Pi Catalyst Research Articles

Hierarchical Co-Pi Clusters/Fe2O3 Nanorods/FTO Micropillars 3D Branched Photoanode for High-Performance Photoelectrochemical Water Splitting.

In this study, an efficient hierarchical Co–Pi cluster/Fe2O3 nanorod/fluorine-doped tin oxide (FTO) micropillar three-dimensional (3D) branched photoanode was designed for enhanced photoelectrochemical performance. A periodic array of FTO micropillars, which acts as a highly conductive “host” framework for uniform light scattering and provides an extremely enlarged active area, was fabricated by direct printing and mist-chemical vapor deposition (CVD). Fe2O3 nanorods that act as light absorber “guest” materials and Co–Pi clusters that give rise to random light scattering were synthesized via a hydrothermal reaction and photoassisted electrodeposition, respectively. The hierarchical 3D branched photoanode exhibited enhanced light absorption efficiency because of multiple light scattering, which was a combination of uniform light scattering from the periodic FTO micropillars and random light scattering from the Fe2O3 nanorods. Additionally, the large surface area of the 3D FTO micropillar, together with the surface area provided by the one-dimensional Fe2O3 nanorods, contributed to a remarkable increase in the specific area of the photoanode. Because of these enhancements and further improvements facilitated by decoration with a Co–Pi catalyst that enhanced water oxidation, the 3D branched Fe2O3 photoanode achieved a photocurrent density of 1.51 mA cm−2 at 1.23 VRHE, which was 5.2 times higher than that generated by the non-decorated flat Fe2O3 photoanode.

Efficient removal TC by Zn@SnO2/PI via the synergy of adsorption and photocatalysis under visible light

Zn@SnO2/PI material was synthesized by solid-state polymerization and hydrothermal method. The catalyst used for the firstly to remove tetracycline hydrochloride (TC) via the synergistic effect of adsorption and photocatalysis. Zn@SnO2 in the composites provides surface adsorption sites, make the composites have high and rapid adsorption capacity for TC. The PI in the catalyst have good visible light response, which provides the possibility for the composites to have excellent visible light response. Among all the prepared composites, 35Zn@SnO2/PI (the ratio of Zn@SnO2 mass to total mass is 35%, marked as 35Zn@SnO2/PI) has the largest specific surface area (121.77 m2/g) and the largest pore volume (0.167 cm3/g) and has good stability. Meanwhile, the composites of PI and Zn@SnO2 increased the lifetime of electron-hole pairs and heightened the photocatalytic activity of composite for the removal of TC. The results show that the composites can adsorb 40% of TC (20 mg/L, 50 mL) in 30 min under shading reaction. Under visible light irradiation, the removal efficiency of TC can reach 86% in 30 min through adsorption and photocatalysis. Furthermore, the probable photocatalytic removal pathway of TC on Zn@SnO2/PI catalyst was researched based on HPLC-MS analysis. The study may afford a few aiming for the construction of catalysts for the synergistic and efficient treatment of environmental pollutants by adsorption and photocatalysis to solve the increasingly serious environmental problems.

Transparently Passivating Catalyst of Hydrated Manganese Phosphate for Photoelectrochemical O2 Generation

A transparently hydrated manganese phosphate (“Mn–Pi”) catalyst is coated onto a Ti-doped hematite (Ti:Fe2O3) photoanode by the photo-assisted electrochemical deposition. Under a visible light illumination of 100 mW cm–2, the photocurrent density of Ti:Fe2O3/Mn–Pi reaches 0.28 mA cm–2 at 1.2 VRHE, which is around two times higher than that obtained with the pristine Ti:Fe2O3 electrode in a neutral electrolyte, and the “onset” potential reduces by 100 mV. The passivating role of the Mn–Pi catalyst is revealed experimentally. The surface states of hematite are significantly passivated after Mn–Pi deposition, which relieves the degree of Fermi level pinning of hematite and enhances the surface band bending of hematite. Therefore, the photovoltage and the charge separation are improved. Moreover, due to the valence change of the Mn element in Mn–Pi, the charge transfer through the Mn–Pi layer is favorable, which has a low charge transfer resistance for photogenerated holes. The suitability of this transparently passivating catalyst for concentrating light is also identified. This work demonstrates that Mn–Pi can be applied for photoelectrochemical water splitting as a transparently passivating catalyst.

Insight into the improvement mechanism of Co−Pi-modified hematite nanowire photoanodes for solar water oxidation

The composite photoanodes composed by cobalt phosphate catalyst (Co−Pi) modified semiconductor have been widely used for solar water splitting, but the improvement mechanism has not been experimentally confirmed. Here we use transient photoelectrochemical measurements and impedance spectroscopy to investigate the effect of Co−Pi catalyst on hematite nanowire photoanode. It is found that under illumination the Co−Pi catalyst can efficiently promote the transfer of photo-generated holes to the Co−Pi layer by increasing the electrical conductivity of the composite structure under a low potential. The Co−Pi catalyst can recombine with photo-generated electrons to reduce the surface recombination efficiency of photo-generated holes and electrons under a high potential. These results provide important new understanding of the performance improvement mechanism for the Co−Pi-modified semiconductor nanowire composite photoanodes.

The morphology and electrochemical properties of WO3 and Se-WO3 films modified with cobalt-based oxygen evolution catalyst

In this paper, we have studied morphology and electrochemical properties of novel (photo)electrocatalyst of selenium doped tungsten trioxide (Se-WO3) modified with cobalt-based oxygen evolution catalyst (Co–Pi). Se-WO3 films were electrodeposited on fluorine doped tin oxide coated glass (FTO) support. Then photochemical deposition was employed to modify the surface of produced films with Co–Pi catalyst in order to increase their performance in photo(electrochemical) water splitting. Morphological and (photo)electrochemical properties were identified using X-ray powder diffraction (XRD), Scanning electron microscopy (SEM) with Energy dispersive X-ray analysis (EDX) and Fourier-transform infrared spectroscopy (FTIR), as well as electrochemical techniques in order to determine photon-to-current efficiency (IPCE) and photoconversion efficiency. The results indicated that the presence of cobalt-based catalyst effectively changes the activity of Se-WO3 films: the incident photon-to-current efficiency and photoconversion efficiency increased nearly twice and the onset potential for O2 evolution in the dark was also shifted towards lower values.

Optimization of the Co–Pi/α-Fe2O3 Catalyst Synthesis Parameters Using the Taguchi Method for Water-Splitting Application

The use of hematite as a photoanode has been limited due to its optoelectronic properties and a large overpotential for photo-assisted water splitting. In this study, a cobalt-phosphate (Co–Pi) layer was deposited on hematite photoanodes (α-Fe2O3) in order to increase the lifetime of the photogenerated holes and to lower the applied bias. The influence of the Co–Pi catalyst thickness, the pH of deposition, and the scan rate of the photoelectrochemical (PEC) measurements were optimized using the Taguchi statistical method. An L9 orthogonal array, with the three factors of control at three levels, was employed to determine the optimum conditions for water splitting of neutral water with minimum applied bias. After depositing Co‒Pi layer on the α-Fe2O3 at different conditions, the PEC was measured via the linear sweep voltammetry (LSV) and chronoamperometry. The analysis of variance of the mean overpotential in comparison with the bare hematite and the signal-to-noise ratio indicated the great influence of catalyst thickness on the decreasing of overpotential for photo-assisted water splitting in comparison with the bare α-Fe2O3. The characterization of the catalyst and the α-Fe2O3 morphology were carried out by SEM, XRD, and EDS. The optimized photoanode showed an enhancement of around two times for photocurrent and 1.5 times in incident photon-to-current conversion efficiency (IPCE) compared to the bare α-Fe2O3. These findings result in an understanding of the role of the Co–Pi catalyst role in improving the hematite photoanode.

Photo-deposition of cobalt-phosphate group modified hematite for efficient water splitting

In this work, Cobalt Phosphate Group (Co−Pi) Modified nanostructured hematite (α-Fe2O3) photoanodes films were deposited on fluorine-doped tin oxide (FTO) coated glass by means of the hydrothermal process. The α-Fe2O3 film morphology can be tuned from nanoparticle – based patterns to high-density cabbage-like structures as Co−Pi deposition time is increased.Systematic studies were performed to clarify the mechanisms that enhance water-splitting performance of α-Fe2O3 electrodes as Co−Pi deposition time increases. All samples followed morphological and structural studies using field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. Chronoamperometry measurements showed that Co-Pi/α-Fe2O3 photoanode exhibited higher photoelectrochemical activity than the α-Fe2O3 films. The maximum photocurrent density and incident photon conversion efficiencies (IPCE) were obtained for 30 s Co-Pi/α-Fe2O3 photoanode. All samples reached their best IPCE at 400 nm. IPCE values for 30 s Co-Pi hematite films were 5 times higher than that of α-Fe2O3 sample. For the α-Fe2O3 photoanode, the H2 evolution rate was ∼5.14 μmol cm−2 h−1. After decoration by Co-Pi, the rate increases to ∼7.31 μmol cm−2 h−1, showing an enhancement of about ∼ 42.2%. It was found that under illumination, the Co−Pi catalyst can efficiently collect and store photogenerated holes from the α-Fe2O3 electrode. This charge separation reduces surface state recombination, leading to the enhancement of the efficiency of water oxidation. Thicker catalytic Co−Pi films were found to increase charge separation and surface area, which in turn increase water oxidation efficiency. These results provide important new understanding of the catalytic enhancement and limitations of the Co−Pi catalyst once coupled to semiconductor electrodes for water-splitting applications.

Fe/N/C Cathode Catalyst Prepared from a Low Content of Fe Precursor to Obtain Atomically Dispersed Metal Center

A Fe/N/C oxygen reduction catalyst was prepared from polyimide nano-particles containing a low content of Fe additive. The new preparation method can provide highly active ORR catalyst but can avoid the use of HCl, which was necessary to wash out extraordinary Fe species. The kinetics was investigated in detail by the RRDE voltammetry associated with the correction of the quasi-four electron reduction, and the contribution of 2+2 electron pathway is considerable. The obtained Fe(0.5)/PI catalyst was tested in MEA and exhibited a quite promising fuel cell performance.

Enhancing Water Oxidation Catalysis on a Synergistic Phosphorylated NiFe Hydroxide by Adjusting Catalyst Wettability

Surface wettability is very important for designing and developing heterogeneous electrocatalysts that can be applied in an aqueous environment. Here, by adjusting the surface wettability of the catalyst using a facile two-step strategy, a porous nanosheet electrocatalyst composed of NiFe hydroxide and NiFe phosphate (denoted as NiFe/NiFe:Pi), has been designed and developed. The two-step strategy not only allows us to successfully control the morphology but also provides an approach to modify the surface chemical property of a catalyst. After phosphorylation, the surface wettability of the NiFe/NiFe:Pi catalyst is significantly enhanced (contact angle 44 ± 3°) in comparison to that of NiFe hydroxide electrode without phosphorylation (contact angle 129 ± 5°). Serving as an oxygen evolution catalyst in 1 M KOH aqueous solution, the NiFe/NiFe:Pi electrode yields strong synergistic oxygen evolution activity to deliver a current density of 10 mA cm–2 with an overpotential merely of 290 mV. The catalyst also e...

Engineering Disorder into Heterogenite‐Like Cobalt Oxides by Phosphate Doping: Implications for the Design of Water‐Oxidation Catalysts

AbstractAmongst the most promising materials designed to catalyse water oxidation from earth‐abundant materials are the metal oxides. Despite the success of these materials, understanding the relationship of structure to function has been very challenging. It has been noted that many metal oxide water‐oxidation catalysts function best in a proton‐accepting electrolyte, such as a borate or phosphate buffer. However, these same electrolytes are known to significantly affect the metal oxide structures by imparting a level of “disorder” or “molecular nature” to the materials. The most well‐known case is that of Nocera's Co–Pi catalyst (Pi: inorganic phosphorus). In this study, we have synthesised a series of “heterogenite‐like” cobalt oxides with different levels of phosphate doping (0–9 %P). Our synthetic method enables us to make “bulk materials”, the structural properties of which (as observed by X‐ray absorption spectroscopy and transmission electron microscopy) mimic those observed directly on electrode surfaces. The changes made to the bulk phases were directly correlated with the reactivity for water‐oxidation catalysis and the ability of the CoOx materials to act as sacrificial oxidants. The most disordered materials were most reactive for sacrificial oxidation but were less effective as water‐oxidation catalysts. These results help us understand how disorder changes the thermodynamic stability of metal oxides and how this impacts on efficiency for water oxidation.

A novel Pt@Te-reduced graphene oxide/polyimide composite catalyst for hydrogen evolution

A promising Pt@Te nanoparticles catalyst has been synthesized on the surface of the reduced graphene oxide/polyimide (rGO/PI) substrate. First of all, Te was deposited on the rGO/PI using electrochemical deposition method to obtain Te-rGO/PI that has various morphologies. Secondly, a monolayer Pb film was deposited on the Te-rGO/PI by the similar electrochemical method to get a transition state material of Pb@Te-rGO/PI. Finally, the target Pt@Te catalyst on the rGO/PI with large surface area was obtained by a chemical oxidation method, in which Pb was stripped off from the Pb@Te-rGO/PI by an oxygenation of H2PtCl6, while the oxygenation process, Pt replaced Pb to be deposited on the surface of Te-rGO/PI to form Pt@Te-rGO/PI. Then the prepared Pt@Te-rGO/PI was used as a working electrode to study the hydrogen evolution behavior in 0.5 mol/l H2SO4. By comparison the catalyst of sole Pt-rGO/PI nanoparticles, Pt@Te-rGO/PI catalyst considerably abated the demanding of Pt. And it shows excellent HER activities with a low overpotential of 0.1 V. The Tafel slope of 55 mV dec−1 reveals most likely the Volmer–Heyrovsky mechanism for hydrogen evolution reaction (HER). On the other hand, the catalyst displays satisfactory stability. This work may pave a potential pathway for synthesizing multi-morphology nano-Pt catalyst with high surface area for application in the study of HER.

Catalytic behavior of Ru supported on Ce0.8Zr0.2O2 for hydrogen production

Ru (2wt.%) on Ce0.8Zr0.2O2 catalysts were investigated to test their ethanol steam reforming (ESR) performance. The supports were obtained through two different synthesis methods: co-precipitation (PI) and mechanochemical reaction (MR). Characterization of the catalysts, before and after ESR, were performed using X-ray powder diffraction (XRPD), nitrogen physisorption, CO chemisorption, temperature programmed reduction (TPR), high resolution transmission electron microscopy (HR-TEM), thermogravimetry, and X-ray photoelectron spectroscopy (XPS) measurements. The Ru (2wt.%) PI catalyst exhibited the highest activity and selectivity to hydrogen and no coke deposition was observed during catalytic operation, even when compared to Ni (8wt.%) PI catalyst. A possible reaction pathway is proposed. The remarkably good catalytic performance of Ru (2wt.%) PI is regarded as being due to a combination of factors: improved metal dispersion on Ce0.8Zr0.2O2, favorable metal-support interaction and water gas shift reaction promotion.

Monopersulfate oxidation of Acid Orange 7 with an iron(III)-tetrakis(N-methylpyridinium-4-yl)porphyrin intercalated into the layers of montmorillonite and pillared clay

A biomimetic catalyst, iron(III)-tetrakis(N-methylpyridinium-4-yl)porphyrin (FeTMPyP), was loaded into montmorillonite (Mt) and pillared clay (Pi). The catalytic activities of the prepared catalysts for the decolorization of Acid Orange 7 (AO7) in the absence and presence of humic acid (HA), a major component of waste water, were compared. Turnover frequencies (TOFs) for the FeTMPyP-loaded Pi (6.7–9.4h−1) were larger than those for the FeTMPyP-loaded Mt (2.9–3.3h−1), while decolorization for the FeTMPyP-loaded Mt was inhibited in the presence of HA (TOFs of 1.3–2.1h−1). These results can be attributed to the fact that the large expansion in basal spacing for the Mt permitted the catalyst to be eluted and HA to be inserted into the Mt layer. However, the levels of catalyst elution and the inhibition of AO7 decolorization by HA were significantly suppressed in the case of the FeTMPyP-loaded Pi catalyst, which was prepared using smaller OH/Al and Al/Mt ratios. These results indicate that the Pi is more suitable support for the FeTMPyP catalyst than the Mt.

A three-dimensional hexagonal fluorine-doped tin oxide nanocone array: a superior light harvesting electrode for high performance photoelectrochemical water splitting

Photonic nanostructures hold great promise in promoting light harvesting. Here we report the first design and construction of a three-dimensional (3D) hexagonal nanocone array of fluorine-doped tin oxide (FTO) on glass as an excellent electrode for photoelectrochemical (PEC) water splitting. The PEC current density with suitably deposited Ti-doped hematite at 1.23 V vs. the reversible hydrogen electrode (RHE) was increased by 86% to 2.24 ± 0.02 mA cm−2 compared to that with the planar counterpart, mainly ascribable to the special light harvesting effect and the electrode surface area provided by 3D FTO. Upon the embedment of a gold layer to concentrate the incident light onto the hematite layer and the deposition of the Co–Pi catalyst with a modified procedure, the photocurrent experienced a large cathodic shift of onset potential by 360 mV and soared to a high value of 3.39 ± 0.01 mA cm−2 (at 1.23 V), yielding a power conversion efficiency of 0.70% at a potential as low as 0.88 V vs. RHE.

Enhanced photoelectrocatalytic performance of nanoporous WO3 photoanode by modification of cobalt–phosphate (Co–Pi) catalyst

Nanoporous WO3 photoanode modified with cobalt–phosphate (Co–Pi) catalyst was synthesized in this study. The nanoporous WO3 was prepared by anodization of W foil following a photo-assisted electrodeposition of Co–Pi catalyst. The presence of Co–Pi catalyst obviously facilitated the charge transfer and reduced the recombination of photoexcited electron/hole by forming an adequate junction between the catalyst and the nanoporous WO3. The photocurrent density of nanoporous WO3/Co–Pi was found to be 1.4 mA/cm2 at 0.8 V which was 20 % higher than nanoporous WO3, and the nanoporous WO3/Co–Pi photoanode is also more stable for long-term application.

EQCM Investigation of Electrochemical Deposition and Stability of Co–Pi Oxygen Evolution Catalyst of Solar Energy Storage

Photoassisted electrolysis of water is considered as an effective way of storing solar energy in the form of hydrogen fuel. This overall reaction involves the oxidation of water to oxygen at the anode and the reduction of protons to hydrogen at the cathode. Cobalt–phosphate-based catalyst (Co–Pi) is a potentially useful material for oxygen evolution reaction. In the present study, electrochemical deposition of Co–Pi catalyst is carried out on Au-coated quartz crystal from 0.1 M phosphate buffer (pH 7) containing 0.5 mM Co2+ ion, along with the simultaneous measurement of mass changes at the electrode surface. Cyclic voltammograms and mass variations are recorded during the course of deposition. A current peak is observed at 0.92 V vs Ag/AgCl, 3 M KCl corresponding to oxidation of Co2+ ion. The mass of the electrode starts increasing at this potential, suggesting the deposition of a Co(III)-based insoluble product on the electrode surface. The stability of the catalyst is also studied at several potentials in both buffered and nonbuffered electrolyte by monitoring the real-time mass variations.

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co–Pi”-Coated Hematite Electrodes

Uniform thin films of hematite (α-Fe(2)O(3)) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, "Co-Pi," were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co-Pi thickness were performed in order to clarify the mechanism by which Co-Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co-Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co-Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co-Pi catalyst coupled with semiconductor electrodes for water-splitting applications.

EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH

Thin-film water oxidation catalysts (Co-Pi) prepared by electrodeposition from phosphate electrolyte and Co(NO(3))(2) have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Co-Pi catalyst films exhibit EPR signals corresponding to populations of both Co(II) and Co(IV). As the deposition voltage is increased into the region where water oxidation prevails, the population of Co(IV) rises and the population of Co(II) decreases. The changes in the redox speciation of the film can also be induced, in part, by prolonged water oxidation catalysis in the absence of additional catalyst deposition. These results provide spectroscopic evidence for the formation of Co(IV) species during water oxidation catalysis at neutral pH.

The Relationship between Hexane-Steam Reaction and Coke Formation on Silica-Supported Nickel Catalysts at High Temperatures

In order to obtain additional information regarding the relationship between hexanesteam reaction and coke formation on silica-supported nickel catalysts at high temperatures, studies have been made using a flow system under atmospheric pressure.The effects of the catalyst preparation methods on the catalyst activity. for the hexane-steam reaction and on the coke yield were examined. The activity sequence is as follows: PI>PII>>W. M. On the other hand, the coke yield on W. M catalyst is found to be much higher than on PI catalyst or PII catalyst.The hexane-steam reaction is approximately first order with respect to steam and zero order with respect to hexane. The coke formation is approximately first order with respect to hexane. The rate of coke-steam reaction is approximately proportional to the amount of coke and to the pressure of steam. The rate of formation of CO plus that of CO2 is over ten times the rate of coke formation or the rate of coke-steam reaction.From these results, it may be supposed that the surface sites on which the hexane-steam reaction occurs are not identical with those on which the coke formation occurs, and the macro reaction scheme may be suggested as: