Polar Layered Intermetallic LaCo2P2 as a Water Oxidation Electrocatalyst

Abstract

We investigate LaCo₂P₂ as an electrocatalytic material for oxygen evolution reaction (OER) under alkaline and acidic conditions. This layered intermetallic material was prepared via Sn-flux high-temperature annealing. The electrocatalytic ink, prepared with the ball-milled LaCo₂P₂ catalyst at the mass loading of 0.25 mg/cm², shows OER activity at pH = 14, reaching current densities of 10, 50, and 100 mA/cm² under the overpotential of 400, 440, and 460 mV, respectively. Remarkably, the electrocatalytic performance remains constant for at least 4 days. Transmission electron microscopy reveals the formation of a catalytically active CoO_x shell around the pre-catalyst LaCo₂P₂ core during the alkaline OER. The core serves as a robust support for the in situ-formed electrocatalytic system. Similar studies under pH = 0 reveal the rapid deterioration of LaCo₂P₂, with the formation of LaPO₄ and amorphous cobalt oxide. This study shows the viability of layered intermetallics as stable OER electrocatalysts, although further developments are required to improve the electrocatalytic performance and increase the stability at lower pH values.

Keywords: core−shell; electrocatalysis; intermetallic; oxygen evolution reaction; precatalyst; water oxidation.

Figure 1

Side-by-side comparison of the crystal structures of AlFe₂B₂ (a) and LaCo₂P₂ (b).

Figure 2

PXRD patterns for the bulk (red) and ball-milled (blue) samples of LaCo₂P₂. The calculated pattern (black) for LaCo₂P₂ is provided for comparison.

Figure 3

TEM analysis of LaCo₂P₂ particles after ball-milling: (a) low-magnification overview HAADF-STEM image and simultaneous STEM–EDX elemental mappings acquired at the L-line of La (red) and K-lines of P (purple), O (blue), and Co (green), and their mixture; (b) high-resolution HAADF-STEM image of a single LaCo₂P₂ particle viewed along the [111] zone axis (the corresponding FT pattern is shown in the inset); (c) bright-field HRTEM image of the LaCo₂P₂ sample, together with the insets showing the FT patterns taken from [100] (A) and [110] (B) oriented particles.

Figure 4

(a) Alkaline OER anodic polarization curves for Ni foam (after 100 activation cycles), Ni foam-supported ball-milled LaCo₂P₂ at 0.25 mg/cm² loading (after 100 activation cycles) and Ni foam-supported reference IrO₂ catalyst at 0.25 and 1 mg/cm² loadings. (b) Respective Tafel plots. (c) Comparison of Nyquist plots for Ni foam-supported LaCo₂P₂ and Ni foam-supported reference IrO₂ at the applied overpotential η = 420 mV. The inset shows an equivalent electrical circuit model used to fit the Nyquist plots. (d) Chronopotentiometric stability tests under alkaline OER for Ni foam-supported LaCo₂P₂ and Ni foam-supported reference IrO₂ at the same loading of 0.25 mg/cm².

Figure 5

Acidic OER anodic polarization curves and the continuous chronopotentiometric profiles (shown as an inset) for the Ti felt, as well as Ti felt-supported ball-milled LaCo₂P₂, and Ti felt-supported reference IrO₂ materials, both at 3 mg/cm² loading.

Figure 6

PXRD patterns of LaCo₂P₂ after 100 h of chronopotentiometric testing at 10 mA/cm² in 1 M NaOH (a) and 0.5 M H₂SO₄ (b). Broad amorphous peaks present in both samples are due to the Nafion ionomer. The calculated patterns for LaCo₂P₂ and LaPO₄ are shown as references.

Figure 7

HAADF-STEM images together with simultaneously collected STEM–EDX elemental mappings of La, P, O, Co, and their mixture for LaCo₂P₂ particles after OER electrocatalysis in alkaline (a) and acidic (c) electrolytes. (b) High-resolution [201] HAADF-STEM image with the corresponding SAED (upper corner inset) and the magnified HAADF-STEM image with an overlaid simulated image (bottom corner inset) for LaCo₂P₂ particles after alkaline OER electrocatalysis. (d) Low-magnification TEM overview image for the LaPO₄ needle-like particles formed after acidic OER electrocatalysis over LaCo₂P₂.