Improved Interface Charge Transfer and Redistribution in CuO‐CoOOH p‐n Heterojunction Nanoarray Electrocatalyst for Enhanced Oxygen Evolution Reaction

Abstract Electron density modulation is of great importance in an attempt to achieve highly active electrocatalysts for the oxygen evolution reaction (OER). Here, the successful construction of CuO@CoOOH p‐n heterojunction (i.e., p‐type CuO and n‐type CoOOH) nanoarray electrocatalyst through an in situ anodic oxidation of CuO@CoS x on copper foam is reported. The p‐n heterojunction can remarkably modify the electronic properties of the space‐charge region and facilitate the electron transfer. Moreover, in situ Raman study reveals the generation of SO4 2− from CoS x oxidation, and electron cloud density distribution and density functional theory calculation suggest that surface‐adsorbed SO4 2− can facilitate the OER process by enhancing the adsorption of OH−. The positively charged CoOOH in the space‐charge region can significantly enhance the OER activity. As a result, the CuO@CoOOH p‐n heterojunction shows significantly enhanced OER performance with a low overpotential of 186 mV to afford a current density of 10 mA cm−2. The successful preparation of a large scale (14 × 25 cm2) sample demonstrates the possibility of promoting the catalyst to industrial‐scale production. This study offers new insights into the design and fabrication of non‐noble metal‐based p‐n heterojunction electrocatalysts as effective catalytic materials for energy storage and conversion.

the above shape represent the CoOOH supported by CuO nanoarrays, the bare arrays represent CuO nanoarrays.
Electric Currents (EC) module and Stationary study has been used to improve the simulation precision. The appropriate simulation precision has been set through using the Finer Mesh.
In planar 2D, the Electric Potential interface assumes that the model has a symmetry where the electric current varies only in the x and y directions and is constant in the z direction. This implies that the electric field, E, is tangential to the xy-plane. In 2D axi-symmetry, the Electric Potential interface considers the situation where the fields and geometry are axially symmetric.
In this case, the electric current is constant in the φ direction, which implies that the electric field is tangential to the rz-plane.
The Potential Conservation was also added to the continuity equation for the electrical current density and provides an interface for defining the electric conductivity as well as the constitutive relation and the relative permittivity for the displacement current.

Chemicals and Reagents.
Copper foam (CF) (thickness: 2 mm, bulk density: 0.58 g/cm 3  All chemicals and reagents were used as received without any further purification.

Pre-treatment of Cu foam
A piece of CF was cut into an area of 1 × 2 cm 2 , and then the CF was washed with acetone, HCl (3 M), ethanol and deionized water, respectively, for 10 min with the assistance of ultrasonication for several times to clean the CF's surface for further use before dried in a vacuum oven.

Synthesis of Cu(OH) 2 /CF and CuO/CF.
The Cu(OH) 2 nanoarrays were prepared by a typical controlled in-situ oxidative etching method at 25 o C. [4] Briefly, A certain amount of NaOH (80 mmol) was added into 30 mL deionized water under stirring to make a transparent solution, in which APS (3 mmol) was added. Then the cleaned CF was immersed into the as-prepared solution at 25 o C (S 2 O 8 2− + Cu → 2SO 4 2− + Cu 2+ ).
After a given reaction time, the sample was taken out of the solution, washed with deionized water three times and alcohol twice, and dried in air. For the preparation of CuO nanoarrays, the as-prepared Cu(OH) 2 nanoarrays were dried and calcined at 180 °C for 3h. Different processing times have been tested to get the best experimental process parameters (Figure S19-S22 and  -S26 and Table S7, Supporting Information).
Additionally, the sample with a larger size (14 × 25 cm 2 ) was prepared using the identical method under laboratory conditions.

Synthesis of CoOOH/CF
CoOOH/CF was prepared using a similar procedure to that for the synthesis of