Selectivity and activity modulation of electrocatalytic carbon dioxide reduction by atomically dispersed dual iron catalysts

Abstract

By regulating the electronic environment of Fe active centers to modulate electrocatalytic CO₂ reduction behavior, an advanced dual Fe₂-site catalyst (Fe₂ DAC) exhibiting CO current density (j_CO) of 10 mA cm⁻² at an overpotential of 330 mV in a CO₂-saturated 0.5 M KHCO₃ electrolyte was designed and characterized by HAADF-STEM microscopy and XAS/EPR spectroscopies. With regard to Fe₂ DAC displaying a higher charge transfer coefficient (α = 0.53), turnover frequency (TOF_CO = 2.03 s⁻¹), and CO faradaic efficiency (FE_CO = 98.6%) at an overpotential of 400 mV compared to that of single iron atom catalyst (Fe SAC, α = 0.31, TOF_CO = 0.25 s⁻¹, FE_CO = 60.1%), the kinetic mechanism was investigated/elucidated by the cation effect and in operando spectroscopy. The higher DMPO–CO₂ EPR intensity (g = 2.0065, a_N = 15.6 G, a_H = 18.9 G) and the smaller separation of ATR-SEIRAS stretching frequencies (1554 (ν_asym), 1288 (ν_sym) cm⁻¹, Δν = 266 cm⁻¹) suggest that the structural type of the [*COOCs]˙/[*COOCs]⁻ intermediate is μ₂–η3 CO₂ coordination (class II) for Fe₂ DAC-triggered electrocatalytic CO₂ reduction in CO₂-saturated CsHCO₃ solution. The strong orbital interaction among the dual Fe₂ site, intermediate [CO₂]˙⁻/[CO₂]²⁻, and Cs⁺ cation (6s orbital) is proposed to accelerate charge transfer kinetics and shift the rate-determining step from the electron transfer step (Li⁺, Na⁺) to the protonation step (K⁺, Cs⁺), as evidenced by Cs⁺-induced increase in the proton reaction order (0.86) and Cs⁺-induced decrease in the kinetic Tafel slope (57.2 mV dec⁻¹) and electrochemical activation energy (23.7 kJ mol⁻¹). In contrast, the structural transformation from the dual Fe^II₂ motif to a single Fe^II site revealed by the disappearance of the Fe–Fe distance (3.10 Å) in operando Fe K edge EXAFS lends support to the absence of stretching frequencies (1429 (ν_asym), 1380 (ν_asym), 1241 (ν_asym) cm⁻¹) ascribed to μ₂–η² CO₂ coordination in a CO₂-saturated LiHCO₃ aqueous medium, demonstrating that the transformation of [*COOCs]⁻/[*COOK]⁻ into the bridge [CO₂]²⁻ [Fe–μ-C(O)O–Fe] is vital for electrocatalytic CO₂-to-CO conversion. In addition to identifying the dinuclear Fe₂^II site as a catalytic center, this study demonstrates that the thermodynamic stabilization effect of both the cation size (large s orbital/soft hydration shell) and dual Fe₂^II motif toward [CO₂]˙⁻/[CO₂]²⁻ intermediate is pivotal to the superior CO₂RR kinetics (activity/selectivity). The proposed pathways (Cs⁺/K⁺/Na⁺vs. Li⁺ and dual Fe₂ site vs. single Fe site) may provide insights into how the orbital interaction and the peculiar electronic structure of the dinuclear Fe₂ site impacts the molecular-level mechanism for efficient electrocatalytic CO₂ reduction.