Photoelectrochemical (PEC) water splitting can be an efficient and economically feasible alternative for hydrogen production if easily processed photoelectrodes made of inexpensive and abundant materials are employed. Here, we present the preparation of porous Cu2O photocathodes with good PEC performance using solely inexpensive electrodeposition methods. Firstly, porous Cu structures with delicate pore networks were deposited on flat Cu substrates employing hydrogen-bubble-assisted Cu deposition. In a second electrodeposition step, the porous Cu structures were mechanically reinforced and subsequently detached from the substrates to obtain free-standing porous frameworks. In a third and final step, photoactive Cu2O films were electrodeposited. The PEC water splitting performance in 0.5 M Na2SO4 (pH ∼6) shows that these photocathodes have photocurrents of up to −2.25 mA cm−2 at 0 V versus RHE while maintaining a low dark current. In contrast, the Cu2O deposited on a flat Cu sample showed photocurrents only up to −1.25 mA cm−2. This performance increase results from the significantly higher reactive surface area while maintaining a thin and homogeneous Cu2O layer with small grain sizes and therefore higher hole concentrations as determined by Mott-Schottky analysis. The free-standing porous Cu2O samples show a direct optical transmittance of 23% (λ = 400–800 nm) and can therefore be used in tandem structures with a photoanode in full PEC cells. Graphical abstract

The use of hydrogen as a chemical fuel is an attractive and promising method as an alternative sustainable green energy carrier. The combination of direct sunlight and semiconductor photoelectrodes in a photo-electrochemical (PEC) cell can generate hydrogen by splitting water molecules into H2 and O2 1. Copper(I) oxide or Cuprous oxide (Cu2O) is a p-type semiconductor with a bandgap of ~2 eV which makes it suitable for solar visible light absorption and it possesses a suitable band position for water reduction to hydrogen in the PEC cell2. The focus here is to increase the surface area of Cu2O photocathode to improve the PEC water splitting performance. Using dynamic hydrogen-bubble assisted electrochemical deposition method, a highly porous Cu substrate was synthesized in an acid copper sulfate bath at current densities above -1 A/cm2. To obtain a free-standing porous copper framework the structure must be reinforced at a lower current density using the same acidic bath and was separated from the substrate via ultrasonication The Cu2O crystal was then deposited on the free-standing porous Cu framework from an alkaline copper bath. The photoelectrochemical analysis of Cu2O was performed using a solar simulator at 1.5 AM and it shows a significant improvement of the photocurrent in comparison to the planar sample. M. Grätzel, Nature, 414, 338–344 (2001). A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, and E. Thimsen, Nat. Mater., 10, 456–461 (2011). Figure 1