CO2 reduction on pure Cu produces only H2 after subsurface O is depleted: Theory and experiment.

We elucidate the role of subsurface oxygen on the production of C2 products from CO2 reduction over Cu electrocatalysts using the newly developed grand canonical potential kinetics density functional theory method, which predicts that the rate of C2 production on pure Cu with no O is ∼500 times slower than H2 evolution. In contrast, starting with Cu2O, the rate of C2 production is >5,000 times faster than pure Cu(111) and comparable to H2 production. To validate these predictions experimentally, we combined time-dependent product detection with multiple characterization techniques to show that ethylene production decreases substantially with time and that a sufficiently prolonged reaction time (up to 20 h) leads only to H2 evolution with ethylene production ∼1,000 times slower, in agreement with theory. This result shows that maintaining substantial subsurface oxygen is essential for long-term C2 production with Cu catalysts.

Development of a photoelectrochemically self-improving Si/GaN photocathode for efficient and durable H2 production.

Development of an efficient yet durable photoelectrode is of paramount importance for deployment of solar-fuel production. Here, we report the photoelectrochemically self-improving behaviour of a silicon/gallium nitride photocathode active for hydrogen production with a Faradaic efficiency approaching ~100%. By using a correlative approach based on different spectroscopic and microscopic techniques, as well as density functional theory calculations, we provide a mechanistic understanding of the chemical transformation that is the origin of the self-improving behaviour. A thin layer of gallium oxynitride forms on the side walls of the gallium nitride grains, via a partial oxygen substitution at nitrogen sites, and displays a higher density of catalytic sites for the hydrogen-evolving reaction. This work demonstrates that the chemical transformation of gallium nitride into gallium oxynitride leads to sustained operation and enhanced catalytic activity, thus showing promise for oxynitride layers as protective catalytic coatings for hydrogen evolution.

Efficient Hole Extraction from a Hole-Storage-Layer-Stabilized Tantalum Nitride Photoanode for Solar Water Splitting.

One of the major hurdles that impedes the practical application of photoelectrochemical (PEC) water splitting is the lack of stable photoanodes with low onset potentials. Here, we report that the Ni(OH)x/MoO3 bilayer, acting as a hole-storage layer (HSL), efficiently harvests and stores holes from Ta3N5, resulting in at least 24 h of sustained water oxidation at the otherwise unstable Ta3N5 electrode and inducing a large cathodic shift of ≈600 mV in the onset potential of the Ta3N5 electrode.

Interface engineering of a CoO(x)/Ta3N5 photocatalyst for unprecedented water oxidation performance under visible-light-irradiation.

Cocatalysts have been extensively used to promote water oxidation efficiency in solar-to-chemical energy conversion, but the influence of interface compatibility between semiconductor and cocatalyst has been rarely addressed. Here we demonstrate a feasible strategy of interface wettability modification to enhance water oxidation efficiency of the state-of-the-art CoO(x)/Ta3N5 system. When the hydrophobic feature of a Ta3N5 semiconductor was modulated to a hydrophilic one by in situ or ex situ surface coating with a magnesia nanolayer (2-5 nm), the interfacial contact between the hydrophilic CoO(x) cocatalyst and the modified hydrophilic Ta3N5 semiconductor was greatly improved. Consequently, the visible-light-driven photocatalytic oxygen evolution rate of the resulting CoO(x)/MgO(in)-Ta3N5 photocatalyst is ca. 23 times that of the pristine Ta3N5 sample, with a new record (11.3%) of apparent quantum efficiency (AQE) under 500-600 nm illumination.

Solar-to-hydrogen efficiency exceeding 2.5% achieved for overall water splitting with an all earth-abundant dual-photoelectrode.

The solar-to-hydrogen (STH) efficiency of a traditional mono-photoelectrode photoelectrochemical water splitting system has long been limited as large external bias is required. Herein, overall water splitting with STH efficiency exceeding 2.5% was achieved using a self-biased photoelectrochemical-photovoltaic coupled system consisting of an all earth-abundant photoanode and a Si-solar-cell-based photocathode connected in series under parallel illumination. We found that parallel irradiation mode shows higher efficiency than tandem illumination especially for photoanodes with a wide light absorption range, probably as the driving force for water splitting reaction is larger and the photovoltage loss is smaller in the former. This work essentially takes advantage of a tandem solar cell which can enhance the solar-to-electricity efficiency from another point of view.

A tantalum nitride photoanode modified with a hole-storage layer for highly stable solar water splitting.

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm(-2) , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole-storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.

Visible light driven overall water splitting using cocatalyst/BiVO4 photoanode with minimized bias.

BiVO4 and many other semiconductor materials are ideal visible light responsive semiconductors, but are insufficient for overall water splitting. Upon loading water oxidation cocatalyst, for example Co-borate (denoted as CoBi) used here, onto BiVO4 photoanode, it is found that not only the onset potential is negatively shifted but also the photocurrent and the stability are significantly improved. And more importantly, PEC overall water splitting to H2 and O2 is realized using CoBi/BiVO4 as photoanode with a rather low applied bias (less than 0.3 V vs. counter electrode) in a two-electrode scheme, while at least 0.6 V is needed for bare BiVO4. This work demonstrates the practical possibility of achieving overall water splitting using the PEC strategy under a bias as low as the theoretical minimum, which is the difference between the flat band and proton reduction potential for a photoanode thermodynamically insufficient for water reduction. As long as the water oxidation overpotential is overcome with an efficient cocatalyst, the applied bias of the whole system is only used for that thermodynamically required for the proton reduction.

Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy.

Progress in the development of plasmon-enabled light-harvesting technologies requires a better understanding of their fundamental operating principles and current limitations. Here, we employ picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in a plasmonic model system composed of 20 nm sized gold nanoparticles (NPs) attached to a nanoporous film of TiO2. The measurement provides direct, quantitative access to transient local charge distributions from the perspectives of the electron donor (AuNP) and the electron acceptor (TiO2). On average, approximately two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%. Back electron transfer from the perspective of the electron donor is dominated by a fast recombination channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns. The findings provide a detailed picture of photoinduced charge carrier generation in this NP-semiconductor junction, with important implications for understanding achievable overall photon-to-charge conversion efficiencies.

Composition-Dependent Functionality of Copper Vanadate Photoanodes.

To understand functional roles of constituent elements in ternary metal oxide photoanodes, essential photoelectrochemical (PEC) properties are systematically analyzed on a series of copper vanadate compounds with different Cu:V elemental ratios. Homogeneous and highly continuous thin films of ß-Cu2V2O7, γ-Cu3V2O8, Cu11V6O26, and Cu5V2O10 are grown via reactive co-sputtering and their performance characteristics for the light-driven oxygen evolution reaction are evaluated. All four compounds have similar bandgaps in the range of 1.83-2.03 eV, though Cu-rich phases exhibit stronger optical absorption and higher charge separation efficiencies. Transient photocurrent analysis reveals a reduction of surface catalytic activity with increasing Cu:V elemental ratio due to competitive charge recombination at Cu-related surface states. This comprehensive analysis of PEC functionalities-including photon absorption, carrier separation, and heterogeneous charge transfer-informs strategies for improving PEC activity in the copper vanadate materials system and provides insights that may aid discovery, design, and engineering of new photoelectrode materials.

Insight into the charge transfer in particulate Ta3N5 photoanode with high photoelectrochemical performance.

Charge separation is one of the most critical factors for generating solar fuels via photoelectrochemical water splitting, but it is still not well understood. This work reveals the fundamental role of charge transfer in photoanodes for achieving high charge separation efficiency. Specifically, we fabricated a particulate Ta3N5 photoanode by a bottom-up method. By improving the charge separation with refined necking treatment, the photocurrent is increased by two orders of magnitude. The charge separation efficiency (ηsep) is analyzed by dividing it into charge generation efficiency (Φgene) and transportation efficiency (Φtrans). Necking treatment is found to substantially improve the electron transfer. Transient photovoltage (TPV) measurements based on the Dember effect is used to confirm the benefit of necking treatment in improving the charge transportation. The superior electron transfer in the necked-Ta3N5 electrode is further evidenced by the facile electron exchange reaction with the ferri/ferrocyanide redox couple. Moreover, cobalt phosphate is found to promote both charge separation and surface reaction, resulting in a photocurrent of 6.1 mA cm-2 at 1.23 V vs. RHE, which is the highest response for a particulate photoanode.

Discovery of Fe-Ce Oxide/BiVO4 Photoanodes through Combinatorial Exploration of Ni-Fe-Co-Ce Oxide Coatings.

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Photoelectrochemical water splitting promoted with a disordered surface layer created by electrochemical reduction.

The recent discovery of colored TiO2 indicated that the disordered surface layer over the TiO2 particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO2 layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO2 nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO2 layer. Photoelectrodes of TiO2 NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm(2) at 1.23 V, which is 2.2 times of that of the pristine TiO2 NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting.

Abnormal effects of cations (Li+, Na+, and K+) on photoelectrochemical and electrocatalytic water splitting.

The electrode-electrolyte interface chemistry is highly important for photoelectrochemical (PEC) and electrocatalytic water splitting where cations in the electrolyte are often crucial. However, the roles of cations in an electrolyte are much debated and not well-understood. This work reports that the PEC and electrocatalytic water oxidation (WO) activities in basic electrolytes with different cations follow an unexpected trend (Li(+) > K(+) > Na(+)) especially for long-term reaction. Such an abnormal order of activity is found to be the balance effect of two factors: the distinct extents of the weakening of O-H bond on electrode surface after interacting with cations in different electrolytes and the different rates of oxygen reduction reaction (ORR) which turns out to be dominant. Li(+) not only brings the most significant decrease of O-H bond strength but also is most effective for avoiding back reaction, while Na(+) shows the most detrimental effect on WO because of ORR. Our results provide important insight into the roles of cations in WO and demonstrate a new strategy of tailoring the electrode-electrolyte interface via judicious choice of cations in electrolyte for more efficient PEC and electrocatalytic water splitting.

A wide visible-light-responsive tunneled MgTa2O(6-x)N(x) photocatalyst for water oxidation and reduction.

We demonstrate for the first time that a nitrogen-doped tunneled oxide MgTa2O(6-x)N(x) with an absorption edge of ca. 570 nm can drive both water oxidation and reduction half reactions in the presence of scavengers under visible light irradiation, showing great potential in solar water splitting.