Phase Transition Assisted Photo-, Electro- and Photoelectrocatalytic Hydrogen Evolution in B/MoS2─Mechanistic Insight

Abstract

Electrocatalytic water splitting is recognized as one of the most effective methods for sustainable hydrogen production, with visible light integration recently emerging as a promising enhancement strategy. To address this, we developed a 2D/2D heterostructure of borophene and molybdenum disulfide (B/MoS₂) to investigate its efficiency in photo-(photo), electro-(HER), and photoelectro-(PEC) catalytic hydrogen evolution reactions under acidic conditions. Optimizing the MoS₂-to-boron mass ratio revealed significantly reduced overpotential, achieving 281.1 mV and a Tafel slope of 56.0 mV/dec in PEC, compared to 312.5 mV and 160.9 mV/dec in conventional HER, indicating boosted activity and kinetics of the hydrogen evolution process. Additionally, a long-term stability test at a constant current density of 10 mA/cm² confirmed the high durability of B/MoS₂ and maintained stable performance for up to 120 h. The B/MoS₂ demonstrated an improved hydrogen evolution rate reaching ∼2.5 mol/g in PEC, representing a 1.4-fold, 1.8-fold, and 3152-fold increase compared to pristine MoS₂ in photoelectro-, electro-, and photocatalytic hydrogen evolution process, respectively. Moreover, comprehensive material characterization elucidated the underlying PEC mechanism, including in situ and ex situ analyses. It highlighted the potential of borophene-enriched MoS₂ as an efficient catalyst for solar and/or electricity-driven hydrogen production, confirming that borophene presence substantially promotes the 2H-to-1T phase transition of MoS₂ by creating strain and defects, destabilizing the 2H phase, and favoring the formation of the 1T phase, thus significantly enhancing catalytic performance. Interestingly, the 2H-to-1T phase transition of MoS₂ is detected in all three processes: photo-, electro-, and photoelectrocatalytic hydrogen evolution reactions. However, its efficiency follows the order: PEC > HER > photo indicating that visible light irradiation is a missing activity descriptor revealing a puzzle of hydrogen evolution mechanism during PEC water splitting.