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

By regulating the electronic environment of Fe active centers to modulate electrocatalytic CO2 reduction behavior, an advanced dual Fe-2-site catalyst (Fe-2 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 Fe-2 DAC displaying a higher charge transfer coefficient (alpha = 0.53), turnover frequency (TOFCO = 2.03 s(-1)), and CO faradaic efficiency (FECO = 98.6%) at an overpotential of 400 mV compared to that of single iron atom catalyst (Fe SAC, alpha = 0.31, TOFCO = 0.25 s(-1), FECO = 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 (nu(asym)), 1288 (nu(sym)) cm(-1), & UDelta;nu = 266 cm(-1)) suggest that the structural type of the [*COOCs]/[*COOCs](-) intermediate is mu(2)-eta 3 CO2 coordination (class II) for Fe-2 DAC-triggered electrocatalytic CO2 reduction in CO2-saturated CsHCO3 solution. The strong orbital interaction among the dual Fe-2 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-2(II) motif to a single Fe-II site revealed by the disappearance of the Fe-Fe distance (3.10 & ANGS;) in operando Fe K edge EXAFS lends support to the absence of stretching frequencies (1429 (nu(asym)), 1380 (nu(asym)), 1241 (nu(asym)) cm(-1)) ascribed to mu(2)-eta(2) CO2 coordination in a CO2-saturated LiHCO3 aqueous medium, demonstrating that the transformation of [*COOCs](-)/[*COOK](-) into the bridge [CO2](2-) [Fe-mu-C( =O)O-Fe] is vital for electrocatalytic CO2-to-CO conversion. In addition to identifying the dinuclear Fe-2(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 Fe-2(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 Fe-2 site vs. single Fe site) may provide insights into how the orbital interaction and the peculiar electronic structure of the dinuclear Fe-2 site impacts the molecular-level mechanism for efficient electrocatalytic CO2 reduction.