Reversible Hydrogen Electrode Potential Research Articles

Heterojunction engineering decorated TiO2/ZnO three‐dimensional hierarchical structure with g‐C3N4 for solar driving water splitting

AbstractImproving the carrier separation efficiency plays a decisive role in designing and constructing a high‐efficiency photocatalysis reaction system. Derived from providing a directional transport channel for photogenerated carriers, three‐dimensional (3D) nanostructures greatly improve the charge separation efficiency. Herein, TiO2/ZnO (TZ)/g‐C3N4 3D hierarchical nanostructure was constructed to artificially simulate photosynthesis of green plants. The optimal TZ/g‐C3N4 photoanode exhibits a photocurrent density of 1.46 mA/cm2 at 1.23 V versus reversible hydrogen electrode potential, 1.6 times that of pure TiO2 (0.9 mA/cm2). Moreover, under constant illumination (100 mW/cm2), the hydrogen production reached 80 μmol/cm2 within 180 min. It is worth noting that the TZ/g‐C3N4 photoanode shows surprising stability, which is an important indicator for the practical application of the photoelectrode. The excellent photoelectrochemical performance benefits from the following two aspects: TZ nanotree structure provides a directional transport channel for photogenerated carriers, and the modification of TZ by g‐C3N4 extends the response range to the visible region.

Electrochemical condition optimization and techno-economic analysis on the direct CO2 electroreduction of flue gas

Direct CO2 electroreduction of flue gas is a technology that simplifies CO2 capture and purification process and directly utilizes CO2 in flue gas. Low CO2 reduction reaction selectivity caused by the low CO2 partial pressure and side reactions competition is the challenge. In this study, a direct CO2 electroreduction system for multicomponent flue gases was established. A two-dimensional steady model was developed by considering major electrode reactions of flue gas in CuxO-based catalysts. General influence criteria and optimum operation window for the potential, temperature, and pressure were proposed. In addition, a techno-economic analysis of the flue gas electrolysis system was conducted. The results demonstrated that the CO2 electroreduction reaction performances are weak at atmospheric pressure owing to the side reaction competition and small reactant concentration. A method involving pressurization and catholyte inlet temperature declination is proposed to break the stalemate of tiny reactant concentrations and ensure the dominance of the CO2 electroreduction reaction. The selectivity and current density of the CO2 reduction reaction were 71% and − 148 mA/cm2 at −1.17 V vs. reversible hydrogen electrode potential (RHE), respectively, under the conditions of an inlet electrolyte temperature of 273 K and an operating pressure of 20 atm. C1 products with excellent yields are the dominant products. Furthermore, the techno-economic analysis showed that the flue gas electrolysis system has higher profitability and lower cost than the purified CO2 electrolysis system. The present results can be used to optimize the electrolyzer operating parameters and construct CO2 reduction systems.

Design and Characterization of a Close Compartment Hydrogen Reference Electrode for Electrocatalytic Studies

The reference electrode plays an important role in the determination of the electrocatalytic activity of a given electrode material. The hydrogen reference electrode has several advantages, as it is stable, can be used in the whole range of pH values, and does not contaminate the system. However, its use is not widespread due to certain difficulties related to its construction and operation. In this context, we report the design of a reversible hydrogen electrode (RHE) characterized by its simplicity, reproducibility, portability, stability, and precision. Moreover, rigorous equations for the variation of the RHE potential on the solution concentration, temperature, and pressure were derived as well as for the conversion of a working electrode potential referred to RHE to the standard hydrogen electrode (SHE).

Doping with Rare Earth Elements and Loading Cocatalysts to Improve the Solar Water Splitting Performance of BiVO4

BiVO4 is a highly promising material for Photoelectrochemical (PEC) water splitting photoanodes due to its narrow band gap value (~2.4 eV) and its ability to efficiently absorb visible light. However, the short hole migration distance, severe surface complexation, and low carrier separation efficiency limit its application. Therefore, in this paper, BiVO4 was modified by loading CoOOH cocatalyst on the rare earth element Nd-doped BiVO4 (Nd-BiVO4) photoanode. The physical characterization and electrochemical test results showed that Nd doping will cause lattice distortion of BiVO4 and introduce impurity energy levels to capture electrons to increase carrier concentration, thereby improving carrier separation efficiency. Further loading of surface CoOOH cocatalyst can accelerate charge separation and inhibit electron–hole recombination. Ultimately, the prepared target photoanode (CoOOH-Nd-BiVO4) exhibits an excellent photocurrent density (2.4 mAcm−2) at 1.23 V versus reversible hydrogen electrode potential (vs. RHE), which is 2.67 times higher than that of pure BiVO4 (0.9 mA cm−2), and the onset potential is negatively shifted by 214 mV. The formation of the internal energy states of rare earth metal elements can reduce the photoexcited electron–hole pair recombination, so as to achieve efficient photochemical water decomposition ability. CoOOH is an efficient and suitable oxygen evolution cocatalyst (OEC), and OEC decoration of BiVO4 surface is of great significance for inhibiting surface charge recombination. This work provides a new strategy for achieving effective PEC water oxidation of BiVO4.

Adaptive Electric Fields Embedded Electrochemical Barrier Calculations.

Electrochemical interfaces are grand canonical ensembles of varying electrons. Simulating them by standard first-principles methods is a challenging task, since the number of electrons (or charge) is fixed in the calculation. Under the constant charge framework, we developed a constant potential simulation method realized by adding an adaptive electric field to a charge neutral system. Electric field is the controlling variable. In addition, we defined an internal reversible hydrogen electrode potential (ϕIRHE), which can ensure the model independence of our method. To validate our method, the reaction energies of some electrochemical reactions are calculated, the results are comparable with the computational hydrogen electrode model and experiments. At last, the evolution of transition state structures and charge transfer coefficients of some electrochemical reactions on Ag(111) surface were discussed by our method.

Monopolar membrane-assisted acid-alkaline amphoteric water electrolysis towards efficient hydrogen generation

Monopolar membrane-assisted electrolyzers enable water electrolysis using acid-alkali asymmetric electrolytes. However, understanding how such an electrolyzer works remains a significant challenge. By assessing the concentration-polarization state in membranes, measuring the ion concentration change in electrolytes, and determining the corresponding transmembrane resistance, we reveal that this electrolyzer can prevent the negative effect of the water dissociation process. The electrolyzer functions by the chemical potential gradient between the asymmetric electrolytes. Briefly, the delta in pH between asymmetric electrolytes significantly modifies the reversible hydrogen electrode potential at both electrolyte compartments and electrodes, and therefore decreases the required external potential. Notably, the unavoidable ion diffusion slightly reduces this positive effect. The electrolyzer performance depends on the membrane property, working temperature, electrolyte compositions as well as electrocatalysts. When adopted with state-of-the-art electrocatalysts, this electrolyzer achieves an industrially relevant current density of 200 mA cm−2 at a cell voltage of only 1.39 V, outperforming most conventional water electrolyzers, and to the best of our knowledge also those fed by asymmetric electrolytes. Overall, this work highlights the promise of coupling chemical potential energy and electrical energy for hydrogen production, which provides a new strategy to lower the potential for driving water splitting.

Investigation of IrO2 Stability As a Cell-Reversal Mitigation Catalyst in PEMFC Anodes

In the recent years, the use of fuel cells for the propulsion of passenger vehicles and heavy-duty vehicles has gained renewed interest as alternative to internal combustion engine-powered vehicles. Particularly for the latter, the long-term durability of proton-exchange-membrane fuel cells (PEMFCs) is still a concern, which in part is governed by the catalyst layer durability during system operating conditions.1 During the operation of a PEM fuel cell system, certain transient conditions can occur that can lead to rapid degradation of the catalyst layers. On one hand, the widely studied start-up/shut-down (SUSD) phenomenon leads to significant degradation of the cathode catalyst layer due to the exposure of the cathode electrode to high potentials (>>1.0 V vs the reversible hydrogen electrode potential (VRHE)) that results from a H2/air-front passing through the anode flow field.2 On the other hand, an intermittent under-supply of hydrogen to one or several cells of a PEMFC stack can lead to a so-called cell-reversal (CR) event, resulting in the oxidation of the carbon support of the anode catalyst and in the subsequent collapse of the anode catalyst layer structure. The latter can be mitigated by the addition of an anode co-catalyst that facilitates the oxygen evolution reaction (OER) at much lower potentials than the carbon-supported platinum catalyst (Pt/C) that serves as anode catalyst for the hydrogen oxidation reaction (HOR).3 However, as shown by our recent study, the stability of the anode co-catalyst for the OER during SUSD events must be considered.4 In the present study, we examine different accelerated stress tests (ASTs) for single-cell PEMFCs that are designed to assess the stability of an anode co-catalyst for the OER in a PEMFC that is subjected to cell-reversal and SUSD events. Thus, we evaluated the stability of an anode co-catalyst under four conditions: i) subjecting the cell to extended SUSD cycles; ii) a cell-reversal protocol where a constant current is being drawn when feeding N2/air to the anode/cathode until a given cut-off cell potential is reached; iii) a protocol where cell-reversal cycling is induced by switching the anode feed gas between hydrogen and nitrogen while applying a constant current and supplying air to the cathode (later on referred as CR cycling); and, iv) an anode potential cycling protocol where the anode is supplied with N2, and its potential is cycled potentiostatically to different upper potential limits, with the cathode purged with air and serving as a counter electrode. With these ASTs, we examine the effect of the different anode potential cycles during SUSD and CR transients in order to elucidate the dependence of the IrO2 dissolution rate on operating potential.The stability of the IrO2 based anode co-catalyst is investigated by electrochemical measurements (cyclic voltammetry and polarization curves) and additionally via ex-situ XPS analysis. For example, Figure 1 shows the ex-situ XPS analysis of pristine and aged membrane electrode assemblies (MEAs). The upper two panels show the XPS data in the Pt 4f (BE>67 eV) and the Ir 4f (BE<67 eV) regions of the pristine Pt/C anode with an IrO2-based anode co-catalyst (Figure 1a) as well as the pristine Pt/C cathode (Figure 1b). The lower two panels show the cathode XPS data of the MEAs after 500 SUSD cycles (Figure 1c) and after 500 CR cycles (Figure 1d). In the case of the SUSD degraded MEA, a higher amount of Ir is observed on the cathode when compared to the CR degraded MEA. The mechanistic implications of these data will be discussed. References USDRIVE Fuel Cell Technical Team Roadmap (2017): https://www.energy.gov/eere/vehicles/downloads/us-drive-fuel-cell-technical-team-roadmap (accessed on 28.10.2021).T. Mittermeier, A. Weiß , F.Hasché, G. Hübner, H. A. Gasteiger. Journal of The Electrochemical Society, 164 (2), F127-F137 (2016).T. R. Ralph, S. Hudson, and D. P. Wilkinson, ECS Transactions, 1 (8), 67-84 (2006).M. Fathi Tovini, A. M. Damjanović, H. A. El-Sayed, J. Speder, C. Eickes, J.-P. Suchsland, A. Ghielmi, and H. A. Gasteiger, Journal of The Electrochemical Society, 168 (6), 064521 (2021). Figure 1

Irreducible IrO2 Anode Co-Catalysts for PEM Fuel Cell Voltage Reversal Mitigation and Their Stability Under Transient Operation Conditions

The cell voltage reversal that can occur during the transient operation of a proton exchange membrane fuel cell (PEMFC) stack leads to a substantial degradation of the anode catalyst. During cell reversal, the anode potential increases (>>1 V vs. the reversible hydrogen electrode potential (RHE)), causing severe oxidation of the anode catalyst carbon support, which leads to a collapse of the anode catalyst layer and to cell failure. One strategy to mitigate the damages of H2 starvation is the addition of a co-catalyst to the anode electrode, which catalyzes the oxygen evolution reaction (OER), so that the non-damaging OER rather than the damaging carbon oxidation reaction (COR) takes place. A commonly used anode co-catalyst to favor the OER over the COR during cell reversal events is iridium oxide (IrO2).1 Recent findings show that the near-surface layer(s) of IrO2 can be completely reduced to metallic Ir upon exposure to H2, e.g., under the operating conditions of a PEMFC anode.2, 3 Such alteration of the near-surface layer(s) of IrO2 drastically affects its stability during the anode potential transients that occur during start-up/shut-down (SUSD) events, where our recent study showed that the dissolution of metallic Ir and crossover of the dissolved Irn+ species through the membrane to the cathode electrode cause iridium deposition on the Pt/C cathode catalyst.3 Such iridium-based contamination on the cathode catalyst surface deteriorates the oxygen reduction reaction (ORR) activity of Pt and results in a significant performance loss during the normal operation of the fuel cell. At the same time, SUSD transients also cause an OER activity loss of the anode co-catalyst, which was shown to be mainly due to the redeposition of Pt dissolved from the anode hydrogen oxidation reaction (HOR) catalyst onto the reduced IrO2 anode co-catalyst, blocking its OER active sites.3 Since these degradation mechanisms are caused by the chemical reduction of the typically employed IrO2 anode co-catalysts in the PEMFC anode, a reduction-resistant OER catalyst would be required.In this contribution, we introduce an unprecedented approach to synthesize IrO2 catalysts that are not reduced in the PEMFC anode environment. The preparation of such irreducible IrO2 (Irr-IrO2) catalysts is based on an industrially scalable procedure consisting of a high-temperature (650-1000 ᵒC) heat treatment step followed by a deagglomeration step and a post annealing step to prepare catalyst powders with specific surface areas of ~25 m2/g. Figure 1 shows the thermogravimetric analysis (TGA) according to our previously proposed evaluation protocol3 under 3.3 vol.% H2/Ar of an as-received commercial benchmark catalyst (IrO2/TiO2 (Umicore)), a self-made IrO2 catalyst heat-treated near the conventional temperature of 500 °C (IrO2-500 °C), and a stabilized IrO2 catalyst prepared by a procedure introduced in this contribution (Irr-IrO2). It can be seen that the temperature that corresponds to the reduction of 20% of the IrO2 phase (α = 20%) of the Irr-IrO2 catalyst is ~38 °C and ~53 °C higher than those of the IrO2-500 °C and the IrO2/TiO2 (Umicore) catalysts, respectively. Considering the fact that all of these catalysts have a specific surface area of ~25 m2/g, the observed reductive stability improvement of Irr-IrO2 catalyst is due to its intrinsic structural distinctions from the typically synthesized IrO2 and the commercial benchmark IrO2 catalysts. As will be shown, this higher stability is reflected by the observation that SUSD cycling of MEAs with the Irr-IrO2 as anode co-catalyst does not result in iridium dissolution and the associated iridium poisoning of the ORR activity of the cathode catalyst. Furthermore, while its OER activity is lower than that of conventional IrO2 catalysts, it still dramatically increases the cell reversal tolerance time of a conventional Pt/C anode. References T. R. Ralph, S. Hudson, and D. P. Wilkinson, ECS Transactions, 1 (8), 67-84 (2006).P. J. Rheinländer and J. Durst, Journal of the Electrochemical Society, 168 (2), 024511 (2021).M. Fathi Tovini, A. M. Damjanovic, H. A. El-Sayed, J. Speder, C. Eickes, J.-P. Suchsland, A. Ghielmi, and H. A. Gasteiger, Journal of The Electrochemical Society, 168 (6), 064521 (2021). Figure 1. TGA (5 K min-1) experiments under 3.3 vol.% H2/Ar with the as-received IrO2/TiO2 (Umicore), the IrO2-500 °C, and the Irr-IrO2 catalyst powder samples (following a drying/cleaning step; see Ref. 3 for further details). The y-axis represents the fraction of the IrO2 phase in the catalyst powder samples that is reduced to metallic Ir (α). The black horizontal dashed line illustrates a reaction extent of α= 20%, to compare the stability of different samples in a H2-containing atmosphere. Figure 1

Numerical Simulation and Modeling of Hydrogen Gas Evolution on Planar and Microwire Array Electrodes

The impact of gas evolution on the electrochemical characteristics of planar electrodes and microwire array electrodes has been analyzed using modeling and simulation. The impacts can mainly be broken into three phenomena: a) a shift in the local reversible hydrogen electrode potential; b) hyperpolarization; and c) an increase in the solution resistance of the electrolyte. The local reversible hydrogen electrode potential shift was found to play the most important role, constituting >40% of the total potential drop between the cathode and reference electrode, following correction for cell resistance. Compared to planar electrodes, a microwire array structure reduces the impact of bubbles on the solution conductance, but the shift in the local reversible hydrogen electrode potential varies with distance from the actual electrode surface.

Degradation Mechanism of an IrO2 Anode Co-Catalyst for Cell Voltage Reversal Mitigation Under Transient Operation Conditions of a PEM Fuel Cell

The cell voltage reversal which can occur during the transient operation of PEM (proton exchange membrane) fuel cells on account of a short under-supply of hydrogen to one or several cells in a fuel cell stack can lead to the substantial degradation of anode catalysts and anode electrodes. This damage can be substantial even if the cell reversal only occurs over time scales of tens of milliseconds, which puts a high requirement on the fuel cell controls system. During cell reversal, the anode potential rises to far above 1 V vs. the reversible hydrogen electrode potential (RHE), causing platinum dissolution and oxidation of the carbon support; the latter leads to a collapse of the anode catalyst layer and to cell failure. One strategy to mitigate the damages of H2-starvation is the addition of a co-catalyst to the anode electrode which catalyzes the oxygen evolution reaction (OER), so that the non-damaging OER rather than the damaging carbon oxidation reaction (COR) will take place. A commonly used anode co-catalyst to facilitate the OER over the COR during cell reversal events is iridium oxide (IrO2).1 Many studies have investigated the effect of IrO2 co-catalyst on mitigating cell reversal damages in PEMFC anodes.2 However, there is no comprehensive study on the effect of the strongly reducing hydrogen atmosphere in a PEMFC anode on the chemical stability of IrO2, which could ultimately affect the stability of an IrO2 anode co-catalyst particularly during the repetitive transitions between normal operating conditions where the anode potential is at ≈0 V vs. RHE and the high anode potentials (≥1 V vs. RHE) during cell voltage reversal and start-up/shut-down (SUSD) transients. That these voltage cycles might be damaging is indicated in a recent publication from our group, where we had shown that H2 permeating through the membrane from the hydrogen electrode to the oxygen electrode of a PEM water electrolyzer during open circuit conditions voltage (OCV) leads to a reduction of the surface of IrO2 OER catalyst to metallic Ir, which in turn gets oxidized to an amorphous IrOx during the subsequent normal operation period of the electrolyzer at the high oxygen evolution potentials. The repetitive cycling between reducing conditions during OCV periods and oxidizing conditions during operation were shown to cause the enhanced dissolution of Ir, which could result in a loss of active material.3 In this contribution, we will discuss the consequences of using an IrO2 based anode co-catalyst to mitigate cell voltage reversal degradation in PEMFCs. Thermal gravimetric analysis (TGA) in reductive atmosphere (5 volume% H2/Ar) is a powerful tool to simulate the chemical environment of PEMFC anode during normal operation. By utilizing this tool, it is shown that the near-surface region of IrO2 can be completely reduced to metallic Ir under the operating condition of PEMFC anode. The formation of metallic Ir has been proven by post characterization of the reduced powders after the TGA experiments using X-ray photoelectron spectroscopy (XPS) and by cyclic voltammetry, where the development of hydrogen under potential deposition (HUPD) features of metallic Ir reveals the time-dependent reduction of IrO2 upon extended exposure to PEMFC anode conditions. Finally, it will be shown that anode potential transients caused, e.g., by SUSD events, leads to the dissolution of iridium from the IrO2 based anode co-catalyst. The consequences of this on the long-term PEMFC performance will be discussed.

Tracking Electrical Fields at the Pt/H2O Interface during Hydrogen Catalysis.

We quantify changes in the magnitude of the interfacial electric field under the conditions of H2/H+ catalysis at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed nonfaradaic reaction, H2 addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H+ at a Pt surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H+ concentration across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the Pt/solution interface and establish that it increases by ∼60 mV per unit increase in pH. These results provide direct insight into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs acidic conditions.

In-Situ Electrode Potential Measurements in Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) cells typically consist of a proton-conducting membrane sandwiched by two porous carbon-fiber based electrodes. The V2+/V3+ and V4+/V5+ redox couples, dissolved in acidic electrolytes, flow through the negative and positive electrodes, respectively. And, the primary reactions during charge and discharge are the redox reactions of these two vanadium couples. The round-trip efficiency (RTE) of a VRFB cell is a complex combination of multiple factors, including the electrodes, membrane, electrolytes, and cell-design configuration, as well as the operating conditions, i.e. charge/discharge current, range of the electrolyte state-of-charge (SOC) and electrolyte flow rates.1,2 The ohmic losses in the membrane and overpotentials on the two electrodes are two major factors that contribute to the cell’s efficiency losses. Undesired side reactions, including hydrogen evolution on the negative electrode and carbon corrosion on the positive electrode, may also occur, especially when the cell is overcharged, which may lead to a change in the electrolyte average vanadium oxidation state and degradation of the carbon electrodes and bipolar plates.3-5 Since both the vanadium redox reactions and these side reactions are strongly potential driven, it is useful to have a reliable reference electrode integrated into the VRFB system so that the electrode potentials vs. reversible hydrogen electrode (RHE) can be measured under in-situ conditions. The use of reference electrodes, such as the dynamic hydrogen electrode, have been reported in VRFB cells to measure in-situ electrode potentials.6,7 However, a significant challenge is the vanadium ion contamination that shifts the RHE potential higher, up to ~ 0.3V by 1.6M vanadium electrolyte. Here we describe a specially designed reference-electrode apparatus (REA), which is depicted in Fig.1 and consists of a small VRFB cell, a RHE, and a continuously acid-washed region between the VRFB cell and the RHE to prevent vanadium contamination of the RHE. This unique REA enables a reliable RHE for a VRFB cell operating under various operating conditions. For example, the REA can be used to measure both the overpotentials and electrochemical potentials (vs. RHE) of the working electrodes in a VRFB cell during a complete charge and discharge cycle, as shown in Fig. 2. The REA also provides real-time measurement of the electrolyte OCV, which can be used to determine the SOC of the posolyte and negolyte individually. The REA can also be used to provide insights into VRFB-electrolyte rebalancing options. Therefore, the REA that will be introduced here can be a useful tool to guide the selection of both electrode materials and the optimization of VRFB operating protocols, and examples of both will be presented. Acknowledgements This work was made possible by financial supported from Vionx Energy.

Pt Nanoparticles Loaded on W18O49 Nanocables-rGO Nanocomposite as a Highly Active and Durable Catalyst for Methanol Electro-Oxidation.

Highly active and durable electrocatalysts are vital for commercialization of direct methanol fuel cells. In this work, a three-dimensional nanocomposite consisting of platinum nanoparticles, W18O49 nanocables, and reduced graphene oxide composite (Pt/W18O49 NCs–rGO) has been prepared as an electrocatalyst for methanol oxidation reaction (MOR). The catalyst is prepared through a two-step method. The W18O49 nanocables and the reduced graphene oxide composite are prepared by a solvothermal process. Then, Pt nanoparticles are loaded on the W18O49 nanocables and the reduced graphene oxide composite by a hydrogen reduction at ambient condition. The obtained catalyst has a special three-dimensional architecture consisting of two-dimensional nanosheets, assembled one-dimensional nanocables, and the loaded nanoparticles on their surface. The Pt/W18O49 NCs–rGO catalyst shows 1.56 time mass activities than the Pt/C, with the current density of the forward anodic peak reaching 1624 mA/mgPt at 0.854 V versus reversible hydrogen electrode potential in 0.1 M HClO4 and 0.5 M CH3OH mixed electrolyte. It also shows a strong antipoisoning property toward CO. For the durability testing, the current density of Pt/W18O49 NCs–rGO shows a 37% decay, whereas the current of Pt/C catalyst shows a 41% degradation from 600 to 3600 s at 0.7 V. The high activity toward MOR, good antipoisoning for intermediate products, and excellent stability are ascribed to strong metal–support interaction effects between the Pt nanoparticles and the W18O49 NCs.

Photocurrent Enhancement by a Rapid Thermal Treatment of Nanodisk-Shaped SnS Photocathodes.

Photocathodes made from the earth-abundant, ecofriendly mineral tin monosulfide (SnS) can be promising candidates for p/n-type photoelectrochemical cells because they meet the strict requirements of energy band edges for each individual photoelectrode. Herein we fabricated SnS-based cell that exhibited a prolonged photocurrent for 3 h at -0.3 V vs the reversible hydrogen electrode (RHE) in a 0.1 M HCl electrolyte. An enhancement of the cathodic photocurrent from 2 to 6 mA cm-2 is observed through a rapid thermal treatment. Mott-Schottky analysis of SnS samples revealed an anodic shift of 0.7 V in the flat band potential under light illumination. Incident photon-to-current conversion efficiency (IPCE) analysis indicates that an efficient charge transfer appropriate for solar hydrogen generation occurs at the -0.3 V vs RHE potential. This work shows that SnS is a promising material for photocathode in PEC cells and its performance can be enhanced via simple postannealing.

Amorphous MoS2 nanosheets grown on copper@nickel-phosphorous dendritic structures for hydrogen evolution reaction

In this study, we introduce highly active, efficient, and Pt-free electrodes (Cu@Ni-P@a-MoS2) fabricated on Ni foils, which have good conductivity and several a-MoS2 edge sites for the hydrogen evolution reaction (HER). A porous Cu dendritic structure on Ni foil was made by electro-plating utilizing hydrogen bubbles, and a Ni-P film that covered the dendritic structure was made by electro-less plating. Vertically grown a-MoS2 nanosheets were formed on the Cu@Ni-P dendritic structure by thermolysis. The structure has abundant active sites because of its particular structure, which was examined by SEM and TEM. XPS analysis was used to confirm that MoS2 was reduced completely, and the a-MoS2 nanosheet layer was characterized by Raman spectroscopy. Electrochemical experiments demonstrated that the electrode was highly effective for the HER with a low onset potential of 118.5mV, and a current density of 10 and 100mA/cm2 for 186 and 345mV versus the reversible hydrogen electrode potential (vs RHE), respectively, a small Tafel slope of 60.5mV/dec, and was stable after 2000 cycles. This study demonstrates that highly porous Cu@Ni-P@a-MoS2 electrodes, possessing a huge surface, are desirable for the HER and these findings will pave the way for a new form of highly efficient electrocatalysts.

Assessing the Suitability of Iron Tungstate (Fe2WO6) as a Photoelectrode Material for Water Oxidation

Orthorhombic iron tungstate (Fe2WO6), with a reported bandgap of ∼1.5–1.7 eV, is a potentially attractive material as the top absorber in a tandem photoelectrochemical (PEC) device. Few studies have been carried out on this material, and most of the important optical, electronic, and PEC properties are not yet known. We fabricated thin film Fe2WO6 photoanodes by spray pyrolysis and identified the performance limitations for PEC water oxidation. Poor charge separation is found to severely limit the photocurrent, which is caused by a large mismatch between the light penetration depth (∼300 nm) and carrier diffusion length (<10 nm) of the material. In addition, the conduction band of Fe2WO6 lies 0.65 V positive of the reversible hydrogen electrode potential, which means that a large external bias potential is required for water oxidation. On the basis of these observations, we critically discuss the suitability of Fe2WO6 as a novel photoelectrode material for photoelectrochemical and photocatalytic applications.

Enhancing the Performances of P3HT:PCBM-MoS3-Based H2-Evolving Photocathodes with Interfacial Layers.

Organic semiconductors have great potential for producing hydrogen in a durable and economically viable manner because they rely on readily available materials and can be solution-processed over large areas. With the objective of building efficient hybrid organic-inorganic photoelectrochemical cells, we combined a noble-metal-free and solution-processable catalyst for proton reduction, MoS3, and a poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ). Different interfacial layers were investigated to improve the charge transfer between P3HT:PCBM and MoS3. Metallic Al/Ti interfacial layers led to an increase of the photocurrent by up to 8 mA cm(-2) at reversible hydrogen electrode (RHE) potential with a 0.6 V anodic shift of the H2 evolution reaction onset potential, a value close to the open-circuit potential of the P3HT:PCBM solar cell. A 50-nm-thick C60 layer also works as an interfacial layer, with a current density reaching 1 mA cm(-2) at the RHE potential. Moreover, two recently highlighted1 figures-of-merit, measuring the ratio of power saved, Φsaved,ideal and Φsaved,NPAC, were evaluated and discussed to compare the performances of various photocathodes assessed in a three-electrode configuration. Φsaved,ideal and Φsaved,NPAC use the RHE and a nonphotoactive electrode with an identical catalyst as the dark electrode, respectively. They provide different information especially for differentiation of the roles of the photogenerating layer and catalyst. The best results were obtained with the Al/Ti metallic interlayer, with Φsaved,ideal and Φsaved,NPAC reaching 0.64% and 2.05%, respectively.

The Electrochemistry of Metallic Nickel: Oxides, Hydroxides, Hydrides and Alkaline Hydrogen Evolution

Ni-based catalysts in aqueous alkaline media are low-cost electrode materials for electrolytic hydrogen generation, a renewable method of producing fuel and industrial feedstock. However, Ni cathodes show a significant decrease in their hydrogen evolution reaction (HER) activity after several hours of electrolysis. Further, industrial electrolysers are often subjected to transient anodic currents, the effects of which on Ni-based catalysts are not well-known. We consider the source of electrode deactivation and the effects of temporary anodic currents on smooth metallic Ni electrodes in alkaline solutions by cyclic voltammetry (CV), galvanostatic and potentiostatic polarization, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Polished surfaces are covered by a bilayer composed of α-Ni(OH)2 underlaid by non-stoichiometric NiOx. Below the reversible hydrogen electrode (RHE) potential, the air-formed layer mostly reduces to Ni metal and H atoms incorporate deep into the electrode material. Under industrial conditions, i.e., concentrated NaOH/KOH solutions and large cathodic current densities, α-NiHx and β-NiHx can form at the electrode surface. Above the RHE potential, NiOx, α-Ni(OH)2, β-Ni(OH)2 and β-NiOOH form reversibly and mostly reduce back to Ni on subsequent cathodic polarization. However, repeated oxidation and reduction will introduce strain on a catalyst material, which may lead to its mechanical failure.

Facile Solution Synthesis of α-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application

We report for the first time the facile solution growth of α-FeF(3)·3H(2)O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe(2)O(3) (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF(3)·3H(2)O NWs (typically <100 nm in diameter) are converted to single-crystal α-Fe(2)O(3) NWs with internal pores, while thick ones (typically >100 nm in diameter) become polycrystalline porous α-Fe(2)O(3) NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ~400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm(2) at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm(2), pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.

Eight-electron oxidation of borohydride at potentials negative to reversible hydrogen electrode

This is the first report of a novel anode catalyst, Pt/Ti 2O, and its use for the complete 8-electron oxidation of borohydride without hydrogen evolution in a potential region negative to reversible hydrogen electrode (RHE). A major obstacle to the development of direct borohydride fuel cells (DBFC) has been the hydrogen evolution at the negative electrode, which causes low coulombic efficiency and safety problems. Attempts to prevent hydrogen evolution during the oxidation of borohydride on a fuel cell anode have always resulted in a shift of the anode potential to values more positive than the RHE potential, thus losing the advantages of DBFC in comparison to conventional hydrogen fuel cells. Pt supported on conductive titanium oxides (Ti 2O) has been found to catalyze the 8-electron oxidation of borohydride without hydrogen evolution in a potential range more negative than RHE, which is desired in order to have a DBFC that is practically applicable. Mechanism of the direct 8-electron oxidation process is discussed on the basis of a synergistic effect at the surface of Pt/Ti 2O catalyst.