Bulk Charge Transfer Research Articles

Synergistic effect of titanium oxide underlayer and interlayer on zirconium-doped zinc ferrite photoanode for photoelectrochemical water splitting

Here, we report the synergistic effect of dual TiO2 layers to enhance the PEC performance of Zirconium-doped zinc ferrite (ZZFO) photoanode by improving the charge carrier density and suppressing the photogenerated charge recombination. The TiO2 underlayer works as a blocking layer to remarkably suppress the back-injection of electrons from the fluorine-doped tin oxide (FTO) leading to reducing the bulk charge recombination. While interlayer TiO2 improves the bulk charge transfer property of ZZFO photoanodes. The optimal TiO2 double-layer modified ZZFO photoanode exhibits an enhanced photocurrent of 0.435 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE), which is 2.5 times higher than that of the ZZFO photoanode. The effect of each layer was deeply investigated by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and time-resolved photoluminescence studies (TRPL) with the aim of gaining a clear picture of the interface modifications and their impact on the efficiency of the ZZFO photoanode.

Green electrodeposition synthesis of NiFe-LDH/MoOx/BiVO4 for efficient photoelectrochemical water splitting

Herein, a facile green synthetic protocol for nanoporous NiFe-LDH/MoOx/BiVO4 had been established via an electrochemical deposition method for enhanced photoelectrochemical cell (PEC) performance. The rational design of nanoporous NiFe-LDH/MoOx/BiVO4 played a vital role in improving the photocurrent density and achieving 2.7 mA /cm2 at 1.23 VRHE (3.9 - fold higher than BiVO4) with a negative onset potential of 267 mV offset. Moreover, the holes were efficiently consumed for water splitting through the cyclic reaction of NiFe-LDH layer. Thus, the nanoporous NiFe-LDH/MoOx/BiVO4 photoanode dramatically improved bulk charge transfer efficiency and surface charge injection efficiency reaching nearly 50% and 95% at 1.23 VRHE, respectively. In addition, the accumulated charge test proved that Mo oxide had the function of transferring holes. And the highest photovoltage and lowest charge recombination kinetics of composite photoanode also presented that the oxide species of Mo and NiFe-LDH had the properties of a passivation layer which were characterized by OCP (Open Circuit Potential) and IMPS (Intensity Modulated Photocurrent Spectroscopy) test. The excellent photocurrent density and facile layer-by-layer synthesis of NiFe-LDH/MoOx/BiVO4 nanocomposite made it a promising photocatalytic material for practical applications. This newly designed strategy was anticipated to be applied in future promising photoanodes for PEC water splitting.

Enhanced charge transfer with tuning surface state in hematite photoanode integrated by niobium and zirconium co-doping for efficient photoelectrochemical water splitting

Niobium and zirconium co-doping was introduced into a hematite (Fe2O3) photoanode by a facile two-step synthesis. The hydrothermally prepared zirconium-doped photoanode shows a reduction in the crystallite size of hematite, with H(104) being the dominant photoactive phase. The incorporation of niobium ions by drop-casting and high-temperature annealing does not alter the crystallinity. The core 3d spin-orbit splitting shows the Nb4+ oxidation state forming NbO2 in the hematite lattice. The Nb4+–Zr4+ co-doped hematite photoanode, prepared on a fluorine-tin oxide glass substrate, shows an enhanced photocurrent density of 2.05 mA cm−2 with no co-catalyst. This enhanced performance is attributed to the Zr4+ doping, which improves the bulk charge transfer in hematite, and Nb4+ suppressed charge recombination in the surface state holes at the electrode–electrolyte interface. This synergistic improvement of bulk and surface properties leads to stable water splitting at the water oxidation potential (1.23 VRHE) of the Nb–Zr co-doped hematite photoanode.

Efficient charge transfers in hematite photoanode integrated by fluorine and zirconia co-doping for photoelectrochemical water splitting

Dual elemental doping strategies were employed in this study to fabricate fluorine and zirconia co-doped hematite (F–Zr:Fe2O3/FTO) photoanodes. Such F–Zr:Fe2O3/FTO photoanodes had a (110)-oriented structure with a remarkable photocurrent density of 1.91 mA cm−2 without any catalyst support at water oxidation potential of 1.23 VRHE. The enhancement in net photocurrent density was attributed to the synergistic effect between the two doping elements. When in situ doped Zr4+ ions were used to substitute iron ions, they increase photogenerated free electrons within the bulk hematite and resulted in lower bulk charge transfer resistance, and a minimal concentration of F was doped into hematite lattice, it would go into the oxygen site rather than the iron site, resulting in more positive charges on iron site. The charge compensation contributed to reduced recombination of free electron–hole pairs and improved surface charge injection efficiency at the semiconductor–electrolyte interface by promoting hole transfer via surface states.

Asymmetric charge carrier transfer and transport in planar lead halide perovskite solar cells

Understanding charge carrier extraction from the perovskite photoactive layer is critical to optimizing the design of perovskite solar cells. Herein, we demonstrate a simple time-resolved photoluminescence method to characterize the kinetics of charge transport across the bulk perovskite and charge transfer from the perovskite layer to the interlayers, elucidating the dependence of these dynamics on film thickness, grain boundaries (GBs), and interlayers. Using asymmetric laser excitation, we selectively probe charge transport by generating charges both close to and far from the heterojunction interface and correlate these results with device performance. We observe that both film thickness and GBs affect the asymmetry between electron and hole charge transport across the bulk perovskite and charge transfer from the bulk perovskite to the respective interlayers. • Time-resolved photoluminescence with spatially localized excitation • Asymmetries of charge carrier transfer and transport are quantified • Lateral grain boundaries result in unbalanced charge transport • Aerosol-assisted solvent treatment removes lateral grain boundaries effectively Xu et al. demonstrate a simple time-resolved photoluminescence method to selectively probe charge transfer and charge transport in perovskite films with interlayers. The results show both film thickness and grain boundaries affect the asymmetry between electron and hole charge transfer and charge transport.

Analysis of Pseudo-Homogeneous and Bulk Charge Transfer in Dye-Sensitized Solar Cells

Analysis of Pseudo-Homogeneous and Bulk Charge Transfer in Dye-Sensitized Solar Cells

Molecular Engineering of Photocathodes based on Polythiophene Organic Semiconductors for Photoelectrochemical Hydrogen Generation.

Organic semiconductors provide significant potentials for the construction of photoelectrochemical (PEC) cells for solar hydrogen production because of their highly tunable properties. Herein, on carbon fiber paper (CFP) surface, pyridyl (Py), and 4,4'-bipyridin-1-ium (Py2+) groups were introduced into polythiophene (pTH) semiconductor by electrochemical copolymerization, respectively. After assembly with the Co(dmgBF2)2 type catalyst (CoB, dmgBF2 = difluoroboryldimethylglyoximate), the CoB@Py2+-pTH/CFP photocathode displayed nearly twice the photocurrent enhancement (550 μA cm-2 at 0.15 V vs reversible hydrogen electrode, RHE) comparing to that generated by the CoB@Py-pTH/CFP photocathode (290 μA cm-2 at 0.15 V vs RHE) for light-driven H2 generation under AM 1.5 solar illumination. Investigation of the mechanism revealed that the introduction of the positively charged pyridinium groups could improve the intrinsic Co(dmgBF2)2 catalyst activity for the H2 generation reaction. Meanwhile, the positively charged pyridinium groups serve as p-type dopants to increase the semiconductor bulk charge transfer rate and act as electron transfer mediators to promote the interfacial charge transfer kinetics between the catalyst and the pTH-based organic semiconductor.

Improving photoelectrochemical water oxidation activity of BiFeO3 photoanode via surface passivation

Bismuth iron oxide (BiFeO3) has attracted widespread attention in the field of photoelectrochemistry due to its special ferroelectric properties. However, its high charge recombination from defects and sluggish water oxidation kinetics limit the potential application. Herein, oxygen vacancies are first introduced in BiFeO3 thin films by Ar annealing to improve the bulk charge transfer efficiency. Subsequently, a p-type conductive poly(3, 4-ethylenedioxythiophene) (PEDOT) is employed to suppress the surface-defect-related recombination of photogenerated carriers and to form a p–n junction to increase the charge extraction efficiency. As a result, the photocurrent density of the optimized NiCoFe-Bi/PEDOT/BiFeO3 photoanode reaches 0.17 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) under simulated 1 sun irradiation for water oxidation, which is enhanced by six times compared with that of the photoanode without PEDOT modification. The photocurrent density is further improved to 0.23 mA cm−2 at 1.23 V vs RHE through polarization of the ferroelectric material. This study strongly suggests that surface passivation is an effective way to improve the activity of the BiFeO3 photoanode for solar water splitting.

Communication—Anode-Free Lithium Metal Batteries: A Case Study of Compression Effects on Coin Cell Performance

Here, we demonstrate that altering the cell configuration can increase the compression level inside a coin cell, resulting in superior charge and discharge cycle performance. A normal anode-free coin cell’s capacity drops to zero within 20 cycles, while the more compressed cell retains 150 mAh g−1 at the 20th cycle, which is 87.3% of the first cycle discharge capacity. EIS shows that the compressed cell possesses lower bulk resistance and charge transfer resistance through SEI. It also leads to homogeneous Li deposition free of dendrites. Our finding enables a more accurate assessment of anode-free Li-metal batteries in a coin cell.

Three-dimensional nanoporous SnO2/CdS heterojunction for high-performance photoelectrochemical water splitting

The porous structure is one of the effective structures to improve the photoelectrochemical (PEC) performance due to its high surface area to volume ratio. In this study, a porous carbon black (CB) film was prepared by a simple and inexpensive flame method, which was used as a template to grow three-dimensional (3D) nanoporous SnO2 through the atomic layer deposition (ALD) method, followed by CdS thin film coating. The photocurrent density of the optimized 3D nanoporous SnO2/CdS heterojunction (11.56 mA/cm2, 1.23 V vs. RHE) shows 9.7 times higher than that of planar SnO2/CdS film (1.20 mA/cm2, 1.23 V vs. RHE), meanwhile, the photocurrent stability was also 2.2 times improved. The maximum incident photon-to-current conversion efficiency (IPCE) and applied bias photo-to-current efficiency (ABPE) values of 35% (around 490 nm at 0 V vs. RHE) and 1.85% (at 0.90 V vs. RHE) were observed for the nanoporous SnO2/CdS. Furthermore, the surface charge separation and bulk charge transfer efficiencies have been achieved by 45.4% and 98.8% (1.23 V vs. RHE). As a whole, the results indicated that the 3D nanoporous SnO2/CdS heterojunction has been demonstrated as an efficient strategy that has been realized to show high performance in PEC water splitting.

Unravelling the bulk and interfacial charge transfer effects of molybdenum doping in BiVO4 photoanodes

The role of Mo6+ doping on the photoelectrochemical (PEC) performance of BiVO4 photoanodes was investigated both in the presence and in the absence of sulfite as hole scavenger. Optically transparent, flat BiVO4 photoanodes containing different amounts of Mo6+ dopant were synthesized by spin coating. An increase of Mo6+ dopant amount was found to both improve the electron transport in the BiVO4 bulk by increasing its conductivity, as unequivocally ascertained when employing a Ni/Fe oxyhydroxide co-catalyst, and facilitate the charge transfer at the electrode/electrolyte interface in water oxidation, in the absence of hole scavenger. On the other hand, increasing amounts of the Mo6+ dopant in BiVO4 induced an unexpected decrease in PEC performance per unit surface area in sulfite oxidation, resulting from enhanced interfacial charge transfer resistance, as demonstrated by electrochemical impedance spectroscopy. First evidence is thus provided of a different behaviour observed upon Mo6+ doping of BiVO4 depending on the nature of the involved electron donor species, together with an intriguing multifaceted role played by Mo6+ doping in enhancing the PEC performance of modified BiVO4 electrodes.

Boosting the Photoelectrochemical Performance of BiVO4 Photoanodes by Modulating Bulk and Interfacial Charge Transfer

BiVO4 is an attractive candidate that is used as a photoanode for solar water splitting. Unfortunately, its photoelectrochemical (PEC) performance is restricted by the slow charge transfer ability in the bulk and the interface. In this study, we accelerated bulk charge transfer via introducing oxygen vacancies by Ar plasma treatment and facilitated interfacial carrier transfer by loading a WCoFe oxyhydroxide electrocatalyst. A larger enhancement was achieved compared to the improvement results from the individual modification, and a photocurrent of 5.49 mA/cm2 was obtained under light irradiation in a potassium phosphate (KPi) buffer solution. The results demonstrate that the PEC performance of the BiVO4-based film photoanode is enhanced by integrating synergy effects of favorable factors by introducing oxygen vacancies and loading electrocatalysts.

Insight into bulk charge transfer of lithium metal anodes by synergism of nickel seeding and LiF-Li3N-Li2S co-doped interphase

Cycling lithium (Li) metal anode with a low negative/positive capacity (N/P) ratio is critical to build high-energy-density Li metal batteries (LMBs). Many strategies focusing on interfacial protection are limited to shallow conditions using excessive Li with a high negative/positive capacity (N/P) ratio > 50. Herein, we demonstrate a deeply enhanced charge transfer kinetic by a rational incorporation of nickel (Ni) seeds in the Li anodes with a tailored interfacial chemistry. The Ni seeds in the bulk Li anode not only act as well-dispersed nucleation sites to enable Ni-seeded growth of granular Li deposits, but also adsorb acetonitrile (AN) molecules when the Ni-seeded Li anodes are paired with an AN-based electrolyte using lithium bis-(fluorosulfonyl)imide as salt, forming a fast Li+ conducting interphase with co-produced LiF, Li3N and Li2S components. These benefits conduct an exchange current density ~ 20 times higher than that of the bare Li in conventional carbonate electrolytes. Stable cycling of LMBs with a practical cathode loading (~ 5 mAh cm−2) and a low N/P capacity ratio of 2 is also demonstrated, showing the prominence of this remedy for developing high-energy-density LMBs.

Highly efficient photocatalytic H2 evolution over NiCo2S4/Mn0.5Cd0.5S: Bulk twinned homojunctions and interfacial heterojunctions

A twinned Mn0.5Cd0.5S (T-MCS) homojunction, consisting of wurtzite and zinc-blende Mn0.5Cd0.5S with different energy band structures, was fabricated using a facile hydrothermal method, resulting in the formation of a type-II bulk phase twinned homojunction. Furthermore, NiCo2S4 nanoparticles were deposited on the surface of T-MCS to form a surface heterojunction. The activities of T-MCS and NiCo2S4/T-MCS were tested in the photocatalytic H2 evolution reaction. T-MCS exhibits a superior H2 evolution rate of 61.4 mmol∙g−1∙h−1 under visible light (λ > 420 nm) irradiation owing to faster bulk phase charge separation, which is 8.2 and 1.9 times higher than those of wurtzite and zinc-blende Mn0.5Cd0.5S, respectively. Moreover, NiCo2S4 can facilitate interfacial electron transfer and can lower the H2 evolution overpotential; the H2 evolution rate is boosted to 127.3 mmol∙ g−1∙h−1 with an apparent quantum yield (AQY) of 23.4% with irradiation of 2 wt%-NiCo2S4/T-MCS under 400 ± 7.5 nm light. This work demonstrates that bulk phase twinned homojunctions and a surface heterojunction can combine to promote bulk and interfacial charge transfer and separation, simultaneously improving the kinetics of photocatalytic H2 evolution.

Effect of oxygen concentration and distribution on holes transfer and photoelectrocatalytic properties in hematite

The holes transfer efficiency is a key issue in the process of photoelectrocatalytic water splitting. Here, we found that the oxygen concentration and distribution in hematite evidently affected the bulk and surface holes transfer, and thus influenced the performance of photoelectrocatalytic water splitting. Two macro-changes on oxygen were observed during the thermal treatment. The first-stage led to a lower bulk O/Fe ratio of only 1/1 and the generation of adsorbed oxygen, obtaining porous nanorods (NRs) with a ~1.4 nm FeOx layer. Bulk charge recombination centers and surface deep traps caused a lower bulk and surface charge transfer. The second-stage led to a bulk O/Fe ratio of nearly 3/2 and single high crystallization NRs. Major increases in charge transfer constant from 0.27 to 4.5 s−1 and in photocurrent density (vs.1.23 VRHE) from 0.0235 to 1.20 mA/cm2 were observed, which demonstrates the higher holes transfer efficiency. Furthermore, the connection of oxygen migration and charge transfer was derived. This work could provide effective guidance for further optimization of semiconductor photoanodes.

Ion Solvation Engineering: How to Manipulate the Multiplicity of the Coordination Environment of Multivalent Ions.

Free energy analysis of solvation structures of free divalent cations, their ion pairs, and neutral aggregates in low dielectric solvents reveals the multiplicity of thermodynamically stable cation solvation configurations and identifies the micro- and macroscopic factors responsible for this phenomenon. Specifically, we show the role of ion-solvent interactions and solvent mixtures in determining the cation solvation free energy landscapes. We show that it is the entropic contribution of solvent degrees of freedom that is responsible for the solvation multiplicity, and the mutual balance between enthalpic and entropic forces or their concerted contributions is what ultimately defines the most stable ion solvation configuration and creates new ones. We show general consequences of ion solvation multiplicity on thermodynamics of complex electrolytes, specifically in the context of homogeneous or interfacial charge transfer. Identified factors and their interplay provide a pathway to formulation of solvation design rules that can be used to control bulk solvation, interfacial chemistry, and charge transfer. Our findings also suggest experimentally testable predictions.

Gap width modification on fully screen-printed coplanar Zn|MnO2 batteries

Fully printed primary zinc-manganese dioxide (Zn|MnO2) batteries in coplanar configuration were fabricated by sequential screen printing. While electrode dimensions and transferred active masses were kept at constant levels, electrode separating gaps were incrementally enlarged from 1 mm to 5 mm. Calendering of solely zinc anodes increased interparticle contact of active material within the electrodes while the porosity of manganese dioxide based electrodes was maintained by non-calendering. Chronopotentiometry revealed areal capacities for coplanar batteries up to 2.8 mAh cm−2. Galvanostatic electrochemical impedance spectroscopy measurements and short circuit measurements were used to comprehensively characterise the effect of gap width extension on bulk electrolyte resistance and charge transfer resistance values. Linear relationships between nominal gap widths, short circuit currents and internal resistances were evidenced, but showed only minor impact on actual discharge capacities. The findings contradict previous assumptions to minimise gap widths of printed coplanar batteries to a sub-millimetre range in order to retain useful discharge capacities. The results presented in this study may facilitate process transfer of printed batteries to an industrial environment.

Phase boundary propagation kinetics predominately limit the rate capability of NASICON-type Na3+xMnxV2-x(PO4)3 (0≤x≤1) materials

NASICON-type Na3V2(PO4)3 cathode materials can be regarded as promising candidates for high-power Na-ion batteries due to the observed facile kinetics of Na-ion de/intercalation. Substitution of V for Mn provides additional advantages related to the increase in the average operating potential and reduced cost of the active material. In this work, we explore the kinetics of Na+ intercalation into Mn-substituted Na3+xMnxV2-x(PO4)3 (x = 0, 0.1, 0.5, 1) materials with a primary focus on the impact of Mn content on the rate capability of the materials. We demonstrate that Mn substitution results in quite subtle changes in bulk ionic diffusivity and charge transfer rates, while more significant impact is observed on the nucleation kinetics, which induces large hysteresis between charge and discharge curves for Mn-rich materials. The increase in hysteresis between charge and discharge curves does not limit the specific energy retention at high C-rates significantly, yet the performance losses are mainly related to the slow phase boundary propagation for biphasic processes. The Mn-rich materials, which demonstrate wider single-phase regions, are shown to outperform the unsubstituted materials in terms of rate-capability and should be preferred for high-power applications.

Preparation of (Ti, Zr) co-doped hematite photoanode for enhanced photoelectrochemical water splitting

Hematite (α-Fe2O3) is one of the promising photoanode materials for photoelectrochemical (PEC) water splitting, but its poor bulk charge transfer has been challenging towards its practical application. Herein, we have synthesized the (Ti, Zr) co-doped hematite by a facile hydrothermal method. The cooperation of (Ti, Zr) co-doped hematite yields photocurrent density of 1.51 mA cm−2 at 1.23 V vs. RHE and IPCE of 17.3% at 400 nm. The structural and PEC analysis indicate that the considerably PEC performance improvement is contributed to the higher carrier density and enhanced charge transfer of the synergistic effect of Ti and Zr co-doping.

Unraveling the role of Ti3C2 MXene underlayer for enhanced photoelectrochemical water oxidation of hematite photoanodes

Hematite is regarded as a promising photoanode for photoelectrochemical (PEC) water splitting. However, the charge recombination occurred at the interface of FTO/hematite strictly limits the PEC performance of hematite. Herein, we reported a Ti3C2 MXene underlayer modified hematite (Ti–Fe2O3) photoanode via a simple drop-casting followed by hydrothermal and annealing processes. Owing to the bifunctional role of Ti3C2 MXene underlayer in improving the interfacial properties of FTO/hematite and providing Ti source for the construction of Fe2TiO5/Fe2O3 heterostructure in hematite nanostructure, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently enhancing the PEC performance. Compared with the pristine hematite, the as-prepared Ti–Fe2O3 photoanode shows an increased photocurrent density from 0.80 mA/cm2 to 1.30 mA/cm2 at 1.23 V vs. RHE. Moreover, a further promoted PEC performance including a dramatically increased photocurrent density of 2.49 mA/cm2 at 1.23 V vs. RHE and an obviously lowered onset potential is achieved for the Ti–Fe2O3 sample after the subsequent surface F-treatment and the loading of FeNiOOH cocatalyst. Such results suggest that the introduction of Ti3C2 MXene underlayer is a facile but effective approach to improve the PEC water splitting activity of hematite.