Electrochemical investigation of the Bunsen reaction in the sulfur–iodine cycle

The Bunsen reaction, as a part of the sulfur–iodine thermochemical cycle, was studied using an electrochemical cell. The effects of current density, operating temperature, H₂SO₄ concentration in the anolyte, HI concentration and I₂/HI molar ratio in the catholyte were explored. Both the H₂SO₄ in anolyte and the HI in catholyte were concentrated during electrolysis. Increasing current density amplified this H₂SO₄ and HI concentration, while the other operating parameters also varied the anolyte and catholyte concentration. The transport properties of the cation exchange membrane were examined. The electrode current efficiency remained close to 100% for most runs except those at high current density. Both the average cell voltage and the heat equivalent of electric energy were determined at different conditions.     

Effect of operating variables on performance of membrane electrolysis cell for carrying out Bunsen reaction of I–S cycle

Membrane electrolysis is coming up as one of the alternatives to direct contact mode of carrying out Bunsen reaction of I–S cycle. It has potential to reduce the use of excess iodine and water. A two-compartment membrane electrolysis cell with graphite electrodes and Nafion 117 membrane was used for Bunsen reaction. Effect of six independent variables on cell voltage was determined for current density values of up to 80 A/dm². The variables were anolyte pressure, catholyte pressure, temperature, sulphuric acid concentration, HI concentration, and I₂/HI molar ratio in catholyte. Flow rate of anolyte and catholyte were identified where mass transfer resistance was not significant before performing experiments with different independent variables. Cell voltage was analysed by identifying three different regimes based on its variation with current density and current density ranges where electrode resistance or ohmic resistance dominated are identified. Current efficiency was measured for 1 A/dm² and was found to be close to 100% irrespective of values of the independent variable. Minimum amount of heat equivalent of electric energy required for membrane electrolysis was calculated and increase in its value with increase in sulphuric acid concentration was compared with estimate of reduction in heat required for concentration of sulphuric acid.     

Decomposition of hydriodic acid by electrolysis in the thermochemical water sulfur–iodine splitting cycle

Although several technologies, such as reactive distillation and catalytic membrane reactor, have been proposed to improve HI conversion efficiency, they still experience several challenges for the application in HI section. In this study, an electrochemical cell was employed for hydriodic acid decomposition under the presence of iodine. Several commercial proton-exchange membranes (PEMs), namely, Nafion 117 and Nafion 115, were used as separators for the electrochemical cell. Anodization of iodide anion occurred at the graphite electrode in the anode compartment. Hydrogen was generated by the reduction reaction of hydrogen cations, which migrated from anolyte to catholyte. In electrolysis experiments, PEM showed good performance in terms of high transport number of proton and low iodine permeation. Several parameters, such as operating temperature, HI molarity, and I₂ molarity in anolyte, which affected current efficiency, iodine permeance, and electric resistance of test cell, were investigated. High operating temperature and I₂ molarity in anolyte enhanced the permeability of iodine, which had several negative influences on electrochemical cell performance. Although current efficiency was negatively affected by increasing temperature and I₂ molarity, it still remained above 0.85 in the range of 30 °C–75 °C. Ohmic resistance, which is a component of cell resistance, offered by PEM was investigated with Nafion 117 and 115. Apart from graphite plates, activated carbon papers were adopted as electrodes to reduce the overpotentials due to their high specific surface characteristic.     

Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells

The hydrogen production rate in a microbial electrolysis cell (MEC) using a non-buffered saline catholyte (NaCl) can be optimized through proper control of the initial anolyte pH and catholyte NaCl concentration. The highest hydrogen yield of 3.3 ± 0.4 mol H₂/mole acetate and gas production rate of 2.2 ± 0.2 m³ H₂/m³/d were achieved here with an initial anolyte pH = 9 and catholyte NaCl concentration of 98 mM. Further increases in the salt concentration substantially reduced the anolyte pH to as low as 4.6, resulting in reduced MEC performance due to pH inhibition of exoelectrogens. Cathodic hydrogen recovery was high (r_cat > 90%) as hydrogen consumption by hydrogenotrophic methanogens was prevented by separating the anode and cathode chambers using a membrane. These results show that the MEC can be optimized for hydrogen production through proper choices in the concentration of a non-buffered saline catholyte and initial anolyte pH in two chamber MECs.     

Electrochemical conversion of carbon dioxide to formic acid on Pb in KOH/methanol electrolyte at ambient temperature and pressure

The electrochemical reduction of CO₂ in KOH/methanol-based electrolyte has been investigated on a lead wire electrode at ambient temperature and pressure. The major products of electrochemical reduction of CO₂ were formic acid, CO and methane. The formation of formic acid from CO₂ predominated in the potential range −1.8 to −2.5 V vs Ag/AgCl (saturated KCl). Hydrogen evolution in competition with CO₂ reduction was observed at only 3.5% faradaic efficiency. The partial current density for CO₂ reduction was more than 22 times larger than that for hydrogen evolution. Study of the Tafel plot showed that the reduction of CO₂ to formic acid and CO was not limited by mass transfer in this potential range.     

Electrolysis of iodine solution in a new NaHCO3-I2 hybrid cycle

The electrolysis step (6) in the following hybrid cycle was studied. Reaction (7) has been confirmed experimentally in an earlier paper [1].

Decomposition voltage of the electrolysis is lowered by introducing CO₂ into the catholyte to decrease the pH. Polarization characteristics and related phase diagrams of NaI-I₂-H₂O and Na₂CO₃-NaHCO₃-H₂O systems were examined. Both I₂ formed in the anolyte and NaHCO₃ formed in the catholyte could be separated by cooling and carbonating each electrolyte, respectively. Polarizations of anodic and cathodic reaction are very small in a range up to about 100 mA cm^?2. Cell voltage at current density of 100 mA cm^?2 on platinum-black at 50°C is estimated to be about 1.6 V. Increasing temperature is effective in reducing concentration polarization.     

The importance of OH− transport through anion exchange membrane in microbial electrolysis cells

In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR = 0.7 m³-H₂ m^?3 d^?1) where the catholyte rapidly increased to pH = 12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at ?1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in the performance. These results show that maintaining charge transfer to control anolyte pH is more important than obtaining a more neutral pH catholyte.     

Bipolar polymer electrolyte interfaces for hydrogen–oxygen and direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) using liquid hydrogen peroxide as the oxidant are safe and attractive low temperature power sources for unmanned underwater vehicles (UUVs) as they have excellent energy and power density and do not feature compressed gases or a flammable fuel stream. One challenge to this system is the disparate pH environment between the anolyte fuel and catholyte oxidant streams. Herein, a bipolar interface membrane electrode assembly (BIMEA) is demonstrated for maintaining pH control of the anolyte and catholyte compartments of the fuel cell. The prepared DBFC with the BIMEA yielded a promising peak power density of 110 mW cm⁻². This study also investigated the same BIMEA for a hydrogen–oxygen fuel cell (H₂–O₂ FC). The type of gas diffusion layer used and the gas feed relative humidity were found to impact fuel cell performance. Finally, a BIMEA featuring a silver electrocatalyst at the cathode in a H₂–O₂ FC was successfully demonstrated.     

H2 production by PEM electrolysis,assisted by textile effluent treatment and a solar photovoltaic cell

In this paper a new approach for H₂ production by PEM electrolysis, assisted by effluent treatment in the anolyte is proposed. H₂ is produced, in the catholyte, by proton reduction at a Fe-cathode, in an acid medium (1 M H₂SO₄). While in the anolyte, a mixture of Fe²⁺/Fe³⁺ is produced from the oxidation of an iron anode. The overall energy required (≤1.00 V) is less than that required by conventional water electrolysis, and is delivered by solar panels. In the anolyte, iron ions can be used in favor of a Fenton-type process, in the presence of H₂O₂. This approach is used in effluent treatment. The oxidation efficiency of dyes reactive black 5 (RB 5) and acid green 25 (AG 25) was investigated, in mild conditions, during H₂ production. The main experimental results show that it is possible to oxidize 0.00024 M RB 5 or 0.0002 M AG 25 in the anolyte, in 20 min.     

Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process

An experimental study of applying electro-electrodialysis (EED) for improved HI concentration in the HIx solution, a mixture of HI–I₂–H₂O of approximately quasi-azeotropic compositions has been carried out in the conditions of around 90 °C and using Nafion 117 and graphite electrodes. A range of 25–80% increase in initial current efficiency of HI molality in catholyte is measured with the use of EED. In general, the efficiency increases with increasing iodine molality and weight ratio of anolyte solution to catholyte solution. The EED performance degrades in time. In some cases, the HI concentration limits are observed. Electric conductivity of the HIx solution, overvoltage of electrode reaction, and the membrane voltage drop is measured in a temperature range of 20–120 °C. It is found that the EED cell voltage, which is an important cell performance parameter, is governed by the membrane voltage drop.     

A development of direct hydrazine/hydrogen peroxide fuel cell

A direct hydrazine fuel cell using H₂O₂ as the oxidizer has been developed. The N₂H₄/H₂O₂ fuel cell is assembled by using Ni-Pt/C composite catalyst as the anode catalyst, Au/C as the cathode catalyst, and Nafion membrane as the electrolyte. Both anolyte and catholyte show significant influences on cell voltage and cell performance. The open-circuit voltage of the N₂H₄/H₂O₂ fuel cell reaches up to 1.75 V when using alkaline N₂H₄ solution as the anolyte and acidic H₂O₂ solution as the catholyte. A maximum power density of 1.02 W cm⁻² has been achieved at operation temperature of 80 °C. The number of electrons exchanged in the H₂O₂ reduction reaction on Au/C catalyst is 2.     

Sodium borohydride hydrolysis-mediated hydrogenation of carbon dioxide,towards a two-step production of formic acid

One of the levers to mitigate the amount of carbon dioxide (CO₂) released into the atmosphere is to capture and use it as cheap, abundant, and safe carbon source, that is, as feedstock in order to produce valuable chemicals like formic acid (HCOOH) that is known to have the potential for high environmental impact reduction. It is in this context that we have developed a two-step process to produce HCOOH by hydrogenation of CO₂ at ambient conditions while using sodium borohydride (NaBH₄) in aqueous solution. Our process can be described as follows. In a first step, CO₂ is bubbled in an aqueous solution of NaBH₄; nickel-catalyzed hydrolysis of NaBH₄ takes place and the reaction is accelerated in the presence of CO₂, resulting in the formation of, among other products, sodium formate (NaHCOO) and a HCOO group containing borate B(OH)₂(OOCH). In a second step, these products are dissolved in alkaline aqueous solution and heated at 130 °C; in such conditions, HCOOH is produced and recuperated as distillate, and a solid ‘residue’ consisting mainly of Na₂CO₃, NaHCOO and NaB(OH)₄ is recovered. Our two-step process aiming at capturing and transforming CO₂ has proven to be effective, importantly at ambient conditions. Our main results and the remaining challenges are reported and discussed herein.     

Photoelectrochemical oxygen evolution using polysulfide as sacrificial electron acceptor

A two-compartment photoelectrochemical cell consisting of a TiO₂ photoanode, Nafion membrane and platinized tin oxide glass as the cathode was constructed. The anolyte and catholyte were 1 M NaOH and 1 M Na₂S₄, respectively. Illumination of the photoanode resulted in oxygen evolution in the anodic compartment simultaneously with the reduction of tetrasulfide in the cathodic compartment. The mechanism of such a photoelectrochemical oxygen evolution is discussed throughout and the possible application of this reaction in solar water splitting is mentioned.     

Determination of the operating range of CO2 conversion and syngas production in dry auto-thermal reforming

A porous medium-catalyst hybrid reformer for CO₂ conversion by dry auto-thermal reforming (DATR) was investigated in this study, and its operating range was discovered. The hybrid design was used to enhance the oxidative heat release by internal heat recirculation during exothermic reaction conditions, thereby increasing the CO₂ conversion efficiency. The experimental results show that the CO₂ conversion could be enhanced with higher catalyst inlet temperatures. The examination of the operating range of DATR showed that the CO₂ conversion efficiency increased at higher reaction temperatures and CO₂/CH₄ ratios (≧1). Moreover, DATR in high temperature conditions must be carried out with high O₂/CH₄ ratios. Under these conditions of high oxygen content, CO₂ generation and reduction reactions occur simultaneously. Overall, optimal CO₂ conversion can be obtained with an O₂/CO₂ ratio of approximately 0.5. At these conditions, CO₂ conversion efficiency can reach approximately 13% without external heat addition.     

Electrochemical conversion of CO2 to methanol using a glassy carbon electrode,modified by Pt@histamine-reduced graphene oxide

The electrochemical reduction of CO₂ to value-added products is one of the useful approaches to reducing the effects of global climate change. Herein, a novel electrocatalyst consisting of platinum nanoparticles on histamine-reduced graphene oxide plates (Pt@His-rGO) supported by a glassy carbon (GC) substrate for the electrochemical conversion of CO₂ to methanol has been developed. The nanocomposite was optimized in terms of pH, applied potential, CO₂ purging time and platinum loading for the highest current densities and faradaic efficiencies toward methanol production. The best results were obtained in a solution containing KNO₃ 0.1 mol L⁻¹ at the pH of 2.0, the applied potential of −0.3 V vs Ag/AgCl (KCl_sat), CO₂ purging duration of 30 min and Pt loading of 5.17 × 10⁻⁷ mol cm⁻². The faradaic efficiency of 37% was obtained for methanol production. The prepared nanocomposite requires a lower applied potential and serves as an intermediate stabilizer through the production of methanol.     

Direct electrolysis of bunsen reaction product for hydrogen production: The continuous-flow operation

Hydrogen sulfide (H₂S) emitted from oil industry's hydrotreating processes can be converted into hydrogen and used back to the same processes through a H₂S splitting cycle, where the Bunsen reaction and HI decomposition are two participating reactions. To overcome the difficulties and complications posted in the scaling up of the cycle, direct electrolysis of the Bunsen reaction product solution was proposed and has been studied in a batch electrolysis cell in our earlier work. This paper studies the direct electrolysis using a customer-made, continuous-flow electrolysis cell. The effects of the operating parameters including the current density, the entering HI concentration and flow rate of the anolyte, the toluene to aqueous phase ratio and stirring speed in anolyte cell, the H₂SO₄ concentration and circulation rate of the catholyte on the performing parameters such as the conversion of iodide ions, the yield of iodine transferred to toluene, and the anodic and cathodic current efficiencies for iodide conversion and hydrogen production were carefully investigated. The results show that the cathodic current efficiency for hydrogen production is nearly 100% for all the runs and that the anodic current efficiency for iodide ion conversion to iodine is relatively low (20%–70%) and varies with the changes in operating parameters. Running at high levels of the current density, the volumetric ratio of toluene to aqueous phase in anolyte, or the stirring speed in anolyte, and low levels of the entering concentration of I^? in anolyte or the flow rate of anolyte in electrolysis operation are in favor of having a high iodide conversion and high I₂-toluene yield. Iodide anions at a few mmol L^?1 level (a few thousandths of the entering concentration) are found in the cathodic chamber caused by its diffuse against the electric field and the proton exchange membrane. The continuous, direct electrolysis of the Bunsen product solution can be considered being adapted in the sulfur-iodine (S–I) water splitting cycle for hydrogen production.     

Hydrogen from electrochemical reforming of C1–C3 alcohols using proton conducting membranes

This study investigates the production of hydrogen from the electrochemical reforming of short-chain alcohols (methanol, ethanol, iso-propanol) and their mixtures. High surface gas diffusion Pt/C electrodes were interfaced to a Nafion polymeric membrane. The assembly separated the two chambers of an electrochemical reactor, which were filled with anolyte (alcohol + H₂O or alcohol + H₂SO₄) and catholyte (H₂SO₄) aqueous solutions. The half-reactions, which take place upon polarization, are the alcohol electrooxidation and the hydrogen evolution reaction at the anode and cathode, respectively. A standard Ag/AgCl reference electrode was introduced for monitoring the individual anodic and cathodic overpotentials. Our results show that roughly 75% of the total potential losses are due to sluggish kinetics of the alcohol electrooxidation reaction. Anodic overpotential becomes larger as the number of C-atoms in the alcohol increases, while a slight dependence on the pH was observed upon changing the acidity of the anolyte solution. In the case of alcohol mixtures, it is the largest alcohol that dictates the overall cell performance.     

Indirect fuel cell based on a redox-flow battery with a new structure to avoid cross-contamination toward the non-use of noble metals

An indirect fuel cell system is constructed. The system is composed of a redox flow battery (RFB) to extract electrical energy and two chemical reactors (anolyte and catholyte regenerators). A quinone as a redox mediator is reduced by a mixture of hydrogen and carbon monoxide in the anolyte regenerator, whereas a polyoxometalate as another redox mediator is oxidized in the catholyte regenerator, followed by a steady-state power generation at the RFB using the two redox mediators as active materials. This system demonstrates how to reduce the amount of platinum required in a proton-exchange membrane fuel cell (PEMFC), especially when using a fuel other than pure hydrogen. The RFB in our system contains two gas-diffusion electrodes (GDEs) with a platinum electrocatalyst to insert a “pure hydrogen gas phase” between the anolyte and catholyte to avoid cross-contamination. These two GDEs participate in the hydrogen evolution reaction and hydrogen oxidation reaction, respectively, and require only a small amount of platinum. In addition, the catalysts used in the anolyte regenerator are rhodium complexes. However, these catalysts are in a dissolved state (molecular catalysts) with micromolar-order concentrations, and very little noble metal is used. A carbonaceous catalyst without platinum is used in the catholyte regenerator. This eliminates the need for a noble metal for the oxygen reduction reaction, which is the main reason why platinum is used in a large amount in a conventional PEMFC. Steady-state operations of the anode side, the cathode side, and the total system are demonstrated in this work. Although a small amount of noble metal is still required at this stage, this work may contribute to the complete elimination of noble metals from a PEMFC.     

Anode reaction mechanism and crossover in direct dimethyl ether fuel cell

The anode reaction mechanism and the crossover of a direct dimethyl ether fuel cell (DDMEFC) have been investigated. This was done by considering the anode products of the half-cell and DDMEFC experiments. It was found that the CO₂ current efficiency of the DDMEFC was almost 1 at 30–80 °C and that this value was higher than that of a DMFC. The main by-products of the DDMEFC were methyl formate and methanol whose amounts are negligibly small compared to CO₂. With respect to crossover, the influence of DME on the oxygen reduction reaction (ORR) was examined with a half-cell, and the amount of crossover of DME was measured while operating an actually constructed DDMEFC. From these experiments, it was found that DME does not influence the ORR as much as methanol under similar conditions. Furthermore, the amount of crossover of DME decreased with an increase in temperature and current density and it was one-half that of methanol on open circuit and at 80 °C.The CO₂ current efficiency of the DDMEFC is higher than that of a DMFC, and the influence of crossover in the DDMEFC is less than that in the DMFC. Since the temperature dependence of the reactivity of DME is larger than that of methanol, the higher output is expected for the DDMEFC at the elevated temperature. Therefore, the DDMEFC has a promising potential as a portable power source in the future.     

The use of products from CO2 photoreduction for improvement of hydrogen evolution in water splitting

CuO/TiO₂ photocatalysts were prepared and shown to enhance the rate of CO₂ photoreduction and the production of total organic carbon (TOC), including HCOOH, HCHO and CH₃OH. Resulting TOC could act as electron donors for enhancing visible light hydrogen evolution from Pt/TiO₂ photocatalysts. The impacts on CO₂ photoreduction were investigated including the effect of Cu dopant, pH, irradiation time and using Na₂SO₃ as a sacrificial agent, and those on hydrogen evolution was also studied including TOC concentration and Pt doping. The CO₂ photoreduction mechanisms with respect to pH and CO₂ reduction potentials were discussed. CuO/TiO₂ and Pt/TiO₂ photocatalysts were characterized by X-ray diffraction, Raman spectroscopy and diffuse reflection UV-vis spectrophotometry. Both photocatalysts showed a visible light response in comparison with pure TiO₂. The photocatalytic experiments and FT-IR spectra indicated that photoproduct desorption was the rate-limiting step in the CO₂ photoreduction.