pH-neutral Electrolyte Research Articles

Na2so4-Polyacrylamide Neutral pH Polymer Electrolytes for Electrochemical Capacitors

Polymer electrolytes are the key enablers for solid-state electrical double layer capacitors (EDLCs) that are thin, light-weight, and flexible. Neutral pH electrolytes are particularly interesting due to their wide operating voltage window for higher energy density and non-corrosive properties for enhanced safety [1-2]. To date, polymer electrolytes comprised of Li2SO4 and polyacrylamide (PAM) have exhibited promising results [3]. However, their ionic conductivity and industrial applications are still limited by the lower mobility of Li+ ions [4] and high cost of lithium salts. Among various sulfate salts, Na2SO4 is a promising candidate as ion conductors for its higher cation mobility, relatively high solubility in water, and low cost from its earth-abundance. In this work, our objectives are to (a) develop a high-performance, low cost neutral pH polymer electrolyte using Na2SO4 and PAM, (b) understand the origin of the ionic conductivity and material properties of Na2SO4-PAM electrolytes, and (c) demonstrate its application in solid EDLC devices. Metallic cells were constructed to study the ionic conductivity, whereas cells with commercial activated carbon electrodes were fabricated to demonstrate the viability in solid EDLCs. The ionic conductivities of the polymer electrolytes with different Na2SO4:PAM ratios were tracked over 30 days period in 45 %RH storage environment (Fig. 1). The optimized electrolyte with salt-to-polymer molar ratio of 10,000:1 demonstrated a high ionic conductivity >30 mS cm-1, considerably higher than previously developed neutral pH electrolytes. With activated carbon electrodes, the polymer electrolytes enabled a stable operating 1.8 V voltage window with good cycling performance >10,000x.

Origin of the Electrochemical Stability of Aqueous Concentrated Electrolyte Solutions

The electrochemical stability of an electrolyte solution, its so-called "potential window", is simply determined by the oxidative potential and reductive potential of the solvent, if solutes dissolved in the electrolyte solution are electrochemically stable within the potential window of the solvent. Water is the most conventional solvent in the field of electrochemistry, and its potential window is as narrow as 1.23 V, thermodynamically. Practically, however, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) need certain overpotentials, which extends the potential windows of aqueous solutions. Besides the catalytic activities of the electrode materials, the properties of aqueous solutions such as hydration structures and salt concentrations also affect the potential window.Recently, aqueous rechargeable lithium batteries (ARLBs) have attracted much attention due to their low cost and safety. However, the working voltage of ARLBs is limited to below 1.5 V due to the narrow potential window of water. Recently, a few papers have discussed the potential window of concentrated aqueous solutions with neutral pH. However, there are still fundamental unanswered questions regarding the pH-dependence of the potential window of water. In this study, the potential window of concentrated electrolyte solutions with neutral pH and the expansion of the potential window were investigated from the viewpoint of the local pH change and water concentration. Here, we shed light on the dependence of the potential window of water on local pH changes and water concentrations. Next, we focused on the water concentration, since the potential windows are determined by the water electrolysis reactions (OER and HER). The potential windows are shown in Fig. 1 as a function of the water concentrations. In this study, the onset potential of the OER/HER was defined when the current density was ± 0.1 mA cm-2 in CVs at 1 mV s-1. There are two clear tendencies in Fig. 1. First, the windows in neutral pH electrolyte solutions (closed symbols in Fig. 1) were obviously wider than those in acidic/alkaline electrolyte solutions (open symbols in Fig. 1), even at a dilute salt concentration (ca. 55 M water concentration). Second, the windows in neutral pH electrolyte solution (closed symbols in Fig. 1) were not affected by the kind of the electrolyte salt, but depended linearly on the water concentration. The windows were expanded when the water concentration decreased, i.e., the electrolyte salt concentrations increased. Potential windows of aqueous solutions were investigated systematically using various salts at different concentrations. Cyclic voltammetry measurements of Pt electrodes revealed two important points. First, the potential window in unbuffered neutral pH solution was broader than that in acidic/alkaline solutions. This expansion of potential windows can be explained by the shift in the reaction potential with local pH changes in the vicinity of the electrode. Second, the potential windows were not affected by electrolyte salts, but rather depended linearly on the water concentration. The difference for OER overpotentials was much larger than that for HER overpotentials. While HER overpotentials were derived from a local pH change, OER overpotentials were derived from both a reduced water concentration and local pH change. This study highlights the importance of these two main factors (water concentration and local pH change) in determining the potential windows of concentrated electrolyte solutions. Figure 1

Corrosion inhibition of pure Mg containing a high level of iron impurity in pH neutral NaCl solution

Three sodium salts of organic acids (2,5-pyridinedicarboxylic, 3-methylsalicylic and fumaric) were studied as corrosion inhibitors for pure magnesium containing a high level (342 ppm) of Fe impurity. The inhibition effect of the three inhibitors was evaluated by means of hydrogen evolution, electrochemical and surface analysis techniques. Significant reduction in corrosion rate was revealed for of the tested inhibitors in pH neutral NaCl electrolyte. 2,5-pyridinedicarboxylate and fumarate showed the effects characteristic for mixed-type inhibitors, while 3-methylsalicylate performed more as cathodic inhibitor, complying with the iron re-deposition model, where the electrochemical activity of iron-rich particles was effectively inhibited.

Electrochemical CO2 reduction to formate at indium electrodes with high efficiency and selectivity in pH neutral electrolytes

Electrochemical reduction of CO2 is promising for a bio-based economy as it combines utilization of CO2 as feedstock and provides a pathway for the utilization and (temporary) storage of electric energy. Among different products formate (HCOO-) can be produced with high rates and selectivity using indium as electrocatalyst. This can be achieved at mild biocompatible reaction conditions, e.g. ambient temperature, ambient pressure and neutral pH. Formate can serve as a source of carbon and energy for the biosynthesis of energy carriers or chemicals. However, the in situ interfacing of electrochemical CO2 reduction and biosynthesis creates challenges for electrochemical engineering. It is demonstrated that the electrode potential is the main steering parameter affecting the columbic efficiency, selectivity and rate of formate production in NaHCO3 electrolyte solution at biocompatible conditions. Coulombic efficiencies and formate production rates of 94.5 ± 2% and 0.136 ± 0.016 mmol h-1 cm-2 (at -2.2 vs. Ag/AgCl and ĸ = 10 mS cm-1), respectively, were achieved. Further, increasing the conductivity using inert electrolytes can enhance formate space-time yields up to 0.254 ± 0.031 mmol h-1 cm-2. Surprisingly, high NaHCO3 concentrations do not further increase formate production which supports that HCO3- is not electrochemically converted but only acting as CO2/H+ reservoir. Based on kinetic modeling insight on the inter-conversion of the carbonaceous species by CO2 sparging of the electrolyte solution is provided. Importantly, the influence of O2 on the electrochemical CO2 reduction was revealed to be marginal. This study, providing principles on the engineering of electrochemical CO2 reduction to formate for future interfacing to biosynthesis, demonstrates its feasibility to become technologically relevant.

Enhanced Electrochemical Activity of Polyaniline-Ferricyanide Films in Neutral Electrolyte

We have successfully developed a two-step chronopotentiometric routine for electropolymerizing a polyaniline-ferricyanide film with stable electroactivity in neutral pH electrolyte containing 1 M KCl. Here we present a spectroscopic, electrochemical, and gravimetric analysis of these films to determine the nature of doping and role of the ferricyanide ion. The experimental evidence currently supports the hypothesis that the presence of ferricyanide alters the electrochemical behavior of the films in 1 M KCl, specifically the films are found to be conducting in non-acidic media. The spectroscopic evidence suggests that the Fe(CN)6 interacts with the nitrogen atoms on the PANI chain, resulting in the increased presence of polarons (mostly localized) even while the film is in the emeraldine base form. Gravimetric measurements provide insight into the role of anion and proton exchange during voltammetric cycling.

High Seebeck Coefficient Electrochemical Thermocells for Efficient Waste Heat Recovery

An electrochemical thermocell realizes thermal to electric energy conversion when two electrodes operate the same reversible reaction but at different temperatures. Its Seebeck coefficient is determined by the entropy change of the redox reaction. Here we report a thermocell containing acetone and iso-propanol as the redox couple, which can achieve the highest reported Seebeck coefficient of −9.9 mV K–1 when the hot side is above the boiling point of acetone. Vaporization entropy of acetone increases the total entropy change in the conversion of iso-propanol to acetone. In addition, a concentration gradient of acetone caused by evaporation and condensation increases the cell voltage significantly. Stable performance of the thermocell is enabled by a Pt–Sn catalyst operating in a neutral pH electrolyte solution. The possibility of utilizing a liquid–gas phase change to increase the Seebeck coefficient of thermocells opens a new venue for exploration.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes.

We report on a self‐assembled system comprising a molecular copper‐porphyrin photoelectrocatalyst, 5‐(4‐carboxy‐phenyl)‐10,15,20‐triphenylporphyrinatocopper(II) (CuTPP‐COOH), covalently bound to self‐organized, anodic titania nanotube arrays (TiO2 NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin‐covered TiO2 NTs under cathodic polarization up to −0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H2O2 in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H2O2 upon irradiation is proven and quantified by direct colorimetric detection using 4‐nitrophenyl boronic acid (p‐NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO2 NTs may ultimately allow for the preparation of a low‐cost H2O2 evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Enhancing and stabilizing α-Fe2O3 photoanode towards neutral water oxidation: Introducing a dual-functional NiCoAl layered double hydroxide overlayer

Hematite (α-Fe2O3) is a promising candidate as photoanode for photoelectrochemical (PEC) water splitting. However, it still suffers from low efficiency and serious hydrolysis in neutral pH electrolyte. Herein, NiCoAl layered double hydroxides (NiCoAl-LDH) is successfully in situ constructed onto hematite (α-Fe2O3) nanoarrays. The resulting NiCoAl-LDH/α-Fe2O3 photoanode exhibits an onset potential as low as 0.55 V vs. reversible hydrogen electrode (RHE) and a significantly enhanced photocurrent density of 2.56 mA cm−2 at 1.23 V vs. RHE for PEC water oxidation in neutral electrolyte, which is ca. 13, 2 and 1.4 times as high as that of α-Fe2O3, NiAl-LDH/α-Fe2O3 and CoAl-LDH/α-Fe2O3. Comprehensive electrochemical and structural analysis reveal the synergistic effect of Ni and Co should be responsible for the enhanced PEC performance. Ni and Co could cause local structure distortion of MO6 (M = Ni or Co) in NiCoAl-LDH, which (1) improve the conductivity of MAl-LDH, thus enhance the separation and the transfer of photogenerated charges and (2) slightly change the chemical environment of Co and Ni and facilitate the formation of low-valent oxidation states, thus lower the energy barrier of Co2+ or Ni2+ to high oxidation states and accelerate the kinetics of water oxidation. The versatile skeleton and interlayered anions as well as the thicker decoration of NiCoAl-LDH enable NiCoAl-LDH/α-Fe2O3 to possess excellent stability in neutral electrolyte. This work reveals the partial substitution of Co (or Ni) with Ni (or Co) in MAl-LDH could produce more efficient materials toward high PEC water splitting and will stimulate further development on ternary LDHs as excellent cocatalysts for other electrode in neutral electrolyte.

Derivation of Guidelines for Reliable Finishing of Aluminium Matrix Composites by Jet Electrochemical Machining

The Collaborative Research Centre SFB 692 at Chemnitz University of Technology is focused on aluminium-based light-weight materials in safety-related parts and components. One field of activity are aluminium matrix composites (AMCs). Thereby, the manufacturing, the mechanical properties, the economic opportunities and challenges as well as the machinability are investigated. One of the research focusses is the finish-machining of AMCs by Jet Electrochemical Machining (Jet-ECM).Therefore, one objective is to achieve guidelines for reliable machining of AMCs by Jet-ECM. In a first step the dissolution characteristic of EN AW 2017 with 10% of SiC particles, which was manufactured within the SFB 692, was analysed by help of a microcapillary cell, applying a sodium nitrate electrolyte. Furthermore, Jet-ECM was used to generate point erosions using different pH-neutral electrolytes, namely NaNO3 and NaCl. AMCs made of the aluminium alloy EN AW 2017 with 0%, 5% and 10% SiC particles were machined with a maximal voltage of 25V.In the current study the parameter field is extended further. The dissolution characteristic by using the pH-neutral electrolyte sodium-bromide is analysed and the voltage range is set to 60V. Those experiments show, that each electrolyte causes a specific dissolution characteristic. Based on the experimental data, guidelines for a reliable finishing of AMCs by Jet-ECM are derived. The resulting guidelines can be used to predict the depth of the point erosions as function of particle fraction, the applied voltage and the machining time, with respect to the used electrolyte.The statistical evaluation of the guidelines show, that the applied voltage and the machining time are the dominant factors to control the dimensions of the point erosions. Whereas the particle fraction has a minor effect on depth and width.

Behavior of Manganese-Rich Layered Sodium Oxides in Aqueous Electrolytes

Aqueous sodium-ion batteries operating in neutral pH electrolytes offer the potential for low cost, low toxicity, high safety, and high power density energy storage for large-scale applications, such as those needed to store electricity from renewable energy sources. However, the mechanisms of ion intercalation into solid-state electrodes operating in neutral pH aqueous electrolytes are not well understood. Layered, Mn-rich sodium oxides are promising electrode materials because of the abundance and safety of manganese. Initial studies of the moisture-stable, layered P2-type oxide, NaxNi0.22Mn0.66Co0.11O2 (NaNMC) show an irreversible phase transition to a hydrated birnessite-like structure during electrochemical cycling in aqueous electrolytes with approximately 25 mAh/g reversible capacity. This work investigates the transformation mechanism of the NaNMC through ex situ X-ray diffraction, in situ Raman spectroscopy, and ex situ transmission electron microscopy. A systematic investigation is then conducted on the effect of transition metal doping in NaNMC to reduce the amount of Ni and Co, and to tune the redox potential and capacity for electrochemical cycling in aqueous electrolytes. In addition, aqueous solutions with both high and low salt concentrations are investigated to determine the effect of the electrolyte on the electrochemical behavior of layered sodium oxides

Surface Roughening of Nickel Cobalt Phosphide Nanowire Arrays/Ni Foam for Enhanced Hydrogen Evolution Activity

Development of earth-abundant, efficient, and stable electrocatalysts for hydrogen evolution reactions (HER) in alkaline or even neutral pH electrolyte is very important for hydrogen production from water splitting. Construction of bimetal phosphides via tuning the bonding strength to hydrogen and increasing effective active sites through nanostructuring and surface engineering should lead to high HER activity. Here, ternary NiCoP nanowires (NWs) decorated by homogeneous nanoparticles have been obtained on Ni foam for a highly efficient HER property via long-term cyclic voltammetric (CV) sweeping. The electron density transfer between the positively charged Ni and Co and negatively charged P atoms, one-dimensional electron transfer channel of the NWs, and abundant active sites supplied by the nanoparticles and NWs endow the catalyst with low overpotentials of 43 and 118 mV to achieve the respective current densities of 10 and 100 mA cm-2 together with long durability for at least 33 h in 1 M KOH. A cycled anodic dissolution-redeposition mechanism is disclosed for the formation of the NiCoP nanoparticles during the CV sweeping process. Such a surface roughening method is found to be adaptable to enhance the HER property of other phosphides, including Ni2P nanoplates/NF, NiCoP nanoparticles/NF, and CoP NW/NF.

Biomass-Derived Porous Fe3C/Tungsten Carbide/Graphitic Carbon Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction.

The oxygen-reduction reaction (ORR) draws an extensive attention in many applications, and there is a growing interest to develop effective ORR electrocatalysts. Iron carbide (Fe3C) is a promising alternative to noble metals (e.g., platinum), but its performances need further improvement, and the real role of the Fe3C phase remains unclear. In this study, we synthesize Fe3C/tungsten carbide/graphitic carbon (Fe3C/WC/GC) nanocomposites, with waste biomass (i.e., pomelo peel) serving as carbon source, using a facile, one-step carbon thermal-reduction method. The nanocomposite is characterized by a porous structure consisting of uniform Fe3C nanoparticles encased by graphitic carbon (GC) layers with highly dispersed nanosized WC. The Fe3C provides the active sites for ORR, while the graphitic layers and WC nanoparticles can stibilize the Fe3C surface, preventing it from dissociation in the electrolyte. The Fe3C/WC/GC nanocomposite is highly active, selective, and stable toward four-electron ORR in pH-neutral electrolyte, which results in a 67.82% higher power density than that of commercial Pt/C and negligible voltage decay during a long-term phase of a 33 cycle (2200 h) operation of a microbial fuel cell (MFC). The density functional theory (DFT) calculations suggest high activity for splitting the O-O bond of molecular oxygen on the surface of Fe3C.

Studies of Earth Abundant Metal Complexes for Near Neutral Aqueous Redox Flow Battery (RFB) for Grid Storage

Among the many options available for Large-Scale Grid Energy Storage, Redox Flow Batteries (RFB) are leading candidates because of the decoupled design of power and energy1 which offers tremendous design flexibility and modularity. As part of the Department of Energy’s Office of Electricity (OE) energy storage program at Sandia National Laboratories, we are investigating earth abundant transition metal coordination complexes for aqueous redox flow batteries at close to pH 7. The aqueous-based systems offer complete freedom from the thermal instabilities that plague non-aqueous systems, such as lithium-ion. Furthermore, the use of neutral pH electrolytes engenders intrinsic safety and low/no corrosion, unlike the highly corrosive and toxic electrolytes used for the vanadium redox flow battery or the zinc-bromine systems. In this abstract we discuss the electrochemical properties of some of the complexes under development, and the performance of full cells containing the optimized redox couples will be presented at the meeting. Performance of Fe(2+)(tris-BPY)Cl2-0.4M Na2SO4 We have performed dc and ac voltammetry to characterize a number of iron coordination complexes, and Figure 1 shows the dc voltammetry curves at a 10 mV/s scan rate of one of these. As seen, the peak separation is ~60 mV, close to the theoretical value. We have also performed DC voltammetry at different scan rates up to 200 mV/s, and the peak separation varies between 60 and 64 mV which clearly demonstrate highly reversible behavior as well. Additionally, this couple exhibits higher redox voltage which will assure a RFB with a higher cell voltage and energy when appropriately configured with another redox couple. Figure 2 shows a plot of redox peak potentials vs. square root of scan rate. The linear increase of peak value vs. square root of san rate indicates that this couple is highly reversible. We also performed AC voltammetry, which is more sensitive than the DC measurement, just to make sure that there are no other redox peaks buried around the solvent decomposition voltage. We observed only one reversible peak (Figure 3) which corroborates the DC measurement. Similar observations were observed for other couples and will be described in detail at the meeting. Reference: B. Dunn, B., H. Kamath, H. & Tarascon, J.-M. “Electrical energy storage for the grid: a battery of choices” Science 334, 928–935(2011). Acknowledgment: Sandia Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation for the U. S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL 85000. The authors would like to thank Dr. Imre Gyuk of the DOE Office of Electricity for the support. Figure 1

Carbon treated commercial aluminium alloys as anodes for aluminium-air batteries in sodium chloride electrolyte

Abstract An easy treatment based in carbon layer deposition into aluminium alloys is presented to enhance the performance of Al-air primary batteries with neutral pH electrolyte. The jellification of aluminate in the anode surface is described and avoided by the carbon covering. Treated commercial Al alloys namely Al1085 and Al7475 are tested as anodes achieving specific capacities above 1.2 Ah g−1 vs 0.5 Ah g−1 without carbon covering. The influence of the binder proportion in the treatment as well as different carbonaceous materials, Carbon Black, Graphene and Pyrolytic Graphite are evaluated as candidates for the covering. Current densities of 1–10 mA cm−2 are measured and the influence of the alloy explored. A final battery design of 4 cells in series is presented for discharges with a voltage plateau of 2 V and 1 Wh g−1 energy density.

Jet Electrochemical Machining of Particle Reinforced Aluminum Matrix Composites with Different Neutral Electrolytes

Conventional mechanical machining of particle reinforced aluminum matrix composites (AMCs) is challenging because the hard ceramic particles in the soft aluminum matrix lead to an increased tool wear. Furthermore, the mechanical and thermal impact during conventional machining affects the microstructure of the AMCs. Electrochemical machining (ECM) is an alternative method to machine AMCs. Based on anodic dissolution, ECM has a slight influence on the work piece material structure and is independent of material strength and hardness. So the microstructure of the work piece remains unaffected. One method of ECM is electrochemical machining with continuous electrolytic free jet (Jet-ECM). Hereby the electrochemical removal is localized by the geometry of the electrolyte jet. By moving the electrolyte jet micro-structures and microgeometries can be generated quickly and flexibly in metallic parts [1]. Another advantage of Jet-ECM is the low consumption of electrolyte which allows an easy and inexpensive change of electrolyte for investigations with different types of electrolyte. In this study AMCs reinforced with different amounts of SiC-particles are machined with two pH-neutral electrolytes using Jet-ECM. The results provide information about the suitability of the selected electrolytes for the machining of AMCs. In addition, the influence of the particle content on the electrochemical removal result will be evaluated.

Photoelectron Spectroscopy (XPS) and Cyclic Voltammetry (CV) Investigation of UxTh1-xO2 Thin Films

Thin film samples of UO2 and U0.55Th0.45O2 have been prepared by sputter co-deposition under argon atmosphere in presence of oxygen (reactive sputtering) onto gold foil. Films were characterized by X-Ray Photoelectron Spectroscopy (XPS). Cyclic Voltammetry (CV) studies have been made by using the thin films as electrodes in 0.01M NaCl pH neutral non-purged electrolyte. The effect of thorium in the UO2 lattice is observed by comparing the U0.55Th0.45O2 electrode to the UO2 electrode. The results indicate that uranium develops enhanced resistance to the oxidation when thorium is added to the lattice. After the CV measurements, the films were again characterized by XPS. The surface is enriched in thorium by 11%. Uranium is in a higher oxidation state; however, uranium is less oxidized in U0.55Th0.45O2 than in UO2.

Improved photoelectrochemical water oxidation kinetics using a TiO2 nanorod array photoanode decorated with graphene oxide in a neutral pH solution.

We prepared TiO2 nanorod (NR) arrays on a fluorine-doped tin oxide substrate and decorated with graphene oxide (GO) to study their photoelectrochemical (PEC) water oxidation activities in two different electrolytes. The PEC performances of GO-decorated TiO2 NR photoanodes were characterized by optical and electrochemical impedance spectroscopy measurements. In 1 M KOH, the photocurrent density of the TiO2 NR film decreased after deposition of GO, while in the neutral pH electrolyte (phosphate buffered 0.5 M Na2SO4), the TiO2 NR photoanode showed enhanced performance after deposition with the 2 wt% GO solution. This was a consequence of the decrease in charge transfer resistance between the electrode surface and the electrolyte. The improvement of photocurrents by GO decoration was obvious near the onset potential of the photocurrents in the neutral pH electrolyte. These opposite contributions of GO on the TiO2 NR photoanodes suggest that GO can promote water oxidation effectively in a neutral electrolyte because depending on the pH of the electrolyte, different chemical species interact with the surface of the photoanode in the water oxidation reaction.

Light Harvesting Proteins for Solar Fuel Generation in Bioengineered Photoelectrochemical Cells

The sun is the primary energy source of our planet and potentially can supply all societies with more than just their basic energy needs. Demand of electric energy can be satisfied with photovoltaics, however the global demand for fuels is even higher. The direct way to produce the solar fuel hydrogen is by water splitting in photoelectrochemical (PEC) cells, an artificial mimic of photosynthesis. There is currently strong resurging interest for solar fuels produced by PEC cells, but some fundamental technological problems need to be solved to make PEC water splitting an economic, competitive alternative. One of the problems is to provide a low cost, high performing water oxidizing and oxygen evolving photoanode in an environmentally benign setting. Hematite, α-Fe2O3, satisfies many requirements for a good PEC photoanode, but its efficiency is insufficient in its pristine form. A promising strategy for enhancing photocurrent density takes advantage of photosynthetic proteins. In this paper we give an overview of how electrode surfaces in general and hematite photoanodes in particular can be functionalized with light harvesting proteins. Specifically, we demonstrate how low-cost biomaterials such as cyanobacterial phycocyanin and enzymatically produced melanin increase the overall performance of virtually no-cost metal oxide photoanodes in a PEC system. The implementation of biomaterials changes the overall nature of the photoanode assembly in a way that aggressive alkaline electrolytes such as concentrated KOH are not required anymore. Rather, a more environmentally benign and pH neutral electrolyte can be used.

Extraordinary Hydrogen Evolution and Oxidation Reaction Activity from Carbon Nanotubes

The hydrogen evolution reaction, 2H+ + 2e- -> H2, has utility in the electrochemical production of hydrogen from water and is perhaps the most studied reaction in all of electrochemistry. Its converse, the hydrogen oxidation reaction, H2 -> 2H+ + 2e-, is the half cell reaction occurring in hydrogen fuel cells, which, by direct conversion of chemical to electrical energy avoids the Carnot efficiency limits intrinsic to heat pumps. These reactions are central to any realization of a hydrogen economy. Various forms of carbon have been used for decades as the precious metal catalyst support in these reactions. Here we report the unexpected result that single wall carbon nanotubes (SWNTs), can be activated by a simple process to exhibit an activity for these reactions, in acidic and pH neutral electrolytes, that exceeds that of all known non-precious metal catalysts. We have labored to exclude conventional metals as the source of this activity and concluded that it is due to the SWNTs themselves.

Robust production of purified H2in a stable, self-regulating, and continuously operating solar fuel generator

The development of practical solar-driven electrochemical fuel generators requires the integration of light absorbing and electrochemical components into an architecture that must also provide easy separation of the product fuels. Unfortunately, many of these components are not stable under the extreme pH conditions necessary to facilitate ionic transport between redox reaction sites. By using a controlled recirculating stream across reaction sites, this work demonstrates a stable, self-regulating and continuous purified solar-hydrogen generation from near neutral pH electrolytes that yield continuous nearly pure H2 streams with solar-fuel efficiencies above 6.2%.