Atomically dispersed Pt–N4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction

Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt−N4 sites doped on a carbon nanotube (Pt1/CNT) can catalyse the CER with excellent activity and selectivity. The Pt1/CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt1/CNT exhibits near 100% CER selectivity even in acidic media, with low Cl− concentrations (0.1 M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl− on Pt−N4 sites during the CER. Density functional theory calculations suggest the PtN4C12 site as the most plausible active site structure for the CER.

The spin-orbit splitting and area ratio for 4f5/2 (dashed lines) and 4f7/2 (solid lines) peaks are 3.34 eV and 3:4, respectively. The peak of Pt 4f7/2 in the spectrum of Pt1/CNT catalyst was observed at 73.1 eV, which is close to the value of Pt porphyrin in a previous report 1 .       (Fig. 1c). An electrolyte was stirred at a rotation speed of 300 rpm. The electrolyte was replaced with a fresh electrolyte after the stability test.

Supplementary Note 2 | Construction of Pourbaix diagrams of model systems.
The Pourbaix diagram represents the thermodynamically stable surface structures in electrochemical systems as a function of pH and electrode potential (U). Note that the detailed description about the Pourbaix diagram is reported elsewhere 18−20 . In this study, we constructed the Pourbaix diagrams for Pt−N4 sites and PtO2 (110) surface by calculating the adsorption free energies for all plausible adsorbates (i.e., H * , O * , OH * , OOH * , Cl * , ClO * , and ClO2 * , see Supplementary Figs. 24 and 29). Note that * denotes the adsorbed states on the surface. A generalised description for the adsorbates (denoted as OmHnClk) on the site (denoted as S) can be written as By rewriting equation (1) where S * denotes the bare site without adsorbates. Then, the adsorption free energy (G) for OmHnClk species can be defined as follows.
where m, n, and k denote the number of oxygen, hydrogen, and chlorine atoms, respectively. From DFT calculation, the G for each adsorbate species can be calculated as follows.
where E is the binding energy for each adsorbate, ZPE is the change in zero-point vibrational enthalpy, and -TS is the entropic correction at room temperature. By rewriting equation (9), the G for each species can be represented as follows. Herein, + and Cl − is defined as a function of USHE, pH, and Cl .
where USHE, kB, and T denote the electrode potential (vs. theoretical standard hydrogen electrode, SHE), Boltzmann constant, and temperature, respectively. Considering that the Cl − concentration is nearly constant under reaction condition by using NaClO4 as a buffer solution, ln Cl was assumed to be negligible for our calculation 17 . At finite USHE and pH, the G can be expressed as By rewriting equation (20), the G for each species as a function of USHE and pH can be defined as follows at standard conditions (T = 298 K). In this study, G's for all species as a function of USHE were initially found to determine the phase boundaries at pH = 0 (Supplementary Figs. 25 and 30). Subsequently, by applying the effect of pH to the G's, the Pourbaix diagrams for the Pt−N4 sites and PtO2 (110) surface were finally constructed (Supplementary Figs. 26, 27, and 31).

Supplementary Note 3 | Free energy diagram of model systems for CER and OER.
To theoretically investigate the CER activity, we calculated the free energy diagrams for the CER in the Pt−N4 sites and PtO2 (110) surface (Fig. 4c). To accomplish this, two possible reaction mechanisms including different intermediates (i.e., Cl * or ClO * ) were considered.
(I) Pathway mediated by the Cl * species * + Cl ( ) ⇌ -Cl * + -Cl * + Cl ( ) ⇌ * + Cl ( ) + (II) Pathway mediated by the ClO * species where (aq) and (g) represent the aqueous and gaseous phases, respectively. The E's for each reaction intermediate were calculated relative to the Cl2 gas molecule as follows.
(I) For * and Cl * species (II) For * and ClO * species To investigate the thermodynamic overpotential for OER at the zero overpotential (TD(OER)) of Pt−N4 sites, we assumed the conventional four-electron pathway for the OER as follows.
where (l) indicates the liquid phase. The E's for each reaction intermediate were calculated relative to H2O and H2 molecules as follows. S48 where ES-OH*, ES-O*, and ES-OOH* are the total energies of adsorbed state of the Pt−N4 sites (i.e., OH * , O * , and OOH * , respectively); ES* is the total energy of bare state of the Pt−N4 sites; and are the total energies of isolated water molecules and hydrogen gas, respectively.
Finally, TD(OER) can be defined by where Ueq indicates the equilibrium potential for OER (i.e., 1.23 V vs. SHE).

Supplementary Note 5 | Full free energy diagram for CER over Pt 1 /CNT.
By combining the experimental data for the kinetics and theoretical data for thermodynamics, a full free energy diagram along the reaction coordinate of CER over Pt1/CNT was constructed. Details regarding the definition and derivation of this approach are fully given in the earlier works by Exner and co-workers 23,24 . Within the Butler-Volmer formalism, the Tafel slope, b, is defined by the following equation.
where kB is the Boltzmann's constant, T is the temperature, e is the elementary charge, is the integer number of transferred electrons before the rate-determining step (rds), rrds is 0 for chemical (i.e., no charge transfer) and 1 for electrochemical step, and k is the transfer coefficient of the considered reaction step k. With increased overpotential for the CER (CER), the experimental Tafel plot revealed two linear Tafel regions with b of 38 mV dec. −1 (30 mV ≤ CER ≤70 mV) and 79 mV dec. −1 (75 mV ≤ CER ≤102 mV) (Supplementary Fig. 34). At room temperature, the respective , rrds, and k for each Tafel region were determined as  = 0, rrds = 1, 1 = 0.83 (for first region, k = 1) and  = 1, rrds = 1, 2 = 0.58 (for second region, k = 2). The overall current density (j) can be expressed as a function of CER.
where m is the molar number of Pt-catalyst loaded on the electrode (i.e., 14.00 nmol cm −2 for Pt1/CNT) and NA is the Avogadro's number (6.022 × 10 23 ). From the log(j0), we can determine the G # rds as follows.
The G # rds's were determined as 0.75 (for first TS) and 0.80 eV (for second TS) in the Pt1/CNT.
The TS free energies of step #i ( # ( CER ) ) showed that with increasing CER, the rds was switched from the first TS (Heyrovsky step, i = 1) to the second TS (Volmer step, i = 2) due to S51 the larger decrease in free energies by CER in the Volmer step (indicated by the slope, d # ( CER ) d , in the Supplementary Fig. 35a). Additionally, the absolute value of free energy for the IM (|G(CER)|) showed that it reached the thermoneutral state at the point where CER is equal to the thermodynamic overpotential of CER (i.e., TD(CER) = 0.09 V for PtN4C12 species, Supplementary Fig. 35b).