Photoelectrocatalytic Water Splitting Research Articles

Evaluation of local oxygen flux produced by photoelectrochemical hydroxide oxidation by scanning electrochemical microscopy

Several in-situ electrochemical approaches have been developed for performing a localized photoelectrochemical investigation of the photoanode. One of the techniques is scanning electrochemical microscopy (SECM), which probes local heterogeneous reaction kinetics and fluxes of generated species. In traditional SECM analysis of photocatalysts, evaluation of the influence of radiation on the rate of studied reaction requires an additional dark background experiment. Here, using SECM and an inverted optical microscope, we demonstrate the determination of O2 flux caused by light-driven photoelectrocatalytic water splitting. Photocatalytic signal and dark background are recorded in a single SECM image. We used an indium tin oxide electrode modified with hematite (α-Fe2O3) by electrodeposition as a model sample. The light-driven flux of oxygen is calculated by analysis of SECM image recorded in substrate generation/tip collection mode. In photoelectrochemistry, the qualitative and quantitative knowledge of oxygen evolution will open new doors for understanding the local effects of dopants and hole scavengers in a straightforward and conventional manner.

Linking the Photoinduced Surface Potential Difference to Interfacial Charge Transfer in Photoelectrocatalytic Water Oxidation.

Charge transfer at the semiconductor/solution interface is fundamental to photoelectrocatalytic water splitting. Although insights into charge transfer in the electrocatalytic process can be gained from the phenomenological Butler-Volmer theory, there is limited understanding of interfacial charge transfer in the photoelectrocatalytic process, which involves intricate effects of light, bias, and catalysis. Here, using operando surface potential measurements, we decouple the charge transfer and surface reaction processes and find that the surface reaction enhances the photovoltage via a reaction-related photoinduced charge transfer regime as demonstrated on a SrTiO3 photoanode. We show that the reaction-related charge transfer induces a change in the surface potential that is linearly correlated to the interfacial charge transfer rate of water oxidation. The linear behavior is independent of the applied bias and light intensity and reveals a general rule for interfacial transfer of photogenerated minority carriers. We anticipate the linear rule to be a phenomenological theory for describing interfacial charge transfer in photoelectrocatalysis.

Developments and Challenges of Catalytic Materials for Green Hydrogen Production

Water splitting coupled to renewable power systems is an attractive way to generate green hydrogen and achieve zero carbon emissions, and represents a strategic technology to meet the high demand of carbon-neutral development. Catalysts essentially determine the efficiency and cost of water splitting technologies, and are a class of key materials for green hydrogen production. In this review, we summarize the catalyst developments for the mainstream green hydrogen production technologies, including water electrolysis, water photolysis, and photoelectrocatalytic water splitting. We first present basic catalytic mechanisms of these water splitting pathways, as well as emphasize their current research status and challenges for practical application. We subsequently introduce the recent progress in representative catalysts and design strategies toward these photo(electro)catalytic technologies, paying particular attention to water electrolysis, including alkaline water electrolyzer (AWE), proton exchange membrane water electrolyzer (PEMWE), anion-exchange membrane water electrolyzer (AEMWE) and solid oxide electrolysis cell (SOEC). Finally, we propose future prospects to develop more desirable catalysts for green hydrogen production at a large scale.

Enhanced charge transport from Pd-doping in CsPbBr3 quantum dots for efficient photoelectrocatalytic water splitting

The Pd-CsPbBr3 QDs accelerate the separation and transport of photo-induced carriers and exhibit a superior photocurrent of about 2.07 mA cm−2 at 1.23 VRHE.

Plasmonic Copper-activated ZnO Microarrays for Efficient Photoelectrocatalytic Applications.

In the present work, green synthesized plasmonic copper nanostructures derived from carbon quantum dots (PCQDs) activated ZnO microarrays (MAs) based catalyst system is developed and studied for photocatalytic activity and photoelectrocatalytic water splitting. CQDs are synthesized from pharmaceutical waste and used as a reducing agent to synthesize PCQDs of an average size of 10±2 nm. PCQDs decorated ZnO (PCQDs/ZnO) MAs exhibited enhanced photocurrent density of ∼7.1 mA/cm2 at 1.23 V (vs. RHE), which is ∼11 fold to ZnO MAs alone (0.65 mA/cm2 ). The catalyst exhibits an ABPE of 1.07% at 0.7 V (vs. RHE), IPEC of 8.8% for 450 nm, and hydrogen production rate of 435 μmol/h. The enhanced PEC characteristics are assigned to the improved photons collection and better charge transfer for their participation in oxidation/reduction reaction. The same is well supported with DFT studies for the PCQDs/ZnO MAs catalyst for the first time.

Distribution Characteristics of Multiphysics around the Bubble on the Surface of Photoelectrode

In the heterogeneous photocatalytic reaction systems, the evolution of bubbles on the catalyst surface is a complex process that depends on multiple factors, such as the solid–liquid interface structure, solution properties of the reaction system, and multiphysics interactions, and directly affects energy and mass transport processes. This study investigated the multiphysics distribution around a bubble and the bubble detachment characteristics on the surface of a TiO2 photoelectrode during photoelectrocatalytic water splitting. The bubble diameter had an effect on the distribution of the light field in the system, affecting the temperature and flow distributions of the electrolyte around the bubble. Marangoni convection was induced by the inhomogeneous temperature distribution along the bubble interface, which enhanced the heat and mass transfer near the reaction site. It was also demonstrated that an increase in laser power increased Marangoni convection on the bubble surface, thus delaying bubble detachment. A force model for predicting the bubble detachment diameter that considered the coupling effect between the light, temperature, and flow fields was developed. The predicted bubble detachment diameters agreed well with the experimental results within an error of ±10%.

BiVO4/MoSe2 Photocatalyst for the photocatalytic Abatement of tetracycline and photoelectrocatalytic water splitting

Here, BiVO4/MoSe2 (BMS) heterojunction material of various mole ratios was prepared via hydrothermal method. BMS photocatalyst is solar light active, has better charge separation, good surface area, lower recombination rate, and narrow band gap of ∼1.5 eV as affirmed through different characterization techniques. The synthesized photocatalyst possesses fern nano-petal-like morphology. The photocatalytic and photoelectrocatalytic experiments were conducted under solar light for tetracycline (TC) degradation and hydrogen production via water splitting, respectively to test out the efficiency of the photocatalyst. Under the illumination of sunlight for 150 min, the 3:1 BiVO4:MoSe2 (31BMS) had the maximum degradation efficiency of 91.9 % (rate constant 0.0147 min−1). The GC-MS analysis was also conducted to examine the intermediates and products formed during the photocatalytic degradation of TC. Moreover, the photoelectrochemical hydrogen evolution experiment revealed a significantly improved photocurrent density of 0.433 mA/cm2 for 31BMS at 1.23 V vs RHE, under simulated solar light in comparison to BiVO4 (0.0045 mA/cm2) and MoSe2 (0.0322 mA/cm2). No change in current density was found for ∼330 min, thus confirming the stability of the electrode, with constant H2 evolution rate of 3.59 mmol/cm2. The reusability efficacy of the photocatalyst was tested for 7 cycles and the stability was confirmed by XRD, BET and FESEM after the reaction.

Solar light responsive 2D/2D BiVO4/SnS2 nanocomposite for photocatalytic elimination of recalcitrant antibiotics and photoelectrocatalytic water splitting with high performance

A hydrothermal approach (environmentally safe route) was followed to synthesize eco-friendly, non-toxic, sunlight responsive 2D/2D BiVO4/SnS2 (BVSNS) nanocomposites with varying mole ratios. BVSNS exhibits favorable band gap, high charge carrier mobility, good surface area, lower recombining capability rendering it highly efficient photocatalyst. The superiority of photocatalysts was then tested for the degradation of pharmaceutical pollutants, ofloxacin (oflox) and tetracycline (TC), and photoelectrocatalytic (PEC) water splitting under sunlight illumination. The 1:3 BiVO4:SnS2 (13BVSNS) composite had highest decomposition efficiency of 93.7 and 80.8 % with rate constants of 0.0164 and 0.010 min−1 for oflox and TC-degradation, respectively. The GC–MS analysis was also conducted to examine the intermediates and products formed during the photocatalytic degradation of TC. The PEC water splitting experiments revealed that 13BVSNS photoanode exhibited highest photocurrent density of 0.21 mA/cm2 at 1.23 V vs RHE. The Mott-Schottky experiments demonstrated flat band potential of -0.37 V at 1.23 V vs RHE for the 13BVSNS electrode. As a result, ∼21 µmolcm−2sec-1 of H2 was produced and electrode was stable for 6 h. Reusability studies (7 runs) were used to further examine the photocatalyst's efficacy, and post reaction characterization results justified that the materials are highly stable with a long life.

Recent Developments in Conductive Polymer‐Based Electro‐/Photoelectrocatalytic Materials for Effective Hydrogen/Oxygen Evolution Reactions: A Review

AbstractCurrently, a substantially greener energy source such as hydrogen fuel cell is an alternative to fossil fuel sources, and much research has been dedicated to achieving highly efficient hydrogen (H2 gas) generation. In this line, electrochemical water splitting that involves hydrogen and oxygen evolution reactions (HER and OER, respectively) is considered to be the greener and most effective hydrogen‐generating procedure with only water as a by‐product. However, in the process of successful OER/HER, it is extremely demanding for electrocatalysts with high activity, stability and cost‐effectiveness for the water splitting process. Herein, the review proposed promising electrocatalysts such as conductive polymers (CPs) with enhanced catalytic effect with the polymer encapsulation of catalytic material that provides an increase in charge transfer kinetics both in OER/HER. Also, the review highlights the OER and HER mechanism of conducting polymers and their effect on the synergism was discussed in detail. Also, the review detailed the reduced overpotentials, importance and enhancement of catalytic efficiency with various polymers reported in both OER and HER. Here, the choice of polymers can matrix with the inorganic active sites to prevent them from aggregation and the water splitting carried out over the electrode surface results in good conductivity to charge transfer from the electrode to the active sites. In addition, the review discussed the polymers induces the photon effect with photoelectrocatalytic water splitting. Thus, the combing effect with conducting polymers and the materials with catalytic active centers open up a new kind of electrocatalyst for synthesis.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting.

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Mass transfer mechanism during bubble evolution on the surface of photoelectrode

The limitation of the transfer of gas products within the photocatalytic system is the key to the efficiency of the water splitting, which is directly influenced by the bubble evolution process on the catalyst surface. In this study, the entire process of bubble evolution on the surface of a superhydrophilic TiO2 photoelectrode was simultaneously measured visually and electrochemically. The bubble growth and detachment characteristics were investigated under different laser powers and constant bias voltages. An expression for the bubble coverage characterized by the bubble radius and photocurrent, which is applicable to the photoelectrocatalytic water splitting, was proposed to accurately obtain the mass transfer coefficient during bubble evolution. The mass transfer mechanism of gas product transfer in the low current density range dominated by single-phase free convection was revealed. The mass transfer coefficient was found to increase in the inertia-controlled phase and decrease in the chemical reaction-controlled phase of bubble growth. Furthermore, a method to accelerate bubble detachment from the photoelectrode surface by periodic illumination was proposed, which could significantly enhance the gas product transfer process.

S-scheme N-doped carbon dots anchored g-C3N4/Fe2O3 shell/core composite for photoelectrocatalytic trimethoprim degradation and water splitting

Photoelectrocatalysis is a promising technique for energy conversion and environmental treatment. This study describes the photoelectrochemical (PEC) degradation of trimethoprim and hydrogen evolution using a photoanode prepared by N-doped carbon dots (NCD) incorporated g-C3N4/α-Fe2O3 (CNFO) shell/core nanocomposite. The electrochemical analysis reveals that the photocurrent density of NCD@CNFO photoanode reached 3.07 mA cm−2 at 1.6 V vs. NHE, which is 4 and 15 times greater than that of CNFO and intact α-Fe2O3, respectively. In the presence of peroxymonosulfate (PMS), the NCD@CNFO photoanode enabled 95 % and 90 % of trimethoprim (TMP) degradation in aqueous solution and lake water, respectively. Hydrogen generation coupled with TMP degradation was also observed in the PEC system, where the H2 generation rate was 550 µmol cm−2 h−1. Both superoxide (•O2−) and hydroxyl (•OH) radicals played a significant role in the degradation of TMP. The achievements could be assigned to the excellent photoabsorption and electron transfer properties of NCD, which enhanced the PEC activity of CNFO by enabling the S-scheme heterojunction to reduce electron-hole recombination. Moreover, PMS served as a cathodic electron acceptor to improve the catalytic properties of NCD@CNFO photoanode, demonstrating its contribution to both water treatment and hydrogen production. Such superior efficiency offers great potential to develop a PEC system using carbon dots/semiconductor hybrid catalysts for antibiotic degradation and synchronous photocatalytic H2 evolution from wastewater, providing an alternative solution to environmental pollution and energy crisis issues.

Plasmon-induced hole-depletion layer on p-n heterojunction for highly efficient photoelectrochemical water splitting

The photoelectrocatalytic (PEC) water splitting efficiency of semiconductor photoelectrodes is mainly limited by the effective separation and transfer of photogenerated charges. Zinc indium sulfide-cuprous oxide (ZnIn2S4-Cu2O) p-n heterojunction is constructed to enhance the PEC properties of ZnIn2S4. The nickel hydroxide iron oxide (NiFeOOH) layer on the surface of the heterojunction can be used as a hole depletion layer under the induction of plasmon resonance of the most surface silver (Ag) (the holes transferred from Cu2O valence band to NiFeOOH layer can be excited by Ag to produce hot electron consumption, which makes the last remaining hot holes participate in the water oxidation reaction) to further promote the carrier separation and transfer. The results exhibit that ZnIn2S4/Cu2O/NiFeOOH/Ag photoelectrode with dramatically enhanced photocurrent density of 1.22 mA/cm2 at 1.23 V versus the reversible hydrogen electrode (VRHE), which is 9.4 times higher than the pure ZnIn2S4. This work provides a promising concept to design photoelectrodes efficiently in PEC water splitting.

Multistage charge redistribution constructing heterostructured WO3@RuSe2 on Si for enhanced photoelectrochemical hydrogen evolution

Producing green hydrogen by photoelectrocatalytic water splitting is one of the most promising approaches to conversion of renewable energy to chemical energy, which is restricted by the unsuitable energy barrier between the conduction band of the semiconductor and the H+/H2 level as well as the unsuitable binding energy of active sites with the intermediates in hydrogen evolution reaction. Herein, we rationally construct an n+p-Si/Ti/WO3@RuSe2 heterostructured photocatalyst, which significantly improves the photovoltage and photocurrent by charge redistribution between p-Si and heterostructured WO3@RuSe2. The representative n+p-Si/Ti/WO3@RuSe2 exhibits a photovoltage of 0.54 VRHE with a high short-circuit photocurrent density of −36 mA cm−2 and solar-to-hydrogen conversion efficiency of 9.43%, which is much higher than that of n+p-Si/Ti (0.45 VRHE, −0.22 mA cm−2, 0.015%). The characterizations and density functional theory calculations reveal that the heterostructured interface moves the d-band center of WO3@RuSe2 (−0.699 eV) to the Fermi level through the multistaged charge redistribution, thus reducing the barrier between the conduction band of p-Si and H+/H2 level as well as improving the charge separation efficiency.

Ni Foam Supported TiO2 Nanorod Arrays with CdS Branches: Type II and Z‐Scheme Mechanisms Coexisted Monolithic Catalyst Film for Improved Photocatalytic H2 Production

TiO2 has garnered a flourish of interest in the field of energy storage and energy conversion. Herein, a new generation of 3D Ni foam supported in situ grown 1D TiO2 nanoarray (Ni/TiO2) is successfully prepared, through an easy solution‐phase hydrothermal process, and then CdS nanorods branched Ni/TiO2 nanoarrays are fabricated (Ni/TiO2@CdS). Their formation parameters and growth mechanism are carefully studied. Both Ni/TiO2 and Ni/TiO2@CdS monolithic catalysts exhibit advantaged photocatalytic (PC) and photoelectrocatalytic (PEC) water splitting properties compared with that of most previously reported TiO2‐based film photocatalysts. Mechanism investigation reveals that both type II and Z‐scheme charge transport mechanisms coexist in Ni/TiO2@CdS sample, and an additional Z‐scheme charge transfer effect accelerates the photogenerated charge separation and transport efficiency. This study may open a new avenue for the utilization of TiO2‐based film catalysts in a broader field of energy and environmental catalysis areas.

Conversion of Interfacial Chemical Bonds for Inducing Efficient Photoelectrocatalytic Water Splitting.

Sp-C-hybridized alkyne bonds present the natural advantages of interacting with metal atoms and have the ability to generate a large number of new catalytic active sites on the surface and the interfaces, thus greatly promoting the efficient progress of various light/electrochemical reactions. In this work, we have successfully fabricated a novel type of interfacial structure containing sp-C-Mo/O bonds and mixed Mo valence states with outstanding catalytic activity and stability for photoelectrocatalytic (PEC) overall water splitting in a wide pH range (0-14), due to the presence of sp-carbon-rich graphdiyne. For example, in alkaline conditions (pH = 14), the overpotentials of oxygen and hydrogen evolution reactions at 10 mA cm-2 are 165 and 8 mV. When being used as an electrolyzer, the cell voltage of this catalyst is only 1.40 V to achieve 10 mA cm-2. The high PEC activity of graphdiyne@molybdenum oxide originates from the conversion of chemical bonds at the sp-C hybrid interface and the coexistence of multivalent states of molybdenum, triggering a large number of catalytic active sites, greatly promoting charge transfer and lowering water dissociation energy.

Bi2S3 entrenched BiVO4/WO3 multidimensional triadic photoanode for enhanced photoelectrochemical hydrogen evolution applications

The catalytic reactivity and photoactivity of WO 3 and BiVO 4 oxide semiconductors have general obstacles as electrodes in emergent photo-electrochemical (PEC) hydrogen evolution applications. The present work comprises the integration of photocatalyst with wide visible photon absorption material which is vital for hydrogen evolution in photo-electrocatalytic water splitting. Herein, the 1D WO 3 NWs have been integrated with stable water oxidation photocatalysts of BiVO 4 and Bi 2 S 3 as a photoanode (Bi 2 S 3 /BiVO 4 /WO 3 ) for photoelectrochemical hydrogen evolution reactions. The morphological variations in the Bi 2 S 3 /BiVO 4 /WO 3 heterostructure manifest catalytic activity and rapid charge transfer characteristics owing to band alignment and a wide range of visible photon absorption. The optimized Bi 2 S 3 /BiVO 4 /WO 3 multidimensional photoanode accomplishes a superior photocurrent density of 1.52 mA/cm 2 , a seven-fold higher than pristine WO 3 photoanode counterpart (0.2 mA/cm 2 ) at 1 V vs. RHE. A prodigious lowest onset potential of −0.01 V vs. RHE) has been achieved which enables very high solar to hydrogen conversion. The photoelectrode with entangled morphology such as nanosheets, nanocrystals and nanorods expanded their surface to volume ratio having enhanced catalytic performance. The hybrid photoanodes have demonstrated the lowest charge transfer resistance of 360 Ohm/cm 2 with a 7-fold rise in hydrogen evolution performance. The resultant triadic Bi 2 S 3 /BiVO 4 /WO 3 heterostructure appeared to be an emerging stable photo-electro catalyst for hydrogen evolution applications. • A multidimensional Bi 2 S 3 /BiVO 4 /WO 3 is prepared by hydrothermal and SILAR method. • The hetero-structure upholds photo-excitation and enhances carriers' kinetics. • The open triadic structure improves the hydrogen evolution. • A type II band alignment in Bi 2 S 3 /BiVO 4 /WO 3 hetero-structure is proposed.

Operando Photo-Electrochemical Catalysts Synchrotron Studies.

The attempts to develop efficient methods of solar energy conversion into chemical fuel are ongoing amid climate changes associated with global warming. Photo-electrocatalytic (PEC) water splitting and CO2 reduction reactions show high potential to tackle this challenge. However, the development of economically feasible solutions of PEC solar energy conversion requires novel efficient and stable earth-abundant nanostructured materials. The latter are hardly available without detailed understanding of the local atomic and electronic structure dynamics and mechanisms of the processes occurring during chemical reactions on the catalyst–electrolyte interface. This review considers recent efforts to study photo-electrocatalytic reactions using in situ and operando synchrotron spectroscopies. Particular attention is paid to the operando reaction mechanisms, which were established using X-ray Absorption (XAS) and X-ray Photoelectron (XPS) Spectroscopies. Operando cells that are needed to perform such experiments on synchrotron are covered. Classical and modern theoretical approaches to extract structural information from X-ray Absorption Near-Edge Structure (XANES) spectra are discussed.

Can Li: A Career in Catalysis

Prof. Can Li is distinguished for his seminal achievements in fundamental and applied researches in catalysis, especially in advancing the characterization of catalysts and catalytic reactions related to solar energy conversion and fine chemical synthesis. The scope of his research is wide ranging, and he is leading some of the fields. This account highlights his major scientific achievements in advancing the fundamental sciences of characterizing catalysts and understanding the mechanisms of catalytic reactions by developing in situ spectroscopic techniques (e.g., infrared (IR), ultraviolet (UV), Raman, space- and time-resolved spectroscopies and photoelectric imaging), and applying the obtained knowledge in the rational design and synthesis of catalysts for practical applications in photocatalytic, electrocatalytic and photoelectrocatalytic water splitting and CO2 reduction, chiral synthesis in nanoreactors and ultradeep desulfurization of fuels, and fine chemical synthesis. In particular, his demonstration of the kiloton scale solar fuel production project will be introduced as a successful model for converting fundamental science into practical and impactful applications to convert CO2 and water into solar fuels. This account also highlights his services to the catalysis communities and international collaborations for promoting catalysis science worldwide.

In-situ Synthesis of the Thinnest In2 Se3 /In2 S3 /In2 Se3 Sandwich-Like Heterojunction for Photoelectrocatalytic Water Splitting.

The efficient utilization of solar energy for photoelectrocatalytic (PEC) water splitting is a feasible solution for developing clean energy and alleviating environmental issues. However, as the core of PEC technology, the existing photoanode catalysts have disadvantages such as poor photoelectrocatalytic conversion efficiency, low conductivity of photogenerated carriers, and instability. Here, we report the ultrathin two-dimensional sandwich-like (SW) heterojunction of In2 Se3 /In2 S3 /In2 Se3 (SW In2 S3 @In2 Se3 ) for the first time for PEC water splitting. Our findings identify the efficient separation of electrons and holes by constructing SW In2 S3 @In2 Se3 heterojunction. The in situ synthesis of ultrathin nanosheet arrays by using surface substitution of Se atom to epitaxially grow cell In2 Se3 maximizes the contact area of heterogeneous interface and accelerates the transmission of charge carrier. Benefitting from the unique structure and composition characteristic, SW In2 S3 @In2 Se3 displays excellent performance in PEC water splitting. The photocurrent density of SW In2 S3 @In2 Se3 reaches 8.43 mA cm-2 at 1.23 VRHE . Compared with In2 S3 , the SW In2 S3 @In2 Se3 photoanode has nearly 12 times higher PEC performance, which represents the best performance among the In2 S3 -based photoanode heterojunction reported so far. The evolution rate of O2 reaches 78.8 μmol cm-2 h-1 , and the photocurrent has no apparent variety within 24 h.