Kinetically Accelerating Ni and C Dual Active Sites Via Pyrrolic-N for Water Dissociation and Alkaline Hydrogen Production
Authors: Kim Jiwon , Choi Hyung Wook , Yoon Dae Ho 
Published: December 2024

Green hydrogen, which is one of the most promising solutions to environmental issues and energy crises, can be produced through water electrolysis using renewable electricity as eco-friendly and sustainable route. However, the cathodic hydrogen evolution reaction (HER) has been a subject of extensive research due to the slow kinetics of proton-coupled electron transfer, which lowers the overall efficiency. In particular, the rate of HER in alkaline media appears two to three times slower than that in acidic or neutral media. This is because in order to form adsorbed hydrogen, additional water dissociation must first proceed in the Volmer step (H2O + e- → Had + OH-), followed by the coupling of adsorbed hydrogen in the Heyrovsky or Tafel steps (H2O + Had + e- → H2 + OH- / Had + Had → H2). The hydrogen adsorption-free energy (ΔGH*) has traditionally been considered the primary factor influencing the kinetics of the HER at the Heyrovsky and Tafel steps. However, it is important to note that sufficient proton supply to the catalytic sites relies on water adsorption capacity and the ability to cleave the HO-H bond. As a results, a proper strategy for achieving high hydrogen coverage (M-Had) must incorporate both water dissociation and hydrogen combination. Unfortunately, the activation of O- and H- containing intermediates for water dissociation and proton combination requires different catalytic properties, which highly presents a challenge. Therefore, it is crucial to understand the correlation between the catalytic chemical state, adsorption energy of intermediate species (H2O, OH, Had), and the kinetic mechanism of the HER from a theoretical standpoint.

Pt-based materials are currently recognized as the most efficient catalysts for the HER. However, they exhibit unsatisfactory intrinsic kinetics under alkaline conditions due to the slow dissociation rate of water caused by the additional energy barrier of HO-H bond dissociation. Furthermore, their scarcity and high price limit their industrial applications. As an alternative, Ni-based materials are considered promising catalysts due to their Pt-like hydrogen adsorption-free energy and the possibility of optimizing water dissociation through chemical state modulation. Recent studies have shown that modulating the surface electronic structure of Ni-based materials through synergistic interactions is an effective way to activate intermediates for alkaline HER. However, most studies on chemical state reorganization focus on a single site, which underestimates the mechanism-based kinetics and makes it difficult to simultaneously control water dissociation and proton adsorption. Therefore, introducing dual active sites that optimize O- and H- affinity independently can be an effective strategy to reduce the energy barrier for Volmer, Heyrovsky and Tafel steps. Moreover, considering the proton pathway within Ni-based catalysts to accelerate alkaline HER kinetics, it is essential to simultaneously govern thermodynamics and kinetics through a kinetic-oriented design.

In this study, we present a novel design approach that focuses on alkaline HER kinetics by incorporating dual active sites using Ni nanoparticles embedded in pyrrolic-N-rich carbon nanoplates (Ni@NCP), resulting in robust alkaline HER kinetics. The introduction of N-functional-rich polydopamine (PDA) serves two purposes: it ensures the uniform distribution of reduced Ni nanoparticles within the carbon substrate, and it enriches the ratio of pyrrolic-N in defective carbon, thereby regulating the properties of the Ni metal and the C atoms near the pyrrolic-N. Consequently, the Ni@NCP catalyst exhibits remarkable alkaline HER performance, with a low overpotential of 42 mV at a current density of 10 mA/cm2 and a Tafel slope of 94.9 mV/dec. These results outperform those of previously reported transition metal-based or single metal phase-based HER electrocatalysts. Further validation through the utilization of in-situ/operando Surface Enhanced Raman Spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations confirmed that the alkaline HER kinetics were significantly optimized by the pyrrolic-N-induced asymmetric electronic distribution, specifically by (ⅰ) shifting the d-band center (εd
) of the nearby Ni atoms towards the Fermi energy level to enhance the interaction with -H in H2O molecules and optimize the proton adsorption energy, (ⅱ) increasing the adsorption affinity of nearby C atoms towards H2O and -OH to optimize H2O adsorption, and (ⅲ) elongating the HO-H bonds, facilitating bond cleavage, and promoting simultaneous proton supply into Ni atoms. The kinetic-oriented design of dual active sites significantly reduces the kinetic energy barrier during Volmer step of water dissociation, resulting in facilitates the supply of sufficient protons for accelerated alkaline HER kinetics, ultimately enhancing the overall catalytic activity. Moreover, in a two-electrode system with Ni@NCP as the cathode for water electrolysis, Ni@NCP||IrO2 outperforms the commercial Pt/C||IrO2 system at the current density over 2.3 A/cm2, which shows superior potential for the practical application.

Figure 1