Water Oxidation Reaction Kinetics Research Articles

Tuning electrochemical water activation over NiIr single-atom alloy aerogels for stable electrochemiluminescence

The anodic oxygen evolution reaction is a well-acknowledged side reaction in traditional aqueous electrochemiluminescence (ECL) systems due to the generation and surface aggregation of oxygen at the electrode, which detrimentally impacts the stability and efficiency of ECL emission. However, the effect of reactive oxygen species generated during water oxidation on ECL luminophores has been largely overlooked. Taking the typical luminol emitter as an example, herein, we employed NiIr single-atom alloy aerogels possessing efficient water oxidation activity as a prototype co-reaction accelerator to elucidate the relationship between ECL behavior and water oxidation reaction kinetics for the first time. By regulating the concentration of hydroxide ions in the electrolyte, the electrochemical oxidation processes of both luminol and water are finely tuned. When the concentration of hydroxide ions in electrolyte is low, the kinetics of water oxidation is attenuated, which limits the generation of oxygen, effectively mitigates the influence of oxygen accumulation on the ECL strength, and offers a novel perspective for harnessing side reactions in ECL systems. Finally, a sensitive and stable sensor for antioxidant detection was constructed and applied to the practical sample detection.

Simultaneously Regulating charge separation and proton supply-demand in polyphenol amine for hydrogen peroxide photosynthesis

In the H2O2 photosynthesis, apart from the charge separation, the balance of proton supply and demand was also required between the kinetically faster oxygen reduction reaction (ORR) (μs ∼ ms) and the sluggish kinetics of water oxidation reaction (WOR) (s). Faced by this challenge, the article designed the heterojunction photocatalyst formed by in-situ polymerization of polyphenol amine on the surface of CdS under alkaline conditions. As a result, the synthesized CdS-Poly(catechol-hexamethylenediamine) (PCHA) produced 6.1 mM H2O2 after 8 h of visible light irradiation, which is 29 times that of CdS. The experimental results evidenced that the coating of PCHA increased the charge separation and mobility, inhibited the photocorrosion of CdS, and thus enhanced the stability and photoactivity. Moreover, the catechol units of PCHA could function as a proton supplier and regulate local protons concentrations around the catalytic active sites to balance the proton supply–demand in the ORR and WOR at different reaction time scales.

Fe–Ni–F electrocatalyst for enhancing reaction kinetics of water oxidation

Highly active and low-cost oxygen evolution reaction (OER) catalytic electrodes are extremely essential for exploration of green hydrogen via water splitting. Herein, an advanced Fe–Ni–F electrocatalyst is fabricated by a facile annealing strategy using ammonium fluoride, of which the structure feature is unveiled by XRD, FESEM, TEM, EDS, BET, and XPS measurements. The as-prepared Fe–Ni–F addresses a low overpotential of 277 mV and a small Tafel slope of 49 mV dec−1 at a current density of 10 mA cm−2, significantly outperforming other control samples as well as the state-of-the-art RuO2. The advanced nature of our Fe–Ni–F catalyst could also be further evidenced from the robust stability in KOH alkaline solution, showing as 5.41% degradation after 24 h continuous working. Upon analysis, it suggests that the decent catalytic activity should be attributed to the formed bimetallic (oxy)hydroxides because of the introduction of fluoride and the synergistic effect of iron and nickel towards oxygen generation. This work represents the potential of Fe- and/or Ni-based fluoride as efficient catalyst for low-energy consumption oxygen generation.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting.

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

Accurate design of spatially separated double active site in Bi4NbO8Cl single crystal to promote Z - Scheme photocatalytic overall water splitting

The efficiency of photocatalytic overall water splitting was mainly limited by the slow reaction kinetics of water oxidation. How to design effective surface active site to overcome the slow water oxidation reaction was a major challenge. Here, we propose a strategy to accelerate surface water oxidation through the fabrication spatially separated double active sites. FeCoPi/Bi4NbO8Cl-OVs photocatalyst with spatially separated double active site was prepared by hydrogen reduction photoanode deposition method. Due to the high matching of the spatial loading positions of FeCoPi and OVs with the photogenerated charge distribution of Bi4NbO8Cl and corresponding reaction mechanisms of substrate, the FeCoPi and OVs on the (001) and (010) crystal planes of Bi4NbO8Cl photocatalyst provided surface active site for water oxidation reaction and electron shuttle reaction (Fe3+/Fe2+), respectively. Under visible light irradiation, the evolution O2 rate of FeCoPi/Bi4NbO8Cl OVs was 16.8 μmol h−1, as 32.9 times as Bi4NbO8Cl. Furthermore, a hydrogen evolution co-catalyst PtRu@Cr2O3 was prepared by sequential photodeposition method. Due to the introduction of Ru, the Schottky barrier between PbTiO3 and Pt was effectively reduced, which promoted the transfer of photogenerated electrons to PtRu@Cr2O3 thermodynamically, the evolution H2 rate on PtRu@Cr2O3/PbTiO3 increased to 664.8 times. On based of the synchronous enhancement of the water oxidation performance on FeCoPi/Bi4NbO8Cl-OVs and water reduction performance on PtRu@Cr2O3/PbTiO3, a novel Z - Scheme photocatalytic overall water splitting system (FeCoPi/Bi4NbO8Cl-OVs) mediated by Fe3+/Fe2+ had successfully constructed. Under visible light irradiation, the evolution rates of H2 and O2 were 2.5 and 1.3 μmol h−1, respectively. This work can provide some reference for the design of active site and the controllable synthesis of OVs spatial position. On the other hand, the hydrogen evolution co catalyst (PtRu@Cr2O3) and the co catalyst FeCoPi for oxygen evolution contributed to the construction of an overall water splitting system.

Determining the Transformation Kinetics of Water Oxidation Intermediates on Hematite Photoanode.

The oxygen evolution reaction (OER) from water is a sequential oxidation reaction process, involved in transformation of multiple reaction intermediates. For photo(electro)catalytic OER, revealing the intermediates transformation kinetics is quite challenging due to its coupling with photogenerated charge dynamics. Herein, we specifically study the transformation kinetics of the OER intermediates in rationally thin hematite photoanodes through increasing the ratio between surface intermediates and photogenerated charges in bulk. We directly identify the formation and consumption kinetics of one-hole OER intermediate (FeIV═O) in photoelectrochemical water oxidation using operando transient absorption (TA) spectroscopy. The microsecond formation kinetics of the FeIV═O species are sensitively changed by the excitation mode of Fe2O3. The subsecond consumption kinetics are closely dependent on surface FeIV═O species density, demonstrating that the cooperation of FeIV═O intermediates is the key to accelerating water oxidation kinetics on the Fe2O3 surface. This work provides insight into understanding and controlling water oxidation reaction kinetics on Fe2O3 surface.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO4 (BV) with In2O3 (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm−2 at 1.23 VRHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

Enhancement of charge separation and hole utilization in a Ni2P2O7-Nd-BiVO4 photoanode for efficient photoelectrochemical water oxidation

It is necessary for photoelectrochemical (PEC) water splitting to reduce the electron-hole recombination rate and enhance the water oxidation reaction kinetics. Here, we prepared Ni2P2O7-Nd-BiVO4 composite photoanodes by coupling Ni2P2O7 co-catalysts to neodymium (Nd)-doped BiVO4 surfaces through photo-assisted electrodeposition. The Ni2P2O7-Nd-BiVO4 photoanode exhibits a high photocurrent density of 3.6 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE), which is three times higher than that of the bare BiVO4 (1.2 mA cm−2). Detailed characterizations demonstrate that Nd doping reduces the band gap, significantly increases the carrier density and effectively reduces the charge transfer resistance. More importantly, the Ni2P2O7 co-catalyst has multiple roles. Specifically, it can act as a hole extraction layer to accelerate hole migration and inhibit hole-electron recombination. At the same time, it significantly improves the water oxidation reaction kinetics. In addition, it also provides more water oxidation active sites. This work provides ideas for the design and study of efficient BiVO4-based photoanodes.

NiFePB-modified ZnO/BiVO4 photoanode for PEC water oxidation.

Photoelectrochemical (PEC) water splitting has been recognized as the most promising approach for directly converting solar energy into chemical energy, and substantial efforts have been made to develop a highly efficient and low-cost photoanode for enhancement of PEC water splitting efficiency due to sluggish water oxidation reaction kinetics. A ternary NiFePB-modified ZnO/BiVO4 heterojunction photoanode was simply assembled by low-temperature hydrothermal, metal-organic decomposition and electrodeposition methods to improve the water splitting efficiency; its photocurrent density for water oxidation reached 1.66 mA cm-2 at 1.23 V (vs. RHE); in comparison, that of ZnO is only 0.4 mA cm-2. The onset potential manifests a cathodic shift of ∼283 mV compared to ZnO. The IPCE and the ABPE respectively are 3.1 and 6.4 times those of ZnO, respectively. This improvement is ascribed to the efficient separation of photogenerated electrons and holes by the formation of a heterojunction between ZnO and BiVO4 and the enhancement in the oxygen evolution reaction kinetics by the decoration of the co-catalyst NiFePB as a hole acceptor.

α-MnO2/FeCo-LDH on Nickel Foam as an Efficient Electrocatalyst for Water Oxidation.

The ever-expanding human societies on the one hand and the diminishing fossil fuel resources on the other have driven man to find a suitable, cheap, clean, and accessible source of energy. Water splitting is a good solution to this crisis. Because of the slow kinetics of water oxidation reaction, it is important to select efficient and durable electrocatalysts to improve the reaction kinetics. In this research, α-MnO2/FeCo-LDH catalysts on nickel foam were developed for water oxidation, which exhibited good catalytic performance and stability in a 0.1 M KOH solution. The electrocatalysts were synthesized by hydrothermal methods and characterized by XRD, FTIR, Raman, SEM, TEM, EDS, and MAP techniques. The proposed modified electrode has large exchange current, low overpotential, and small Tafel slope. Here, only an overpotential of 210 mV is required to achieve a current density of 5 mA cm2 with a Tafel slope of 70.4 mV dec-1 in an alkaline solution.

Surface defects engineering of BiFeO3 films for improved photoelectrochemical water oxidation

As a promising material for photoelectrochemical (PEC) water oxidation, the fast recombination rate and sluggish surface reaction for BiFeO3 (BFO) still hinder its practical application. In this work, nitrogen plasma was first used to engineer surface defects of the pulsed laser deposition (PLD)-fabricated BFO films to suppress the surface charge recombination and improve the surface water oxidation activity, as well as enhance the stability. A remarkable photocatalytic activity enhancement was observed after the surface modification. Specifically, the photocurrent density is 95 μA/cm2 at 0.6 V (vs. Ag/AgCl, pH=12.8) for BFO–N (after nitrogen plasma treatment), which is about 8 times higher than as-grown BFO (12 μA/cm2) at the same potential. Meanwhile, the onset potential of BFO–N shifts toward the negative value by 120 mV compared to BFO. Also, the BFO–N reveals superior stability to BFO, which shows nearly no decay after the 1-h test. Complete experimental characterization and deep mechanistic analysis indicate that the enhanced performance could be due to the newly-formed Fe-enriched oxynitride layer on the surface after nitrogen plasma treatment, which suppresses the fast recombination, improves the water oxidation reaction kinetics, and protects the photoanode against photocorrosion. Our work offers a simple, but effective way to improve the PEC performance of BFO, which is instructive for fabricating similar high-performance photoelectrodes for PEC applications.

Tunable Carrier Transfer of Polymeric Carbon Nitride with Charge-Conducting CoV2O6∙2H2O for Photocatalytic O2 Evolution.

Photocatalytic water splitting is one of the promising approaches to solving environmental problems and energy crises. However, the sluggish 4e− transfer kinetics in water oxidation half-reaction restricts the 2e− reduction efficiency in photocatalytic water splitting. Herein, cobalt vanadate-decorated polymeric carbon nitride (named CoVO/PCN) was constructed to mediate the carrier kinetic process in a photocatalytic water oxidation reaction (WOR). The photocatalysts were well-characterized by various physicochemical techniques such as XRD, FT-IR, TEM, and XPS. Under UV and visible light irradiation, the O2 evolution rate of optimized 3 wt% CoVO/PCN reached 467 and 200 μmol h−1 g−1, which were about 6.5 and 5.9 times higher than that of PCN, respectively. Electrochemical tests and PL results reveal that the recombination of photogenerated carriers on PCN is effectively suppressed and the kinetics of WOR is significantly enhanced after CoVO introduction. This work highlights key features of the tuning carrier kinetics of PCN using charge-conducting materials, which should be the basis for the further development of photocatalytic O2 reactions.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts.

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(μ-O)]2+ (IrMolecular, pyalc = 2-(2′pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)–Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with potential-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

Improved Photoelectrochemical Performance of Chemically Grown Pristine Hematite Thin Films

The alpha phase of hematite (α-Fe2O3) is one of the most promising catalysts for photoelectrochemical (PEC) water splitting among several photoanode materials due to its suitable band gap and stability in aqueous solutions. The surface structure and morphology of films play pivotal roles in the enhancement of water oxidation reaction kinetics. In this work, α-Fe2O3 films were produced via either spray pyrolysis (SP), chemical vapor deposition (CVD), or aerosol-assisted chemical vapor deposition (AACVD). Their structural and morphological properties were subsequently characterized by powder x-ray diffraction (PXD), scanning electron microscopy (SEM), and Raman spectroscopy. High-quality thin films were best achieved by AACVD annealed at 525 °C, possessing an average thickness of 0.75 µm with 85% transmittance and an optical absorption onset at 650 nm. The results showed that the thermal oxidation process achieved at 525 °C eliminated undesired impurity phases, such as FeO and Fe3O4 , and enabled the microstructure to be optimized to facilitate the generation and transport of photogenerated charge carriers. The optimized α-Fe2O3 film showed a stable PEC water oxidation current density of ~1.23 mA cm-2 at 1.23 V (vs. RHE), with an onset potential of 0.76 V, under AM 1.5 irradiation. The obtained higher current density of pristine α-Fe2O3 thin films obtained by the AACVD method is unique, and the films presented good photocurrent stability with 92% retention after 6 h. Data from electrochemical impedance spectroscopy (EIS) corroborated these results, identifying fast charge transfer kinetics with decreased resistance and an electron lifetime of 175 µs. Quantitative measurements showed that 1.2 μmol cm-2 of oxygen could be produced at the photoanode in 6 h.Graphical Abstract

Enhancing the Photoelectrochemical Water Oxidation Activity of α‐Fe2O3 Thin Film Photoanode by Employing rGO as Electron Transfer Mediator and NiFe‐LDH as Cocatalyst

AbstractThe shortcomings of poor charge transfer ability and slow surface water oxidation kinetics in the single component photoanode limit the performances of photocatalytic water splitting. Herein, a α‐Fe2O3/rGO/NiFe‐LDH composite photoanode was designed for enhancing photoelectrochemical (PEC) water oxidation activity. The rGO nanosheets serve as an efficient electron transfer mediator for promoting charge transport and separation. The NiFe‐LDH is well anchored on rGO nanosheets and provides additional active sites for accelerating water oxidation reaction kinetics. By the virtue of synergistic effect between rGO and NiFe‐LDH, the composite photoanode achieves a pronounced photocurrent density of 1.46 mA/cm2 at 1.23 VRHE (V vs. reversible hydrogen electrode), which is 2.86 times of α‐Fe2O3. Moreover, the onset potential exhibits a significant cathodic shift of 230 mV. This work provides a valuable strategy for the design of photoanode with light absorber, rGO electron transfer mediator and other cocatalysts to achieve efficient and stable PEC water splitting.

Photocatalytic H2O2 generation assisted photoelectrochemical water oxidation for enhanced BiVO4 photoanode performance

A method is devised to integrate a photocatalytic and photoelectrochemical (PEC) systems to achieve enhanced water oxidation performance of a BiVO4 photoanode. The reduced gC3N4 (R-gC3N4) photocatalyst suspended in the anode compartment electrolyte of the PEC cell can serve as an agent to integrate the systems. The performance of BiVO4 photoanode in PEC water splitting is hampered by the surface states back electron-hole recombination. This is overcomed by incorporating R-gC3N4 in the electrolyte to produce hydroxyl radicals and hydrogen peroxide reactive intermediates, thereby reducing the onset potential for oxygen evolution reaction in the BiVO4 photoanode. The enhanced charge-transfer for water oxidation reaction kinetics at the photoanode/electrolyte interface is observed by suspending R-gC3N4 photocatalyst in the electrolyte. Approximately 3-fold increase in photocurrent density (105 μA/cm2 at 1.23 V vs. RHE) and a significant reduction in the onset potential (395 mV vs. RHE) of the BiVO4 photoanode is observed in R-gC3N4 + 2-propanol + 0.1 M PBS electrolyte. The application of R-gC3N4 photocatalyst to promote the reactivity of neutral pH electrolytes introduces a new paradigm for hydrogen generation.

Fabrication of BiVO4-Ni/Co3O4 photoanode for enhanced photoelectrochemical water splitting

The low efficiency of charge separation and slow kinetics of water oxidation of BiVO4 limit its application in the PEC water splitting. Here, an effective strategy is explored with Ni-doped Co3O4 (Ni/Co3O4) decorated BiVO4 photoanode by a simple route. The photocurrent of the BiVO4-Ni/Co3O4 (2.23 mA/cm2 at 1.23 VRHE biases) has significantly increased up to 4.4 times of that of bare BiVO4, and the Vonset of BiVO4-Ni/Co3O4 shows a significant negative shift (180 mV) to 0.23 V. The IPCE value of BiVO4-Ni/Co3O4 (27%, 380 nm) is about 5 times that of the BiVO4 (5%). The ηinjection and ηseparation of BiVO4-Ni/Co3O4 at 1.23 VRHE reaches 75.82% and 37.08%. The results demonstrate that the Ni/Co3O4 could enhance the charge transfer and accelerate the kinetics of water oxidation reaction, leading to the excellent PEC water splitting performance of the photoelectrode.

BiVO4 photoanode decorated with cobalt-manganese layered double hydroxides for enhanced photoelectrochemical water oxidation

Bismuth vanadate (BiVO4) is being widely identified as a leading n-type semiconductor material for photoelectrochemical (PEC) water splitting. Nevertheless, achieving efficient PEC water oxidation process through BiVO4 photoanode still faces serious challenge such as severe electron-hole recombination. In this case, PEC activity of BiVO4 photoanode was enhanced by decoration of three-dimensional CoMn-layered double hydroxide (CoMn-LDH) nanoflakes on the BiVO4 surface via a facile electrodeposition process. It was suggested that CoMn-LDH played a synergistic effect on broadening internal light absorption, which accelerated injection of holes carrier to electrolyte and alleviated the electron-hole recombination, resulting in expediting faster PEC water oxidation reaction kinetics. Consequently, the photocurrent density of BiVO4/CoMn-LDH photoanode achieved 2.69 mA cm−2 at 1.23 VRHE, 2.45 times higher than the pristine BiVO4. What's more, 220 mV negative-shift took place on onset potential that was further decreased to 0.31 VRHE. The vastly enhanced PEC performance was also prioritized to those of Co and Mn single relatives. This work demonstrated that the synergistic BiVO4/CoMn-LDH as a capable candidate material, can be utilized for effective PEC water splitting.

Surface Modification of Hematite Photoanodes with CeOx Cocatalyst for Improved Photoelectrochemical Water Oxidation Kinetics.

Hematite is a promising photoanode for solar water splitting by photoelectrochemical (PEC) cells, but its performance is limited by the slow kinetics of water oxidation reaction or oxygen evolution reaction (OER). Surface modification of hematite photoanodes with a suitable water oxidation cocatalyst is a key strategy for improving the kinetics of water oxidation. In this study, a CeOx overlayer is deposited on the surface of the hematite photoanode by a water-based solution method with ceric ammonium nitrate (CAN) followed by heat treatment. The photocurrent of CeOx -modified hematite is 3 times higher than that of pristine hematite (at 1.23 V vs. RHE) under AM 1.5G, 1 sun conditions. Through hole-scavenger measurements, Tafel plot analysis, and electrochemical impedance spectroscopy, it is concluded that CeOx overlayer increases the hole injection efficiency, improves the surface catalytic activity, and enhances charge transfer across the photoanode/electrolyte interface. These observations are attributed to the synergistic effects of Ce3+ /Ce4+ redox species in CeOx and the oxygen vacancies. This work elucidates the role of CeOx as an efficient cocatalyst overlayer to improve the OER kinetics of photoanodes.

Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes.

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.