Fuel Production from Seawater and Fuel Cells Using Seawater

Highly Stable Bifunctional Heterostructured Electrocatalyst Integrated with LDPE-Derived Spherical Carbon for Longevous Alkaline Seawater Splitting

The development of innovative electrocatalysts for seawater splitting shows great potential for large-scale green energy. Specifically, interface engineering plays a vital role in improving surface properties and charge transfer. However, seawater electrolysis encounters considerable challenges like chloride-induced corrosion, impurities, and microorganisms that hinder efficiency. Herein, we design a highly durable electrocatalyst based on selenium-enriched NiMn-Sx  supported on low-density polyethylene-derived spherical carbon-Ni foam (Se-NiMnSx@SC/NF) using combination of pyrolysis and hydrothermal processes. The resulting Se-NiMnSx@SC/NF bifunctional catalyst with hollow cycas cone structure exhibited exceptional electrochemical performance and corrosion resistance in alkaline seawater with an ultralow overpotential of 146 and 262 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to achieve a large current density of 500 mA cm⁻2. In a simulated alkaline seawater splitting setup, the Se-NiMnSx@SC/NF catalyst maintained a cell voltage of 2.07 V at 500 mA cm⁻2, demonstrating outstanding durability for over 100 h with ≈100% Faradaic efficiency. Se and S doping in the heterostructured electrocatalyst refines the electronic structure and boosts reaction kinetics, while the hollow cycas cone design increases the exposure of active sites. Additionally, the carbon layer provided strong resistance to seawater corrosion, making Se-NiMnSx@SC/NF an excellent bifunctional catalyst for alkaline seawater electrolysis.

The Role of Anode Potential in Electromicrobiology

Electroactive microorganisms are capable of exchanging electrons with electrodes and thus have potential applications in many fields, including bioenergy production, microbial electrochemical synthesis of chemicals, environmental protection, and microbial electrochemical sensors. Due to the limitations of low electron transfer efficiency and poor stability, the application of electroactive microorganisms in industry is still confronted with significant challenges. In recent years, many studies have demonstrated that modulating anode potential is one of the effective strategies to enhance electron transfer efficiency. In this review, we have summarized approximately 100 relevant studies sourced from PubMed and Web of Science over the past two decades. We present the classification of electroactive microorganisms and their electron transfer mechanisms and elucidate the impact of anode potential on the bioelectricity behavior and physiology of electroactive microorganisms. Our review provides a scientific basis for researchers, especially those who are new to this field, to choose suitable anode potential conditions for practical applications to optimize the electron transfer efficiency of electroactive microorganisms, thus contributing to the application of electroactive microorganisms in industry.

The Influence of Alkaline Earth Metal Cations on the Performance of Platinum Electrodes during Brine Electrolysis

The performance of platinum electrodes towards the hydrogen evolution reaction (HER) during pH-neutral brine electrolysis was investigated in the presence of alkaline earth metal cations, with a focus on Mg2+ ions. This study aimed to understand the impact of these cations and/or their hydroxide deposits on electrode performance. Electrochemical measurements, including chronoamperometry and polarization curves, were conducted in pH-neutral electrolytes with varying Mg2+ concentrations (0 mM to 55 mM) in 1 M Na2SO4 solution. Contrary to concerns that Mg(OH)2 deposits might impair performance, Pt electrodes exhibited improved HER activity across all Mg2+ concentrations compared to Mg-free electrolytes. To distinguish the effects of hydroxide deposits from solvated cation interactions with the electrocatalyst, other divalent cations (Ca2+, Sr2+, Mn2+) with varying solubility product constants were also tested. All cations enhanced HER performance, including those unlikely to form deposits under the tested conditions. Our results point to cation acidity and Gibbs free energy of hydration as effective descriptors for predicting the role of divalent cations in HER kinetics. In situ image analysis revealed that Mg(OH)2 deposits are periodically removed by evolving hydrogen bubbles, preventing the blockage of active sites. In situ Raman spectroscopy further confirmed the formation of Mg(OH)2 deposits during HER.

Customized Design of Covalent Organic Framework with Asymmetric Dual-Function Hybrid Linkages for Promoting H2O2 Photosynthesis in Air and Water

Efficient sacrificial-agent-free photosynthesis of H2O2 from air and water represents the greenest, lowest-cost, most real-time avenue for H2O2 production but remains a challenging issue. Here, we show a general and effective approach through a structural design on covalent organic frameworks (COFs) with asymmetric dual-function hybrid linkages for boosting the H2O2 photosynthesis of the COFs. Through such design we can equip a COF with not only a catalytic active center but also a special function for isolating the D-A motif, which consequently endows the COF (CI-COF) built on asymmetric dual-function hybrid linkages with a significantly enhanced efficiency in the generation, transmission, and separation of photogenerated carriers, relative to the COF (II-COF and CC-COF) built on symmetric single-function single linkages. Correspondingly, the performance of H2O2 photosynthesis is enhanced by three or five times. Accompanied is the largely promoted O2 utilization and conversion efficiency from 36.6% to 99.9%. A rare dual-channel H2O2 photosynthesis is suggested for the CI-COF.

High Catalytic Selectivity of Electron/Proton Dual-Conductive Sulfonated Polyaniline Micropore Encased IrO2 Electrocatalyst by Screening Effect for Oxygen Evolution of Seawater Electrolysis

Acidic seawater electrolysis offers significant advantages in high efficiency and sustainable hydrogen production. However, in situ electrolysis of acidic seawater remains a challenge. Herein, a stable and efficient catalyst (SPTTPAB/IrO2) is developed by coating iridium oxide (IrO2) with a microporous conjugated organic framework functionalized with sulfonate groups (-SO3H) to tackle these challenges. The SPTTPAB/IrO2 presents a -SO3H concentration of 5.62 × 10-4 mol g-1 and micropore below 2 nm numbering 1.026 × 1016 g-1. Molecular dynamics simulations demonstrate that the conjugated organic framework blocked 98.62% of Cl- in seawater from reaching the catalyst. This structure combines electron conductivity from the organic framework and proton conductivity from -SO3H, weakens the Cl- adsorption, and suppresses metal-chlorine coupling, thus enhancing the catalytic activity and selectivity. As a result, the overpotential for the oxygen evolution reaction (OER) is only 283 mV@10 mA cm-2, with a Tafel slope of 16.33 mV dec-1, which reduces 13.8% and 37.8% compared to commercial IrO2, respectively. Impressively, SPTTPAB/IrO2 exhibits outstanding seawater electrolysis performance, with a 35.3% improvement over IrO2 to 69 mA cm-2@1.9 V, while the degradation rate (0.018 mA h-1) is only 24.6% of IrO2. This study offers an innovative solution for designing high-performance seawater electrolysis electrocatalysts.

Solar Assisted Mitigation of Chloramphenicol and H2 Evolution Using CuNi Alloy Nanoparticles on h-BN Doped g-C3N4: A Comprehensive Approach Combining Synchrotron and DFT Analysis

A simple one-step deposition-precipitation method was used to synthesize highly active and well-defined CuNi alloy bimetallic nanoparticles supported on h-BN/g-C3N4. The nanocomposite was applied for hydrogen gas evolution via seawater splitting and photocatalytic chloramphenicol (CHP) removal. Through TEM and synchrotron studies, the formation of CuNi alloy and uniform distribution of CuNi bimetallic nanoparticles on the h-BN/g-C3N4 surface was observed. The EXAFS analysis verified the successful formation of the alloy, while the XPS and XANES spectra showed that the bimetallic nanoparticles are in a metallic state. Additionally, XANES revealed nanoparticle distortion upon interaction with the support, confirming the effective formation of the nanocomposite. The nanocomposite achieved a maximum hydrogen evolution rate of 3658.9 μmol g-1 h-1 for 5 wt % CuNi(3:1)/h-BN/g-C3N4, outperforming CuNi(3:1) nanoparticles and pristine g-C3N4 by 1.82 and 4.31 times, respectively. Additionally, it degraded chloramphenicol with a rate constant (kapp) of 0.018 min-1. Optical and electrochemical analysis revealed enhanced charge mobility, extended lifetime, improved photostability, and superior photoresponse. X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations attributed the performance to the synergy between the bimetallic nanoparticles and the h-BN/g-C3N4 sheet. DFT calculations demonstrated the effective breakdown of chloramphenicol and the promotion of hydrogen gas evolution, aligning with experimental observations. Cytotoxicity of CHP post-treatment was analyzed using Drosophila melanogaster (fruit fly) and the Oregon-R strain of D. melanogaster.

A scalable approach using a gC3N4-covalent organic framework hybrid catalyst towards sustainable hydrogen production from seawater and wastewater

The photocatalytic generation of H2 using covalent organic frameworks (COFs) is gaining more interest. While numerous reports have focused on the production of H2 from deionized water using COFs, the inability to produce H2 from industrial wastewater or seawater is a common limitation in many reported catalysts. Additionally, many of these reports lack a clear path to scale up the catalyst synthesis. In this study, we explore the prospect of hybridizing a COF with gC3N4 to create a robust photocatalyst for efficient H2 generation. This hybrid exhibits outstanding performance not only in deionized water, but also in wastewater, and simulated seawater. Furthermore, we explore the feasibility of the bulk-scale synthesis and successfully produce a 20 g hybrid catalyst in a single batch, and the synthesis method is scalable to achieve the commercial target. Remarkably, a maximum HER rate of 94 873 μmol g-1 h-1 and 109 125 μmol g-1 h-1 was obtained for the hybrid catalyst from industrial wastewater and simulated seawater, respectively. The performance of bulk-scale batches closely matches that of the small-scale ones. This research paves the way for the utilization of organic photocatalysts on a commercial scale, offering a promising solution for sustainable large-scale H2 production.

Multifunctional Design of Catalysts for Seawater Electrolysis for Hydrogen Production

Direct seawater electrolysis is a promising technology within the carbon-neutral energy framework, leveraging renewable resources such as solar, tidal, and wind energy to generate hydrogen and oxygen without competing with the demand for pure water. High-selectivity, high-efficiency, and corrosion-resistant multifunctional electrocatalysts are essential for practical applications, yet producing stable and efficient catalysts under harsh conditions remains a significant challenge. This review systematically summarizes recent advancements in advanced electrocatalysts for seawater splitting, focusing on their multifunctional designs for selectivity and chlorine corrosion resistance. We analyze the fundamental principles and mechanisms of seawater electrocatalytic reactions, discuss the challenges, and provide a detailed overview of the progress in nanostructures, alloys, multi-metallic systems, atomic dispersion, interface engineering, and functional modifications. Continuous research and innovation aim to develop efficient, eco-friendly seawater electrolysis systems, promoting hydrogen energy application, addressing efficiency and stability challenges, reducing costs, and achieving commercial viability.

Regulating Benzene Ring Number as Connector in Covalent Organic Framework for Boosting Photosynthesis of H2O2 from Seawater

Photosynthesis of H2O2 from seawater represents a promising pathway to acquire H2O2, but it is still restricted by the lack of a highly active photocatalyst. In this work, we propose a convenient strategy of regulating the number of benzene rings to boost the catalytic activity of materials. This is demonstrated by ECUT-COF-31 with adding two benzene rings as the connector, which can result in 1.7-fold enhancement in the H2O2 production rate relative to ECUT-COF-30 with just one benzene ring as the connector. The reason for enhancement is mainly due to the release of *OOH from the surface of catalyst and the final formation of H2O2 being easier in ECUT-COF-31 than in ECUT-COF-30. Moreover, ECUT-COF-31 provides a stable photogeneration of H2O2 for 70 h, and a theoretically remarkable H2O2 production of 58.7 mmol per day from seawater using one gram of photocatalyst, while the cost of the used raw material is as low as 0.24 $/g.

Challenges and progress in oxygen evolution reaction catalyst development for seawater electrolysis for hydrogen production

Production of green hydrogen on a large scale can negatively impact freshwater resources. Therefore, using seawater as an electrolyte in electrolysis is a desirable alternative to reduce costs and freshwater reliance. However, there are limitations to this approach, primarily due to the catalyst involved in the oxygen evolution reaction (OER). In seawater, the OER features sluggish kinetics and complicated chemical reactions that compete. This review first introduces the benefits and challenges of direct seawater electrolysis and then summarises recent research into cost-effective and durable OER electrocatalysts. Different modification methods for nickel-based electrocatalysts are thoroughly reviewed, and promising electrocatalysts that the authors believe deserve further exploration have been highlighted.

Performance and Solution Structures of Side-Chain-Bridged Oligo (Ethylene Glycol) Polymer Photocatalysts for Enhanced Hydrogen Evolution under Natural Light Illumination

Converting solar energy into hydrogen energy using conjugated polymers (CP) is a promising solution to the energy crisis. Improving water solubility plays one of the critical factors in enhancing the hydrogen evolution rate (HER) of CP photocatalysts. In this study, a novel concept of incorporating hydrophilic side chains to connect the backbones of CPs to improve their HER is proposed. This concept is realized through the polymerization of carbazole units bridged with octane, ethylene glycol, and penta-(ethylene glycol) to form three new side-chain-braided (SCB) CPs: PCz2S-OCt, PCz2S-EG, and PCz2S-PEG. Verified through transient absorption spectra, the enhanced capability of PCz2S-PEG for ultrafast electron transfer and reduced recombination effects has been demonstrated. Small- and wide-angle X-ray scattering (SAXS/WAXS) analyses reveal that these three SCB-CPs form cross-linking networks with different mass fractal dimensions (f) in aqueous solution. With the lowest f value of 2.64 and improved water/polymer interfaces, PCz2S-PEG demonstrates the best HER, reaching up to 126.9 µmol h-1 in pure water-based photocatalytic solution. Moreover, PCz2S-PEG exhibits comparable performance in seawater-based photocatalytic solution under natural sunlight. In situ SAXS analysis further reveals nucleation-dominated generation of hydrogen nanoclusters with a size of ≈1.5 nm in the HER of PCz2S-PEG under light illumination.

Recent Advances in Hybrid Seawater Electrolysis for Hydrogen Production

Seawater electrolysis (SWE) is a promising and potentially cost-effective approach to hydrogen production, considering that seawater is vastly abundant and SWE is able to combine with offshore renewables producing green hydrogen. However, SWE has long been suffering from technical challenges including the high energy demand and interference of chlorine chemistry, leading electrolyzers to a low efficiency and short lifespan. In this context, hybrid SWE, operated by replacing the energy-demanding oxygen evolution reaction and interfering chlorine evolution reaction (CER) with a thermodynamically more favorable anodic oxidation reaction (AOR) or by designing innovative electrolyzer cells, has recently emerged as a better alternative, which not only allows SWE to occur in a safe and energy-saving manner without the notorious CER, but also enables co-production of value-added chemicals or elimination of environmental pollutants. This review provides a first account of recent advances in hybrid SWE for hydrogen production. The substitutional AOR of various small molecules or redox mediators, in couple with hydrogen evolution from seawater, is comprehensively summarized. Moreover, how the electrolyzer cell design helps in hybrid SWE is briefly discussed. Last, the current challenges and future outlook about the development of the hybrid SWE technology are outlined.

Photocatalytic hydrogen production from seawater by TiO2/RuO2 hybrid nanofiber with enhanced light absorption

Compared to hydrogen production through pure water photocatalysis, the direct utilization of seawater for hydrogen production aligns better with the principles of sustainable development. Seawater, however, contains impurity ions like Na+ and Cl-, which pose higher demands on photocatalysts. It is widely acknowledged that RuO2 and TiO2 demonstrate excellent stability in seawater. Consequently, this study focuses on the model system of RuO2-modified TiO2 for investigating hydrogen production in simulated seawater. TiO2/RuO2 nanofibers (NFs) were synthesized via a one-step electrospinning method, ensuring intimate contact between the two components. The hydrogen production activity of TiO2/RuO2 NFs in simulated seawater nearly matches that in pure water. Remarkably, RuO2 serves as an oxidation cocatalyst, effectively scavenging holes from the valence band of TiO2 and enhancing the separation efficiency of electrons and holes. Interestingly, Cl- ions, similar to RuO2, contribute to reducing excess holes. This study lays the groundwork for future research into hydrogen production from seawater.

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2

The direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl- upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co2 P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200-2000 mA cm-2 over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO4 3- and Fe(CN)6 3- generated by the electrode play synergistic role in repelling Cl- via electrostatic repulsion and dense coverage, which reduced Cl- adsorption by nearly 5-fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.

An Atomically Dispersed Mn-Photocatalyst for Generating Hydrogen Peroxide from Seawater via the Water Oxidation Reaction (WOR)

In this work, we have fabricated an aryl amino-substituted graphitic carbon nitride (g-C₃N₄) catalyst with atomically dispersed Mn capable of generating hydrogen peroxide (H₂O₂) directly from seawater. This new catalyst exhibited excellent reactivity, obtaining up to 2230 μM H₂O₂ in 7 h from alkaline water and up to 1800 μM from seawater under identical conditions. More importantly, the catalyst was quickly recovered for subsequent reuse without appreciable loss in performance. Interestingly, unlike the usual two-electron oxygen reduction reaction pathway, the generation of H₂O₂ was through a less common two-electron water oxidation reaction (WOR) process in which both the direct and indirect WOR processes occurred; namely, photoinduced h⁺ directly oxidized H₂O to H₂O₂ via a one-step 2e^- WOR, and photoinduced h⁺ first oxidized a hydroxide (OH^-) ion to generate a hydroxy radical (^•OH), and H₂O₂ was formed indirectly by the combination of two ^•OH. We have characterized the material, at the catalytic sites, at the atomic level using electron paramagnetic resonance, X-ray absorption near edge structure, extended X-ray absorption fine structure, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, magic-angle spinning solid-state NMR spectroscopy, and multiscale molecular modeling, combining classical reactive molecular dynamics simulations and quantum chemistry calculations.

Biohybrid Molecule-Based Photocatalysts for Water Splitting Hydrogen Evolution

The problems of resource depletion and environmental pollution caused by the excessive use of fossil fuels greatly restrict the rapid development of human technology and industry, which has led to a high demand for the development of new and clean energy sources. Hydrogen, due to its high calorific value and environmentally friendly combustion products, is undoubtedly a very promising energy carrier. The current methods of industrial hydrogen production are mainly water electrocatalytic decomposition or fossil fuels conversion, which also results in the waste of other energy sources. Since only one-step is involved during the conversion from solar to chemical energy and thus unnecessary energy waste is avoided, solar energy photocatalytic decomposition of water provides a more viable method for hydrogen production. The utilization of biohybrid molecules, which are widely available in nature and environmentally friendly, further reduce the cost of such photocatalytic systems. This Review discusses the research progress on hydrogen production using biohybrid molecules for photocatalytic hydrogen evolution. The basic reaction mechanism, general types and system structures about biohybrid molecule-based photocatalysts are summarized. The current challenges and prospects in the research of water splitting hydrogen evolution by biohybrid molecules photocatalysts are presented.

Probing the Effects of Electrode Composition and Morphology on the Effectiveness of Silicon Oxide Overlayers to Enhance Selective Oxygen Evolution in the Presence of Chloride Ions

Seawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic chlorine alongside the evolution of oxygen, a promising approach is the use of electrochemically inert overlayers. Such thin films can exert a perm-selective effect, allowing the transport of water and oxygen between the bulk electrolyte and the electrocatalytic buried interface while suppressing the transport of chloride ions. In this work, we investigate thin (5-20 nm) overlayer films composed of amorphous silicon oxide (SiO x ) and their application to suppressing the chlorine evolution reaction (CER) in favor of the oxygen evolution reaction (OER) during acidic saltwater electrolysis on three different types of electrodes. While SiO x overlayers are seen to be an effective barrier against the CER on well-defined, smooth Pt thin films, decreasing the CER activity roughly 20-fold, this ability has not been previously explored on Ir-based catalysts with a higher surface area relevant to industrial applications. On amorphous iridium oxide electrodes, the selectivity toward the CER versus the OER was marginally reduced from ∼98 to ∼94%, which was attributed to the higher abundance of defects in overlayers deposited on the rougher electrode. On the other hand, Ir-based anodes consisting of thick mixed metal oxide films supported on Ti showed a significant decrease in CER selectivity, from ∼100 to ∼50%, although this came at the cost of reduced activity toward the OER. These results show that the morphology and composition of the underlying electrode play important roles in the effectiveness of the selective overlayers and provide guidance for further development of high-surface-area OER-selective anodes.

Photocatalytic Hydrogen Evolution from Artificial Seawater Splitting over Amorphous Carbon Nitride: Optimization and Process Parameters Study via Response Surface Modeling

Photocatalytic water splitting has garnered tremendous attention for its capability to produce clean and renewable H₂ fuel from inexhaustible solar energy. Until now, most research has focused on scarce pure water as the source of H₂, which is not consistent with the concept of sustainable energy. Hence, the importance of photocatalytic splitting of abundant seawater in alleviating the issue of pure water shortages. However, seawater contains a wide variety of ionic components which have unknown effects on photocatalytic H₂ production. This work investigates photocatalytic seawater splitting conditions using environmentally friendly amorphous carbon nitride (ACN) as the photocatalyst. The individual effects of catalyst loading (X₁), sacrificial reagent concentration (X₂), salinity (X₃), and their interactive effects were studied via the Box–Behnken design in response surface modeling towards the H₂ evolution reaction (HER) from photocatalytic artificial seawater splitting. A second-order polynomial regression model is predicted from experimental data where the variance analysis of the regressions shows that the linear term (X₁, X₂), the two-way interaction term X₁X₂, and all the quadratic terms (X₁₂, X₂₂, X₂₃) pose significant effects towards the response of the HER rate. Numerical optimization suggests that the highest HER rate is 7.16 µmol/h, achievable by dosing 2.55 g/L of ACN in 45.06 g sea salt/L aqueous solution containing 17.46 vol% of triethanolamine. Based on the outcome of our findings, an apparent effect of salt ions on the adsorption behavior of the photocatalyst in seawater splitting with a sacrificial reagent has been postulated.

High-performance seawater oxidation by a homogeneous multimetallic layered double hydroxide electrocatalyst

Seawater is one of the most abundant resources on Earth. Direct electrolysis of seawater is a transformative technology for sustainable hydrogen production without causing freshwater scarcity. However, this technology is severely impeded by a lack of robust and active oxygen evolution reaction (OER) electrocatalysts. Here, we report a highly efficient OER electrocatalyst composed of multimetallic layered double hydroxides, which affords superior catalytic performance and long-term durability for high-performance seawater electrolysis. To the best of our knowledge, this catalyst is among the most active for OER and it advances the development of seawater electrolysis technology.

Seawater electrolysis is an intriguing technology for sustainable hydrogen production that will not exacerbate the global shortage of freshwater or increase carbon emissions. However, due to the undesirable anodic chlorine evolution reaction and the strong corrosiveness of seawater, this technology is significantly hindered by a lack of robust oxygen evolution reaction (OER) electrocatalysts that exhibit high activity, high selectivity, and good stability. Here, we demonstrate a homogeneous multimetallic catalyst consisting of Ni and Fe coincorporated into CuCo layered double hydroxide (denoted as NiFe-CuCo LDH) that serves as an active and durable OER electrode for high-performance seawater electrolysis. With abundant exposed multimetal sites and well-defined micronanostructures, the NiFe-CuCo LDH catalyst requires overpotentials of only 259, 278, and 283 mV to yield current densities of 100, 300, and 500 mA cm⁻², respectively, in 6 M KOH seawater electrolyte. Moreover, it exhibits very high OER selectivity (Faradaic efficiency of 97.4% for O₂ at 500 mA cm⁻²) and superior durability during operation, working stably under a large current density of 500 mA cm⁻² for up to 500 h in 6 M KOH seawater electrolyte. This multimetallic electrocatalyst is one of the best performing ones among all reported transition-metal-based OER electrocatalysts in alkaline seawater electrolyte, which boosts the development of seawater electrolysis technology.

High Selectivity Electrocatalysts for Oxygen Evolution Reaction and Anti-Chlorine Corrosion Strategies in Seawater Splitting

Seawater is one of the most abundant and clean hydrogen atom resources on our planet, so hydrogen production from seawater splitting has notable advantages. Direct electrolysis of seawater would not be in competition with growing demands for pure water. Using green electricity generated from renewable sources (e.g., solar, tidal, and wind energies), the direct electrolytic splitting of seawater into hydrogen and oxygen is a potentially attractive technology under the framework of carbon-neutral energy production. High selectivity and efficiency, as well as stable electrocatalysts, are prerequisites to facilitate the practical applications of seawater splitting. Even though the oxygen evolution reaction (OER) is thermodynamically favorable, the most desirable reaction process, the four-electron reaction, exhibits a high energy barrier. Furthermore, due to the presence of a high concentration of chloride ions (Cl−) in seawater, chlorine evolution reactions involving two electrons are more competitive. Therefore, intensive research efforts have been devoted to optimizing the design and construction of highly efficient and anticorrosive OER electrocatalysts. Based on this, in this review, we summarize the progress of recent research in advanced electrocatalysts for seawater splitting, with an emphasis on their remarkable OER selectivity and distinguished anti-chlorine corrosion performance, including the recent progress in seawater OER electrocatalysts with their corresponding optimized strategies. The future perspectives for the development of seawater-splitting electrocatalysts are also demonstrated.

Recent Advances in Seawater Electrolysis

Hydrogen energy, as a clean and renewable energy, has attracted much attention in recent years. Water electrolysis via the hydrogen evolution reaction at the cathode coupled with the oxygen evolution reaction at the anode is a promising method to produce hydrogen. Given the shortage of freshwater resources on the planet, the direct use of seawater as an electrolyte for hydrogen production has become a hot research topic. Direct use of seawater as the electrolyte for water electrolysis can reduce the cost of hydrogen production due to the great abundance and wide availability. In recent years, various high-efficiency electrocatalysts have made great progress in seawater splitting and have shown great potential. This review introduces the mechanisms and challenges of seawater splitting and summarizes the recent progress of various electrocatalysts used for hydrogen and oxygen evolution reaction in seawater electrolysis in recent years. Finally, the challenges and future opportunities of seawater electrolysis for hydrogen and oxygen production are presented.

Nickel Boride/Boron Carbide Particles Embedded in Boron-Doped Phenolic Resin-Derived Carbon Coating on Nickel Foam for Oxygen Evolution Catalysis in Water and Seawater Splitting

Electrolysis of seawater can be a promising technology, but chloride ions in seawater can lead to adverse side reactions and the corrosion of electrodes. A new transition metal boride-based self-supported electrocatalyst was prepared for efficient seawater electrolysis by directly soaking nickel foam (NF) in a mixture of phenolic resin (PR) and boron carbide (B₄ C), followed by an 800 °C annealing. During PR carbonization process, the reaction of B₄ C and NF generated nickel boride (Ni_x B) with high catalytic activity, while PR-derived carbon coating was doped with boron atoms from B₄ C (B-C_PR ). The B-C_PR coating fixed Ni_x B/B₄ C particles in the frames and holes to improve the space utilization of NF. Meanwhile, the B-C_PR coating effectively protected the catalyst from the corrosion by seawater and facilitates the transport of electrons. The optimal Ni_x B/B₄ C/B-C_PR /NF required 1.50 and 1.58 V to deliver 100 and 500 mA cm^-2 , respectively, in alkaline natural seawater for the oxygen evolution reaction.

The Critical Role of Additive Sulfate for Stable Alkaline Seawater Oxidation on Nickel-Based Electrodes

Seawater electrolysis to produce hydrogen is a critical technology in marine energy projects; however, the severe anode corrosion caused by the highly concentrated chloride is a key issue should be addressed. In this work, we discover that the addition of sulfate in electrolyte can effectively retard the corrosion of chloride ions to the anode. We take nickel foam as the example and observe that the addition of sulfate can greatly improve the corrosion resistance, resulting in prolonged operating stability. Theoretical simulations and in situ experiments both demonstrate that sulfate anions can be preferentially adsorbed on anode surface to form a negative charge layer, which repulses the chloride ions away from the anode by electrostatic repulsion. The repulsive effect of the adsorbed sulfate is also applicable in highly-active catalyst (nickel iron layered double hydroxide) on nickel foam, which shows ca. 5 times stability of that in traditional electrolyte.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER-related proton exchange membrane (PEM) electrolyzers, ORR-related PEM fuel cells, NRR-driven ammonia electrosynthesis from water and nitrogen, and AOR-related direct ammonia fuel cells.

A metal-free photocatalyst for highly efficient hydrogen peroxide photoproduction in real seawater

Artificial photosynthesis of H2O2 from H2O and O2, as a spotless method, has aroused widespread interest. Up to date, most photocatalysts still suffer from serious salt-deactivated effects with huge consumption of photogenerated charges, which severely limit their wide application. Herein, by using a phenolic condensation approach, carbon dots, organic dye molecule procyanidins and 4-methoxybenzaldehyde are composed into a metal-free photocatalyst for the photosynthetic production of H2O2 in seawater. This catalyst exhibits high photocatalytic ability to produce H2O2 with the yield of 1776 μmol g-1h-1 (λ ≥ 420 nm; 34.8 mW cm-2) in real seawater, about 4.8 times higher than the pure polymer. Combining with in-situ photoelectrochemical and transient photovoltage analysis, the active site and the catalytic mechanism of this composite catalyst in seawater are also clearly clarified. This work opens up an avenue for a highly efficient and practical, available catalyst for H2O2 photoproduction in real seawater.

Alkali Halide Boost of Carbon Nitride for Photocatalytic H2 Evolution in Seawater

Photocatalytic H₂ evolution (PHE) from extremely abundant seawater resources is an ideal way to secure sustainable H₂ for humanity, but the saline in seawater easily competitively absorbs the active sites and poisons the catalyst. Herein, a series of low-cost alkali halide (NaI, KI, RbI, CsI, CsBr, and CsCl), analogous to the saline in natural seawater, was selected to modify carbon nitride (MX-CN) through one-step facile pyrolysis with the assistance of water. MX-CN possesses a large amount of negative charges, which could inhibit anion absorption, to some extent, preventing chloride corrosion. Importantly, it can greatly boost the electron transfer between MX-CN and triethanolamine (TEOA) (sacrificial agent) because the alkali cation in seawater can coordinate with TEOA, and easily come in contact with MX-CN through alkali-cation exchange and electrostatic attraction. Benefiting from it, the PHE performance in seawater is 200 times better than that of original CN in deionized water above, and the apparent quantum efficiency of MX-CN (CsI-CN) under 420 nm light irradiation comes to 72% in seawater, the highest value reported for seawater thus far. This work provides a new research direction for engineering the electron transfer pathway between the photocatalyst and sacrificial agent (e.g., pollutant) in natural seawater.

Common-Ion Effect Triggered Highly Sustained Seawater Electrolysis with Additional NaCl Production

Developing efficient seawater-electrolysis system for mass production of hydrogen is highly desirable due to the abundance of seawater. However, continuous electrolysis with seawater feeding boosts the concentration of sodium chloride in the electrolyzer, leading to severe electrode corrosion and chlorine evolution. Herein, the common-ion effect was utilized into the electrolyzer to depress the solubility of NaCl. Specifically, utilization of 6 M NaOH halved the solubility of NaCl in the electrolyte, affording efficient, durable, and sustained seawater electrolysis in NaCl-saturated electrolytes with triple production of H₂, O₂, and crystalline NaCl. Ternary NiCoFe phosphide was employed as a bifunctional anode and cathode in simulative and Ca/Mg-free seawater-electrolysis systems, which could stably work under 500 mA/cm² for over 100 h. We attribute the high stability to the increased Na⁺ concentration, which reduces the concentration of dissolved Cl⁻ in the electrolyte according to the common-ion effect, resulting in crystallization of NaCl, eliminated anode corrosion, and chlorine oxidation during continuous supplementation of Ca/Mg-free seawater to the electrolysis system.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis

Photoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl^- ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O₂ and HClO production in aqueous solution containing Cl^- on photoanodes modified with various metal oxides was investigated. The modification of MnO_x resulted in the promotion of the O₂ evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Optimal Sizing of a Real Remote Japanese Microgrid with Sea Water Electrolysis Plant Under Time-Based Demand Response Programs

Optimal sizing of power systems has a tremendous effective role in reducing the total system cost by preventing unneeded investment in installing unnecessary generating units. This paper presents an optimal sizing and planning strategy for a completely hybrid renewable energy power system in a remote Japanese island, which is composed of photovoltaic (PV), wind generators (WG), battery energy storage system (BESS), fuel cell (FC), seawater electrolysis plant, and hydrogen tank. Demand response programs are applied to overcome the performance variance of renewable energy systems (RESs) as they offer an efficient solution for many problems such as generation cost, high demand peak to average ratios, and assist grid reliability during peak load periods. Real-Time Pricing (RTP), which is deployed in this work, is one of the main price-based demand response groups used to regulate electricity consumption of consumers. Four case studies are considered to confirm the robustness and effectiveness of the proposed schemes. Mixed-Integer Linear Programming (MILP) is utilized to optimize the size of the system’s components to decrease the total system cost and maximize the profits at the same time.

Compositional Control as the Key for Achieving Highly Efficient OER Electrocatalysis with Cobalt Phosphates Decorated Nanocarbon Florets

Compositional interplay of two different cobalt phosphates (Co(H₂ PO₄ )₂ ; Co-DP and Co(PO₃ )₂ ; Co-MP) loaded on morphologically engineered high surface area nanocarbon leads to an increased electrocatalytic efficiency for oxygen evolution reaction (OER) in near neutral conditions. This is reflected as significant reduction in the onset overpotential (301 mV) and enhanced current density (30 mA cm^-2 @ 577 mV). In order to achieve uniform surface loading, organic-soluble thermolabile cobalt-bis(di-tert-butylphosphate) is synthesized in situ inside the nanocarbon matrix and subsequently pyrolyzed at 150 °C to produce Co(H₂ PO₄ )₂ /Co(PO₃ )₂ (80:20 wt%). Annealing this sample at 200 or 250 °C results in the redistribution of the two phosphate systems to 55:45 or 20:80 (wt%), respectively. Detailed electrochemical measurements clearly establish that the 55:45 (wt%) sample prepared at 200 °C performs the best as a catalyst, owing to a relay mechanism that enhances the kinetics of the 4e^- transfer OER process, which is substantiated by micro-Raman spectroscopic studies. It is also unraveled that the engineered nanocarbon support simultaneously enhances the interfacial charge-transfer pathway, resulting in the reduction of onset overpotential, compared to earlier investigated cobalt phosphate systems.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Electricity Production from Marine Water by Sulfide-Driven Fuel Cell

While there is a universal trend to replace fossil fuels at least partially, renewable fuels seem to impose new solutions. Hydrogen sulfide, typical for closed water ponds such as the Black Sea, seems to offer one namely, a new sulfide-driven fuel cell providing for exchange of OH− anions across the membrane by use of hydrogen sulfide in natural marine water. When tested in batch and continuous operation modes, this solution showed that the initial sulfide concentration needed to achieve results of practical value was within 200 to 300 mg dm−3. The predominating final products of the energy production process were sulfite and sulfate ions. Very low overpotentials and mass transfer resistances were observed. The mass balance and the electrochemical parameters showed about 30% efficiency in sulfate ions as the final product. Efforts should be made to enhance sulfide to sulfate conversion. The observed current and power density were comparable and even better than some of the results so far reported for similar systems. Three types of ion exchange membranes were tested. Comparison of their ion conductivity to literature data shows good performance. At higher initial sulfide concentrations polysulfides and thio-compounds were formed with considerably low current yield.

MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

The oxygen evolution reaction (OER) and chlorine evolution reaction (CER) are electrochemical processes with high relevance to water splitting for (solar) energy conversion and industrial production of commodity chemicals, respectively. Carrying out the two reactions separately is challenging, since the catalytic intermediates are linked by scaling relations. Optimizing the efficiency of OER over CER in acidic media has proven especially difficult. In this regard, we have investigated the OER versus CER selectivity of manganese oxide (MnO_x), a known OER catalyst. Thin films (∼5–20 nm) of MnO_x were electrodeposited on glassy carbon-supported hydrous iridium oxide (IrO_x/GC) in aqueous chloride solutions of pH ∼0.9. Using rotating ring–disk electrode voltammetry and online electrochemical mass spectrometry, it was found that deposition of MnO_x onto IrO_x decreases the CER selectivity of the system in the presence of 30 mM Cl^– from 86% to less than 7%, making it a highly OER-selective catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnO_x film is in fact not a catalytically active phase, but functions as a permeable overlayer that disfavors the transport of chloride ions.