Metal–Organic Framework-Derived Cu-Doped Co₉S₈ Nanorod Array with Less Low-Valence Co Sites as Highly Efficient Bifunctional Electrodes for Overall Water Splitting

Abstract

One of the main goals of current renewable energy research is continuously developing low-cost, robust, and environmentally friendly electrocatalysts to replace scarce and unstable precious metal catalysts. Hence, a novel highperformance electrocatalytic water-splitting material based on a metal−organic framework (MOF)-derived Cu-doped Co 9 S 8 nanorod array was successfully prepared. Using 1,3,5benzenetricarboxylic acid (H 3 BTC) as a ligand, a CuCo MOF (CuCo-MOF) nanobelt array precursor was prepared and then transformed into a Cu-Co 9 S 8 nanorod by a simple sulfurization process using thioacetamide as a sulfur source. It is worth noting that the intermediate CuCo-MOF nanobelts play a key role in the generation of nanostructures of the Cu-doped Co 9 S 8 nanorod array. As one of the most promising bifunctional electrocatalysts reported, the Cu-doped Co 9 S 8 -6h only needs 260 mV to drive 50 mA cm −2 for oxygen evolution reaction and 62 mV to drive 10 mA cm −2 for hydrogen evolution reaction. When the Cu-doped Co 9 S 8 -6h was used as an electrode in a self-made two-electrode system, it requires only 1.49 V of cell voltage to drive 10 mA cm −2 , which is one of the smallest open-circuit voltages reported. Density functional theory results show that the superior performance of the Cu-doped Co 9 S 8 -6h is attributed to increased conductivity and increased water adsorption energy because of Cu doping. By comparing the water adsorption energy of Co 2+ , Cu 2+ , and Co 3+ , it is proved that the active Cu 2+ replaces the inert Co 2+ , and less low-valence Co 2+ sites make the Cu-doped Co 9 S 8 -6h show superior catalytic activity. These results demonstrate that the Cu-doped Co 9 S 8 -6h nanorod array can be used as an excellent bifunctional electrocatalyst for overall water splitting, and this work will provide an excellent synergistic strategy to energy storage and conversion.

Hierarchical CuCo2S4 Nanoflake Arrays Grown on Carbon Cloth: A Remarkable Bifunctional Electrocatalyst for Overall Water Splitting

“…In the deconvoluted Cu 2p XPS spectrum (Figure 2b), the peaks at 952.6 and 932.6 eV are attributed to Cu 2 p 1/2 and Cu 2 p 3/2 of Cu + , respectively. Meanwhile, two peaks at 954.6 and 933.9 eV are ascribed to Cu 2 p 1/2 and Cu 2 p 3/2 with one satellite peak at 942.7 eV, indicating the presence of Cu 2+ [14,60] . In high‐resolution Co 2p XPS spectrum (Figure 2c), the doublet peaks centered at 778.4 and 797.9 eV can be ascribed to the Co 2p 3/2 and Co 2p 1/2 of Co 2+ , respectively.…”

“…Meanwhile, two peaks at 954.6 and 933.9 eV are ascribed to Cu 2 p 1/2 and Cu 2 p 3/2 with one satellite peak at 942.7 eV, indicating the presence of Cu 2 + . [14,60] In high-resolution Co 2p XPS spectrum (Figure 2c), the doublet peaks centered at 778.4 and 797.9 eV can be ascribed to the Co 2p 3/2 and Co 2p 1/2 of Co 2 + , respectively. While the peaks at 775.6 and 790.9 eV correspond to the Co 2p 3/2 and Co 2p 1/2 spin-orbit peaks of Co 3 + , respectively.…”

“…[11] Therefore, developing earth-abundant and low-cost transition metal compounds (oxides, sulfides, carbides, and phosphides) has become urgent. [12][13][14][15][16] However, it is not advisable to simply develop HER or OER electrocatalyst and ignore the development of bifunctional catalysts. [17][18] From practical application, fabricating bifunctional electrode materials combining both the remarkable HER and OER performance has become the promise approach to resolve the issues in this field.…”

“…While these noble metals are high cost and rare, which greatly hamper their practical applications [11] . Therefore, developing earth‐abundant and low‐cost transition metal compounds (oxides, sulfides, carbides, and phosphides) has become urgent [12–16] . However, it is not advisable to simply develop HER or OER electrocatalyst and ignore the development of bifunctional catalysts [17–18] .…”

Metal‐organic frameworks‐derived novel nanostructured electrocatalysts for oxygen evolution reaction

“…Using similar elements, Du et al 107 prepared a novel high‐performing water‐splitting electrocatalyst based on CuCo bimetallic MOF‐derived Cu‐doped Co 9 S 8 nanorod arrays. Utilizing 1,3,5‐benzene tricarboxylic acid (H 3 BTC) as ligands, a CuCo MOF nanobelt array precursor was formed and transformed by sulfurization into a Cu‐Co 9 S 8 nanorod.…”