Ti-Mesh supported porous CoS2 nanosheet self-interconnected networks with high oxidation states for efficient hydrogen production via urea electrolysis

Recent Development of Electrodes Construction for HER in Electrocatalytic Water Splitting

Production of hydrogen (H2) fuel using the hydrogen evolution reaction (HER) through electrocatalysis of water splitting is inexpensive, has optimal performance, and offers remarkable stability. Developing electrocatalysts with excellent stability and high efficiency has been a significant and challenging factor for practical applications of HER for decades. Hydrogen generation occurs on the HER electrode due to the emission of bubbles, proton diffusion, and the transfer of electrons. These considerations should be taken into account during the construction and development of the electrode. This review offers a synopsis of recent advancements in various electrodes used as a base for electrocatalysts, such as nickel foam, titanium foil, copper foam, carbon foam, and others, and discusses their HER catalytic activity, with a focus on the emission of bubbles, diffusion of ions, the structure of the electrode, and the formulation and preparation process. In conclusion, we provide an overview of ideas to further improve and address the significant issues in the manufacture of HER electrodes.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Polypyrrole regulates Active Sites in Co-based Catalyst in Direct Borohydride Fuel Cells

Direct borohydride fuel cells (DBFCs) convert borohydride (NaBH4) chemical energy into clean electricity. However, catalytic active site deactivation in NaBH4 solution limits their performance and stability. We propose a strategy to regulate active sites in Co-based catalysts using polypyrrole modification (Co-PX catalyst) to enhance electrochemical borohydride oxidation reaction (eBOR). As an anode catalyst, the synthesized Co-PX catalyst exhibits excellent eBOR performance in DBFCs, with current density of 280 mA ⋅ cm-2 and power density of 151 mW ⋅ cm-2, nearly twice that of the unmodified catalyst. The Co-PX catalyst shows no degradation after 120-hour operation, unlike the rapidly degrading control. In-situ electrochemical attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIRS) and density functional theory (DFT) suggest that polypyrrole-modified carbon support regulate the charge distribution, increasing oxidation state and optimizing adsorption/desorption of intermediates. A possible reaction pathway is proposed. This work presents a promising strategy for efficient polymer-modulated catalysts in advanced DBFCs.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Development of starch-based amorphous CoOx self-supporting carbon aerogel electrocatalyst for hydrogen evolution

Hydrogen energy is turning into a major research topic in this complex and changing world. In recent years, more and more research has been done on transition metal oxides and biomass composites. In this study, potato starch and amorphous cobalt oxide were assembled into carbon aerogel by sol-gel method and high-temperature annealing (CoOx/PSCA). The connected porous structure of the carbon aerogel is conducive to HER mass transfer, and its structure can avoid the agglomeration of transition metals. It also has great mechanical properties and can be directly used as a self-supporting catalyst for electrolysis with 1 M KOH for hydrogen evolution, which showed excellent HER activity and yielded the effective current density of 10 mA cm^-2 at 100 mV overpotential. Electrocatalytic experiments further showed that the better performance of CoOx/PSCA for HER can be attributed to the high electrical conductivity of carbon and the synergistic effect of unsaturated catalytic sites on the amorphous CoOx cluster. The catalyst comes from a wide range of sources, is easy to produce, and has good long-term stability, so it can be used in large-scale production. This paper provides a simple and easy method to make biomass-based transition metal oxide composites for electrolyzing water to produce hydrogen.

Construction of Nitrogen-Doped Biphasic Transition-Metal Sulfide Nanosheet Electrode for Energy-Efficient Hydrogen Production via Urea Electrolysis

Urea-assisted hybrid water splitting is a promising technology for hydrogen (H₂ ) production, but the lack of cost-effective electrocatalysts hinders its extensive application. Herein, it is reported that Nitrogen-doped Co₉ S₈ /Ni₃ S₂ hybrid nanosheet arrays on nickel foam (N-Co₉ S₈ /Ni₃ S₂ /NF) can act as an active and robust bifunctional catalyst for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), which could drive an ultrahigh current density of 400 mA cm^-2 at a low working potential of 1.47 V versus RHE for UOR, and gives a low overpotential of 111 mV to reach 10 mA cm^-2 toward HER. Further, a hybrid water electrolysis cell utilizing the synthesized N-Co₉ S₈ /Ni₃ S₂ /NF electrode as both the cathode and anode displays a low cell voltage of 1.40 V to reach 10 mA cm^-2 , which can be powered by an AA battery with a nominal voltage of 1.5 V. The density functional theory (DFT) calculations decipher that N-doped heterointerfaces can synergistically optimize Gibbs free energy of hydrogen and urea, thus accelerating the catalytic kinetics of HER and UOR. This work significantly advances the development of the promising cobalt-nickel-based sulfide as a bifunctional electrocatalyst for energy-saving electrolytic H₂ production and urea-rich innocent wastewater treatment.

Modulating the electronic structure of CoS2 by Sn doping boosting urea oxidation for efficient alkaline hydrogen production

Urea electrocatalytic oxidation afforded by renewable energies is highly promising to replace the sluggish oxygen evolution reaction in water splitting for hydrogen production while realizing the treatment of urea-rich waste water. Therefore, the development of efficient and cost-effective catalysts for water splitting assisted by urea is highly desirable. Herein, Sn-doped CoS₂ electrocatalysts were reported with the engineered electronic structure and the formation of Co-Sn dual active sites for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), respectively. Consequently, the number of active sites and the intrinsic activity were enhanced simultaneously and the resultant electrodes exhibited outstanding electrocatalytic activity with a very low potential of 1.301 V at 10 mA·cm^-2 for UOR and an overpotential of 132 mV at 10 mA·cm^-2 for HER. Therefore, a two-electrode device was assembled by employing Sn(2)-CoS₂/CC and Sn(5)-CoS₂/CC and the constructed cell required only 1.45 V to approach a current density of 10 mA·cm^-2 along with good durability for at least 95 h assisted by urea. More importantly, the assembled electrolyzer can be powered by commercial dry battery to generate numerous gas bubbles on the surface of the electrodes, demonstrating the high potential of the as-fabricated electrodes for applications in hydrogen production and pollutant treatment at a low-voltage electrical energy input.

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Rapidly reconstructing the active surface of cobalt-based perovskites for alkaline seawater splitting

As a potential oxygen evolution reaction (OER) catalyst, Co-based perovskites have received intensive attention. However, Sr readily accumulates on their surface, and makes them inert toward the OER. Herein, we propose a simple but versatile electrochemical reduction method to reconstruct the active surface of Co-based perovskites within a few seconds. By this method, Sr rapidly precipitates from Co-based perovskites, accompanied by the introduction of Sr and oxygen vacancies. After reconstruction, the electrochemical active surface areas of Co-based perovskites greatly increase, and the OER overpotential of the optimized SrNb_0.1Co_0.7Fe_0.2O_3-δ (ER-SNCF-20s) reaches 278 mV at 10 mA cm^-2. This can be explained by the decrease of overpotentials at the rate-determining step. Using ER-SNCF-20s, the splitting voltage of alkaline natural seawater can reach 1.56 V at 10 mA cm^-2, and remains steady for 300 h. This effort offers a feasible method for reconstructing the active surface of Co-based perovskites.

The effect of heteroatom doping on the active metal site of CoS2 for hydrogen evolution reaction

The exploration of cost-effective hydrogen evolution reaction (HER) electrocatalysts through water splitting is important for developing clean energy technology and devices. The application of CoS2 in HER has been drawing more and more attention due to its low cost and relatively satisfactory HER catalytic performance. And CoS2 was found to exhibit excellent HER catalytic performance after appropriate doping according to other experimental investigations. However, the theoretical simulation and the intrinsic catalytic mechanism of CoS2 remains insufficiently investigated. Therefore, in this study, density functional theory is used to investigate the HER catalytic activity of CoS2 doped with a heteroatom. The results show that Pt-, N- and O-doped CoS2 demonstrates smaller Gibbs free energies close to that of Pt, compared with the original CoS2 and CoS2 doped with other atoms. Furthermore, HER catalytic performance of CoS2 can be improved by tuning d-band centers of H adsorption sites. This study provides an effective method to achieve modified CoS2 for high-performance HER and to investigate other transition metal sulfides as HER electrode.

Construction of MnxCoyO4/Ti electrocatalysts for efficient bifunctional water splitting

In this work, we report the design and synthesis of non-noble metal-based electrocatalysts for effective overall water splitting in alkaline solutions for the development of hydrogen energy. The electrocatalysts were synthesized by a one-step hydrothermal method similar to microflower structure electrocatalysts. The synergistic effect between the special Echinops sphaerocephalus nanostructure and the nanowire can greatly improve the conductivity of the nanomaterial due to its high activity quality, fast ion transport, and exposure of more active sites, thus resulting in a better catalytic activity and a longer material stability of the electrocatalyst. For Mn_xCo_yO₄/Ti in alkaline aqueous solutions, a current density of 10 mA cm^-2 is required when the voltage is only 1.60 V. In addition, the hydrogen evolution activity of electrocatalysts is 168 mV at 10 mA cm^-2, the Tafel slope is 174 mV dec^-1, and the oxygen evolution activity of electrocatalysts is 229 mV at 10 mA cm^-2, which showed good long-term stability within 12 h, even better than that of previously reported electrocatalysts.

Advanced Nickel-Based Catalysts for Urea Oxidation Reaction: Challenges and Developments

The electrochemical urea oxidation reaction (UOR) is crucial for determining industrial and commercial applications of urea-based energy conversion devices. However, the performance of UOR is limited by the dynamic complex of the six-electron transfer process. To this end, it is essential to develop efficient UOR catalysts. Nickel-based materials have been extensively investigated owing to their high activity, easy modification, stable properties, and cheap and abundant reserves. Various material designs and strategies have been investigated in producing highly efficient UOR catalysts including alloying, doping, heterostructure construction, defect engineering, micro functionalization, conductivity modulation, etc. It is essential to promptly review the progress in this field to significantly inspire subsequent studies. In this review, we summarized a comprehensive investigation of the mechanisms of oxidation or poisoning and UOR processes on nickel-based catalysts as well as different approaches to prepare highly active catalysts. Moreover, challenges and prospects for future developments associated with issues of UOR in urea-based energy conversion applications were also discussed.

Boosting alkaline water splitting and the urea electrolysis kinetic process of a Co3O4 nanosheet by electronic structure modulation of F, P co-doping

Designing non-precious metal electrocatalysts for accelerated electron transfer and richer active site exposure is necessary and challenging to achieve the versatility of electrocatalysts. In this research, a self-grown nanosheet array electrocatalyst on nickel foam with high structural stability is first rationally designed through suitable anionic doping. The combined experimental and theoretical calculations reveal that the F-P-Co₃O₄/NF material optimizes the adsorption energy of hydrogen/water through electron coupling, and its nanosheet structure provides abundant active sites, accelerating the mass and electron transfer in the reaction process. It is worth noting that the as-developed F-P-Co₃O₄/NF materials exhibit outstanding catalytic activity for overpotentials of 192 and 110 mV at a current density of 10 mA cm^-2 for the oxygen evolution reaction and the hydrogen evolution reaction in 1 M KOH, respectively. More notably, an assembled F-P-Co₃O₄/NF//F-P-Co₃O₄/NF alkaline electrolytic cell requires only an ultra-low cell voltage of 1.53 V to achieve a current density of 10 mA cm^-2, which is one of the best activities reported so far. Furthermore, F-P-Co₃O₄/NF also shows excellent performance for urea electrolysis. Theoretical calculations show that the superior activity of the F-P-Co₃O₄/NF catalyst is attributed to the optimal electron configuration and the lower Gibbs free energy of hydrogen adsorption due to co-doping of P and F. The work provides an alternative solution for the preparation of electrocatalysts with high structural stability, high catalytic activity and multifunction for alkaline water splitting and urea electrolysis.

Electrocatalyst based on Ni2P nanoparticles and NiCoP nanosheets for efficient hydrogen evolution from urea wastewater

Urea electrolysis is a promising approach to produce hydrogen while simultaneously purifying urea-rich wastewater. In practice, it is highly desirable but still challenging, through the structure construction strategy, to implement a method with controllable synthesis of ultra-thin nanosheet arrays with rich interfaces, and then apply them into the catalysis operations of hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). In this work, the bifunctional electrocatalyst Ni₂P/NiCoP nanosheets anchored nickel foam (NF) were prepared with ultra-thin rich interfaces by regulating the Co- and P-doping. The results showed that the elaborated Ni₂P/NiCoP/NF electrode delivered the excellent electrocatalytic activities for both UOR and HER operations. Particularly for UOR, it required only a cell voltage of 1.41 V at 100 mA cm^-2, which was 400 mV lower than that in the traditional overall water splitting operation.

A Nickel–Salen as a Model for Bifunctional OER/UOR Electrocatalysts: Pyrolysis Temperature–Electrochemical Activity Interconnection

The nickel derivative of a salen-type Schiff base is pyrolyzed under controlled conditions to form a nano-size Ni/NiOx core-shell species that serves as a model for a proof-of-concept investigation of...

A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting

Electrolyzing water technology to prepare high-purity hydrogen is currently an important field in energy development. However, the preparation of efficient, stable, and inexpensive hydrogen production technology from electrolyzed water is a major problem in hydrogen energy production. The key technology for hydrogen production from water electrolysis is to prepare highly efficient catalytic, stable and durable electrodes, which are used to reduce the overpotential of the hydrogen evolution reaction and the oxygen evolution reaction of electrolyzed water. The main strategies for preparing catalytic electrodes include: (i) choosing cheap, large specific surface area and stable base materials, (ii) modulating the intrinsic activity of the catalytic material through elemental doping and lattice changes, and (iii) adjusting the morphology and structure to increase the catalytic activity. Based on these findings, herein, we review the recent work in the field of hydrogen production by water electrolysis, introduce the preparation of catalytic electrodes based on nickel foam, carbon cloth and new flexible materials, and summarize the catalytic performance of metal oxides, phosphides, sulfides and nitrides in the hydrogen evolution and oxygen evolution reactions. Secondly, parameters such as the overpotential, Tafel slope, active site, turnover frequency, and stability are used as indicators to measure the performance of catalytic electrode materials. Finally, taking the material cost of the catalytic electrode as a reference, the successful preparations are comprehensively compared. The overall aim is to shed some light on the exploration of high-efficiency and economical electrodes in energy chemistry and also demonstrate that there is still room for discovering new combinations of electrodes including base materials, composition lattice changes and morphologies.

Novel WS2 /Fe0.95 S1.05 Hierarchical Nanosphere as a Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

Fe_0.95 S_1.05 with high reactivity and stability was incorporated into WS₂ nanosheets via a one-step solvothermal method for the first time. The resulted hybrid catalyst has much higher catalytic activity than WS₂ and Fe_0.95 S_1.05 alone, and the optimal WS₂ /Fe_0.95 S_1.05 hybrid catalyst was found by adjusting the feed ratio. The addition of Fe_0.95 S_1.05 was proven to be able to enhance the hydrogen evolution reaction (HER) activity of WS₂ , and vice versa. At the same time, it was found that the catalytic effect of the hybrid catalyst was the best when the feed ratio was W : Fe=2 : 1. In other words, we confirmed that there is a synergistic effect between W- and Fe-based sulfide hybrid catalysts, and validated that the reason for the improved HER performance is the strong interaction between the two in the middle sulfur. WS₂ /Fe_0.95 S_1.05 -2 hybrid catalyst leads to enhanced HER activity, which shows a low overpotential of ∼0.172 V at 10 mA cm^-2 , low Tafel slope of ∼53.47 mV/decade. This study supplies innovative synthesis of a highly active WS₂ /Fe_0.95 S_1.05 hybrid catalyst for HER.

Hydrogen and Potassium Acetate Co-Production from Electrochemical Reforming of Ethanol at Ultrathin Cobalt Sulfide Nanosheets on Nickel Foam

The sluggish reaction kinetics of the anodic oxygen evolution reaction increases the energy consumption of the overall water electrolysis for high-purity hydrogen generation. In this work, ultrathin cobalt sulfide nanosheets (Co₃S₄-NSs) on nickel foam (Ni-F) nanohybrids (termed as Co₃S₄-NSs/Ni-F) are synthesized using cyanogel hydrolysis and a sulfurization two-step approach. Physical characterizations reveal that Co₃S₄-NSs with a 1.7 nm thickness have abundant holes, implying the big surface area, abundant active edge atoms, and sufficient active sites. Electrochemical measurements show that as-synthesized Co₃S₄-NSs/Ni-F have excellent electrocatalytic activity and selectivity for ethanol oxidation reaction and hydrogen evolution reaction. Due to their bifunctional property of Co₃S₄-NSs/Ni-F nanohybrids, a symmetric Co₃S₄-NSs/Ni-F∥Co₃S₄-NSs/Ni-F ethanol electrolyzer can be effectively constructed, which only requires a 1.48 V electrolysis voltage to reach a current density of 10 mA cm^-2 for high-purity hydrogen generation at the cathode as well as value-added potassium acetate generation at the anode, much lower than the electrolysis voltage of traditional electrochemical water splitting (1.64 V).