Cu2O Photocathode Research Articles

Effect of morphology on the photoelectrochemical performance of nanostructured Cu2O photocathodes

Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph ), onset potential (E onset), band edges, carrier density (NA ), and interfacial charge transfer resistance (R ct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and R ct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (E fb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. E onset of 0.48 ± 0.04 eV obtained for these systems is close to the E fb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.

(Invited) Photoelectrochemical Reduction of CO2 at Poly(4-vinylpyridine)-Stabilized Copper(I) Oxide Semiconductor Decorated with Palladium Cocatalyst

Poly(4-vinylpyridine)-modified copper(I) oxide photocathodes, bare or decorated with palladium nanoparticles, can be used for the visible-light-driven selective reduction of CO2, mostly to carbon monoxide (in the absence of Pd co-catalyst) and methanol (in the presence of Pd co-catalyst). The photocathode materials, which are composed of hierarchically deposited (onto transparent fluorine-doped conducting glass electrode) Cu2O (inner layer) and poly(4-vinylpyridine), P4VP, polyelectrolyte film (outer-layer), with or without dispersed Pd-nanoparticles, have been fabricated and, subsequently, examined using electrochemical methodology, atomic force microscopy and various spectroscopic techniques, including Raman approach. While the physicochemical characteristics of Cu2O, including the oxide identity and its semiconducting properties, are not largely affected by the interfacial modification with P4VP, the photostability of the hybrid photocathode is significantly improved. The P4VP-modified Cu2O photocathodes exhibit reasonable durability during photoelectrochemical reduction of CO2 upon illumination with sunlight in semi-neutral medium (Na2SO4). On the whole, the photoelectrochemical reduction of CO2 proceeds at potentials starting from ca 0.55 V (vs RHE), which are substantially (almost 800 mV) more positive, in comparison to those characteristic of the conventional electrochemical reduction in the dark. Introduction of palladium co-catalyst seems to improve distribution (collection and transport) of photoelectrons at the photoelectrochemical interface and affects the CO2-electroreduction mechanism.

Electrodeposition of cuprous oxide on a porous copper framework for an improved photoelectrochemical performance

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

Optimal n-Type Al-Doped ZnO Overlayers for Charge Transport Enhancement in p-Type Cu2O Photocathodes.

An effective strategy for improving the charge transport efficiency of p-type Cu2O photocathodes is the use of counter n-type semiconductors with a proper band alignment, preferably using Al-doped ZnO (AZO). Atomic layer deposition (ALD)-prepared AZO films show an increase in the built-in potential at the Cu2O/AZO interface as well as an excellent conformal coating with a thin thickness on irregular Cu2O. Considering the thin thickness of the AZO overlayers, it is expected that the composition of the Al and the layer stacking sequence in the ALD process will significantly influence the charge transport behavior and the photoelectrochemical (PEC) performance. We designed various stacking orders of AZO overlayers where the stacking layers consisted of Al2O3 (or Al) and ZnO using the atomically controlled ALD process. Al doping in ZnO results in a wide bandgap and does not degrade the absorption efficiency of Cu2O. The best PEC performance was obtained for the sample with an AZO overlayer containing conductive Al layers in the bottom and top regions. The Cu2O/AZO/TiO2/Pt photoelectrode with this overlayer exhibits an open circuit potential of 0.63 V and maintains a high cathodic photocurrent value of approximately −3.2 mA cm−2 at 0 VRHE for over 100 min.

Complete solar-driven dual-photoelectrode fuel cell for water purification and power generation in the presence of peroxymonosulfate

This study reports the development of complete solar-driven dual-photoelectrode fuel cell (PFC) based on WO3 photoanode and Cu2O photocathode with peroxymonosulfate (PMS) serving as cathodic electron acceptor. As indicated by photoelectrochemical measurements, the PMS was able to improve thermodynamic properties of photocathode, achieving an increased open circuit potential from 0.42 V to 0.65 V vs standard hydrogen electrode (SHE). Under simulated sunlight irradiation (~100 mW cm−2), the maximum power density of 0.12 mW cm−2 could be obtained at current density of 0.34 mA cm−2, which was 8.57 times of that produced by PFC without PMS (0.014 mW cm−2). Correspondingly, adding PMS (1.0 mM) increased overall removal efficiency of 4-chlorophenol (4-CP) from 39.8% to 96.8%, accounting for the first-order kinetic constant (k=0.056 min−1) being 6.67 times of that in the absence of PMS (k=0.0084 min−1). Radical quenching and electron spin-resonance (ESR) results suggested the contribution of free radicals (•OH and SO4•-) and non-radical pathway associated with direct activation of PMS by Cu2O photocathode. Fourier transformed infrared (FTIR) analysis confirmed the strong non-radical interaction between Cu2O photocathode and PMS, resulting in 4-CP removal via activation of PMS by surface complex on Cu2O. The proof-in-concept complete solar-driven dual-photoelectrode fuel cell may offer an effective manner to realize water purification and power generation, making wastewater treatment more economical and more sustainable.

Stable Unbiased Photo-Electrochemical Overall Water Splitting Exceeding 3% Efficiency via Covalent Triazine Framework/Metal Oxide Hybrid Photoelectrodes.

Photo-electrochemical (PEC) water splitting systems using oxide-based photoelectrodes are highly attractive for solar-to-chemical energy conversion. However, despite decades-long efforts, it is still challenging to develop efficient and stable photoelectrodes for practical applications. Here, thin layers of covalent triazine frameworks (CTF-BTh) containing a bithiophene moiety are conformably deposited onto the surfaces of a Cu2 O photocathode and a Mo-doped BiVO4 photoanode via electropolymerization to construct new hybrid photoelectrodes, successfully addressing the efficiency and stability issues. The CTF-BTh possesses a suitable band structure to form favorable band edge alignment with each metal oxide, creating a p-n junction and a staggered type-II heterojunction with Cu2 O and Mo-doped BiVO4 , respectively. Thus, the as-fabricated hybrid photoelectrodes exhibit substantially increased PEC performances. Meanwhile, the CTF-BTh film also serves as an effective corrosion-resistant overlayer for both photoelectrodes to inhibit photocorrosion and enable long-term operation for 150 h with only ≈10% loss in photocurrent densities. Furthermore, a stand-alone unbiased PEC tandem device comprising CTF-BTh-coated photoelectrodes exhibits 3.70% solar-to-hydrogen conversion efficiency. Even after continuous operation for 120 h, the efficiency can still retain at 3.24%.

Cu vacancies enhanced photoelectrochemical activity of metal-organic gel-derived CuO for the detection of l-cysteine

Defect engineering in the photoelectrochemical (PEC) process of photoelectrodes has been extensively studied. But insufficient attention has been received about the impact of metal vacancies (VM) in PEC process. Herein, the influence of Cu vacancies (VCu) on PEC performance of copper oxide (CuO) derived from Cu-based metal-organic gel (Cu-MOG) precursor was reported. It can be found that the presence of more VCu can improve the PEC activity of CuO photocathode by facilitating the charge separation and transfer. Moreover, the as-prepared CuO was presented as a new PEC sensor to detect l-cysteine (L-Cys) on the basis of the excellent PEC performance, which showed high sensitivity and selectivity. Good linear response of L-Cys within the range of 0.1–6 μM was performed with a detection limit of 0.04 μM. This work not only provides insights into the role of VM in the PEC process of photocathodes, but also proved the high potential applicability of CuO as a PEC device for biomolecule detection.

Operando Analysis of Semiconductor Junctions in Multi‐Layered Photocathodes for Solar Water Splitting by Impedance Spectroscopy

AbstractAlthough electrochemical impedance spectroscopy (EIS) is a powerful technique for investigating optoelectronic devices, realistic equivalent circuit (EC) models suitable for multi‐layered water splitting electrodes have rarely been reported due to their complex nature. In the present study, the utility of the EIS method for investigating multi‐layered photocathodes for photoelectrochemical water splitting is demonstrated. By analyzing the EIS data of TiO2‐coated Sb2Se3 photocathodes, one is able to obtain information about the constituent semiconductors and interfaces such as recombination processes, carrier lifetimes, doping densities, and flat band potentials under operando conditions. The charge transfer time to the electrolyte is also extracted from the EIS data and confirmed by transient photocurrent decay measurements. In addition, the method is successfully applied to other photocathodes with different classes of light absorber, such as metal oxides (Cu2O) and crystalline Si, to compare the device characteristics under real operational conditions. It is shown that the lifetime of photo‐generated carriers in the Si photocathode is much higher than those of the Sb2Se3 and Cu2O photocathodes. It is believed that the EIS analysis method presented in this study will become a powerful routine characterization technique for discovering the limiting factors in a wide range of photo‐electrosynthetic as well as photovoltaic devices.

Nanostructured CuO with a thin g-C3N4 layer as a highly efficient photocathode for solar water splitting.

A g-C3N4/CuO nanostructure featuring improved photoelectrochemical properties was successfully prepared using a facile and cost-effective method involving electrodeposition and thermal oxidation. The improved photoelectrochemical properties were mainly ascribed to the increased surface area and improved charge transportation of the g-C3N4/CuO photocathode. This photocathode can be used in novel strategies for resolving problems associated with low-efficiency CuO photocathodes.

Solar/photoelectrocatalytic cell development for H2 production and simultaneous organic dye degradation

The situation of alternative energy and environmental management is an urgent problem that must be solved. This research aims to improve photoelectrocatalytic (PEC) cells for hydrogen (H2) production and simultaneous organic degradation as an alternative way to solve this problems. We developed Cu2O film fabrication using a cyclic voltammetry deposition (CVD) method for optimal H2 production efficiency. We designed the PEC cell using a Cu2O photocathode developed in conjunction with a WO3/BiVO4 photoanode for simultaneous H2 production and organic dye degradation. The PEC cell was then designed to separate H2 gas from the system, including the solar cells as bias potential units to save energy in the operation process. The Cu2O photocathode shows high efficiency of H2 production related to its absorption properties and morphology, as well as a charge transfer rate improvement. The optimum conditions for FTO/Cu2O electrode fabrication was the precursor solution's original pH value and a scan rate of 50 mV/s for 20 cycles with hydrogen generation of 5.9 mL/h. The developed solar/PEC cell presents high efficiency for methylene blue (MB) degradation up to 97% for 3 h under an applied potential of 0.4 V and visible light irradiation. The highlight of this developed cell is a high-efficiency Cu2O cathode for H2 production, and the designed PEC cell can simultaneously eliminate organic waste. Notably, the design of the solar cell panel as an applied potential unit can reduce the energy consumption of the system. This developed solar/PEC cell suitable for the alternative energy and waste water treatment applications.

Electrodeposition of Cuprous Oxide on a Free-Standing Porous Cu Framework for Photoelectrochemical Water Splitting

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

A Photoelectrochemical Fuel Cell Based on a CuO Photocathode for Sustainable Resources Utilization

AbstractNew techniques for sustainable energy resources utilization are highly desirable to meet the continuously increasing energy demand and address growing environmental concerns. Herein, we report the fabrication of a CuO photocathode through a simple and scalable two‐step method and, by combining it with a glucose oxidase (GOD) bioanode, a photoelectrochemical fuel cell (PFC) has also been further developed. In the presence of 60 mM glucose and under visible‐light illumination, the PFC gives maximum power output densities of 14.4 μW cm−2 and 8.9 μW cm−2 at 0.1 V in air‐ and nitrogen‐saturated solutions together with open‐circuit potentials of 0.34 V and 0.38 V, respectively, demonstrating the capability of sourcing energy from visible light and glucose. The PFC shows promise as an efficient, user‐friendly option for sustainable energy resources utilization, and the CuO photocathode can be used for the development of other PFCs in the future.

In-situ decoration of unsaturated Cu sites on Cu2O photocathode for boosting nitrogen reduction reaction

Photoelectrochemical N2 reduction reaction (PEC NRR) into chemical energy source is a promising technique for solving environmental challenges and the energy crisis, which depends on the exploration of p-type photocathodes. Cu2O is a p-type semiconductor, which can be applied as a photocathode for PEC NRR. However, the adsorption and activation ability of N2 over the pristine Cu2O is still not ideal, causing the overall efficiency far below the theoretical value. In this work, we first proposed a feasible strategy to in-situ modify the surface of Cu2O with Cu-MOF, and the Cu-MOF/Cu2O heterostructure exhibited the significantly boosted NH3 production rate (7.16 mmol/m2/h). The ultra-thin Cu-MOF film takes a nature of porous structure and unsaturated Cu sites, contributing to a favorable interface environment for the adsorption and activation of N2. The N2 temperature programmed desorption and contrast photocurrent curves revealed the desirable selectivity for PEC NRR, and the isotope NMR further confirmed the accuracy of NH3 evolution process. Therefore, the organic–inorganic hybrid systems built from Cu-MOF and Cu2O would offer a new insight for establishing the efficient PEC NRR system.

An efficient optofluidic photocatalytic fuel cell with dual-photoelectrode for electricity generation from wastewater treatment

A novel optofluidic photocatalytic fuel cell consisted of a Cu2O photocathode and a BiVO4 photoanode for efficient electricity generation from wastewater treatment. The dual-photoelectrodes design can greatly improve the separation efficiency of the photoexcited charge. Notably, the photoelectrodes were shoulder-to-shoulder placed in a microchamber, not only providing easy fabrication but also offering a large contact area between electrolyte and photoelectrodes. Additionally, the reduced inherent ohmic loss of as-established optofluidic PFC increased the maximum theoretic power density by ~60%. The photoelectricity conversion in using glucose as model organic waste has been entirely recorded under 1 sun illumination. The open-circuit voltage and short-circuit photocurrent were 0.463 ​V and 0.113 ​mA cm-2, respectively, which were ~2.5 times higher than that of the traditional large-dimension PFC. The PFC proposed in this study opened up a new insight into developing an efficient PFC system for pollutant treatment and electricity generation.

A highly efficient photocatalytic methanol fuel cell based on non-noble metal photoelectrodes: Study on its energy band engineering via experimental and density functional theory method

Photocatalytic fuel cell (PFC) is considered as a new energy source due to its cooperative conversion capacity of both solar energy and chemical energy. In this work, a highly efficient ZnO/BiVO4 photoanode is synthesized on fluorine-doped tin oxide (FTO) by continuous electrodeposition and hydrothermal method. Then, a visible-light driven dual photoelectrode photocatalytic methanol fuel cell (PMFC) consisted of the ZnO/BiVO4 photoanode and Cu2O photocathode is designed and constructed. Electrochemical results show that the short-circuit current density (Jsc), the open-circuit voltage (Voc) and the maximum power output (Pmax) of the prepared PMFC can reach 5.18 mA/cm2, 0.82 V and 0.91 mW/cm2 under simulated solar irradiation, respectively. Surface photovoltage (SPV) transients reveal the charge transporting dynamics in semiconductor heterojunction, and photoelectrochemical experiments further confirmed the light-induced methanol oxidation and oxygen reduction process. Under illumination, an interior potential difference between n-type ZnO/BiVO4 and p-type Cu2O actuate the electrons on the photoanode to transfer through the external circuit to photocathode, combining with the holes and generating electricity. The built-in electric field from the ZnO (002) surface to the BiVO4 (001) surface is confirmed based on the theoretical calculation of density functional theory (DFT).

A low overpotential photoelectrochemical reduction of carbon dioxide to methanol with highly photoactive hierarchical structured cuprous oxide

In this study, two different highly scalable hierarchical structured photocathodes of Cu2O nanowire (NW) and microyarn (MY), were prepared and evaluated for their photoelectrochemical (PEC) reduction of CO2 (PECRC) to methanol activity. Notably, Cu2O NW showed a considerably higher photoactivity than Cu2O MY, and the highest ever reported in terms of photoconversion efficiency in comparison with similar material. The enhancement is ascribed to the increased in light entrapment, carrier density, and reduced charge transfer resistance as a result of highly dense and uniform NW arrays. Additionally, the PECRC to methanol for both Cu2O photocathodes in this study was operated at considerably low positive bias potential (+0.2 V vs. Ag/AgCl). The yield for methanol achieved in this study is also comparable to that of surface-modified Cu2O photocathode prepared by using a more costly technique. The physicochemical characterization of the photocathodes after the PECRC reaction revealed a periodical appearance of CuCO3.Cu(OH)2, Cu, and CuO on the Cu2O photocathode after the PECRC reaction, which is hypothesized to work synergistically during the PECRC to methanol. Without surface modification, bare Cu2O was adequate to catalyze PECRC to methanol. Overall, the integration of highly scalable and photoactive Cu2O NW photocathode and considerably low overpotential for PECRC will be a promising approach to realize the PECRC to methanol in the future.

Electrochemical Synthesis of Uniform Cu2O Film and Its Photoelectrochemical Properties

Cu2O films as photocathodes were synthesized using direct current electrodeposition method. Utilizing a cyclic voltammetry (CV) test before potentiostatic electrodeposition increased the uniformity and transparency of the Cu2O film. The optical band gap of the Cu2O film obtained through electrodeposition for 7200 s was 2.1 eV, while the one electrodeposited for 1000 s showed a blue shift (2.4 eV). The high transparency of the Cu2O film electrodeposited after the CV test and increased thickness of the electrodeposited film for 7200 s did not affect the value of the photocurrent density. The photocurrent density of the electrodeposited films for time periods more than 200 s did not change considerably. The thickness of the Cu2O film electrodeposited for 200 s was measured to be 70 nm. Electrodeposition for above 200 s resulted in Cu2O films with thicknesses greater than the electron collection length, i.e. 20–100 nm, which was the reason why the photocurrent density was not improved. Electrochemical impedance spectroscopy (EIS) was used to explain charge transfer characteristics of the Cu2O photocathode. The Nyquist plot showed two semicircles, which can be attributed to the charge transfer process across the electrode/electrolyte interface and inside the electrode. The EIS data was fitted with an equivalent circuit and the parameters value was derived. Using the Mott–Schottky plot, the flat band potential and the carrier density were obtained to be 0.19 V vs. Ag/AgCl and 1.3 × 1018 cm–3, respectively.

Extending lifetime of photoinduced charge carriers in CuO photocathode by Zn doping for photoelectrochemical water reduction

CuO is a potential photocathode material with p-type conductivity and suitable bandgap of 1.8 ​eV. However, its energy conversion efficiency is low, which is suffered from the slow intrinsic charge transport properties. Herein, a Zn doping strategy has been proposed used to solve the above problems. For the Zn doped CuO photocathode, an increased photocurrent of −17.70 ​μA/cm2 is observed with the optimized doping amount of 0.3%. The capacitance measurements show that Zn dopant may serves as charge trapped states, which promotes more electrons accumulated on the surface of photocathode. Most importantly, the lifetime of the photoinduced charge carriers is extended, which provides more possibilities for the carriers to participate the following catalytic reaction. Density functional theory (DFT) calculations further reveal that Zn doping induced change of electronic structure, including density of state (DOS), as well as more thermos-neutral free energy of H∗ adsorption, which implies the high catalytic activity toward water reduction.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction.

SummaryPhotoelectrochemical CO2 reduction into syngas (a mixture of CO and H2) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H2-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H2 ratios (0–10) under one-sun illumination. Moreover, unassisted solar CO2 reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

A study on tandem photoanode and photocathode for photocatalytic formaldehyde fuel cell

We report a fuel cell device with double-side semiconductor photoelectrode, in which TiO2 nanorods (NRs) array serves as photoanode for formaldehyde oxidation, and Cu2O photocathode takes part in oxygen reduction reaction (ORR). Unlike traditional noble metal-based fuel cells, the whole fuel cell reaction is operated from photoelectrocatalytic process due to photogenerated holes and electrons. The best performance is obtained by photoexciting both TiO2 photoanode and Cu2O photocathode through contrast experiments, the short circuit current and open circuit voltage of the fuel cell reach 1.2 mA/cm2 and 0.58 V, respectively. Mott-schottky measurement, surface photovoltage (SPV) transients and linear sweep voltammetry (LSV) test under illumination further reveal the thermodynamic and dynamic feasibility for charge transfer in the formaldehyde oxidation-oxygen reduction process. This study introduces an alternative design in developing non-noble metal photocatalytic fuel cells with low cost and high practicality.