CaCd 2 P 2 : A Visible‐Light Absorbing Zintl Phosphide Stable Under Photoelectrochemical Water Oxidation

Overview

CaCd2P2, a Zintl phase phosphide, was identified through high-throughput computational screening as a visible-light absorbing photoelectrode material for the oxygen evolution reaction. The material exhibits a 1.6 eV bandgap favorable for solar light absorption and demonstrates operational stability under alkaline OER conditions via light-induced surface transformation, addressing a fundamental challenge in photoelectrochemical energy conversion where conventional semiconductors suffer from either weak light absorption or photocorrosion instability.

Methods and approach

High-throughput computational screening was employed to identify CaCd2P2 from the AM2P2 Zintl phase family. Characterization employed photoelectrochemical measurements including current-voltage characteristics and electrochemical impedance spectroscopy under illumination. Surface chemistry and structural transformations were examined using complementary microscopy techniques (scanning and transmission electron microscopy) and spectroscopic methods (X-ray photoelectron spectroscopy, Raman spectroscopy, and absorption spectroscopy). Co-catalyst integration studies evaluated the synergistic interactions between the in situ modified CaCd2P2 surface and cobalt phosphate catalyst under OER conditions.

Results

CaCd2P2 undergoes a light-stabilized surface transformation under alkaline OER conditions that prevents photocorrosion, contrasting sharply with degradation mechanisms typical of low-bandgap semiconductor photoelectrodes. The surface modification processes in situ enhance electrochemical stability without requiring protective overlayers. Cobalt phosphate (CoPi) functions as a co-catalyst in synergistic combination with the chemically modified CaCd2P2 surface, yielding improved catalytic performance for oxygen evolution.

Implications

The demonstrated light-stabilized transformation mechanism represents a departure from conventional photocorrosion pathways and suggests that designed interface chemistry can enable direct utilization of low-bandgap semiconductors for photoelectrochemical applications without requiring passive protection strategies. This finding indicates that transition metal phosphides and related Zintl phases merit reconsideration as photoelectrode materials through the lens of beneficial in situ surface evolution rather than inherent instability. The broader AM2P2 Zintl phase family presents a materials platform for systematic exploration of stabilizing surface chemistry and optical absorption properties relevant to solar fuels generation.