Innovative FeNiHOF Catalyst Boosts Alkaline Water Electrolysis Efficiency

Summary

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The oxygen evolution reaction, or OER, plays a critical role in a wide range of industrial applications, most notably in the context of alkaline water electrolyzers. These devices are pivotal for the sustainable production of hydrogen, a clean fuel with the potential to significantly reduce carbon emissions in various sectors. The efficiency of these electrolyzers hinges on the performance of the electrocatalysts used to drive the OER, as this reaction is inherently slow and energy-intensive. Recent advancements have introduced the FeNi hydroxide-organic framework, abbreviated as FeNiHOF, as a highly innovative electrocatalyst. This new catalyst has shown remarkable promise in enhancing the efficiency and performance of OER. The FeNiHOF electrocatalyst is synthesized through a straightforward hydrothermal method, where FeNi nanosheets are grown on conductive nickel foam. This method is not only scalable but also cost-effective, making it a viable option for large-scale industrial applications. The unique structure of FeNiHOF provides several advantages. The nanosheets form a vertical array architecture, which significantly increases the surface area and porosity. These characteristics are essential for the efficient dynamic transport of reactants and the swift release of gaseous products, particularly at high current densities. The structural integrity and high surface area of FeNiHOF ensure that it can sustain the rigorous conditions of industrial-scale OER. Moreover, the FeNiHOF electrocatalyst undergoes a cyclic voltammetry activation process. This process leads to the formation of highly active and stable redox species, which further enhance the catalytic performance. The activation process induces a self-reconstruction within the catalyst, transforming it into a more reactive and efficient form for the OER. In summary, the FeNiHOF electrocatalyst represents a significant advancement in the field of electrolysis. Its innovative design and exceptional performance metrics make it a strong candidate for improving the efficiency and sustainability of hydrogen production through alkaline water electrolyzers. The potential for widespread industrial application of FeNiHOF could mark a pivotal step towards more sustainable energy solutions. The synthesis of FcNiOF nanosheets is achieved through a scalable hydrothermal method, which involves growing these nanosheets on a conductive nickel foam substrate. The synthesis process is straightforward and conducive to industrial implementation. During this process, the nickel foam undergoes a noticeable color change to dark yellow, signifying the successful growth of FcNiOF nanosheets on its surface. To understand the structural properties of the synthesized FcNiOF, several advanced characterization techniques are employed. Scanning electron microscopy, or SEM, provides detailed images showing that the nanosheets uniformly cover the nickel foam substrate, forming a vertical array structure. This unique architecture contributes to the high porosity and large surface area of the nanosheets, which are crucial for efficient catalysis, particularly at high current densities. Transmission electron microscopy, or TEM, further reveals that these unit nanosheets possess thin, leaf-like structures with smooth surfaces. The average thickness of these nanosheets is approximately twenty-eight point nine nanometers, with radial dimensions extending to several hundred nanometers. The thin and expansive nature of these nanosheets is beneficial for maximizing the exposure of active sites, thereby enhancing catalytic activity. X-ray diffraction, or XRD, analysis of FcNiOF confirms the crystalline structure of the material. The diffraction peaks align closely with simulated patterns based on the FcZn-MOF structure, indicating a well-defined crystal formation. Each nickel atom within the structure is coordinated by six oxygen atoms from two carboxylate groups on different Fc units, creating a two-dimensional lamellar structure. This crystallinity is essential for the stability and performance of the catalyst under operational conditions. Energy-dispersive X-ray spectroscopy, or EDS, is used to analyze the elemental composition of FcNiOF. The EDS results indicate the presence of iron, nickel, carbon, and oxygen elements, with their corresponding elemental mapping visualizing a uniform distribution within the nanosheets. This uniformity in composition is critical for ensuring consistent catalytic performance across the entire material. Further probing into the chemical composition and coordination involves Fourier-transform infrared spectroscopy, or FT-IR, Raman spectroscopy, and X-ray photoelectron spectroscopy, or XPS. The FT-IR spectra indicate successful conjugation of nickel atoms on the carboxyl groups of Fc units, as evidenced by the disappearance of the C = O stretching vibration band. Raman spectra identify characteristic vibration modes of Fc units, while XPS analysis confirms the presence of nickel, iron, carbon, and oxygen, consistent with the EDS results. The high-resolution spectra reveal specific binding energies associated with Ni2+ and Fe2+ ions, indicative of their coordination within the structure. These comprehensive characterization techniques collectively highlight the significant structural attributes of FcNiOF nanosheets. The high porosity, large surface area, and well-defined crystalline structure are pivotal for their enhanced catalytic performance. The uniform elemental distribution and robust chemical coordination further contribute to the stability and efficiency of FcNiOF as an electrocatalyst, making it a promising candidate for industrial applications in alkaline water electrolyzers. The cyclic voltammetry, or CV, activation process is a crucial step in deriving highly active and stable redox species from FcNiOF. This process involves subjecting the FcNiOF nanosheets to a series of cyclic voltammetric scans, which induces dynamic reconstruction within the material. The CV curves show a gradual increase in the closed areas with each scan cycle, reaching a steady maximum by the tenth cycle. This increase indicates the proliferation of active sites, as evidenced by the growing metal nickel and iron redox peaks, which signify increased pseudocapacitance. One of the significant changes observed during the CV activation process is the decrease in overpotential. Initially, the overpotential at zero point two amperes per centimeter squared is two hundred eighty-four millivolts. However, after ten cycles, this value drops to two hundred fifty-two millivolts, indicating a more efficient oxygen evolution reaction, or OER, activation. This reduction in overpotential highlights the enhanced catalytic activity of the material. In situ electrochemical impedance spectroscopy, or EIS, further demonstrates the improvements in electrochemical properties. The charge transport resistance decreases significantly from eleven point zero two ohms to one point five seven ohms as the CV activation progresses. This reduction in resistance indicates more favorable charge transfer dynamics, which are essential for high-performance catalysis. The structural changes in FcNiOF during CV activation are critical for its transformation into the highly active FeNiHOF. SEM images reveal that the vertical nanosheet arrays retain their morphology, ensuring mechanical adhesion and a large specific surface area. However, TEM images show that the surfaces of the nanosheets become rough and uneven, a notable change from their original smooth appearance. High-resolution TEM identifies the emergence of local small crystal domains, with lattice fringes corresponding to Fe-Ni layered double hydroxides, or FeNiLDH. XRD patterns corroborate these findings, as the characteristic peaks of the FcNiOF phase weaken and eventually disappear, indicating a transition to a quasi-amorphous texture. Raman spectroscopy further confirms this transformation, with the disappearance of bands associated with Fc units and the appearance of new bands corresponding to Ni-O and Fe-O vibrations. XPS analysis reveals significant changes in the chemical states of the elements. The Ni-2p spectrum shows an increase in Ni3+ content, while the Fe-2p spectrum transforms from peaks associated with Fe2+ in Fc units to broader peaks indicative of a mix of Fe2+ and Fe3+ in FeNiLDH. FT-IR spectra show that the vibration bands of Fc groups vanish, while the characteristic bands of carboxyl groups remain, alongside new bands corresponding to Ni-O-H and Fe-O-H bending modes. These findings indicate the formation of FeNiLDH with carboxyl ligands. EDS mapping images display homogeneous distributions of nickel, iron, carbon, and oxygen within the nanosheets, although with slightly decreased metal contents compared to the initial FcNiOF. This consistent distribution is crucial for maintaining the materials catalytic efficiency. The self-reconstruction process transforms FcNiOF into FeNiHOF, a highly active catalyst for OER. This transformation enhances the materials catalytic performance by reducing the energy barriers associated with the rate-determining steps in the OER pathway. The carboxyl conjugation within FeNiHOF plays a significant role in optimizing the electronic structure, improving OER kinetics, and enhancing peroxide resistance. Overall, the CV activation process is instrumental in converting FcNiOF into FeNiHOF, endowing the material with superior catalytic properties. The structural and chemical changes induced during this process result in a highly efficient and stable electrocatalyst, capable of performing exceptionally well in industrial applications such as alkaline water electrolyzers. The electrocatalytic performance of FeNiHOF is rigorously evaluated to highlight its capabilities in high-current-density oxygen evolution reaction, or OER. When compared to other catalysts such as FeHOF, NiHOF, and commercial RuO2, FeNiHOF exhibits superior OER activity. This is evident from the quasi-steady linear sweep voltammetry scans conducted in one molar potassium hydroxide solution, where FeNiHOF demonstrates the minimum onset potential and the fastest polarization behavior among the tested catalysts. FeNiHOF reaches an impressive current density of up to two amperes per centimeter squared. Specifically, it requires low overpotentials of two hundred seventy-three, two hundred eighty, and two hundred eighty-four millivolts to deliver current densities of zero point five, one, and two amperes per centimeter squared, respectively. These overpotentials are significantly lower than those required by FeHOF, NiHOF, and RuO2 catalysts, underscoring the enhanced catalytic efficiency of FeNiHOF. The Tafel plots provide further insights into the OER kinetics of FeNiHOF. The Tafel slope for FeNiHOF is thirty-four point eight millivolts per decade, which is considerably smaller than those of FeHOF, NiHOF, and nickel foam, indicating more favorable OER kinetics. Notably, FeNiHOF maintains this small Tafel slope even as the current density increases, whereas the other catalysts show a notable increase in Tafel slopes, particularly beyond zero point five amperes per centimeter squared. This behavior demonstrates that FeNiHOF can maintain its intrinsic OER kinetics without being restricted by mass or charge transfer limitations, even at high current densities. Electrochemical impedance spectroscopy, or EIS, Nyquist plots further confirm the high charge transfer kinetics of FeNiHOF. The charge transport resistance, or Rct, for FeNiHOF is much smaller than that of FeHOF, NiHOF, and nickel foam, indicating more efficient charge transfer processes. Non-Faraday cyclic voltammetry curves are used to evaluate the double-layer capacitances, or Cdl, revealing that FeNiHOF has the largest Cdl and electrochemical surface area among the compared samples. This large surface area is beneficial for enhancing the catalytic performance by providing more active sites for the OER. Dynamic wetting images show superior hydrophilicity on the surface of FeNiHOF compared to nickel foam, which helps facilitate the detachment of oxygen bubbles during the reaction. Dynamic bubbling images at one ampere per centimeter squared demonstrate that FeNiHOF produces numerous tiny oxygen bubbles, most of which are within the size range of zero point two to zero point three millimeters, and do not adhere to the surface. This efficient bubble release further supports the high mass and charge transfer kinetics of FeNiHOF. To determine the intrinsic activity of FeNiHOF, the linear sweep voltammetry curves are normalized by the electrochemical surface area and the number of active sites. FeNiHOF exhibits higher electrochemical surface area-normalized activity and turnover frequency compared to NiHOF and FeHOF, verifying the synergistic contribution of nickel and iron in enhancing the OER performance. Theoretical calculations provide deeper insights into the OER mechanism for FeNiHOF. Density functional theory, or DFT, calculations show that the rate-determining step in the OER pathway for both FeNiHOF and FeNiLDH involves the transition of HOO* from O* intermediates, with FeNiHOF having a lower overpotential for this step. The charge density difference calculations reveal that the electronic interaction between the carboxyl ligand and the active metal in FeNiHOF mediates the electronic structure around the metal sites, reducing the energy barrier for the rate-determining step. In summary, FeNiHOF demonstrates outstanding OER performance, characterized by low overpotentials, high current densities, and favorable OER kinetics. These attributes, supported by comprehensive electrochemical analyses, position FeNiHOF as a highly effective and efficient electrocatalyst for industrial applications in alkaline water electrolyzers. The long-term stability of FeNiHOF at high current densities is a critical factor for its practical application in industrial processes. Durability tests reveal that FeNiHOF operates steadily without any decay during a continuous chronopotentiometry test conducted at one ampere per centimeter squared for one thousand hours. Remarkably, there is no increase in overpotential; instead, a slight decrease is observed, potentially attributed to temperature fluctuations during the operation. Real-time inductively coupled plasma mass spectrometry, or ICP, measurements conducted during a one hundred-hour chronopotentiometry test at zero point five amperes per centimeter squared confirm the stability of the active metal sites in FeNiHOF. The results show considerable resistance to metal dissolution, with only an initial minor loss of iron, likely due to a small number of iron atoms not bound by carboxyl groups. In contrast, typical FeNi layered double hydroxides exhibit significant decay, with a thirty-eight percent increase in overpotential and noticeable leakage of metal ions into the electrolyte. This comparison underscores the robustness of FeNiHOF, which benefits from carboxyl conjugation that stabilizes iron sites against peroxidation. The practical application of FeNiHOF is demonstrated in an alkaline water electrolyzer, or AWE. The hydrothermal synthesis method allows for scalable and cost-effective production of FeNiHOF electrodes. A piece of synthesized FeNiHOF electrode, measuring nine centimeters by ten centimeters, shows a uniform dark yellow color, indicating consistent growth of nanosheets. The spontaneous reconstruction of FcNiOF into FeNiHOF under OER conditions eliminates the need for additional procedures, contributing to the cost-effectiveness of the electrode, which is estimated at approximately one hundred forty-seven dollars per square meter. Mechanical strength tests further validate the practical viability of FeNiHOF. The flexible nickel foam loaded with the catalyst withstands one hundred bending tests without detectable mass loss, indicating reliable adhesion essential for high-performance catalysis in gas-bubbling environments. In an industrial context, FeNiHOF is used as the anode in a FeNiHOFandR-Ni alkaline water electrolyzer, with commercial RANEY nickel on nickel wire mesh as the cathode. This electrolyzer configuration is tested for its performance metrics. It requires a cell voltage of one point eight seven volts to achieve a high current density of one ampere per centimeter squared in one molar potassium hydroxide at room temperature, significantly outperforming a commercial electrolyzer with nickel wire mesh electrodes. The cell voltage further decreases to one point eight one volts in six molar potassium hydroxide, compared to the two point zero four to two point two six volts required by commercial electrolyzers to reach industrial-level current densities. Long-term tests show that the FeNiHOFandR-Ni electrolyzer maintains stability for five hundred hours at one ampere per centimeter squared, even with twenty ON/OFF cycles, demonstrating negligible changes in cell voltage. These tests highlight the electrolyzers durability and resilience to operational stresses, including reverse currents generated during shutdowns. In terms of power consumption and energy efficiency, the FeNiHOFandR-Ni electrolyzer consumes approximately four point two three kilowatt-hours per normal cubic meter and achieves an energy efficiency of eighty-three point six percent at zero point five amperes per centimeter squared at one point seven seven volts. These figures are superior to those of commercial electrolyzers, which exhibit higher power consumption and lower energy efficiency. The hydrogen yield rate for the FeNiHOFandR-Ni electrolyzer is around three point one eight normal cubic meters per hour per square meter at one point eight volts, significantly higher than that of the commercial setup. The cost of producing hydrogen using the FeNiHOFandR-Ni electrolyzer is estimated to be approximately one dollar and one cent per gallon of gasoline equivalent, well below the two dollar technical goal set by the United States Department of Energy for two thousand twenty-six. This cost-effectiveness, combined with high performance metrics, positions FeNiHOF as a highly promising catalyst for industrial-scale alkaline water electrolyzers. In summary, FeNiHOF demonstrates exceptional long-term stability at high current densities, making it a practical and cost-effective solution for industrial alkaline water electrolyzers. Its outstanding performance in terms of cell voltage, stability, power consumption, and energy efficiency surpasses that of commercial alternatives, underscoring its potential for widespread industrial application.