Pt nanoclusters on GaN nanowires for solar-asssisted seawater hydrogen evolution

Abstract

Seawater electrolysis provides a viable method to produce clean hydrogen fuel. To date, however, the realization of high performance photocathodes for seawater hydrogen evolution reaction has remained challenging. Here, we introduce n⁺-p Si photocathodes with dramatically improved activity and stability for hydrogen evolution reaction in seawater, modified by Pt nanoclusters anchored on GaN nanowires. We find that Pt-Ga sites at the Pt/GaN interface promote the dissociation of water molecules and spilling H* over to neighboring Pt atoms for efficient H₂ production. Pt/GaN/Si photocathodes achieve a current density of -10 mA/cm² at 0.15 and 0.39 V vs. RHE and high applied bias photon-to-current efficiency of 1.7% and 7.9% in seawater (pH = 8.2) and phosphate-buffered seawater (pH = 7.4), respectively. We further demonstrate a record-high photocurrent density of ~169 mA/cm² under concentrated solar light (9 suns). Moreover, Pt/GaN/Si can continuously produce H₂ even under dark conditions by simply switching the electrical contact. This work provides valuable guidelines to design an efficient, stable, and energy-saving electrode for H₂ generation by seawater splitting.

Fig. 1. Pt nanoclusters on GaN nanowires grown on n⁺-p Si wafer.

a Schematic illustration of the fabrication of Pt/GaN/Si photocathode by epitaxial growth of GaN nanowires (NWs) on Si p-n wafer and photodeposition of Pt nanoclusters (NCs). b 45°-tilted-view SEM image of Pt/GaN/Si. c HAADF-STEM image of Pt/GaN NWs. Bright contrast regions due to Pt NCs are marked with red circles. STEM-EDS elemental maps of d Ga, e N, and f Pt. g Ga 2p_3/2, h N 1 s, and i Pt 4f XPS spectra for GaN/Si and Pt/GaN/Si electrodes.

Fig. 2. Photoelectrochemical seawater hydrogen evolution.

a LSV curves of Si, Pt/Si, GaN/Si, and Pt/GaN/Si measured with 3-electrode configuration in 0.5 M NaCl solution under AM 1.5 G 1 sun illumination or in dark. b LSV curves of Pt/GaN/Si in six different aqueous solutions. Acidic solutions (pH = 0): 0.5 M H₂SO₄ and 0.5 M NaCl + 0.5 M H₂SO₄; Neutral solutions (pH = 7.4): 1 M PBS and 0.5 M NaCl + 1 M PBS; Weak alkaline solution: 0.5 M NaCl (pH = 9.1) and seawater (pH = 8.2). c ABPE of Pt/GaN/Si in seawater, 0.5 M NaCl, and 0.5 M NaCl + 1 M PBS. d LSV curves of Si, Pt/Si, GaN/Si, and Pt/GaN/Si measured with 2-electrode configuration in 0.5 M NaCl. Inset shows the onset potential of each electrode. e Amount of H₂ produced and faradaic efficiency of Pt/GaN/Si in 0.5 M NaCl at −3 V vs IrO_x. The faradaic efficiency is nearly 100%. f Stability of Pt/GaN/Si in 0.5 M NaCl and seawater at −3 V. The photocurrent density retains >85% of initial value after 15 h reaction. Inset shows the LSV curves before and after the stability test.

Fig. 3. Theoretical study of water dissociation.

The Pt/GaN interface promotes water splitting. Optimized structures and calculated energy changes of a water dissociation on Pt(111), b proton transfer from surface N–H to a Pt cluster, c water dissociation at a Pt-Ga site at the Pt/GaN interface and subsequent H spillover to Pt surface. All energy changes are in the unit of eV. The white, red, blue, green, and gray spheres represent H, O, N, Ga, and Pt atoms, respectively.

Fig. 4. Concentrated solar light PEC water splitting.

a Schematic illustration and b photographs of the liquid flow cell for PEC water splitting. Light illuminates on the backside of n⁺-p Si wafer and HER takes place on the front side of Pt/GaN nanowires (NWs). LSV curves of Pt/GaN/Si measured with c 3-electrode and d 2-electrode configurations in 0.5 M NaCl under different light intensities. e Chronoamperometric curve and f amount of H₂ produced and faradaic efficiency measured under light intensities of 1, 3, 6, and 9 suns at −3 V.

Fig. 5. PEC/EC switchable electrode.

a Schematic of switchable dual contact electrode. The front contact is placed on the front side of GaN nanowires (NWs) and the back contact is placed on the back side of n⁺-p Si wafer. GaIn eutectic was sandwiched between the Cu contacts and the substrate for Ohmic connection. Illustrations of working principles of b front contact for electrochemical HER under dark and c back contact for photoelectrochemical HER under light. d LSV curves of electrode measured with 2-electrode configuration in 0.5 M NaCl solution. e Onset potential and current density at −2 and −3 V for front and back contacts. Onset potential was defined as the potential at −10 mA/cm². f Amount of H₂ produced and faradaic efficiency measured at −2 and 3 V under dark (front contact) and light (back contact) conditions.