Superlattice blotting constructs ordered mesoporous carbon with high nickel single atom support for efficient electrocatalysis

Yuanyuan Wang and Wenlei Zhu's group at Nanjing University, China, and Yuehe Lin at Washington State University, USA, recently reported the development of a three-dimensional ordered mesoporous carbon skeleton with Ni single atom support using the superlattice blotting strategy for efficient electrocatalytic hydrogen production. Firstly, they derived an ordered mesoporous framework based on finite element simulation, which is of great significance for promoting uniform gas distribution, stabilizing the gas-liquid-solid interface of nanoscale hydrophilic surfaces, and enhancing mass transfer kinetics. Then, the proposed superlattice blotting strategy integrated confined oxidation to achieve thermal stability, ligand carbonization to maintain superlattice-derived porosity, acid etching to improve hydrophilicity, high-temperature graphitization to enhance conductivity, and in-situ heteroatom doping to optimize Ni coordination for the successful preparation of a three-dimensional ordered mesoporous carbon skeleton with Ni single atom support. The resulting Ni-N₂S₂ and Ni-N₃P catalysts exhibited excellent electrocatalytic activity, reaching overpotents of 239 mV (OER, 20 mA cm⁻²) and 90 mV (HER, 10 mA cm⁻²), respectively. Ni-N₂S₂⁽⁺⁾Ni-N₃P^(-) electrolysis pairs can achieve stable electrolysis performance for more than 100 hours. This work, published in CCS Chemistry, introduces a finite element simulation guidance framework to customize three-phase equilibrium, as well as confined oxidation pathways to design highly active and durable single-atom electrocatalysts.

Background:

Porous materials are attracting attention as versatile platforms in energy conversion and storage applications, especially in the field of electrocatalysis, due to their inherently high specific surface area and three-dimensional interconnected frame structure. However, despite their advantages, porous catalysts still face many key challenges in the practical application of electrocatalytic systems. These challenges include slow active site reaction kinetics, inefficient gas-liquid separation, and compromised structural integrity during long-term operation. In addition, the convoluted channels in disordered porous networks often hinder the rapid and continuous transport of gas molecules, leading to bubble buildup, clogging of catalytic sites, and ultimately performance degradation. Yuanyuan Wang, Wenlei Zhu, Yuehe Lin, and their research teams developed a superlattice blotting method to construct ordered mesoporous carbon with high nickel single-atom loads, achieving efficient electroelectrolysis of hydrogen from water.

Highlights of this article:

1. Design based on finite element simulation: ultra-thin liquid film theory for enhancing electrolyte transport

Through finite element simulation, the authors compared the air pressure distribution of gases in ordered mesoporous and disordered porosity, and concluded that the ordered mesoporous framework is of great significance for promoting uniform gas distribution, stabilizing the gas-liquid-solid interface of nanoscale hydrophilic surfaces, and enhancing mass transfer kinetics. In a system with uniform size and ordered mesoporosity, the gas flowing at a controllable rate shows a relatively stable velocity distribution and pressure distribution, and the impact stress generated by the gas on the pore wall is evenly distributed. Based on the hydrophilic pore wall and homogeneously dispersed active sites, the electrolyte tends to diffuse along the pore surface and stabilize within the quadrilateral constraint region, forming a nanometer-thick liquid film, thereby establishing a coherent gas-liquid interface and thus achieving enhanced gas transport efficiency. In contrast, the disordered porous structures show gas flow rates near the large pores that are significantly lower than those near the small pores, resulting in uneven gas impact stress distribution. This gap that can reach two orders of magnitude. Such extreme pressure fluctuations disrupt the stability of the interface, making it difficult to maintain a stable and uniformly distributed ultra-thin liquid film. Therefore, the discontinuous gas channels and poor gas-liquid interface stability significantly reduce the catalytic contact area, resulting in poor electrocatalytic activity.

2. Superlattice trace method to construct ordered mesoporous carbon with high nickel single atom load

The authors first self-assembled Ni nanocrystals to achieve confined oxidation with the help of superlattices, increasing the sintering resistance temperature of Ni nanocrystals while maintaining their morphology and size. The ligand was then calcined at 500 °C to achieve ligand carbonization and maintain the superlattice structure. Further acid etching exposes a three-dimensional ordered mesoporous skeleton, and at the same time, the impregnation and adsorption of Ni ions are spontaneously formed. Finally, after 900 °C high-temperature graphitization, heteroatoms were introduced to adjust the coordination structure and improve the conductivity.

Spherical aberration electron microscopy results showed the successful introduction of Ni single atoms. The Fourier transform extended X-ray absorption fine structure (EXAFS) fitting results showed that the nickel atoms in Ni-S/N-C were coordinated with 2 nitrogen atoms and 2 sulfur atoms (Ni-N₂S₂), while each nickel atom in Ni-P/N-C was coordinated with 3 nitrogen atoms and 1 phosphorus atom (Ni-N)₃P).  In addition, wavelet transform (WT)-EXAFS imaging in k-space and radial distance further confirmed the isolated coordination of nickel atoms with non-metallic atoms in the two samples, rather than aggregated nickel atoms. The comprehensive morphological and structural characterization results fully verify the coordination of Ni single atoms and heteroatoms.

3. Exploration of catalyst performance

Ni-S/N-C had the highest oxygen evolution activity. The unique disulfide coordination gives it excellent water oxidation ability, with an overpotential of only 239 mV at a current density of 20 mA cm^-2.  At the same current density, its catalytic potential is 91 mV lower than that of commercial RuO₂ catalysts. In addition, due to the well-designed ordered porous substrate, the gas-liquid mass transfer capacity is improved, and its water oxidation performance is enhanced, making it superior to the recently reported Ni-based single-atom electrocatalyst. Ni-P/N-C has the highest hydrogen evolution activity, and only an overpotential of 90 mV can drive a current density of 10 mA cm^-2. The Tafel slope obtained by fitting is 51.96 mV/dec, which is in line with the Heyrovsky reaction path. The excellent hydrogen evolution performance is mainly due to the fast reaction kinetics and high reaction efficiency.

In a two-electrode system, Ni-N₂S₂ was used as an anode to facilitate the release of oxygen, while the Ni-N₃P acts as a cathode to enhance hydrogen production. The Ni-N₂S₂⁽⁺⁾ // Ni-N₃ P^(-) electrode pair exhibits excellent catalytic activity and stability during the total hydrolysis reaction, reaching a current density of 10 mA cm^-2 with a battery potential of only 1.59 volts. Stable operation was maintained for 100 hours, and the current did not weaken significantly.

Summary and outlook:

The authors used finite element modeling to reveal the mechanism of how the ordered mesoporous structure controls the three-phase equilibrium, and they developed a superlattice blotting method to successfully prepare an ordered mesoporous carbon-supported Ni single-atom catalyst that achieved excellent hydrogen and oxygen evolution performance. By combining a three-dimensional ordered mesoporous carbon skeleton with precise heteroatomic coordination with fine heteroatomic coordination, the stability, mass transport, and overall catalytic efficiency of the active site were significantly improved. The hydrophilic ordered mesoporous framework promotes the formation of ultra-thin liquid films, improving electrolyte transport and accelerating gas release. In addition, the highly dispersed monoatomic nickel enabled fine-tuning of the electronic structure after heteroatomic coordination, optimizing the adsorption and desorption behavior of key reaction intermediates. This work provides a new mechanistic perspective for coordinating gas-liquid-solid three-phase interface catalysis, while also establishing a feasible solution to solve the long-term challenges in sustainable energy conversion systems through the rational design of catalytic microenvironments.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.