Immobilization of Fe-Doped Ni2P Particles Within Biomass Agarose-Derived Porous N,P-Carbon Nanosheets for Efficient Bifunctional Oxygen Electrocatalysis

Waste-Derived Catalysts for Water Electrolysis: Circular Economy-Driven Sustainable Green Hydrogen Energy

The sustainable production of green hydrogen via water electrolysis necessitates cost-effective electrocatalysts. By following the circular economy principle, the utilization of waste-derived catalysts significantly promotes the sustainable development of green hydrogen energy. Currently, diverse waste-derived catalysts have exhibited excellent catalytic performance toward hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water electrolysis (OWE). Herein, we systematically examine recent achievements in waste-derived electrocatalysts for water electrolysis. The general principles of water electrolysis and design principles of efficient electrocatalysts are discussed, followed by the illustration of current strategies for transforming wastes into electrocatalysts. Then, applications of waste-derived catalysts (i.e., carbon-based catalysts, transitional metal-based catalysts, and carbon-based heterostructure catalysts) in HER, OER, and OWE are reviewed successively. An emphasis is put on correlating the catalysts' structure-performance relationship. Also, challenges and research directions in this booming field are finally highlighted. This review would provide useful insights into the design, synthesis, and applications of waste-derived electrocatalysts, and thus accelerate the development of the circular economy-driven green hydrogen energy scheme.

Facile Synthesis of Carbon Cloth Supported Cobalt Carbonate Hydroxide Hydrate Nanoarrays for Highly Efficient Oxygen Evolution Reaction

Developing efficient and low-cost replacements for noble metals as electrocatalysts for the oxygen evolution reaction (OER) remain a great challenge. Herein, we report a needle-like cobalt carbonate hydroxide hydrate (Co(CO₃)_0.5OH·0.11H₂O) nanoarrays, which in situ grown on the surface of carbon cloth through a facile one-step hydrothermal method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations demonstrate that the Co(CO₃)_0.5OH nanoarrays with high porosity is composed of numerous one-dimensional (1D) nanoneedles. Owing to unique needle-like array structure and abundant exposed active sites, the Co(CO₃)_0.5OH@CC only requires 317 mV of overpotential to reach a current density of 10 mA cm⁻², which is much lower than those of Co(OH)₂@CC (378 mV), CoCO₃@CC (465 mV) and RuO₂@CC (380 mV). For the stability, there is no significant attenuation of current density after continuous operation 27 h. This work paves a facile way to the design and construction of electrocatalysts for the OER.

Nanostructured Carbon-Nitrogen-Sulfur-Nickel Networks Derived From Polyaniline as Bifunctional Catalysts for Water Splitting

The development of reliable production routes for sustainable hydrogen (H₂), which is an essential feedstock for industrial processes and energy carrier for fuel cells, is needed. It appears to be an unavoidable alternative to significantly reduce the dependence on conventional energy sources based on fossil fuels without increasing the atmospheric CO₂ levels. Among the different power-to-X scenarios to access high purity H₂, the electrochemical approach based on electrolysis looks to be a promising sustainable solution at both the small and large industrial scales. However, the practical realization of this important opportunity faces several challenges, including the efficient design of cost-effective catalytic materials to be used as a cathode with improved intrinsic and durable activity. In this contribution, we report the design and development of efficient nanostructured catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in aqueous media, whereby noble metal-free elements are embedded in a matrix of a conducting polymer, polyaniline (PANI). To increase the electrical conductivity and further the electrocatalytic ability toward HER of the chemically polymerized PANI in the presence of nickel (II) salt (nitrate), the PANI-based materials have first been stabilized at a mild temperature of 250-350°C in air and then carbonized at 800-1,000°C under nitrogen gas to convert the chemical species into nitrogen, sulfur, nickel, and carbon nanostructured networks (CNNs). Different physicochemical (TGA-DSC, Raman spectroscopy, XRD, SEM, EDX, ICP, CHNS, BET, and XPS) and electrochemical (voltammetry and electrochemical impedance spectrometry) methods have been integrated to characterize the as-synthesized CNNs materials and interrogate the relationship of material-to-performance. It has been found that those synthesis conditions allow for the substantial increase of the electrocatalytic performance toward HER and OER in alkaline media in terms of the onset potential and charge transfer resistance and overpotential at the specific activity of 10 milliamps per square centimeter, thus ranking the present materials among the most efficient noble metal-free catalysts and making them possible candidates for integration in practical low-energy consumption alkaline electrolyzers.

Co/CoP Nanoparticles Encapsulated Within N, P-Doped Carbon Nanotubes on Nanoporous Metal-Organic Framework Nanosheets for Oxygen Reduction and Oxygen Evolution Reactions

Herein, Co/CoP nanoparticles encapsulated with N, P-doped carbon nanotubes derived from the atomic layer deposited hexagonal metal-organic frameworks (MOFs) are obtained by calcinations and subsequent phosphating and are employed as electrocatalyst. The electrocatalytic performance evaluations show that the as-prepared electrocatalyst exhibits an overpotential of 342 mV at current density of 10 mA cm-2 and the Tafel slope of 74 mV dec-1 for oxygen evolution reaction (OER), which is superior to the most advanced ruthenium oxide electrocatalyst. The electrocatalyst also shows better stability than the benchmark RuO2. After 9 h, the current density is only decreased by 10%, which is far less than the loss of RuO2. Moreover, its onset potential for oxygen reduction reaction (ORR) is 0.93 V and follows the ideal 4-electron approach. After the stability test, the current density of the electrocatalyst retains 94% of the initial value, which is better than Pt/C. The above results indicate that the electrocatalyst has bifunctional activity and excellent stability both for OER and ORR. It is believed that this strategy provides guidance for the synthesis of cobalt phosphide/carbon-based electrocatalysts.