Free-base porphyrin polymer for bifunctional electrochemical water splitting

Abstract

Water splitting is considered a promising approach for renewable and sustainable energy conversion. The development of water splitting electrocatalysts that have low-cost, long-lifetime, and high-performance is an important area of research for the sustainable generation of hydrogen and oxygen gas. Here, we report a metal-free porphyrin-based two-dimensional crystalline covalent organic polymer obtained from the condensation of terephthaloyl chloride and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin which is stabilized by an extensive hydrogen bonding network. This material exhibits bifunctional electrocatalytic performance towards water splitting with onset overpotentials, η, of 36 mV and 110 mV for HER (in 0.5 M H₂SO₄) and OER (in 1.0 M KOH), respectively. The as-synthesized material is also able to perform water splitting in neutral phosphate buffer saline solution, with 294 mV for HER and 520 mV for OER, respectively. Characterized by electrochemical impedance spectroscopy (EIS) and chronoamperometry, the as-synthesized material also shows enhanced conductivity and stability compared to its molecular counterpart. Inserting a non-redox active zinc metal center in the porphyrin unit leads to a decrease in electrochemical activity towards both HER and OER, suggesting the four-nitrogen porphyrin core is the active site. The high performance of this metal-free material towards water splitting provides a sustainable alternative to the use of scarce and expensive metal electrocatalysts in energy conversion for industrial applications.

Scheme 1. Synthesis procedure of Porphvlar.

Fig. 1. (A) Scanning electron microscopy (SEM) imaging of Porphvlar in various scales, and (B) pXRD pattern.

Fig. 2. Spectroscopic characterization of Porphvlar. (A) FT-IR spectra of Porphvlar and molecular porphyrin (H₂TAPP). (B) Comparison UV-vis spectra of Porphvlar and H₂TAPP.

Fig. 3. Polarization curves of molecular porphyrin (H₂TAPP) (blue), Porphvlar (red) and blank carbon paper electrode under different conditions. (A) Oxidation in 1.0 M KOH aqueous solution; (B) reduction in 0.5 M H₂SO₄ aqueous solution, (C) oxidation in 1.0 M PBS buffer solution; (D) reduction in 1.0 M PBS buffer solution, scan rate: 5 mV s⁻¹.

Fig. 4. Electrochemical impedance spectroscopy of Porphvlar (red) and H₂TAPP (purple) operated at 250 mV overpotential: (A) in 1.0 M KOH aqueous solution (B) in 1.0 M PBS buffer solution and 0.5 M H₂SO₄ aqueous solution (blue), respectively; Tafel plots of Porphvlar constructed by polarization curves: (C) in 1.0 M KOH aqueous solution (red) and 1.0 M PBS buffer (purple); (D) 1.0 M PBS buffer solution (green) and 0.5 M H₂SO₄ aqueous solution (blue).

Fig. 5. Time dependence of the current density for as-synthesized Porphvlar at static potential; blue: HER in 0.5 M H₂SO₄; green: in 1.0 M PBS buffer; teal: OER in 1.0 M PBS buffer; red: OER in 1.0 M KOH.