Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis

Abstract

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu–S bonds and the exchange coupling between CuI and CuII centers. Although electrocatalytic study with digenite Cu9S5 and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu9S5 and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of Cu9S5 to be d(0015) and d(002) for CuS. Tetrahedral (T d) CuII, distorted octahedral (Oh) CuII, and trigonal planar (Tp) CuI sites form the d(0015) surface of Cu9S5, while the (002) surface of CuS consists of only T d CuII. The distribution of CuI and CuII sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of Cu9S5 and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that Cu9S5 is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu2O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu9S5 surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu9S5 and CuS to CuO/Cu2O/Cu(OH)2 electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the Cu9S5/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure–activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.

Evolution of Mn–Bi2O3 from the Mn-doped Bi3O4Br electro(pre)catalyst during the oxygen evolution reaction

“…S10†) to achieve 10 mA cm −2 current density, without much change in the catalytic actvity. 57 In comparison, the Bi 3 O 4 Br/NF anode only reached 50 mA cm −2 current density at 1.72 V, which indicated an enhanced OER activity presumably due to the active contribution of Mn-doping in Bi 3 O 4 Br. The use of a different Mn II and Bi III precursor ratio (see the Experimental section) during synthesis gave a very similar polycrystalline (with a notable amorphous feature) Mn-doped Bi 3 O 4 Br material (Fig.…”

“…Bi 3 O 4 Br and Mn–Bi 3 O 4 Br were initially deposited on the GC electrode to perform the OER under alkaline conditions (1 M KOH). 57 Cyclic voltammetry (CV) cycles were recorded within 0.9–1.9 V ( vs . RHE) in a three-electrode system, using materials deposited on GC, Pt wire and Hg/HgO as working (WE), counter (CE) and reference electrode (RE), respectively.…”

Precursor engineering for soft selective synthesis of phase pure metal-rich digenite (Cu9S5) and djurleite (Cu31S16) nanocrystals and investigation of their photo-switching characteristics

“…Kundu et al , in a recent investigation, achieved tailoring of the electrochemical behavior of mixed valent copper sulfide nanomaterials during water electrolysis. 55 They also correlated the device performance to the atomic arrangements and coordination geometry of the surface exposed sites. Liu et al studied these Cu vacancies within the crystal lattices of metal deficient copper sulfides and concluded that the phenomenon enhances their performance in photodetectors or solar cells.…”