Cathodic Shift Of Onset Potential Research Articles

Cathodic shift in flatband potential of hematite based photoelectrodes: Evaluating and correlating the redshift to solar driven photocatalysis

As a promising metal oxide semi-conductor, hematite (α-Fe2O3) has interesting optical properties to be regarded as photoactive material. In this study, we evaluated the cathodic shift in onset potential (Eonset) and the negative shift in flat band potential (Efb). It was observed that the Eonset value of −0.457 V for α-Fe2O3 based electrode shifted to −0.638 V for the ternary nanocomposite (PNFC) [Pani/Ni-α-Fe2O3/CNT] based electrode. The flat band potential for α-Fe2O3 in 1 M NaOH is at −0.49 V which shifts to −0.68 V for the ternary nanocomposite (PNFC). This photoelectrochemical enhancement appears due to electron-hole pair separation. Therefore, it apparently increases the photocurrent trace for PNFC at −0.60 V to 7.04 mA cm-2, compared to the photocurrent of 4.52 mA cm-2 observed for pristine α-Fe2O3. According to the Mott-Schottky plot, the donor concentration (Nd) for the α-Fe2O3 is 4.83 × 109 cm-3, whereas for the ternary nanocomposite (PNFC) is 7.68 × 109 cm-3, respectively. As evident, the Nd value increase for the ternary nanocomposite PNFC points to the proximity of the Fermi (Ef) level below the CB edge when Ni doped α-Fe2O3 comes in contact with polyaniline (Pani). Indeed, the ternary nanocomposite PNFC with lower Eg value (1.32 eV) and higher photocurrent (7.04 mA cm-2) show higher photo-activity with percent degradation of 98.42 % for photocatalytic degradation of Sunset Yellow under visible light irradiation. The aforementioned correlations of the energetic levels of band edges with the shifting of Efb to more negative potentials provide insight into the high photo-activity observed.

Photo-driven electrochemical etching to construct porous structures with oxygen vacancies on ZnO photoanodes for efficient solar water splitting

Fast charge separation and oxidation kinetics are the key issues for photoelectrochemical (PEC) water splitting. Herein, photo-driven electrochemical etching (PEE) was applied to modify the morphology and surface properties of ZnO photoanodes to improve PEC performance. By regulating the applied potential and treatment time, the PEE generated a lotus root like porous structure with a pore diameter of 20–30 nm and depth of about 300 nm, and produced a large amount (30.1 %) of oxygen vacancies (Ov) on ZnO photoanode. PEE treated-ZnO array demonstrated a charge separation efficiency of 87 %, and achieved 1.8 times larger photocurrent (1.26 mAcm−2 at 1.23 VRHE) with a cathodic shift of onset potential of 0.12 VRHE than pristine ZnO. The introduced Ov and the shortened charge transport distance of porous microstructures by PEE treatment improved the electrode/electrolyte interface charge transfer and water oxidation kinetics. DFT calculations indicate that the construction of oxygen defects on the catalyst surface can reduce the overpotential of oxygen evolution reaction (OER) and further promote the catalytic activity of OER. Moreover, electrode surface pore structure can be tailored by adjusting illumination parameters and electrolytes. This work illustrates a novel and effective approach to tunable photo-corrodible electrode materials for solar water splitting.

Computational fluid dynamics simulation of bubble hydrodynamics in water splitting: Effect of electrolyte inflow velocity and electrode morphology on cell performance

Efficient management of oxygen gas bubbles evolving at the photoanode is important for a good overall performance of a photoelectrochemical (PEC) cell. The process of formation, detachment, coalescence and rise affects the bubble residence time at or near the photoanode. A reduced bubble residence time promotes easier electrolyte access to the photoanode surface, thereby enhancing the oxygen evolution reaction (OER) kinetics. In this work, computational fluid dynamics (CFD) simulations using laminar flow and phase-field model in COMSOL Multiphysics are conducted to study the gas bubble hydrodynamics for different bubble configurations, sizes, and bubble gaps. CFD results predict enhanced bubble rise in the presence of forced convection, i.e., some electrolyte inflow velocity (u≠0m.s−1). Experimental results using hematite as the model electrode also show a higher photocurrent density (ca. 4.33 mA.cm−2 at 2 V vs. RHE) for u = 0.1 m.s−1, compared to that without flow (3.5 mA.cm−2 at 2 V vs. RHE). This indicates improved access of electrolyte at the electrode surface and faster OER. Further, CFD simulations for electrode morphology effects show a reduced bubble adhesion for hydrophilic hematite nanorod arrays compared to planar photoanode, due to capillary wicking. The continuous contact film detaches and results in a reduced blockage of active sites at the bubble base, thereby enhancing the PEC performance for the nanorod arrayed morphology. Here too, experimental results show a four-fold increase in photocurrent density (ca. 0.1 mA.cm−2 at 1.5 V vs. RHE) and a 210 mV cathodic shift in onset potential for nanorods, compared to planar electrodes (0.025 mA.cm−2 at 1.5 V vs. RHE). A significant reduction in charge transfer resistance is observed for the former, implying better OER kinetics. The results obtained help in the understanding of transient processes involving O2 bubble hydrodynamics and electrolyte usage efficiency in a PEC reactor.

Enhanced photoelectrochemical water oxidation in Hematite: Accelerated charge separation with Co doping

Hematite (α-Fe2O3) is one of the most promising candidates for a photoanode for photoelectrochemical water splitting. However, it has low efficiency because of poor conductivity in the bulk and sluggish oxygen evolution (OER) kinetics on the surface. In this study, a Co-doped α-Fe2O3 photoanode was prepared by the hydrothermal method. It had increased charge separation efficiency and accelerated surface reaction kinetics simultaneously. At the optimal Co content, the photocurrent density of the Co-doped α-Fe2O3 increased by 23 times to 0.54 mA/cm2 at 1.23 VRHE in 1 M NaOH electrolyte; this increase was attributed to the improvement in carrier density and charge transfer behavior. Moreover, the cathodic shift of onset potential for Co-doped α-Fe2O3 reached 290 mV owing to the accelerated OER kinetics. This work reveals the key roles of Co doping and provides a suitable photoanode for solar hydrogen technology.

Fabrication of tin‐doped hematite modified with NiFe‐LDH nanoflakes for highly efficient solar water splitting

Photoelectrochemical (PEC) water splitting is widely regarded as an effective process for sustainable storage of solar energy as chemical energy in the form of H2. The efficiency of this process depends upon the light absorption, charge injection, and charge separation efficiency of the photoanode material used. In this work, doping hematite with 1% tin and loading with NiFe-layered double hydroxide (LDH) co-catalyst has been done to improve the charge transport properties. To enhance the PEC performance and study the effect of precursor concentration, NiFe-LDH at two different concentrations (low and high) was coated on Sn-doped hematite. A high photocurrent density ca. 2.9 mA/cm2 at 1.8 V vs reversible hydrogen electrode (RHE) and a 170 mV cathodic shift in onset potential compared to pristine hematite (0.22 mA/cm2 at 1.8 V vs RHE) was recorded for Sn-doped hematite coated with low precursor concentration. The enhanced PEC activity can be attributed to the synergistic effect of Sn doping and NiFe-LDH co-catalyst on hematite, which contributes to an efficient separation of the photogenerated charge carriers and reduces hole accumulation at the surface. However, for the high precursor concentration case, the formation of a thick film of NiFe-LDH results in a comparatively lower current density (0.85 mA/cm2 at 1.8 V vs RHE). The results obtained show that NiFe-LDH can be used as an effective co-catalyst to significantly enhance the charge transport properties of tin-doped hematite. A charge transfer mechanism of the developed photoanodes is also discussed in detail.

Defect-Free Single-Layer Graphene by 10 s Microwave Solid Exfoliation and Its Application for Catalytic Water Splitting.

Mass production of defect-free single-layer graphene flakes (SLGFs) by a cost-effective approach is still very challenging. Here, we report such single-layer graphene flakes (SLGFs) (>90%) prepared by a nondestructive, energy-efficient, and easy up-scalable physical approach. These high-quality graphene flakes are attributed to a novel 10 s microwave-modulated solid-state approach, which not only fast exfoliates graphite in air but also self-heals the surface of graphite to remove the impurities. The fabricated high-quality graphene films (∼200 nm) exhibit a sheet resistance of ∼280 Ω/sq without any chemical or physical post-treatment. Furthermore, graphene-incorporated Ni–Fe electrodes represent a remarkable ∼140 mA/cm2 current for the catalytic water oxidation reaction compared with the pristine Ni–Fe electrode (∼10 mA/cm2) and a 120 mV cathodic shift in onset potential under identical experimental conditions, together with a faradic efficiency of >90% for an ideal ratio of H2 and O2 production from water. All these excellent performances are attributed to extremely high conductivity of the defect-free graphene flakes.

Cathodic shift of onset potential on TiO2 nanorod arrays with significantly enhanced visible light photoactivity via nitrogen/cobalt co-implantation* *Project supported by the National Natural Science Foundation of China (Grant No. 11875211), the Major Science and Technology Program of Changsha, China (Grant No. kq1902046), and the Fundamental Research Funds for the Central Universities, China.

Despite anionic doping has been widely implemented to increase the visible light activity of TiO2, it often gives rise to a dramatical anodic shift in current onset potential. Herein, we show an effective method to achieve the huge cathodic shift of TiO2 photoanode with significantly enhanced visible light photo-electrochemical activity by nitrogen/cobalt co-implantation. The nitrogen/cobalt co-doped TiO2 nanorod arrays (N/Co-TiO2) exhibit a cathodic shift of 350 mV in onset potential relative to only nitrogen-doped TiO2 (N-TiO2). Moreover, the visible-light (λ > 420 nm) photocurrent density of N/Co-TiO2 reaches 0.46 mA/cm2, far exceeding 0.07 mA/cm2 in N-TiO2 at 1.23 V versus reversible hydrogen electrode (RHE). Systematic characterization studies demonstrate that the enhanced photo-electrochemical performance can be attributed to the surface synergic sputtering of high-energy nitrogen/cobalt ions.

Improving water oxidation performance by implementing heterointerfaces between ceria and metal-oxide nanoparticles

The main technical challenge for the electrolytic production of hydrogen via water splitting lies in realizing a very stable material that effectively oxidizes water under low overpotential (η). Of all materials, metal oxides hold the greatest promise due to their inherited chemical stability in aqueous solutions; however, electrolytic effectiveness in water oxidation reactions (OERs) is limited to precious metals. In this study, we designed metal oxide/metal oxide (MO/MO) nanoparticle heterointerfaces to offer more active sites and enhance the overall performance of the OER. To demonstrate this improvement, we synthesized and characterized CeO2/Co3O4, CeO2/CuO, and CeO2/NiO nanoparticles. In these structures, onset potential and photoactivity were significantly improved relative to a single MO. A cathodic shift of onset potential as high as ~0.4 or 0.3 V was recorded for CeO2/Co3O4 relative to CeO2 or Co3O4, respectively. This improvement was further investigated using density functional theory calculations, upon which adsorption preferability and reaction free energy at the CeO2/Co3O4 heterointerface were found to play significant roles in OER enhancement.

Fabrication of WO3/RGO/Ni:FeOOH heterostructure for synergistically enhancing photoelectrochemical water oxidation

The instability and low activity of WO3 is the hot-topic for photoelectrochemical (PEC) water splitting, which is decreased by a sluggish interfacial kinetics and incomplete water oxidation. Aiming at such issues, we designed a ternary WO3/RGO/Ni:FeOOH photoanode, which exhibits 2.05 times larger photocurrent (1.32 mA cm−2) than the WO3 NFs in 0.5 M Na2SO4 electrolyte solution, accompanied with 83 mV cathodic shift of onset potential. These results demonstrates PEC response can occurs only in Ni:FeOOH but not FeOOH. Therefore, Ni:FeOOH acting as bi-functional modifier, can not only increase light absorption but also promote charge transfer process by forming p-n heterojunction with the WO3 NFs. Besides, RGO forming a continuously conducting network can further improve charge separation process. Owing to their synergistic effects, the Faradaic efficiency and stability are both improved compared to its counterparts and the bare WO3 NFs. This work may inspire the PEC application in other co-catalyst decorated systems.

Elucidating the role of surface states of BiVO4 with Mo doping and a CoOOH co-catalyst for photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical (PEC) water splitting, however, its PEC performance is limited by the high surface and bulk charge recombination rates. Here we present a comprehensive study to elucidate a recombination phenomenon of BiVO4 that arises with Mo doping. The Mo doping produces multiple effects including the formation of MoOx (reduced form of Mo6+) species and oxygen vacancies (VOs) on the surface of the BiVO4 that work in tandem with V4+ species (and MoOx) acting as surface-active intermediates (i-SS) providing improved hole transfer to the electrolyte. In contrast, in the absence of V4+ species, the VOs can act as recombination centers (r-SS). Further, CoOOH co-catalyst coating is used to minimize such recombination centers. Eventually, a photocurrent enhancement of ~37 times (1.1 mA/cm2 at 1.23 V vs. RHE) and a cathodic shift in onset potential of ~500 mV compared to that of pristine BiVO4 (0.03 mA/cm2 at 1.23 V vs. RHE) is obtained. We carried out in-depth PEC analysis using hole scavenger measurements, PEC impedance spectroscopy, and intensity-modulated photocurrent spectroscopy to elucidate the effect of the surface reduction process upon doping, the impact of Vos, MoOx species and CoOOH layer on the enhanced PEC performance.

Valence modulation on zinc-cobalt-vanadium layered double hydroxide nanosheet for accelerating BiVO4 photoelectrochemical water oxidation

This work puts forward an exploration of a new ternary ZnCoV–layered double hydroxide (ZnCoV–LDH) as a highly efficient oxygen evolution co-catalyst to enhance the photoelectrochemical (PEC) water splitting performance of BiVO4 photoanode. ZnCoV-LDH nanosheet was prepared by a fast and simple electrodeposition method. Compared to the bare BiVO4, the composite photoanode showed a 370-mV cathodic shift in onset potential and an enhancement in photocurrent density by a factor of around 4, which delivering 2.7 mA cm−2 at 1.23 V versus a reversible hydrogen electrode. In addition, the modified electrode was of greater steadiness for water oxidation. It is postulated that the enhanced PEC performance results from the efficient utilization of photogenerated holes in water oxidation due to the improved oxygen evolution reaction (OER) kinetics in the presence of the ZnCoV-LDH cocatalyst. More importantly, the role of each component in the electrocatalyst was also explored, in which Co species provide active sites to capture photogenerated holes for the water oxidation reaction while Zn species serve as structural support. It was found that the introduction of strongly electron-withdrawing V5+ can not only facilitate the in-situ formation of highly active Co3+ species for the OER, but can also adjust the electronic conductivity, thereby leading to the improved intrinsic activity and accelerated OER reaction kinetics. This work provides not only a demonstration of a novel and promising co-catalyst for PEC water splitting, but also a facile method for the rational design other hydroxide-based materials.

Improved water oxidation performance of ultra-thin planar hematite photoanode: Synergistic effect of In/Sn doping and an overlayer of metal oxyhydroxides

Hematite is a promising photoanode candidate with many favorable material properties, such as stability and suitable band-gap. However, there are some severe challenges, including high losses due to charge recombination and slow oxidation kinetics, which can be addressed by doping and addition of co-catalysts. Here, the effects of temperature driven diffusion of substrate impurities (doping) and subsequent surface modification by metal oxy-hydroxides (co-catalysts) have been studied for enhanced water-oxidation performance in photoelectrochemical (PEC) measurements. Diffusion of indium and tin from the indium-doped tin oxide (ITO) substrate into planar films of α-Fe2O3 photoanodes results in a photocurrent density (Jph) of 0.09 mA/cm2, corresponding to an approximate 9-fold enhancement over the control pristine α-Fe2O3 (0.01 mA/cm2) at 1.23 VRHE. A thin amorphous FeOOH coating over the In/Sn co-doped α-Fe2O3 photoanode improves the water oxidation performance further, with a 211 % enhancement in Jph at 1.23 VRHE and a 0.21 V cathodic shift in onset potential. Thin layers of NiOOH and FeNiOOH co-catalysts exhibit 100 and 155 % enhancement in Jph, respectively. Characterization and electrochemical measurements reveal that the enhanced performance is a result of reduced bulk recombination by temperature driven In/Sn substrate impurity doping and improved surface oxidation kinetics by the metal oxy-hydroxide overlayer. Especially deposition of FeOOH onto In/Sn co-doped α-Fe2O3 significantly reduces resistance at the semiconductor/electrolyte interface, leading to the shift in onset potential. Further, the results indicate that all the samples exhibit a quantitative correlation between the cathodic shift in photocurrent onset potential (Vonset) and flat band potential (Vfb).

Photocharged molybdenum-doped BiVO4 photoanodes for simultaneous enhancements in charge transport and surface passivation

The two main drawbacks of monoclinic bismuth vanadate (BiVO4: BVO) as a photoanode, i.e., poor charge transport property and fast electron-hole recombination, were improved by simultaneous Mo doping and photocharged surface passivation methods. First, molybdenum-doped BiVO4 (BiVO4: Mo) was prepared to control the donor concentration without producing any secondary phase. Subsequently, photocharging treatment of the Mo-BVO photoanode induced a cathodic shift in onset potential, ~0.15 V (0.75–0.6 V vs RHE) and improved the photocurrent density (~2.2-fold). Moreover, the photocharged 2% Mo-BVO photoanode exhibited an enhanced incident photon current efficiency (IPCE) of about 50% compared to only around 30% (440 nm > λ > 330 nm range) for the non-photocharged photoanode. Mott-Schottky analysis revealed that 2% Mo doping greatly increased the donor concentration (~1000-fold), whereas the photocharging technique only slightly increased the donor concentration (~1.5-fold).

A “surface patching” strategy to achieve highly efficient solar water oxidation beyond surface passivation effect

The hole trapping sites at the photoanode/electrolyte interface seriously detract from the positive effect of oxygen-vacancy on photoelectrochemical (PEC) water oxidation. In this work, a “patching” strategy is put forward to eliminate those charge trapping sites of the oxygen-deficient in disordered overlayer (DL) of WO3 photoanode by inactive pieces of oxygen-rich carbon nitride quantum dots (CNQDs). The “patching” leads to a 1.5-fold enhancement in PEC performance and a 100 mV cathodic shift of onset potential compared to the pre-optimized DL-WO3 photoanode. The remarkably raised charge transfer efficiency from 60% to 87% at 1.23 V vs RHE is an indication of boosting hole transfer. Density function theory (DFT) calculations reveal that DL-WO3/CNQDs produces a stepped valence band alignment together with the removal of charge trapping sites, is capable of overcoming the hole transfer limitation at the photoanode/electrolyte interface. This study might open a window to pursue a simple but highly efficient strategy on modification of solid/liquid interface for solar to fuel conversion.

Charge Carrier Dynamic Study on Different Catalyst-Modified BiVO4 Photoanodes

Transient absorption measurement is developed to investigate the charge carrier dynamic of different catalyst-modified BiVO4 and the origin of photocurrent performance enhancement. In-situ IPCE under conditions of transient absorption measurement showed the IPCE enhancement and cathodic shift of onset potential on both Co9POM and CoPi modified of BiVO4 compared to bare BiVO4. The normalized transient absorption kinetics averaged from 500-530 nm exhibited similar slower kinetic decay on both catalysts treated BiVO4 compared to bare BiVO4. The power law decay feature of kinetic decay from 10 ns – 500 ns indicates a trap-limited recombination process occurs in all investigated BiVO4. The slower bulk recombination kinetic decay observed on Co9POM/BiVO4 and CoPi/BiVO4 suggests that both co-catalyst treatments on BiVO4 enhance the PEC performance by suppressing the bulk recombination process.

An effective strategy to promote hematite photoanode at low voltage bias via Zr4+/Al3+ codoping and CoOx OER co-catalyst

Herein, we report the surface treatment on Zr4+/Al3+ codoped α-Fe2O3 photoanode for high-performance photoelectrochemical water splitting. A high-temperature quenching exhibits the Zr4+/Al3+ codoping in α-Fe2O3 photoanode without damaging morphology. The presence of Zr4+/Al3+ codoping shows a cathodic shift in onset potential, but lack of increment in photocurrent reveals the major role of passivation and the minimum doping effect of aluminum. Additionally, CoOx cocatalyst exhibits increment in photocurrent with the greater cathodic shift in onset potential than the pristine α-Fe2O3 nanorods. The CoOx surface-reworked Zr4+/Al3+ codoped α-Fe2O3 photoanode displays the highest photocurrent of 1.5 mA/cm2 at 1.23 V vs. RHE (76% increment over the pristine α-Fe2O3) and 0.7 mA/cm2 at 1.0 V vs. RHE (102% increment over the pristine α-Fe2O3). The systematic characterization carried out using x-ray diffraction and scanning electron microscopy confirms that after Zr4+/Al3+ codoping, and surface treatment, the crystalline structure, and morphology of the photoanodes remains unchanged. X-ray photoelectron spectroscopy confirmed the existence of Zr4+/Al3+ codopants in the hematite nanostructure. The electrochemical properties of the photoanode suggest that Al3+ and Zr4+ codoping, as well as surface treatment with CoOx, cocatalyst lowers charge transfer resistance across the FTO/hematite interface, and hematite/electrolyte interface. This designs not only lowers onset potential but also offers the blueprint for the development of an efficient catalyst for solar water oxidation.

Energy level alignment and interfacial dipole layer formation at the P3HT:PCBM-Electrolyte interface in organic photoelectrochemical cells

The influence of PCBM on the electronic energy levels of P3HT:PCBM photocathodes was investigated in a non-aqueous electrolyte using a benzoquinone redox couple. The presence of PCBM introduces a density of interface states at the semiconductor-electrolyte interface, consequently leading to the formation of an interfacial dipole layer and decrease in the built-in potential developed at the semiconductor-electrolyte interface. Photoelectrochemical measurements reveal a decrease in photovoltage from 0.26 V to 0.16 V and cathodic shift in onset potential from -0.27 V to -0.34 V (vs. Ag/Ag+) as PCBM content of the P3HT:PCBM photocathode is increased from 0 through 50 wt%. Additionally, depletion layer widths decreased from 90 nm to 50 nm. When the thickness of the active layer was controlled to coincide with the depletion layer thickness (e.g., 90 nm), photocurrent densities at pristine P3HT photocathodes were found to be higher than for P3HT:PCBM photocathodes. This suggests that the built-in electric field within the active layer is seemingly capable of effective exciton dissociation in the absence of a fullerene acceptor.

An ultrathin cobalt–iron oxide catalyst for water oxidation on nanostructured hematite photoanodes

An optically transparent cobalt-iron oxide catalyst improves the photoelectrochemical performance of nanostructured hematite photoanodes.

Interface Engineering of Hematite with Nacre-like Catalytic Multilayers for Solar Water Oxidation.

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalate (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (∼369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

Fe2TiO5 as an Efficient Co-catalyst To Improve the Photoelectrochemical Water Splitting Performance of BiVO4.

Fe2TiO5 was synthesized via the solvothermal method and adopted as co-catalyst to improve the photoelectrochemical (PEC) water splitting performance of BiVO4 photoanode. After surface modification by Fe2TiO5, the BiVO4/Fe2TiO5 photoanode shows a 300 mV cathodic shift in onset potential and 3 times enhancement in photocurrent, which delivers a photocurrent density of 3.2 mA/cm2at 1.23 V vs reverse hydrogen electrode. Systematic optical, electrochemical, and intensity-modulated photocurrent spectroscopy characterizations were performed to explore the role of Fe2TiO5 and reveal that the enhanced PEC performance is mainly caused by the surface passivation effect of Fe2TiO5.