Switchable molecular electrocatalysis

Abstract

We demonstrate a switchable electrocatalysis mechanism modulated by hydrogen bonding interactions in ligand geometries. By manipulating these geometries, specific electrochemical processes at a single catalytic site can be selectively and precisely activated or deactivated. The α geometry enhances dioxygen electroreduction (ORR) while inhibiting protium redox processes, with the opposite effect seen in the β geometry. Intramolecular hydrogen bonding in the α geometry boosts electron density at the catalytic center, facilitating a shift of ORR to a 4-electron pathway. Conversely, the β geometry promotes a 2-electron ORR and facilitates electrocatalytic hydrogen evolution through an extensive proton charge assembly; offering a paradigm shift to conventional electrocatalytic principles. The expectations that ligand geometry induced electron density modulations in the catalytic metal centre would have a comparable impact on both ORR and HER has been questioned due to the contrasting reactivity exhibited by α-geometry and β-geometry molecules. This further emphasizes the complex and intriguing nature of the roles played by ligands in molecular electrocatalysis.

Fig. 1. Molecular structures of isomeric (a) α-TACoPc and β-TACoPc molecules. (b) UV-vis spectra. (c) ATR-FTIR spectra, and (d) Raman spectra of isomeric α-TACoPc and β-TACoPc molecules. HRTEM images and elemental mapping of (e) β-TACoPc and (f) α-TACoPc molecules. (g) ¹H NMR spectra in DMSO-d₆ for the isomeric molecules before and after D₂O addition.

Fig. 2. (a) Simultaneous linear sweep voltammograms of α-TACoPc and β-TACoPc modified NPG electrodes in oxygen saturated 0.1 M H₂SO₄ electrolyte at a scan rate of 5 mV s⁻¹. (b) Rotating ring disk electrode studies (at 5 mV s⁻¹ scan rate) at a rotation rate of 1600 rpm. (c) Number of electrons involved in ORR. In situ electrochemical Raman spectra of (d) β-isomer and (e) α-isomer in 0.1 M H₂SO₄ solution saturated with oxygen (blue to red trace)/argon (black trace at the open circuit voltage), when the voltage is scanned from 1 V to 0.3 V vs. RHE. The change in intensities for different bands of (f) β-isomer and (g) α-isomer during the reduction scan from 1 V to 0.3 V vs. RHE in 0.1 M H₂SO₄ solution saturated with oxygen (orange trace for β and blue trace for α)/argon (grey trace).

Fig. 3. (a) Voltammograms of α-TACoPc and β-TACoPc modified NPG electrodes in 0.1 M H₂SO₄ at a scan rate of 5 mV s⁻¹, and (b) the corresponding chronopotentiometry profiles at −0.2 mA cm⁻². (c) Tafel plots of α-TACoPc and β-TACoPc isomers in 0.1 M H₂SO₄. In situ electrochemical Raman spectra of (d) β-isomer and (e) α-isomer in 0.1 M H₂SO₄ solution (blue to red trace) when the voltage is scanned from 0.16 to −0.38 V vs. RHE. The change in intensities for different bands of (f) β-isomer and α-isomer during the reduction scan from 0.16 to −0.38 V vs. RHE in 0.1 M H₂SO₄ solution. (g) Percentage change in intensity of 744 cm⁻¹ band at different potentials vs. RHE (i_v) with respect to the initial intensity of 756 cm⁻¹ band at 0.16 V vs. RHE (i₀) (orange trace for β-isomer and blue trace for α-isomer).

Fig. 4. (a) Co 2p XPS spectra of α-TACoPc and β-TACoPc molecules. (b) Co²⁺/Co³⁺ area ratio which is extracted from (a). N 1s XPS spectra of (c) pristine and protonated forms of α-TACoPc and β-TACoPc isomers. (d) Bar plot showing the difference of N 1s binding energies between protonated β-TACoPc and protonated α-TACoPc molecules (ΔBE_β–α). (e) Zeta potential profiles of α-TACoPc and β-TACoPc isomeric molecules. (f) Raman spectra of α-TACoPc (bottom panel) and β-TACoPc (top panel) isomers and (g) the intensity ratio of i₁₅₄₂/i₁₃₅₀ peaks in the absence and presence of various solvents.