Chirality enhances oxygen reduction

Abstract

Controlled reduction of oxygen is important for developing clean energy technologies, such as fuel cells, and is vital to the existence of aerobic organisms. The process starts with oxygen in a triplet ground state and ends with products that are all in singlet states. Hence, spin constraints in the oxygen reduction must be considered. Here, we show that the electron transfer efficiency from chiral electrodes to oxygen (oxygen reduction reaction) is enhanced over that from achiral electrodes. We demonstrate lower overpotentials and higher current densities for chiral catalysts versus achiral ones. This finding holds even for electrodes composed of heavy metals with large spin-orbit coupling. The effect results from the spin selectivity conferred on the electron current by the chiral assemblies, the chiral-induced spin selectivity effect.

Keywords: chirality; fuel cells; oxygen; respiration; spin.

Fig. 1.

The experimental setup and the effect of chiral monolayers on oxygen reduction. (A) A schematic layout of the electrochemical setup, which includes a gold working electrode coated with a SAM of either chiral or achiral molecules. The potentials reported here were converted to the standard RHE reference scale. Note that the working electrode was static during these measurements. (B) As a demonstration, the linear sweep voltammogram that was measured when the electrode was coated with a monolayer of achiral molecules (3-mercaptopropionic acid, blue curves) or with chiral molecules (l-cysteine, red curves) in N₂ (dashed) and O₂ (solid)-saturated 0.1 M KOH aqueous solution. The onset potential is defined as the potential when the current reaches 0.1 mA/cm², as shown by the dotted black line. Current densities were normalized in reference to the geometric area of the working electrode.

Fig. 2.

The molecular length-dependent oxygen reduction. The ORR polarization curves of (A) achiral and (B) chiral monolayers of various molecular lengths in O₂-saturated 0.1 M KOH at room temperature and a potential scan rate of 50 mV/s. (C) The onset potentials (at 0.1 mA⋅cm⁻²) versus molecular length for working electrodes modified with monolayers of different lengths. For chiral monolayers, the onset potential becomes more positive as the length increases, indicating a lower barrier for reduction. On the contrary, the onset potential of the achiral monolayers become more negative as the molecular length increases, indicating a higher barrier for reduction. Both hydrophobic (achiral-CH₃) and hydrophilic (achiral-COOH) monolayers were studied and revealed a similar behavior as a function of length. (D) ORR polarization curves obtained by the RDE method for l-ala7 and l-ala3 in air-saturated 0.1 M KOH at a sweep rate of 5 mV⋅s⁻¹ from 500 rpm to 2,000 rpm. (Inset) The kinetic current density (j_k) of l-ala7 and l-ala3 for O₂ reduction at 0.4 V.

Fig. 3.

Chirality enhances ORR activities of inorganic NPs. (A) The static ORR polarization curves were measured with chiral and achiral Au NPs in O₂-saturated 0.1 M KOH solution at a sweep rate of 50 mV/s. (B) RDE measurements of the ORR polarization curves of chiral and achiral Pt NPs and the commercial catalysts Pt/C in air-saturated 0.1 M KOH at 1,500 rpm and a sweep rate of 5 mV⋅s⁻¹. (C) Comparisons of onset potentials (identified as the potential at −0.1 mA⋅cm⁻²), half-wave potentials (E_1/2), and current density at 0.9 V (versus RHE, right scale). (D) A comparison of mass activity and specific activity at 0.9 V versus RHE.

Fig. 4.

The effect of chiral molecules. (A) The dependence of the spin polarization on the length of chiral oligopeptides, adapted with permission from ref. . (B) The splitting in the spin states of the triplet oxygen upon interaction with the spin-polarized electrons residing on the chiral molecules. The possible spin states in the case of (C) a chiral system and in the case of (D) an achiral one. In the case of an achiral system the two electrons can have four possible configurations from which only one of them leads to reaction. There is only one possible configuration in the chiral system, and the electrons are strongly coupled to the molecular frame. As a result, this is the only configuration that can lead to the reaction. (E) The calculated triplet energy levels on the oxygen presented as a function of the distance between the chiral molecule and the oxygen. The Zeeman splitting, which causes the stabilization of one triplet state on the oxygen, is about 100 meV (5 kT at room temperature) at a distance of 0.55 nm between the interacting electrons.