Cu2O Photocathode for Low Bias Photoelectrochemical Water Splitting Enabled by NiFe-Layered Double Hydroxide Co-Catalyst

Layered double hydroxides (LDHs) are bimetallic hydroxides that currently attract considerable attention as co-catalysts in photoelectrochemical (PEC) systems in view of water splitting under solar light. A wide spectrum of LDHs can be easily prepared on demand by tuning their chemical composition and structural morphology. We describe here the electrochemical growth of NiFe-LDH overlayers on Cu2O electrodes and study their PEC behavior. By using the modified Cu2O/NiFe-LDH electrodes we observe a remarkable seven-fold increase of the photocurrent intensity under an applied voltage as low as −0.2 V vs Ag/AgCl. The origin of such a pronounced effect is the improved electron transfer towards the electrolyte brought by the NiFe-LDH overlayer due to an appropriate energy level alignment. Long-term photostability tests reveal that Cu2O/NiFe-LDH photocathodes show no photocurrent loss after 40 hours of operation under light at −0.2 V vs Ag/AgCl low bias condition. These improved performances make Cu2O/NiFe-LDH a suitable photocathode material for low voltage H2 production. Indeed, after 8 hours of H2 production under −0.2 V vs Ag/AgCl the PEC cell delivers a 78% faradaic efficiency. This unprecedented use of Cu2O/NiFe-LDH as an efficient photocathode opens new perspectives in view of low biasd or self-biased PEC water splitting under sunlight illumination.

The accelerated depletion of fossil fuel reserves combined with increasing demand of energy across the world has triggered a tremendous effort toward alternative energy sources and various technologies are currently being explored. Among those, photoelectrochemical (PEC) water splitting under solar light irradiation has shown their potential by the successful integration of abundant energy source and high efficient catalysts. In order to realize high efficiency PEC systems in view of water splitting, a number of photocatalysts have been investigated including the widespread TiO 2 [1][2][3] , ZnO 4-7 , WO 3 [8][9][10][11] , Fe 2 O 3 12 , as well as more innovative catalysts such as Cu 2 O [13][14][15] , and BiVO 4 16 . Cu 2 O is a promising photocathode material due to its suitable band gap (1.9-2.2 eV) and high absorption coefficient in the visible region [17][18][19][20] . Its conduction band is located above the reduction potential of water, making it suitable for mediating H 2 production with little or no external bias in a PEC cell. Among different methods, electrodeposition has proven to be the most convenient and reliable to prepare nanostructured Cu 2 O 17,21 . Recent studies show that the Cu 2 O morphology and orientation can be controlled by judiciously tuning the deposition parameters such as the electrolyte pH, temperature, applied potential and current density 17,21 . The morphology and crystal structure determine the material performances through their role in light absorption and charge transport.
Although Cu 2 O possesses an intrinsic potential as a photocathode, the reported photocurrents remain well below the theoretical values, in particular under low external biases. These poor performances arise essentially from structural defects at the semiconductor-electrolyte interface. It has been reported that at low external Scientific RepoRts | 6:30882 | DOI: 10.1038/srep30882 voltages, Cu 2 O does not transfer electrons efficiently towards the electrolyte due to an inappropriate band bending 22 . In order to improve the photocurrent of Cu 2 O electrodes, various approaches have been recently developed .  For example Cu 2 O can be combined with another catalyst such as TiO 2   23   , or by depositing a protective layer on its  surface such as Cu 2 S 13,14 , RuO 2   24 , or polyoxometallates 25 3+ 26,27 . The variety in chemical composition and structural morphology of LDH materials make them suitable for a wide range of applications as electrocatalysts [28][29][30][31][32] . In particular, LDHs can act as efficient photocatalysts for improving the charge separation of photogenerated electrons and holes 33 . Moreover, the hierarchical morphology of LDHs provides convenient charge transfer at the electrolyte interface in PEC systems. Several types of LDHs such as CoNi 33 , or ZnCo 34 , have been widely investigated to enhance PEC water splitting.
We describe here the fabrication of Cu 2 O/NiFe-LDH via a facile two-step electrodeposition method and the use of NiFe-LDH to enhance the performances of Cu 2 O photocathodes. The NiFe-LDH layers grow as nanoplatelets that are uniformly anchored onto the Cu 2 O surface. Our Cu 2 O/NiFe-LDH exhibits greatly improved photocurrent intensities particularly at low applied voltages. Moreover we demonstrate that modified Cu 2 O/ NiFe-LDH photocathodes allow more efficient H 2 production at low applied voltages. The Cu 2 O/NiFe-LDH photocathodes reveal to be highly stable with no degradation under low bias after 40 hours of illumination, making Cu 2 O/NiFe-LDH an excellent photoelectrode for low bias PEC water splitting.

Results and Discussion
Morphology of Cu 2 O/NiFe-LDH materials. Images recorded by field-emission scanning electron microscopy (FESEM) show that electrodeposited Cu 2 O consists of compact and highly homogeneous layers made of cubic particles having ~400 nm in size (Fig. 1a) and thicknesses of ~1 μ m after 1,500 s of deposition time (Fig. 1b). NiFe-LDH overlayers are then grown on the Cu 2 O samples using different deposition times (Fig. 1b,c). After 60 seconds, the NiFe-LDH material adopts a uniform nanoflake morphology ( Fig. 1d) with individual flakes of size ~5 nm ( Figure S1). After 300 s it self-assembles into sponge-like structures (Fig. 1f). AFM images show that after deposition of NiFe-LDH, the surface roughness is greatly increased (Table S1 and Figure S2) and may thus increase substantially the number of photons absorbed by the photocathode. Besides, the XRD pattern of NiFe-LDH after 150 s exhibits typical (003), (006), and (104) reflection peaks, while Cu 2 O shows the known dominant (111) reflection ( Figure S3 The Mott-Schottky plots comparison of Cu 2 O photocathode with and without NiFe-LDH (20 s) reveals the greatly increased number of charge carriers that has reached the interface is induced by NiFe-LDH cocatalyst ( Figure S7). This unveils the function of NiFe-LDH as an efficient cocatalyst that improves the charge separation of Cu 2 O photocathode. Furthermore, in order to eliminate the possibilities that this photocurrent enhancement is induced by the metal valence state changes in NiFe-LDH, we have investigated the cyclic voltammetry behaviour of NiFe-LDH ( Figure S8). Results indicate that at the low bias condition (less than − 0.2 V vs Ag/AgCl), there is no reduction/oxidation behaviour on NiFe-LDH. This excludes the likelihoods of metal valence state changes-induced photocurrent enhancement of Cu 2 O/NiFe-LDH photocathode in low bias condition. The photoresponse of pure NiFe-LDH (300 s) has also been explored ( Figure S9). Results show that NiFe-LDH itself generates very limited charge carrier upon illumination. This excludes the possibilities that the photocurrent enhancement of Cu 2 O/NiFe-LDH photocathode is induced by the increased number of photon-generated charge carriers induced by NiFe-LDH.
Another point worth notice is that the difference in photocurrent of Cu 2 O photocathode and Cu 2 O/NiFe-LDH photocathode becomes smaller with increased bias voltage. This is because the increased high applied voltage is able to efficiently separate the majority of photon-generated charge carriers. The effects of NiFe-LDH become less pronounced. As there is no difference for the total number of charge carriers as Cu 2 O is the only photon absorber ( Figure S9). Thus, the maximum photocurrent of both electrodes have very little difference. Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) provides information about the interfacial property of the synthesized photocathode, which further reveals the efficient charge separation effect and improved electron injection property brought by NiFe-LDH layer (Fig. 3). The semi-circular diameter of the measured EIS stands for the charge carrier transfer resistance (R ct ) that controls the electron transfer kinetics at the electrode/electrolyte interface 37 . The resistance of Cu 2 O/NiFe-LDH in the dark is much larger than that under illumination, indicating a higher number of charge carriers at the electrode interface. (Figure S10). In addition, as compared with bare Cu 2 O, the radius of the semicircle of Cu 2 O/NiFe-LDH is smaller under all the conditions (Fig. 3a,b and Figure S11).
An interesting observation is that by increasing the applied bias from − 0.02 V to − 0.12 V vs Ag/AgCl, the interface resistance increases for bare Cu 2 O but decreases for the Cu 2 O/NiFe-LDH samples. This phenomenon arises from the difference between the electrode/electrolyte interfaces with either Cu 2 O or NiFe-LDH. Under illumination over the voltage range − 0.02/− 0.12 V vs Ag/AgCl, the surface of bare Cu 2 O is predominantly charged with holes essentially because of the blockage of the electron transfer path from Cu 2 O to the electrolyte (see Fig. 4a). At more negative voltages, the number of holes decreases, thus resulting in a resistance increase of bare Cu 2 O. This is evidenced by the larger semi-circle diameter in the EIS (Fig. 3a). In contrast, deposition of a NiFe-LDH overlayer induces an appropriate energy level alignment with respect to the electrolyte redox levels so that electrons are efficiently transported towards the surface where they reduce water into H 2 ( Fig. 4b and Figure S12b). Electrons become predominant and the NiFe-LDH surface is negatively charged. The number of electrons increases with increasing negative voltage, as confirmed by the smaller semi-circle diameter in Fig. 3b. This demonstrates the key role of NiFe-LDH in introducing appropriate energy levels at the interface and the subsequent higher electron injection rate into the electrolyte. Overall, the inefficient surface charge separation efficiencies and insufficient electron transfer in the electrolyte of bare Cu 2 O induces an overwhelming number of photogenerated electrons in the bulk. On the contrary, after coating by NiFe-LDH, electrons are able to transfer from electrode to electrolyte under very low external bias (Fig. 4). This results in a reduced number of electrons in the Cu 2 O layer, driving electrons from the counter-electrode to the Cu 2 O photocathode. As a result, the sample with and without NiFe-LDH have different electron flow directions for a given bias ( Fig. 4 and Figure S12). This superior property makes Cu 2 O/NiFe-LDH an excellent candidate for photocathode in low or non-bias PEC systems.
Photostability. In order to evaluate the potential of Cu 2 O/NiFe-LDH photocathodes for H 2 production, we tested their long-term stability under illumination at low voltages. It is well-known that Cu 2 O suffers from poor stability under illumination. The photogenerated electrons reduce Cu 2 O into Cu and photogenerated holes oxidize Cu 2 O into CuO. When oxidized, the illuminated area rapidly turning black. Various attempts have been made to solve this problem [38][39][40][41] , but the stability of Cu 2 O under low bias is rarely mentioned because of too low photocurrents. However, in the presence of the NiFe-LDH co-catalyst, the photostability of Cu 2 O under low bias can be explored. Observations indicate that Cu 2 O instability is mostly caused by the highly negative applied voltage, under which electrons that accumulate at the photocathode reduce Cu 2 O into metallic copper ( Figure S13). The testing conditions in the literature are usually under ~0V vs RHE 17 , under which environment Cu 2 O tends to be reduced to a most stable state -metallic Cu. Whereas, our testing condition is − 0.2 V vs Ag/AgCl under pH 6.5, which fell exactly in its stable condition range. This is supported by the fact that Cu 2 O samples under − 0.2 V vs Ag/AgCl is also surprisingly stable after 40 hours of continuous illumination (Fig. 5). The stability tests of Cu 2 O with and without NiFe-LDH under − 0.6 V vs Ag/AgCl show that NiFe-LDH also lengthens the stability of Cu 2 O under relatively high voltage ( Figure S14). Whereas, the purpose of this study to improve Cu 2 O photocathode's performance under its stable condition (green shadowed area in Figure S13) instead of spend numerous efforts in improving its stability under extreme conditions such as under strong negative bias. H 2 production. H 2 evolution tests using Cu 2 O/NiFe-LDH (20 s) photocathodes have been conducted in a 25% methanol solution as a sacrificial reagent under visible light irradiation according to literature 42 . The faradaic efficiency is calculated according to ρ = n H2 /(Q/2F), where n H2 is the amount of hydrogen generated, Q is the total amount of charge passed through the cell (C), and F is the faraday constant. As shown in Fig. 6, under low bias − 0.2 V vs Ag/AgCl, the initial faradaic efficiency is 61%, and it increases with time up to 78% before decreasing slowly after 800 minutes of illumination. There is no significant decrease in the performance during the first 12 hours. Over the following 10 hours the efficiency slightly decreases to 59%. When a higher bias of − 0.8 V vs Ag/AgCl is applied, it shows good H 2 evolution in the first 40 minutes. However, after 60 minutes H 2 stops evolving and a progressive decrease of the faradaic efficiency takes place to become less than 5% after 1,000 minutes of illumination.    Electrodeposition of the NiFe-LDH co-catalyst. Electrodeposition of NiFe-LDH is realized using Morphological characterizations. The crystal phase of the synthesized photocathodes were studied using a Shimadzu thin film X-ray diffractometer with a Cu Ka excitation (λ = 1.54 Å). Microscopic morphologies including lattice analysis of scraped particles were obtained using field emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and high resolution transmission electron microscopy (HRTEM, JEOL-2100F) operating at 200 kV.
Photoelectrochemical measurements. All PEC measurements were conducted in a 0.5 M Na 2 SO 4 electrolyte using a three-electrode configuration with synthesized sample as the working electrodes, Pt and Ag/ AgCl electrodes being used as the counter and reference electrodes, respectively. The inter-electrode spacing was ~1 cm. Photocurrents were recorded using a PCI4/300 ™ potentiostat equipped with PHE200 ™ software (Gamry Instruments, Inc.). The working electrodes were exposed to the AM 1.5 light from a solar simulator equipped with a 300 W Xe-lamp (HAL-320, Asahi Spectra Co., Ltd.). The incident light intensity was 100 mW·cm −2 and the sample illumination area 0.28 cm 2 . Linear sweep voltammetry (LSV) was carried out under both dark and illumination conditions with a scan rate of 5 mV·s −1 with chopped light irradiation (frequency = 0.2 Hz). Stability tests were conducted by chronoamperometry under a potential of -0.2 V vs Ag/AgCl in a 0.5 M Na 2 SO 4 solution. Mott-Schottky measurements were conducted using the same equipment and configuration with Mott-Schottky mode in 0.5 M Na 2 SO 4 electrolyte with a frequency of 300 Hz in the potential range of chemical stability. Cyclic voltammetry (CV) was carried out using the same equipment and configuration in 0.5 M Na 2 SO 4 electrolyte at the potential window 0.3 V to − 0.4 V vs Ag/AgCl with scan rate of 100 mV S −1 .
Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) was conducted using the same equipment and configuration as for the PEC measurements, which is a PCI4/300 ™ potentiostat equipped with EIS300 ™ software (Gamry Instruments, Inc.). Potentiostatic mode was applied under white light illumination (AM1.5, 100 mW·cm −2 ) at an applied voltage of − 0.2 V vs. Ag/AgCl. AC perturbations of amplitude 5 mV were superimposed with frequency in the range 0.01-100 kHz. Equivalent circuit modeling and curve fitting were performed using the Echem Analyst ™ software (Gamry Instruments, Inc.) Hydrogen production. The amount of hydrogen generated was measured using a well-sealed glass cell (100 ml) mounted with a quartz window. The Cu 2 O and Cu 2 O/NiFe-LDH working electrodes (sample illumination area = 1 cm 2 ) were exposed to the light of a solar simulator equipped with a 300 W Xe-lamp (HAL-320, Asahi Spectra Co., Ltd.), and the incident AM 1.5 light intensity was 100 mW·cm −2 . The working electrode, Pt counter-electrode and Ag/AgCl reference electrode were suspended in a solution (pH = 7) containing 30 ml of 0.1 M Na 2 SO 4 and 10 ml of methanol (25%). Prior to testing, the reactor was repeatedly vacuum-pumped and purged with argon to remove the residual air. Then, an external bias is applied on the working electrode and the lamp is turned on. The amount of generated H 2 gas was quantitatively analyzed by a gas chromatograph (Shimadzu GC-2014; molecular sieve 5 Å, TCD detector, Ar carrier gas).