screened for overall alkaline seawater electrolysis. Co-P-B is found to be the best of the lot, requiring low overpotentials of ≈270mV for hydrogen evolution reaction (HER), ≈410mV for oxygen evolution reaction (OER), and an overall voltage of 2.50V to reach a current density of 2Acm-2 in highly alkaline natural seawater. Furthermore, the optimized electrocatalyst shows formidable stability after 10,000 cycles and 30h of chronoamperometric measurements in alkaline natural seawater without any chlorine evolution, even at higher current densities. A detailed understanding of not only HER and OER but also chlorine evolution reaction (ClER) on the Co-P-B surface is obtained by computational analysis, which also sheds light on the selectivity and stability of the catalyst at high current densities.

Nitrogen and Sulfur Co-Doped Carbon-Coated Ni3S2/MoO2 Nanowires as Bifunctional Catalysts for Alkaline Seawater Electrolysis.

Bifunctional catalysts have inherent advantages in simplifying electrolysis devices and reducing electrolysis costs. Developing efficient and stable bifunctional catalysts is of great significance for industrial hydrogen production. Herein, a bifunctional catalyst, composed of nitrogen and sulfur co-doped carbon-coated trinickel disulfide (Ni3S2)/molybdenum dioxide (MoO2) nanowires (NiMoS@NSC NWs), is developed for seawater electrolysis. The designed NiMoS@NSC exhibited high activity in alkaline electrolyte with only 52 and 191mV overpotential to attain 10mAcm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Significantly, the electrolyzer (NiMoS@NSC||NiMoS@NSC) based on this bifunctional catalyst drove 100mAcm-2 at only 1.71V along with a robust stability over 100h in alkaline seawater, which is superior to a platinum/nickel-iron layered double hydroxide couple (Pt||NiFe LDH). Theoretical calculations indicated that interfacial interactions between Ni3S2 and MoO2 rearranged the charge at interfaces and endowed Mo sites at the interfaces with Pt-like HER activity, while Ni sites on Ni3S2 surfaces at non-interfaces are the active centers for OER. Meanwhile, theoretical calculations and experimental results also demonstrated that interfacial interactions improved the electrical conductivity, boosting reaction kinetics for both HER and OER. This study presented a novel insight into the design of high-performance bifunctional electrocatalysts for seawater splitting.

Enhancing the stability of NiFe-layered double hydroxide nanosheet array for alkaline seawater oxidation by Ce doping

Electrocatalytic hydrogen production from seawater holds enormous promise for clean energy generation. Nevertheless, the direct electrolysis of seawater encounters significant challenges due to poor anodic stability caused by detrimental chlorine chemistry. Herein, we present our recent discovery that the incorporation of Ce into NiFe layered double hydroxide nanosheet array on Ni foam (Ce-NiFe LDH/NF) emerges as a robust electrocatalyst for seawater oxidation. During the seawater oxidation process, CeO2 is generated, effectively repelling Cl− and inhibiting the formation of ClO−, resulting in a notable enhancement in the oxidation activity and stability of alkaline seawater. The prepared Ce-NiFe LDH/NF requires only overpotential of 390 mV to achieve the current density of 1 A cm−2, while maintaining long-term stability for 500 h, outperforming the performance of NiFe LDH/NF (430 mV, 150 h) by a significant margin. This study highlights the effectiveness of a Ce-doping strategy in augmenting the activity and stability of materials based on NiFe LDH in seawater electrolysis for oxygen evolution.

Manipulating the Microenvironment of Single Atoms by Switching Support Crystallinity for Industrial Hydrogen Evolution.

Modulating the microenvironment of single-atom catalysts (SACs) is critical to optimizing catalytic activity. Herein, we innovatively propose a strategy to improve the local reaction environment of Ru single atoms by precisely switching the crystallinity of the support from high crystalline and low crystalline, which significantly improves the hydrogen evolution reaction (HER) activity. The Ru single-atom catalyst anchored on low-crystalline nickel hydroxide (Ru-LC-Ni(OH)2 ) reconstructs the distribution balance of the interfacial ions due to the activation effect of metal dangling bonds on the support. Single-site Ru with a low oxidation state induces the aggregation of hydronium ions (H3 O+ ), leading to the formation of a local acidic microenvironment in alkaline media, breaking the pH-dependent HER activity. As a comparison, the Ru single-atom catalyst anchored on high-crystalline nickel hydroxide (Ru-HC-Ni(OH)2 ) exhibits a sluggish Volmer step and a conventional local reaction environment. As expected, Ru-LC-Ni(OH)2 requires low overpotentials of 9 and 136 mV at 10 and 1000 mA cm-2 in alkaline conditions and operates stably at 500 mA cm-2 for 500 h in an alkaline seawater anion exchange membrane (AEM) electrolyzer. This study provides a new perspective for constructing highly active single-atom electrocatalysts.

Manipulating the Microenvironment of Single Atoms by Switching Support Crystallinity for Industrial Hydrogen Evolution

AbstractModulating the microenvironment of single‐atom catalysts (SACs) is critical to optimizing catalytic activity. Herein, we innovatively propose a strategy to improve the local reaction environment of Ru single atoms by precisely switching the crystallinity of the support from high crystalline and low crystalline, which significantly improves the hydrogen evolution reaction (HER) activity. The Ru single‐atom catalyst anchored on low‐crystalline nickel hydroxide (Ru−LC−Ni(OH)2) reconstructs the distribution balance of the interfacial ions due to the activation effect of metal dangling bonds on the support. Single‐site Ru with a low oxidation state induces the aggregation of hydronium ions (H3O+), leading to the formation of a local acidic microenvironment in alkaline media, breaking the pH‐dependent HER activity. As a comparison, the Ru single‐atom catalyst anchored on high‐crystalline nickel hydroxide (Ru−HC−Ni(OH)2) exhibits a sluggish Volmer step and a conventional local reaction environment. As expected, Ru−LC−Ni(OH)2 requires low overpotentials of 9 and 136 mV at 10 and 1000 mA cm−2 in alkaline conditions and operates stably at 500 mA cm−2 for 500 h in an alkaline seawater anion exchange membrane (AEM) electrolyzer. This study provides a new perspective for constructing highly active single‐atom electrocatalysts.

Microzone-Acidification-Driven Degradation Mechanism of the NiFe-Based Anode in Seawater Electrolysis.

The anode stability is critical for efficient and reliable seawater electrolyzers. Herein, a NiFe-based film catalyst was prepared by anodic oxidation to serve as a model electrode, which exhibited a satisfactory oxygen evolution performance in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) with an overpotential of 348 mV at 100 mA cm-2 and a long-term stability of over 100 h. After that, the effects of the current density and bulk pH of the electrolyte on its stability were evaluated. It was found that the electrode stability was sensitive to electrolysis conditions, failing at 20 mA cm-2 in 0.1 M KOH + 0.5 M NaCl but over 500 mA cm-2 in 0.5 M KOH + 0.5 M NaCl. The electrode dissolved, and some precipitates immediately formed at the region very close to the electrode surface during the electrolysis. This can be ascribed to the pH difference between the electrode/electrolyte interface and the bulk electrolyte under anodic polarization. In other words, the microzone acidification accelerates the corrosion of the electrode by Cl-, thus affecting the electrode stability. The operational performances of the electrode under different electrolysis conditions were classified to further analyze the degradation behavior, which resulted in three regions corresponding to the stable oxygen evolution, violent dissolution-precipitation, and complete passivation processes, respectively. Thereby increasing the bulk pH could alleviate the microzone acidification and improve the stability of the anode at high current densities. Overall, this study provides new insights into understanding the degradation mechanism of NiFe-based catalysts and offers electrolyte engineering strategies for the application of anodes.

Coacervate Phase Evolution and Membrane Formation in Natural Seawater.

Marine organisms produce biological materials through the complex self-assembly of protein condensates in seawater, but our understanding of the mechanisms of microstructure evolution and maturation remains incomplete. Here, we show that critical processing attributes of mussel holdfast proteins can be captured by the design of an amphiphilic, fluorescent polymer (PECHIA) consisting of a polyepichlorohydrin backbone grafted with 1-imidazolium acetonitrile. Aqueous solutions of PECHIA were extruded into seawater, wherein the charge repulsion of PECHIA is screened by high salinity, facilitating interfacial condensation via enhanced "cation-dipole" interactions. Diffusion of seawater into the PECHIA solution caused droplets to form immiscibly within the PECHIA phase (i.e., inverse coacervation). Simultaneously, weakly alkaline seawater catalyzes nitrile cyclization and time-dependent solidification of the PECHIA phase, leading to hierarchically porous membranes analogous to porous architectures in mussel plaques. In contrast to conventional polymer processing technologies, processing of this biomimetic polymer required neither organic solvents nor heating and enabled the template-free production of hollow spheres and fibers over a wide range of salinities.

Borate Anion‐Intercalated NiV‐LDH Nanoflakes/NiCoP Nanowires Heterostructures for Enhanced Oxygen Evolution Selectivity in Seawater Splitting

AbstractResourceful and inexpensive seawater direct splitting omits the desalination process and effectively increases the efficiency of hydrogen energy generation. However, the development of seawater splitting is hampered by the competing selectivity challenges from anodic oxygen evolution reaction (OER) and chlorine evolution reaction and the issues of electrode corrosion. Herein, the borate anion‐intercalated NiV‐LDH nanoflakes/NiCoP nanowires heterostructures supported on Ni foam (2D/1D NiV‐BLDH/NiCoP/NF) is synthesized. Theoretical calculations show that a small amount of V atom doping in Ni(OH)2 is favorable for changing the electronic environment around Ni atoms via bridging Ni─O, which can construct Ni─O─V to accelerate electron transfer and promote catalytic activity. The borate anions (B(OH)4−) intercalation not only results in the good hydrophilicity and high OH− selectivity but also weakens the adsorption of chlorine (Cl−), which effectively restrains the chlorine evolution reaction. Thus, the component optimized NiV0.1‐BLDH/NiCoP/NF electrocatalyst only requires 268 mV overpotential to reach 100 mA cm−2 for OER in an alkaline environment. Particularly, the NiCoP/NF||NiV0.1‐BLDH/NiCoP/NF cell exhibits attractive overall water splitting performance with a low voltage of 1.46 and 1.53 V at 10 mA cm−2 in alkaline freshwater and alkaline seawater, respectively. The design strategy of this electrocatalyst provides a new avenue for seawater splitting.

Dopant-Induced Charge Redistribution on the 3D Sponge-like Hierarchical Structure of Quaternary Metal Phosphides Nanosheet Arrays Derived from Metal-Organic Frameworks for Natural Seawater Splitting.

Dopant-induced electron redistribution on transition metal-based materials has long been considered an emerging new electrocatalyst that is expected to replace noble-metal-based electrocatalysts in natural seawater electrolysis; however, their practical applications remain extremely daunting due to their sluggish kinetics in natural seawater. In this work, we developed a facile strategy to synthesize the 3D sponge-like hierarchical structure of Ru-doped NiCoFeP nanosheet arrays derived from metal-organic frameworks with remarkable hydrogen evolution reaction (HER) performance in natural seawater. Based on experimental results and density functional theory calculations, Ru-doping-induced charge redistribution on the surface of metal active sites has been found, which can significantly enhance the HER activity. As a result, the 3D sponge-like hierarchical structure of Ru-NiCoFeP nanosheet arrays achieves low overpotentials of 52, 149, and 216 mV at 10, 100, and 500 mA cm-2 in freshwater alkaline, respectively. Notably, the electrocatalytic activity of the Ru-NiCoFeP electrocatalyst in simulated alkaline seawater and natural alkaline seawater is nearly the same as that in freshwater alkaline. This electrocatalyst exhibits superior catalytic properties with outstanding stability under a high current density of 85 mA cm-2 for more than 100 h in natural seawater, which outperforms state-of-the-art 20% Pt/C at high current density. Our work provides valuable guidelines for developing a low-cost and high-efficiency electrocatalyst for natural seawater splitting.

Boosting hydrogen evolution reaction activity of Ru anchored binary oxyhydroxide by F-doping in alkaline seawater

Seawater electrolysis still faces harsh challenges especially at elevated current densities, which has to be ensured by highly-efficient and stable electrocatalyst. Recently, strategy involving hydrogen spillover between metal clusters and carriers has emerged as a means to enhance the hydrogen evolution reaction (HER) efficiency. In this study, we present a heterogeneous Ru/F-FeCoOOH catalyst to dissect the mechanism of hydrogen spillover between these two constituents. The DFT calculations and in-situ Raman analysis confirm the spillover of hydrogen species (H*) from Ru to F-FeCoOOH carrier. Furthermore, the introduction of F-doping narrows the work function disparity between the Ru metal and the F-FeCoOOH carrier, fostering a milieu that curtails interface H* capture and augments the potential for interface hydrogen spillover. Moreover, in-situ electrochemical impedance spectroscopy (EIS) and kinetic isotope effects (KIEs) corroborate that Ru/F-FeCoOOH exhibits extensive H* adsorption coverage and hydrogen transfer influences reaction rates. Leveraging the hydrogen spillover mechanism, the resultant Ru/F-FeCoOOH heterogeneous catalyst manifests a low overpotential of 260 mV at 2 A cm−2 and with long-term stability at least for 400 h in alkaline seawater.

Nitrogen-doped carbon layer coated Co(OH)F/CoP2 nanosheets for high-current hydrogen evolution reaction in alkaline freshwater and seawater.

Utilizing renewable energy such as offshore wind power to electrolyze seawater for hydrogen production offers a sustainable development pathway to address energy and climate change issues. In this study, by incorporating nitrogen-doped carbon quantum dots (N-CDs) into precursors, we successfully synthesized a nitrogen-doped carbon (NC)-layer-coated Co(OH)F/CoP2 catalyst NC@Co(OH)F/CoP2/NF loaded on nickel foam (NF). The introduction of N-CDs induced significant morphology change of the catalyst, facilitating the exposure of numerous active sites, ensuring the presence of catalytically active species CoP2 in nanoparticle form and avoiding agglomeration, which was advantageous to enhancing the overall hydrogen evolution reaction (HER) activity of the catalyst. The formation of Co-N bonds accelerated electron transfer, regulated the electronic structure, and optimized the catalyst's adsorption capacity for H* intermediates, which resulted in remarkably improved HER performance. In addition, Co(OH)F can also serve as a structural support, preventing the catalyst from collapsing during the HER catalytic process. NC@Co(OH)F/CoP2/NF exhibited excellent HER activity in alkaline freshwater and alkaline seawater, respectively requiring overpotentials of only 107 and 128 mV to achieve a current density of 100 mA cm-2. More importantly, it also demonstrated excellent HER activity at high current densities, with overpotentials of 189 and 237 mV at a current density of 1000 mA cm-2 in alkaline freshwater and alkaline seawater, respectively. This work provides new insights into the design and construction of highly efficient HER catalysts for applications in alkaline freshwater and seawater.

A hierarchical bimetallic nitride hybrid electrode with strong electron interaction for enhanced hydrogen production in seawater

A hierarchical MoN/Co2N hybrid electrode is fabricated for hydrogen production in alkaline seawater, which shows high catalytic performance due to the appropriate ΔGH*, unique 1D/2D/3D hierarchical structure, and N-doped carbon protection layer.

Reasonable regulation of flexible sulfur-based bifunctional catalytic electrodes for efficient seawater splitting

An iron-regulated three-dimensional nano-conical, economical, and flexible Fe-NiS@HA electrode exhibiting high intrinsic activity is prepared by gentle one-step electroless plating for efficient, durable hydrogen production in alkaline seawater.

Manipulating anion intercalation into layered double hydroxide for alkaline seawater oxidation at high current density

In this work, we propose a convenient strategy to manipulate anion intercalation into layered double hydroxide. The obtained NiFe LDH–Cl– electrode shows outstanding OER performance with both low overpotentials and...

Constructing built-in electric fields in 2D/2D Schottky heterojunctions for efficient alkaline seawater electrolysis

The construction of Schottky heterojunction in Co3(PO4)2@MXene accelerates the directional transfer of electrons at the interface, achieving efficient hydrogen evolution performance under high current density conditions in seawater electrolysis.

Rapid carbothermal shocking fabrication of iron-incorporated molybdenum oxide with heterogeneous spin states for enhanced overall water/seawater splitting.

Molybdenum dioxide (MoO2) has been considered as a promising hydrogen evolution reaction (HER) electrocatalyst. However, the active sites are mainly located at the edges, resulting in few active sites and poor activity in the HER. Herein, we first reported on an efficient strategy to incorporate Fe into MoO2 nanosheets on Ni foam (Fe-MoO2/NF) using a rapid carbothermal shocking method (820 °C for 127 s). Notably, the different spin states between Fe and Mo atoms could lead to rich lattice dislocations in Fe-MoO2/NF, exposing abundant oxygen vacancies and the low-oxidation-state of Mo sites during the rapid Joule heating process. As tested, the catalyst exhibited superior activity with ultralow overpotentials (HER: 17 mV@10 mA cm-2; oxygen evolution reaction (OER): 310 mV@50 mA cm-2) and high OER selectivity in alkaline seawater splitting. Meanwhile, this catalyst was equipped in a home-made anion exchange membrane (AEM) seawater electrolyzer, which achieved a low energy consumption (5.5 kW h m-3). More importantly, Fe-MoO2/NF also coupled very well with a solar-driven electrolytic system and turned out a solar-to-hydrogen (STH) efficiency of 13.5%. Theoretical results also demonstrated that Fe incorporated and abundant oxygen vacancies in MoO2 can distort the distance of the Mo-O bonds and regulate the electronic structure, thus optimizing the binding energy of H*/OOH* adsorption. This method can be extended to other heterogeneous spin states in MoO2-based catalysts (e.g. Ni-MoO2/NF, Co-MoO2/NF) for seawater splitting, and provide a simple, efficient and universal strategy to prepare highly-efficient MoO2-based electrocatalysts.

La doping greatly enhances electrochemical alkaline seawater oxidation over Ni(OH)2 nanosheet

An La–Ni(OH)2/NF catalyst exhibits superior performance for alkaline seawater oxidation, requiring a low overpotential of 434 mV to achieve a current density of 500 mA cm−2.