Design of template-stabilized active and earth-abundant oxygen evolution catalysts in acid

Abstract

Oxygen evolution reaction (OER) catalysts that are earth-abundant and are active and stable in acid are unknown. Active catalysts derived from Co and Ni oxides dissolve at low pH, whereas acid stable systems such as Mn oxides (MnO _x ) display poor OER activity. We now demonstrate a rational approach for the design of earth-abundant catalysts that are stable and active in acid by treating activity and stability as decoupled elements of mixed metal oxides. Manganese serves as a stabilizing structural element for catalytically active Co centers in CoMnO _x films. In acidic solutions (pH 2.5), CoMnO _x exhibits the OER activity of electrodeposited Co oxide (CoO _x ) with a Tafel slope of 70-80 mV per decade while also retaining the long-term acid stability of MnO _x films for OER at 0.1 mA cm^-2. Driving OER at greater current densities in this system is not viable because at high anodic potentials, Mn oxides convert to and dissolve as permanganate. However, by exploiting the decoupled design of the catalyst, the stabilizing structural element may be optimized independently of the Co active sites. By screening potential-pH diagrams, we replaced Mn with Pb to prepare CoFePbO _x films that maintained the high OER activity of CoO _x at pH 2.5 while exhibiting long-term acid stability at higher current densities (at 1 mA cm^-2 for over 50 h at pH 2.0). Under these acidic conditions, CoFePbO _x exhibits OER activity that approaches noble metal oxides, thus establishing the viability of decoupling functionality in mixed metal catalysts for designing active, acid-stable, and earth-abundant OER catalysts.

Fig. 1. Tafel plots of oxygen evolution for unary metal oxides in 0.10 M P_i and 1.0 M KNO₃ at (a) pH 7.0 and (b) pH 2.5 of CoO_x (red , 60 and 82 mV per decade Tafel slope at pH 7.0 and 2.5, respectively), NiO_x (light magenta , 90 mV per decade at pH 7.0), FeO_x (orange , 45 and 51 mV per decade at pH 7.0 and 2.5, respectively), MnO_x (blue , 125 and 650 mV per decade at pH 7.0 and 2.5), PbO_x (brown , 130 and 121 mV per decade at pH 7.0 and 2.5), and IrO_x (purple , 41 and 32 mV per decade at pH 7.0 and 2.5).

Fig. 2. Cyclic voltammograms (CVs) of a 1 cm² FTO electrode at 50 mV s^–1 in 50 mM MeP_i buffer at pH 8.0 with 0.25 mM of each metal: (a) Co²⁺ and Mn²⁺ (red ); and (b) Co²⁺ and Pb²⁺ (light green ) with addition of Fe²⁺ (dark green ). Background CV of metal-free MeP_i buffer (grey ) included for comparison.

Fig. 3. Tafel plots of oxygen evolution for CoMnO_x in 0.10 M P_i and 1.0 M KNO₃ at (a) pH 7.0 and (b) pH 2.5 of: CoMnO_x deposited at 0.90 (dark green , 65 and 81 mV per decade at pH 7.0 and 2.5), 0.65 (light green , 85 mV per decade at pH 2.5), and 1.15 V (cyan , 83 mV per decade at pH 2.5). Unary metal oxides provided for comparison with slopes defined in Fig. 1, CoO_x (red ), MnO_x (blue ), and IrO_x (purple ).

Fig. 4. Tafel plots of oxygen evolution in 0.10 M P_i and 1.0 M KNO₃ at (a) pH 7.0 and (b) pH 2.5 for CoPbO_x (light green , ∼72 mV per decade at both pH 7.0 and 2.5) and CoFePbO_x (dark green , ∼70 mV per decade at both pH 7.0 and 2.5). CoFeO_x (orange , ∼70 mV per decade at both pH 7.0 and 2.5) and unary metal oxides are provided for comparison with slopes defined in Fig. 1, CoO_x (red ), PbO_x (brown ), and IrO_x (purple ).

Fig. 5. Electrochemical stability for acidic OER measured by sustained chronoamperometry at: (a) 0.1 mA cm^–2 in pH 2.5 P_i for CoMnO_x deposited at 0.65 (light green ), 0.90 (dark green ), and 1.15 V (cyan ) along with CoO_x (red ), NiO_x (light magenta ), MnO_x (blue ), IrO_x (purple ), and FTO (grey ) for comparison; (b) 1.0 mA cm^–2 in pH 2.5 P_i for CoPbO_x (light green ) and CoFePbO_x (dark green ) with CoFeO_x (orange ), CoO_x (red ), and PbO_x (brown ) for comparison; and (c) 1.0 mA cm^–2 in pH 2.0 sulfate for CoFePbO_x (dark green ). The inflection of potential in the plots indicates film dissolution.

Fig. 6. FESEM images of CoMnO_x electrodeposited at (a) 0.65, (b) 0.90, and (c) 1.15 V with (d) CoO_x and (e) MnO_x for comparison. All samples were prepared on FTO substrate, and scale bars are 200 nm.

Fig. 7. FESEM images of (a) CoPbO_x and (b) CoFePbO_x with (c) PbO_x and (d) FTO for comparison. All samples were electro-deposited on FTO, and scale bars are 100 nm.

Fig. 8. EDS elemental maps recorded through SEM of (a) CoMnO_x (deposited at 0.90 V) and (b) CoFePbO_x. Individual elemental channels for Co (red), Mn (blue), Fe (orange), and Pb (green) were combined and overlaid on the respective SEM image. All samples were prepared on FTO substrate, and scale bars are 200 nm for CoMnO_x and 100 nm for CoFePbO_x.

Fig. 9. High-resolution EDS elemental maps recorded through STEM of (a) CoMnO_x (deposited at 0.90 V) and (b) CoFePbO_x. Individual elemental channels for Co (red), Mn (blue), Fe (orange), and Pb (green) were combined and overlaid on the respective image. Scale bars are 15 nm for a resolution of 7.4 Å per px.

Fig. 10. High-resolution XPS spectra in the (a) Co 2p and (b) Mn 2p regions for: CoMnO_x (deposited at 0.90 V, dark green ) compared to CoO_x (red ), and MnO_x (blue ). Grey dotted lines are presented as guides.

Fig. 11. High-resolution XPS spectra in the (a) Co 2p and (b) Pb 4f regions for: CoPbO_x (light green ) and CoFePbO_x (dark green ) compared to CoO_x (red ), and PbO_x (brown ). Grey dotted lines are presented as guides.

Fig. 12. Progression of designing an active, stable, and earth-abundant acidic OER catalyst. The first generation system focused on demonstrating film stability at low pH with MnO_x.^, The activity of MnO_x was improved in the second generation by activating MnO_x for OER. The catalyst was reformulated as a mixed metal oxide for the third generation, where functionality was separated into catalytic and structural elements comprising Co and Mn, respectively. Finally, the degradation of Mn oxides at high anodic potentials was solved by replacing it with a FePb oxide structural component to create the fourth-generation catalyst.

Fig. 13. Process for independently optimizing the structural component of mixed metal films to discover a metal oxide that is both stable at high anodic potentials and at acidic pH. Pourbaix diagrams of metals (shown as simplified representations of stability and corrosion, generated from the Materials Project and experimental data) were analysed for stability in the top left region of the plots (corresponding to high anodic potentials and low pH). The candidates were then filtered by removing precious and rare metals; then further refined by excluding oxides that were incompatible with anodic electrodeposition in buffer. In this manner, Pb was identified as a promising replacement for Mn for stabilizing OER catalysts in acid.