Nickel-cobalt nitride nanoneedle supported on nickel foam as an efficient electrocatalyst for hydrogen generation from ammonia electrolysis

Recent progress in non-noble metal nano-electrocatalysts for hybrid water splitting

Hybrid water splitting, which combines thermodynamically favorable inorganic/organic oxidation with hydrogen evolution, typically requires lower cell voltage to achieve the same current density as traditional water splitting. By replacing the sluggish oxygen evolution reaction (OER), the overall energy input required in hybrid water splitting can be greatly decreased. Moreover, by selecting the appropriate anodic substrate, energy-saving hydrogen production can be achieved alongside pollutant degradation or organic upgrading, thereby enhancing its practicality and environmental benefits. Recent advancements in nanostructured non-noble metal catalysts have shown significant potential for enhancing the anodic oxidation reaction performance. These nanocatalysts offer a platform for optimizing the reaction kinetics and selectivity owing to their high surface area and tunable properties, potentially eliminating the need for noble metal catalysts in hybrid water splitting. This review summarizes recent advances in non-noble metal nanocatalysts for diverse alternative anodic oxidation reactions, including pollutants' oxidative degradation and selective organic upgrading. Their performance, mechanism, and practical applications in hybrid water splitting are also highlighted. This review also discusses current challenges and future directions, such as targeted catalyst design, industrial-scale evaluation, electrolyte system optimization, and production collection-related problems. By addressing these issues, hybrid water splitting holds the promise of becoming a transformative technique for sustainable hydrogen production, offering both economic and environmental advantages.

Electrochemically driven inactivation of harmful cyanobacteria assisted with the visible light photocatalyst of CeO2-Fe3O4-CQDs

The application of photoelectrocatalysis in the inactivation of algae is attracting growing attention. In this study, CeO2-Fe3O4-carbon quantum dots (CQDs) were prepared as particle electrodes to assess their effectiveness in inactivating cyanobacteria within a three-dimensional (3D) electrochemical system under visible light. The results showed that both algal cells and extracellular organic matter (EOM) could be efficiently removed. The incorporation of 10 mg/mL CQDs significantly boosts the efficiency of photoelectrocatalysis under the optimal conditions, where the CeO2-Fe3O4-CQDs concentration is 0.20 g/L, j = 21 mA/cm2, under a neutral pH. Notably, the CeO₂-Fe3O4-CQDs exhibit favorable electrochemical catalytic performance, outstanding photocurrent responsiveness, and efficient magnetic separation (for recyclability). The coexistence of Ce3+ and Ce4+ species gives rise to the formation of a redox couple (Ce3+/Ce4+) at the interface of CeO₂, thereby demonstrating favorable performance in terms of oxygen release and storage capabilities. The activities of superoxide dismutase (SOD), catalase (CAT), and malondialdehyde (MDA) confirm the apoptosis of algal cells. Hydroxyl radicals (•OH) and singlet oxygen (1O₂) are participated in the algae inactivation process, with 1O₂ playing a more prominent role. Our research clarifies the complex mechanisms of algal apoptosis and promotes practical algae inactivation from wastewater.

Self-sacrificing and self-supporting biomass carbon anode-assisted water electrolysis for low-cost hydrogen production

Electrooxidation of renewable and CO2-neutral biomass for low-cost hydrogen production is a promising and green technology. Various biomass platform molecules (BPMs) oxidation assisted hydrogen production technologies have obtained noticeable progress. However, BPMs anodic oxidation is highly dependent on electrocatalysts, and the oxidation mechanism is ambiguous. Meanwhile, the complexity and insolubility of natural biomass severely constrain the efficient utilization of biomass resources. Here, we develop a self-sacrificing and self-supporting carbon anode (SSCA) using waste corncobs. The combined results from multiple characterizations reveal that the structure-property-activity relationship of SSCA in carbon oxidation reaction (COR). Theoretical calculations demonstrate that carbon atoms with a high spin density play a pivotal role in reducing the adsorption energy of the reactive oxygen intermediate (*OH) during the transition from OH- to *OH, thereby promoting COR. Additionally, the HER||COR system allows driving a current density of 400 [Formula: see text] at 1.24 V at 80 °C, with a hydrogen production electric consumption of 2.96 kWh Nm-3 (H2). The strategy provides a ground-breaking perspective on the large-scale utilization of biomass and low-energy water electrolysis for hydrogen production.

The Enhanced Performance of NiCuOOH/NiCu(OH)2 Electrode Using Pre-Conversion Treatment for the Electrochemical Oxidation of Ammonia

Electrochemical oxidation of ammonia is an attractive process for wastewater treatment, hydrogen production, and ammonia fuel cells. However, the sluggish kinetics of the anode reaction has limited its applications, leading to a high demand for novel electrocatalysts. Herein, the electrode with the in situ growth of NiCu(OH)2 was partially transformed into the NiCuOOH phase by a pre-treatment using highly oxidative solutions. As revealed by SEM, XPS, and electrochemical analysis, such a strategy maintained the 3D structure, while inducing more active sites before the in situ generation of oxyhydroxide sites during the electrochemical reaction. The optimized NiCuOOH-1 sample exhibited the current density of 6.06 mA cm-2 at 0.5 V, which is 1.67 times higher than that of NiCu(OH)2 (3.63 mA cm-2). Moreover, the sample with a higher crystalline degree of the NiCuOOH phase exhibited lower performance, demonstrating the importance of a moderate treatment condition. In addition, the NiCuOOH-1 sample presented low selectivity (<20%) towards NO2- and stable activity during the long-term operation. The findings of this study would provide valuable insights into the development of transition metal electrocatalysts for ammonia oxidation.

Design of Superior Electrocatalysts for Proton-Exchange Membrane-Water Electrolyzers: Importance of Catalyst Stability and Evolution

By virtue of the high energy conversion efficiency and compact facility, proton exchange membrane water electrolysis (PEMWE) is a promising green hydrogen production technology ready for commercial applications. However, catalyst stability is a challenging but often-ignored topic for the electrocatalyst design, which retards the device applications of many newly-developed electrocatalysts. By defining catalyst stability as the function of activity versus time, we ascribe the stability issue to the evolution of catalysts or catalyst layers during the water electrolysis. We trace the instability sources of electrocatalysts as the function versus time for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acid and classify them into internal and external sources. Accordingly, we summarize the latest studies for stability improvements into five strategies, i. e., thermodynamic stable active site construction, precatalyst design, support regulation, superwetting electrode fabrication, and catalyst-ionomer interface engineering. With the help of ex-situ/ in-situ characterizations and theoretical calculations, an in-depth understanding of the instability sources benefits the rational development of highly active and stable HER/OER electrocatalysts for PEMWE applications.

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.

Developing energy-efficient N-doping technology to controllably construct N-Ru2P@Ru nanospheres for highly efficient hydrogen evolution at an ampere-level current density

The N-doping strategy plays a vital role in optimizing electrocatalytic performance, but it often requires high-temperatures accompanied by the emission of irritating gases, which is contrary to the concept of energy saving and environmental protection. Based on this, this work innovatively uses the quenching of waste heat and the non-equilibrium state of materials to realize controllable N-doping. Notably, N dopants stimulate metal-like electroconductivity and accelerate the alkaline HER kinetics by optimizing the electronic structure of Ru2P. Surprisingly, the hydrophilic Ru core and the N-Ru2P shell with a low HER reaction energy barrier synergistically expedite hydrogen release. As anticipated, the current density of N-Ru2P@Ru (963 mA cm-2) is 2.6-fold that of Pt/C (359 mA cm-2) at 150 mV. Overall, the novel N-doping technology greatly simplifies material preparation procedures and reduces energy consumption. Moreover, this unique N-doping strategy provides a new idea for optimizing the catalyst structure and reaction kinetics.

Recent Progress on Hydrogen Production from Ammonia Decomposition: Technical Roadmap and Catalytic Mechanism

Ammonia decomposition has attracted significant attention in recent years due to its ability to produce hydrogen without emitting carbon dioxide and the ease of ammonia storage. This paper reviews the recent developments in ammonia decomposition technologies for hydrogen production, focusing on the latest advances in catalytic materials and catalyst design, as well as the research progress in the catalytic reaction mechanism. Additionally, the paper discusses the advantages and disadvantages of each method and the importance of finding non-precious metals to reduce costs and improve efficiency. Overall, this paper provides a valuable reference for further research on ammonia decomposition for hydrogen production.

Kirkendall effect Strengthened-Superhydrophilic/superaerophobic Co-Ni3N/NF heterostructure as electrode catalyst for High-current hydrogen production

The development of efficient electrocatalysts for large-scale water electrolysis is crucial and challenging. Research efforts towards interface engineering and electronic structure modulation can be leveraged to enhance the electrochemical performance of the developed catalysts. In this work, a surface-engineered Co-Ni₃N/NF heterostructure electrode was prepared based on Kirkendall effect for high-current water electrolysis. In the experiments, the textural feature and intrinsic activity of the Co-Ni₃N/NF heterostructure were tuned through cobalt-doping and the creation of structural defects. As a result, the increased surface energy endowed Co-Ni₃N/NF heterostructure with superhydrophilic and superaerophobic properties. Meanwhile, the contact area of the gas-liquid-solid three phases was optimized. With a large underwater bubble contact angle (CA) of 169°, the electrolyte solution can infiltrate the Co-Ni₃N/NF electrode within 150 ms. Sequentially, the generated gas bubbles were able to detach at high frequency, which ensured the rapid mass exchange. The performance tests showed that the optimal Co-Ni₃N/NF electrode sample reached current densities of 100 mA cm^-2 and 500 mA cm^-2 at the overpotentials of 98 mV and 123 mV, respectively. Benefiting from the reduction of hydrogen embrittlement, the HER performance of the prepared Co-Ni₃N/NF electrode sample decreased slightly after 100 h durability test, but the overall structure remained well. Those results allowed us to conclude that the prepared Co-Ni₃N/NF electrocatalyst holds the promises for large-scale water electrolysis in industries. More specifically, this work provided a new perspective that the efficiency of electrocatalysts for large-scale water electrolysis can be enhanced by constructing a heterostructure with good wettability and gas repellency.

Self-Terminating Surface Reconstruction Induced by High-Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen

Ammonia (NH₃ ) is a promising hydrogen (H₂ ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO₂ ) polyhedrons with exposed high-index facets are prepared by a one-step NH₃ -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO₂ facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO₂ and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH₃ and exhibit a potential improvement of 260 mV for electrocatalytic AOR at 10 mA cm^-2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH₃ to *NH₂ ) is presented on high-index CuFeO₂ facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.