Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis

Abstract

Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO₂ reduction without loss of selectivity (i.e., H₂ evolution). Fast pre-equilibrium metal-hydride formation, at 10⁸ M^-1 s^-1, boosts the CO₂ electroreduction to formate rate up to 296 s^-1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis.

Scheme 1. Three Common Pathways for the Metal–Hydride Reaction

Scheme 2. Proposed CO₂ Reduction Mechanism by 1^3–

Figure 1

CV of 0.05 mM 1^2– in 0.1 M Bu₄NBF₄ MeCN solution (black): (left) in 0.1 M Bu₄NBF₄ MeCN/H₂O (95:5) under 1 atm N₂ (red); in 0.1 M Bu₄NBF₄ MeCN/H₂O (95:5) under 1 atm CO₂ (blue). (right) with 5.1 mM AnsdH⁺ under 1 atm N₂ (red) and with 0.25 mM AnsdH⁺ under 1 atm CO₂ (blue). Inset: Plot of j versus [H₂O] under 1 atm CO₂.

Figure 2

CVs of 0.1 mM 1^2– in 0.1 M Bu₄NBF₄ MeCN/H₂O (99.3:0.7). (left) Under 1 atm N₂ at variable scan rates (starting from bright green 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.2, 1.4, and 1.6 V s^–1); CV in black is for 1^2– without added H₂O. (right) Under 1 atm CO₂ at variable scan rates (starting from red trace 0.8, 1.2, 1.6, 2, 3, 4, 5, and 6 V s^–1). Insets: plots of (E_p – E_1/2) (F/RT) vs ln(υ). The red line is a linear fit with slope fixed at −0.5.

Figure 3

CVs of 1^2– with (0.05, 0.06, 0.77, 0.09, 0.11-, and 0.12 mM H⁺, colors) in 0.1 M Bu₄NBF₄ MeCN under CO₂, at 100 mV/s and using the GC electrode. (left) with 5% H₂O as the source of H⁺. Inset: plot of j_c versus [1^2–], at −1.054 V. (right) with 2 mM AnsdH⁺ as the source of H⁺. Inset: plot of j_c vs [1^2–], at −0.82 V. Red lines are linear fit to the data, and black CV trace has no added H⁺.

Figure 4

Forward CV traces of (top left) 0.06 mM 1^2– in 0.1 M Bu₄NBF₄ MeCN/H₂O (95:5) under 1 atm CO₂ at the scan rate = 0.1, 0.2, 0.4, 0.6, 0.9, 1.2, 1.5, 2, 3, 4, 6, 8, 12, 14, 18, 21, and 22 V/s; and (bottom left) 3.5 mM AnsdH⁺ and 0.2 mM 1^2– in 0.1 M Bu₄NBF₄ MeCN under CO₂ at various scan rates: 0.3, 0.5. 0.7, 0.8, 1.2, 1.8, 2.5, 3, 4, 5, 5.5, 6, 7, and 8 V/s. (Right) Plots of j_max vs υ at potentials negative to −1.08 V (top) and −0.82 (bottom), after subtraction of the background current value.

Figure 5

Tafel-style plot: Log₁₀[(TOF/s^–1) (EF)²] vs overpotential (η) at E_cat/2, for selected molecular CO₂ to HCO₂^– reduction catalysts. Details of calculations and parameters used to construct the plot are shown in Table S2 and references therein. η = E_CO2/HCOO^– – E_cat/2.

Figure 6

LFER between Log(TOF/s^–1) and E_cat/2 for selected molecular electrocatalysts {1^2–, [Fe₄N(CO)₁₂]^1–,^, [Fe₄N(CO)₁₁(PPh₃)]⁻, [FeN₅Cl₂]⁺, [FeP₄N₂]²⁺, [Co(imino-bpy)]²⁺, [(bipy)Co(PyS)₂]⁺, CpCoP^Cy₂N^Bn₂I₂, [Ni(qpdt)₂]⁻, [Pt(depe)₂]⁺,^, Ir(POCOP), and [Mn(bipy)(CO)₃]/Fe–S}. Data for this plot, see Table S2. The blue shadow highlights correlation of Log₁₀[(TOF/s^–1)(FE_HCOO^–)²] with E_cat/2.