Baohui Wang

Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3

Taking carbon out of the ammonial recipe The reaction used to make ammonia for synthetic fertilizer requires hydrogen. Nowadays, that hydrogen is stripped from methane, creating CO2 as a by-product. Licht et al. demonstrate a relatively efficient electrochemical process in which water and nitrogen react directly to form ammonia. The approach removes the need for an independent hydrogen generation step. The process takes place in molten hydroxide salt and requires a nanostructured iron oxide–derived catalyst. Although the catalyst suspension is currently only stable for a few hours, the protocol points to a way to produce ammonia from purely renewable resources. Science, this issue p. 637 An electrochemical route offers preliminary prospects for making the ammonia in fertilizer purely from renewable resources. The Haber-Bosch process to produce ammonia for fertilizer currently relies on carbon-intensive steam reforming of methane as a hydrogen source. We present an electrochemical pathway in which ammonia is produced by electrolysis of air and steam in a molten hydroxide suspension of nano-Fe2O3. At 200°C in an electrolyte with a molar ratio of 0.5 NaOH/0.5 KOH, ammonia is produced at 1.2 volts (V) under 2 milliamperes per centimeter squared (mA cm−2) of applied current at coulombic efficiency of 35% (35% of the applied current results in the six-electron conversion of N2 and water to ammonia, and excess H2 is cogenerated with the ammonia). At 250°C and 25 bar of steam pressure, the electrolysis voltage necessary for 2 mA cm−2 current density decreased to 1.0 V.

Over 18% solar energy conversion to generation of hydrogen fuel; theory and experiment for efficient solar water splitting

Abstract Contemporary models are shown to significantly underestimate the attainable efficiency of solar energy conversion to water splitting, and experimentally a cell containing illuminated AlGaAs/Si RuO2/Ptblack is demonstrated to evolve H2 and O2 at record solar-driven water electrolysis efficiency. Under illumination, bipolar configured Al 0.15 Ga 0.85 As (E g =1.6 eV ) and Si (E g =1.1 eV ) semiconductors generate open circuit and maximum power photopotentials of 1.57 and 1.30 V, well suited to the water electrolysis thermodynamic potential: H 2 O → H 2 +1/2 O 2 ; E H 2 O °=E O 2 −E H 2 ; E H 2 O °(25° C )=1.229 V . The EH2O°/photopotential matched semiconductors are combined with effective water electrolysis O2 or H2 electrocatalysts, RuO2 or Ptblack. The resultant solar photoelectrolysis cell drives sustained water splitting at 18.3% conversion efficiencies. Alternate dual bandgap systems are calculated to be capable of attaining over 30% solar photoelectrolysis conversion efficiency.

Insoluble Fe(VI) compounds: effects on the super-iron battery

Abstract Cathodes composed of Fe(VI) salts are capable of three-electron reduction, and are useful for energetic super-iron batteries. This study investigates the solubility of BaFeO 4 and K 2 FeO 4 Fe(VI) salts. Electrolytes are determined in which Fe(VI) has a low aqueous or non-aqueous solubility, or is insoluble. Insoluble Fe(VI) salts have the duel benefits of preventing Fe(VI) solution-phase (i) decomposition and (ii) diffusion to the anode; thereby preventing super-iron battery self-discharge. BaFeO 4 is insoluble in water, and has a solubility of less than 2×10 −4 M in 5 M KOH containing Ba(OH) 2 . A BaFeO 4 super-iron battery has a high discharge efficiency when containing an electrolyte of either 12 M KOH, or 6 M KOH saturated in Ba(OH) 2 . Fe(VI) cathodes in non-aqueous media may be useful in providing a high-capacity Li or Li-ion super-iron battery. We illustrate that Fe(VI) salts are insoluble and chemically unreactive with a range of organic electrolytes, and can be discharged as cathodes in non-aqueous electrolytes. In acetonitrile containing 1 M LiClO 4 , the discharge of an Fe(VI) cathode is demonstrated to a capacity over 394 mAh g −1 K 2 FeO 4 .

SOLID PHASE MODIFIERS OF THE FE(VI) CATHODE : EFFECTS ON THE SUPER-IRON BATTERY

Abstract Cathodes comprised of Fe(VI) salts, and capable of three-electron reduction, are useful for the formation of energetic super-iron batteries. Several solid phase modifiers can significantly impact the electrochemical characteristics of the Fe(VI) cathode. The average discharge potential of the super-iron alkaline battery may be increased by ∼150 mV by addition of a few percent by weight of solid Co(III) salts, such as Co 2 O 3 . This potential may be decreased by ∼200 mV, through manganate or permanganate coating of the Fe(VI) particles with small amounts (for example, 1%) of MnO 2 . Evidence indicates that the modifier effects are electrocatalytic in origin. The addition of In, such as In 2 O 3 , to the cathode improves (decreases) the reversible charging potential of the Fe(VI) cathode and improves the cycle life of the super-iron metal hydride battery.

Rapid chemical synthesis of the barium ferrate super-iron Fe(VI) compound, BaFeO4

An alternate rapid synthesis of BaFeO4 is demonstrated. Fe(VI) salts, including BaFeO4, are energetic cathode materials in super-iron batteries ranging from primary to secondary, and including aqueous and non-aqueous cells. Of the Fe(VI) salts, BaFeO4 sustains unusually facile charge transfer, of importance to the high power domain of alkaline batteries. Unlike previous syntheses, BaFeO4 preparation is demonstrated from all solid state room temperature reactants. This eliminates several synthetic procedural steps and improves stability to approach that of the rigorously stable chemically synthesized K2FeO4 salt.

Enhanced Fe(VI) cathode conductance and charge transfer: effects on the super-iron battery

Cathodes comprising Fe(VI) salts are capable of three-electron reduction, and are useful for the formation of energetic ‘super-iron’ batteries. Material additions to the Fe(VI) cathode can be used to enhance the conductance and the efficiency of charge transfer to the cathode, and control the characteristics of the electrochemical storage. Whereas several common carbons are ineffective as conductive matrices for Fe(VI) reduction, several others such as small particle (1 μm) graphite, compressed carbon black, and fluorinated polymer graphites support efficient Fe(VI) 3e− reduction. Several inorganic salts also sustain Fe(VI) reduction, but at lower current densities. Titanates and other salts added to a K2FeO4 cathode improve the faradaic efficiency of Fe(VI) reduction at higher (∼3 mA cm−2) discharge current densities. Fluorinated polymer graphites provide an unusual additive to the Fe(VI) cathode mix, and at a low level (10 wt.%) addition can support efficient Fe(VI) reduction.

STEP wastewater treatment: a solar thermal electrochemical process for pollutant oxidation.

A solar thermal electrochemical production (STEP) pathway was established to utilize solar energy to drive useful chemical processes. In this paper, we use experimental chemistry for efficient STEP wastewater treatment, and suggest a theory based on the decreasing stability of organic pollutants (hydrocarbon oxidation potentials) with increasing temperature. Exemplified by the solar thermal electrochemical oxidation of phenol, the fundamental model and experimental system components of this process outline a general method for the oxidation of environmentally stable organic pollutants into carbon dioxide, which is easily removed. Using thermodynamic calculations we show a sharply decreasing phenol oxidation potential with increasing temperature. The experimental results demonstrate that this increased temperature can be supplied by solar thermal heating. In combination this drives electrochemical phenol removal with enhanced oxidation efficiency through (i) a thermodynamically driven decrease in the energy needed to fuel the process and (ii) improved kinetics to sustain high rates of phenol oxidation at low electrochemical overpotential. The STEP wastewater treatment process is synergistic in that it is performed with higher efficiency than either electrochemical or photovoltaic conversion process acting alone. STEP is a green, efficient, safe, and sustainable process for organic wastewater treatment driven solely by solar energy.

STEP organic synthesis: an efficient solar, electrochemical process for the synthesis of benzoic acid

This study presents the first demonstration of STEP for organic synthesis. The synthesis of benzoic acid was fully driven by solar energy without the input of any other forms of energy. STEP (the Solar Thermal Electrochemical Process) was established to drive chemical reactions by using solar heat to minimize the energy and maximize the rate of a growing portfolio of electrolysis reactions. The use of solar energy can minimize or eliminate the carbon footprint associated with the production of societal staples. To date this has included STEP chemistries for the high solar efficiency generation of hydrogen fuels, carbon dioxide splitting to generate fuels and graphite, hematite (iron ore) splitting for iron metal, of lime from limestone, and the production of bleach, magnesium and other useful chemistries. Benzoic acid is produced by the electrolysis of toluene. Solar thermal and solar electrical energy synergistically drive the electrolysis; solar thermal decreases the requisite electrolysis voltage, and improves the kinetics, selectivity and yield of the reaction, while solar electrical current drives the electrolysis. The low electrolysis potential inhibits over-oxidation of the product. In this STEP organic synthesis of benzoic acid at an applied photopotential of 3 V the reaction conversion of benzoic acid is 3.9% at a temperature of 30 °C, 12.4% at 60 °C, and 32.0% at 90 °C. The results demonstrate that the STEP is suitable for an efficient synthesis of benzoic acid from toluene.

Evolution of confined species and their effects on catalyst deactivation and olefin selectivity in SAPO-34 catalyzed MTO process

Confined species in SAPO-34 cages participate in methanol reaction and affect product selectivity as well as leading to the deactivation of the catalyst during the MTO process. In this work, spatial- and time-evolution of the confined species in a fixed bed are investigated by TG and dissolution–extraction experiment. Results indicate that both methanol and olefins lead to the formation of confined methylbenzenes and methylnaphthalenes, which are active intermediates in the MTO process. These intermediates further transformed into phenathrene and pyrene in the methanol reaction section and led to the deactivation of the catalyst. A pseudo-steady state period, during which selectivities of products are relatively stable, is achieved while most of cages are occupied by naphthalene and methylnaphthalenes. Confined species reduce the cage volume for products and reactants and thus affect product selectivities. Empty cages are likely to form relatively large-size products like C4 and C5+ molecules. Cages occupied by large-size methylnaphthalenes tend to form more ethene, less propene and even less C4+ products than those occupied by relatively small methylbenzenes. Simultaneously, secondary reactions of olefins, which increase the formation of alkanes and C4+ products during the initiation period, are greatly reduced in the pseudo-steady state period since olefins are hindered from entering occupied cages. As a result, ethene and propene selectivities and C2/C3 ratio tend to increase while C4+ and alkane selectivities decrease with the prolonging of time on stream.

Effect of molten carbonate composition on the generation of carbon material

Carbon dioxide, CO2, is thought to be a main culprit leading to global climate change and a wide variety of strategies have been proposed to reduce atmospheric CO2 levels. Here, CO2 is captured and subsequently electrochemically split into carbon materials in an electrolyzer comprising a eutectic mixture of carbonates, an Fe cathode and a Ni anode, at 600 °C and current densities of 50, 100, 200 mA cm−2. SEM, EDS, XRD and BET are employed to analyze the morphology, elemental composition, crystal structure as well as the BET surface area of the synthetic cathodic products. In addition, coulomb efficiency under different electrolytic conditions is measured via the comparison between moles of formed carbon product and the Faradays of charge passed during the electrolysis reaction. This paper investigated the effect of molten carbonate compositions on carbon product generation, and confirmed the visible dependence of produced carbon on the electrolytes.