Impedance analysis of high-entropy alloy for ammonia synthesis

Global energy demand has increased significantly due to world population growth and the industrialization of developing economies. Energy production has been based mainly on fossil-fuel energy, which has increased global warming due to the rise of greenhouse gases in the atmosphere, such as carbon dioxide. According to the Energy Institute, renewable power generation through wind, solar, and other renewable sources, represent only 40,86% of energy sources in 2022 [1]. An alternative energy source for fossil fuels is hydrogen, which can be produced through renewable resources that increase energy efficiency. However, the storage and transportation of hydrogen present a series of technical challenges, resulting in high costs and motivating the development of intermediate technologies. Recently, ammonia generated by renewable energy sources has gained significant attention as an energy carrier, a medium to store and transport chemical energy, and directly as fuel [2].Ammonia is also a primary raw material for making inorganic fertilizers, pharmaceuticals, synthetic fibers, resins, and other applications, benefiting nearly half the world's population. Ammonia can be transported more efficiently and safely than hydrogen in tanker vessels or pipelines due to its relative ease of being liquefied at room temperature and moderate pressure, increasing energy density. Currently, ammonia is produced mainly from hydrogen and nitrogen by the Haber-Bosch process, which utilizes fossil fuel, thus resulting in carbon dioxide emissions. Ammonia production can also involve the non-spontaneous nitrogen reduction reaction by electrochemical techniques, which uses hydrogen that can be provided from the water, reducing energy consumption and carbon dioxide emissions. However, the non-spontaneous nitrogen reduction reaction has low activity, and its voltage is close to that of the hydrogen evolution reaction. Plasma and electrothermal chemical cycle methods have been explored to improve the selectivity of non-spontaneous nitrogen reduction reactions. Several studies have proposed new catalysts to increase the active sites, modify the size and morphology of particles, and introduce defects, such as transition metal-based catalysts, carbon-based catalysts, phosphorus-based catalysts, etc. Nevertheless, traditional catalysts frequently degrade rapidly due to the harsh chemical environment and the inherent corrosiveness of the reactions involved. A highly active catalyst that degrades quickly due to corrosion offers limited practical value. Researchers are exploring novel materials like high-entropy alloys (HEAs) to address this challenge, which also have high corrosion resistance [3].This study focuses on the potential of a HEA, FeCrMnNiCo, as a catalyst for the electrochemical conversion of nitrogen to ammonia via the electrochemical method, evaluating the influence of the microstructure on its mechanical properties, catalytic activity, and corrosion resistance. A ball burnishing deformation was applied to the HEA at different conditions. The X-ray diffraction revealed that the FeCrMnNiCo alloy presented a face-centered cubic crystalline structure, and scanning electron microscopy analysis showed that the alloying elements were segregated in the deformed samples. The mechanical deformation determined both catalytic activity and corrosion resistance of the HEA. This study highlights the importance of considering corrosion resistance as a crucial factor in developing catalysts for green ammonia production.

The Haber-Bosch (HB) process, critical for global ammonia production, is hindered by its high energy consumption and operational demands, requiring extreme pressures and temperatures. Developing catalysts that reduce these demands while maintaining practical efficiency is essential for achieving sustainable ammonia synthesis. Here, we investigate the FeCoNi(AlSi)0.76 high-entropy alloy (HEA) as a catalyst for the HB process using quantum mechanics (QM) and kinetic Monte Carlo (kMC) simulations. Mechanistic analysis revealed significantly lower reaction barriers compared to pure Fe, and kMC simulations predict an NH3 turnover frequency (TOF) that is 65 times higher than pure Fe under industrial conditions. Under reduced pressure (21 atm) and moderate temperature condition (673 K), the HEA retained half the NH3 production rate of pure Fe at extreme industrial conditions, revealing its potential to reduce energy and pressure requirements. This study demonstrates the promise of HEAs in enabling more energy-efficient and sustainable ammonia production technologies.

The successful synthesis of high-entropy alloy (HEA) nanoparticles, a long-sought goal in materials science, opens a new frontier in materials science with applications across catalysis, structural alloys, and energetic materials. Recently, a Co25Mo45Fe10Ni10Cu10 HEA made of earth-abundant elements was shown to have a high catalytic activity for ammonia decomposition, which rivals that of state-of-the-art, but prohibitively expensive, ruthenium catalysts. Using a computational approach based on first-principles calculations in conjunction with data analytics and machine learning, we build a model to rapidly compute the adsorption energy of H, N, and NHx (x = 1, 2, 3) species on CoMoFeNiCu alloy surfaces with varied alloy compositions and atomic arrangements. We show that the 25/45 Co/Mo ratio identified experimentally as the most active composition for ammonia decomposition increases the likelihood that the surface adsorbs nitrogen equivalently to that of ruthenium while at the same time interacting moderately strongly with intermediates. Our study underscores the importance of computational modeling and machine learning to identify and optimize HEA alloys across their near-infinite materials design space.

Herein, this study reports the synthesis of a uniform, sphere-like NiCoFeRuIr high-entropy alloy (HEA) via the pulsed laser irradiation in liquid method. This method enables the rapid, reducing-agent-free, and energy-efficient fabrication of HEAs under ambient conditions, thereby overcoming the limitations associated with conventional high-temperature alloying or chemical reduction methods. The resulting NiCoFeRuIr HEA exhibits outstanding nitrite reduction reaction (NO2 -RR) performance, achieving a high NH3 yield rate of 2.34 mg·h-1·cm-2 and Faradaic efficiency of 95.78% at -0.6V versus the reversible hydrogen electrode. An enhanced electrocatalytic activity is attributed to unique synergistic interactions among multiple metal components in NiCoFeRuIr HEA, optimizing the adsorption energy of intermediates and accelerating reaction kinetics. Mechanistic insights obtained from in situ Raman spectroscopy, ex situ X-ray diffraction, and density functional theory calculations further support the origin of the outstanding NO2 -RR performance of the NiCoFeRuIr HEA electrocatalyst. Furthermore, NiCoFeRuIr HEA serves as an efficient cathode in a Zn-nitrite battery, enabling simultaneous NO2 - remediation, ammonia (NH3) production, and electricity generation with a power density of 3.80 mW·cm-2. Thus, this study highlights the potential of the pulsed laser-driven synthesis of HEA electrocatalysts, paving the way for integrated pollutant remediation, green chemical production, and sustainable energy applications.

Directly electrochemical conversion of nitrate (NO3−) is an efficient and environmentally friendly technology for ammonia (NH3) production but is challenged by highly selective electrocatalysts. High‐entropy alloys (HEAs) with unique properties are attractive materials in catalysis, particularly for multi‐step reactions. Herein, we first reported the application of HEA (FeCoNiAlTi) for electrocatalytic NO3− reduction to NH3 (NRA). The bulk HEA is active for NRA but limited by the unsatisfied NH3 yield of 0.36 mg h−1 cm−2 and Faradaic efficiency (FE) of 82.66 %. Through an effective phase engineering strategy, uniform intermetallic nanoparticles are introduced on the bulk HEA to increase electrochemical active surface area and charge transfer efficiency. The resulting nanostructured HEA (n‐HEA) delivers enhanced electrochemical NRA performance in terms of NH3 yield (0.52 mg h−1 cm−2) and FE (95.23 %). Further experimental and theoretical investigations reveal that the multi‐active sites (Fe, Co, and Ni) dominated electrocatalysis for NRA over the n‐HEA. Notably, the typical Co sites exhibit the lowest energy barrier for NRA with *NH2 to *NH3as the rate‐determining step.

Directly electrochemical conversion of nitrate (NO3 -) is an efficient and environmentally friendly technology for ammonia (NH3) production but is challenged by highly selective electrocatalysts. High-entropy alloys (HEAs) with unique properties are attractive materials in catalysis, particularly for multi-step reactions. Herein, we first reported the application of HEA (FeCoNiAlTi) for electrocatalytic NO3 - reduction to NH3 (NRA). The bulk HEA is active for NRA but limited by the unsatisfied NH3 yield of 0.36 mg h-1 cm-2 and Faradaic efficiency (FE) of 82.66 %. Through an effective phase engineering strategy, uniform intermetallic nanoparticles are introduced on the bulk HEA to increase electrochemical active surface area and charge transfer efficiency. The resulting nanostructured HEA (n-HEA) delivers enhanced electrochemical NRA performance in terms of NH3 yield (0.52 mg h-1 cm-2) and FE (95.23 %). Further experimental and theoretical investigations reveal that the multi-active sites (Fe, Co, and Ni) dominated electrocatalysis for NRA over the n-HEA. Notably, the typical Co sites exhibit the lowest energy barrier for NRA with *NH2 to *NH3as the rate-determining step.

Learning What Makes Catalysts Good

N2 dissociation on AuCoFeMoRu high-entropy alloys: Circumventing scaling relations and step dependencies

Electrocatalytic nitrate reduction reaction provides a broad prospect for developing green electrochemical ammonia production and efficient treatment of industrial wastewater rich in nitrate, but poses a challenge to the high activity and stability of electrocatalysts. Herein, we report the versatile and scalable one-pot solvothermal synthesis of a series of RuFeMMnMo (M═CoNi, Co, and Ni) high-entropy alloy (HEA) nanomaterials, possessing a unique face-centered cubic-hexagonal close-packed-face-centered cubic (fcc-hcp-fcc) heterophase. The highly random distribution of multiple metal components and the tunable diversity of metal atomic arrangements can be realized simultaneously. Significantly, RuFeCoNiMnMo HEAs present a high Faradaic efficiency of 99.3 % and a promising yield rate of 83.35mgh-1mgcat -1 toward ammonia production at -0.6V vs. reversible hydrogen electrode. Ex/in situ characterizations and theoretical calculations have revealed that by high electron coupling of high-entropy effect, heterophase fcc-hcp-fcc RuFeCoNiMnMo HEAs have shown strong electronic modulations with charge redistributions. The positive charges and negative charges for Ru sites and Ni/Co sites promote the adsorption of key intermediates and generation of active protons, respectively, which guarantees efficient nitrate reduction due to the reduced energy barriers.

Computational design of one FeCoNiCuZn high-entropy alloy for high-performance electrocatalytic nitrate reduction

Ammonia (NH₃) is one of the most important materials for human activities, finding versatile applications in areas such as fertilizers, hydrogen carrier, and carbon-free energy fuel. Traditionally, NH₃ is produced through the energy-intensive Haber-Bosch (HB) process, resulting in considerable CO₂ emissions. As an alternative, electrochemical NH₃ production is gaining attentions. The electrochemical nitrate reduction reaction (NO₃RR) has been explored due to its advantages, including a low energy barrier, high nitrogen source solubility in the electrolyte, and high activity. In order to produce NH3 electrochemically, transition metal-based alloys have been employed as catalysts due to their features such as high conductivity, synergy effects, and low cost. However, transition metal catalysts suffer from low NH3 selectivity and high activity of hydrogen evolution reaction (HER), which is a competitive reaction. Recently, high-entropy alloy (HEA) catalysts have been employed as catalysts due to their features such as high entropy effect, lattice distortion, cocktail effect, and sluggish diffusion. To enhance the activity and selectivity of the NO₃RR, we developed FCC-based HEA (AlFeCoNiCu) electrocatalyst. The FCC-based HEA catalysts demonstrated a yield rate exceeding 9.88 mg h-1 cm-2 and a faradaic efficiency (FE) of 90.2 % at -0.6 V vs. RHE in 0.1 M NO₃- and 1 M KOH solution. This research is expected to contribute not only to the efficient production of ammonia under ambient conditions but also to the exploration of HEAs catalysts for enhancing the NO3RR pathway.

To address persistentchallenges of low ammonia yield and Faradaicefficiency (FE) in electrocatalytic nitrogen reduction, this workpioneers a breakthrough strategy utilizing nitrogen-enriched spirulinabiomass as the nitrogen source coupled with high-entropy alloy (HEA)catalysts. This novel electrocatalytic synthesis route establishesa sustainable pathway for green ammonia production. The MnFeNiCuAland MnFeNiCuCo HEAs were synthesized via a coprecipitation method.Structural analysis reveals that MnFeNiCuAl exhibits a pure face-centeredcubic (FCC) phase, and the Al element incorporation suppresses high-temperaturesintering and mitigates compositional segregation. Its oxygen vacancyconcentration is higher than that of the MnFeNiCuCo HEO, which canoptimize the multielectron transfer kinetics, thereby enhancing theelectrocatalytic performance. Electrocatalytic tests using four aminoacid model compounds from spirulina (glutamate, histidine, leucine,and phenylalanine) revealed superior activity of the MnFeNiCuAl catalystcompared to that of MnFeNiCuCo. The MnFeNiCuAl HEA catalyst was thenemployed for the spirulina electrolysis, which achieved a high NH3 yield of 1783.9 μg/(h cm2) at −0.9V vs RHE and an FE up to 80.27% at −0.6 V vs RHE. Meanwhile,cycling stability tests validate the robust stability and durabilityof the MnFeNiCuAl catalyst. The mechanism analysis showed that thecarboxyl groups (−COOH) dissociate to generate H+ as proton sources under the action of Cu/Al sites, while the aminogroups (−NH2) are adsorbed and activated at theFe sites. Subsequently, the activated amino groups undergo continuoushydrogenation to form *NH3, which then desorbs from theFe sites.

Competitive surface adsorption energies on catalytic surfaces constitute a fundamental aspect of modeling electrochemical reactions in aqueous environments. The conventional approach to this task relies on applying density functional theory, albeit with computationally intensive demands, particularly when dealing with intricate surfaces. In this study, we present a methodological exposition of quantifying competitive relationships within complex systems. Our methodology leverages quantum-mechanical-guided deep neural networks, deployed in the investigation of quinary high-entropy alloys composed of Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn. These alloys are under examination as prospective electrocatalysts, facilitating the electrochemical synthesis of ammonia in aqueous media. Even in the most favorable scenario for nitrogen fixation identified in this study, at the transition from O and OH coverage to surface hydrogenation, the probability of N2 coverage remains low. This underscores the fact that catalyst optimization alone is insufficient for achieving efficient nitrogen reduction. In particular, these insights illuminate that system consideration with oxygen- and hydrogen-repelling approaches or high-pressure solutions would be necessary for improved nitrogen reduction within an aqueous environment.

High-entropy alloys (HEAs) show high activities toward oxygen reduction reaction (ORR), Zn-air batteries (ZABs) and nitrate reduction reaction (NO3 -RR). In this work, FeNiCoMnRh HEA supported by N-doped carbon frameworks is prepared and showed excellent ORR performance with a half-wave potential (E1/2) of 0.89V versus RHE, limiting diffusion current (jL) of 5.6mA cm-2 and better current stability. The HEA-assembled ZAB exhibited a high-power density of 103.8mW cm-2 with a specific capacity of 790 mAh gZn -1. Also, its oxides presented 77% Faraday efficiency (FE) for ammonia production at -0.3V versus RHE. Accordingly, our designed ZAB was employed to drive NO3 -RR to construct a self-powered system, which provides an attractive route for low-energy sewage treatment and environmentally friendly preparation of ammonia.

Low-temperature ammonia synthesis by applying an electric field to a solid heterogeneous catalyst was investigated to realize an on-demand, on-site catalytic process for converting distributed renewable energy into ammonia. By applying an electric field to the catalyst, even at low temperatures, the reaction proceeds efficiently by an "associative mechanism" in which proton-conducting species on the support surface promote the formation of N2Had intermediates through surface protonics. Kinetics, isotope exchange, infrared spectroscopy, X-ray spectroscopy, and AC impedance analysis were performed to clarify the effect of metal and catalyst support structure on the reaction, and an evaluation method for the surface protonics of the support was established to analyze the reaction mechanism, and further analysis using computational chemistry was also conducted. The elementary step determining catalytic activity changed from N2 dissociation to N2H formation, and this difference resulted in high activity for ammonia synthesis at low temperatures even when using base metal catalysts such as Fe and Ni.