Tuning the Electrocatalytic Properties of Trimetallic Pentlandites: Stability and Catalytic Activity as a Function of Material Form and Selenium Concentration

Abstract

Pentlandites are one possible cost-effective alternative to platinum group metals for green hydrogen production. This study delves into the catalytic performance of trimetallic pentlandite systems, exploring the influence of selenium concentration and material form on their efficiency by combining the investigation of materials in various forms (powder catalysts, ingots, and highly densified pellets) with a computational investigation. The experimentally observed solubility limit of selenium was clarified based on the formation energies approach. The best and most stable defect combination, namely, Se:S substitution and S vacancy, was identified and correlated with improved catalytic properties of the systems with small Se addition. Further findings highlight the evolving importance of intrinsic material properties, such as bond properties, intermetallic interactions, or electronic structure, over surface effects, including the activation process, as the material density increases. The research contributes valuable insight into the intricate mechanisms governing pentlandite catalysis. Understanding these dynamics allows for intentional modifications, advancing the application of pentlandites in hydrogen production.

Role of Foreign Phases, Synergistic Effects, and Morphology in the HER Performance of Trimetallic Pentlandites with Non-Equimolar Co:Fe:Ni Ratio

“…The density of states plots of the pentlandites (Figure S6) confirm that all the materials were metallic in nature, with strongly overlapping valence and conductive states [21]. Additionally, two main observations can be made.…”

“…The pentlandite structure is characterized by cubic 𝐹𝑚3 𝑚 symmetry with a highly metallic framework. As noted by Lu et al, the structure can be further divided into substructures, ensuring a highly conductive backbone coupled with the presence of numerous bimetallic and Me-S active sites [21]. To showcase the effect of different cationic site occupations, 2 × 2 × 2 supercells were used (Figure 2).…”

“…Previous research on pentlandites has explored various approaches, including a multicomponent approach [16][17][18], the dissolution of additional selenium into the sulfur sublattice [4,16,17], and initial attempts to optimize catalytic performance by modifying the cationic sublattice [19,20]. For example, substituting sulfur with selenium in pentlandite structures has been shown to significantly reduce the overpotential η 120 from 489 [mV] for TM 9 S 8 to 416 [mV] for TM 9 S 7 Se [21]. Similarly, tailoring the cationic sublattice has resulted in notable improvements, such as achieving an overpotential η 50 of 378 [mV] for Fe 2 Co 4 Ni 3 S 8 [19].…”

“…Assemblies employing powder catalysts anchored to conductive electrodes usually feature a highly developed surface and a large number of active centers, leading to high electrochemical surface area (ECSA) values. In the case of sulfur-rich pentlandites, the powder catalyst approach causes an increased formation of sulfur vacancies, which are crucial for creating active sites and enhancing overall activity towards HER [15,16,21,29]. In turn, pentlandites utilized in volumetric forms exhibit reduced surface development and a limited ability to form sulfur vacancies.…”

“…Yet, they may demonstrate remarkable performance due to improved charge-transfer abilities. Additionally, material properties stemming from intrinsic factors such as chemical composition and electronic structure begin to play a dominant role [19,21]. It is also worth investigating the influence of randomly oriented crystallites resulting from spontaneous crystallization (ingot samples) as well as the impact of thermal processing (sintered pellets) on microstructure and overall electrochemical properties.…”