Influences of sodium borohydride concentration on direct borohydride fuel cell performance

In this work, the effects of sodium borohydride concentration on the performance of direct borohydride fuel cell, which consisted of Pd/C anode, Pt/C cathode and Na⁺ form Nafion^® membrane as the electrolyte, have been investigated in steady state/steady-flow and uniform state/uniform-flow systems. The experimental results have revealed that the power density increased as the sodium borohydride concentration increased in the SSSF system. Peak power densities of 7.1, 10.1 and 11.7 mW cm⁻² have been obtained at 0.5, 1 and 1.5 M, respectively. However, the performance has decreased when the sodium borohydride concentration has been increased, and the fuel utilization ratios of 29.8%, 21.6% and 20.4% have been obtained at 0.5, 1 and 1.5 M, respectively in the USUF system.     

Direct borohydride fuel cells

The recent, rapid progress in the development of direct borohydride fuel cells is reviewed. Electrochemical reactions are considered together with the importance of operating parameters on cell performance. The advances in technology necessary for a widespread testing and more application of borohydride fuel cells are highlighted. A comparison of borohydride and methanol fuel cells shows that both system exhibit similar cell voltages, current and power densities despite that methanol cells operate at higher temperatures. The results are encouraging although more research is necessary, particularly in the synthesis of new electrocatalysts for borohydride oxidation.     

Direct borohydride fuel cell using Ni-based composite anodes

In this study, nickel-based composite anode catalysts consisting of Ni with either Pd on carbon or Pt on carbon (the ratio of Ni:Pd or Ni:Pt being 25:1) were prepared for use in direct borohydride fuel cells (DBFCs). Cathode catalysts used were 1 mg cm⁻² Pt/C or Pd electrodeposited on activated carbon cloth. The oxidants were oxygen, oxygen in air, or acidified hydrogen peroxide. Alkaline solution of sodium borohydride was used as fuel in the cell. High power performance has been achieved by DBFC using non-precious metal, Ni-based composite anodes with relatively low anodic loading (e.g., 270 mW cm⁻² for NaBH₄/O₂ fuel cell at 60 °C, 665 mW cm⁻² for NaBH₄/H₂O₂ fuel cell at 60 °C). Effects of temperature, oxidant, and anode catalyst loading on the DBFC performance were investigated. The cell was operated for about 100 h and its performance stability was recorded.     

Improving the direct borohydride fuel cell performance with thiourea as the additive in the sodium borohydride solution

In this study, the effects of the additive thiourea (TU) have been investigated under steady state/steady-flow and uniform state/uniform-flow systems with the aim of minimizing the anodic hydrogen evolution on Pd in order to increase the performance of a direct borohydride fuel cell. The fuel cell has consisted of Pd/C anode, Pt/C cathode and Na⁺ form Nafion membrane as the electrolyte. There has been a small improvement in peak power density and fuel utilization ratio by addition of TU (1.6 × 10⁻³ M) into the sodium borohydride solution; the peak power densities of 14.4 and 15.1 mW cm⁻², and fuel utilization ratios of 21.6% and 23.2% have been obtained without and with TU, respectively.     

Manganese dioxide as a cathode catalyst for a direct alcohol or sodium borohydride fuel cell with a flowing alkaline electrolyte

The oxygen reduction reaction at a manganese dioxide cathode in alkaline medium is studied using cyclic voltammetry and by measuring volume of oxygen consumed at the cathode. The performance of the manganese dioxide cathode is also determined in the presence of fuel and an alkali mixture with a standard Pt/Ni anode in a flowing alkaline-electrolyte fuel cell. The fuels tested are methanol, ethanol and sodium borohydride (1 M), while 3 M KOH is used as the electrolyte. The performance of the fuel cell is measured in terms of open-circuit voltage and current–potential characteristics. A single peak in the cyclic voltammogram suggests that a four-electron pathway mechanism prevails during oxygen reduction. This is substantiated by calculating the number of electrons involved per molecule of oxygen that are reacted at the MnO₂ cathode from the oxygen consumption data for different fuels. The results show that the power density of the fuel cell increases with increase in MnO₂ loading to a certain limit but then decreases with further loading. The maximum power density is obtained at 3 mg cm⁻² of MnO₂ for each of the three different fuels.     

A comparison of sodium borohydride as a fuel for proton exchange membrane fuel cells and for direct borohydride fuel cells

Two types of fuel cell systems using NaBH₄ aqueous solution as a fuel are possible: the hydrogen/air proton exchange membrane fuel cell (PEMFC) which uses onsite H₂ generated via the NaBH₄ hydrolysis reaction (B-PEMFC) at the anode and the direct borohydride fuel cell (DBFC) system which directly uses NaBH₄ aqueous solution at the anode and air at the cathode. Recently, research on these two types of fuel cells has begun to attract interest due to the various benefits of this liquid fuel for fuel cell systems for portable applications.It might therefore be relevant at this stage to evaluate the relative competitiveness of the two fuel cells. Considering their current technologies and the high price of NaBH₄, this paper evaluated and analyzed the factors influencing the relative favorability of each type of fuel cell.Their relative competitiveness was strongly dependent on the extent of the NaBH₄ crossover. When considering the crossover in DBFC systems, the total costs of the B-PEMFC system were the most competitive among the fuel cell systems. On the other hand, if the crossover problem were to be completely overcome, the total cost of the DBFC system generating six electrons (6e-DBFC) would be very similar to that of the B-PEMFC system. The DBFC system generating eight electrons (8e-DBFC) became even more competitive if the problem of crossover can be overcome. However, in this case, the volume of NaBH₄ aqueous solution consumed by the DBFC was larger than that consumed by the B-PEMFC.     

Stability of alkaline aqueous solutions of sodium borohydride

Experimental results regarding long-term stability of the alkaline-water borohydride solutions for hydrogen generation are presented. The influence of the concentration of sodium borohydride and sodium hydroxide on the rate of borohydride hydrolysis is analyzed at various temperatures, such as 25 °C, 40 °C, and 80 °C, and various concentrations of NaOH. The rate of hydrolysis decreases with the increase of the water to sodium borohydride mole ratio. For diluted solutions at H₂O/NaBH₄ >30, the rate of hydrolysis and hydrogen generation at a given temperature remains constant. At room temperature in 1.0 N NaOH, the degree of hydrolysis is 0.01% NaBH₄/h that meets the stability requirements for the borohydride solutions during the long-term storage.     

Water management in an alkaline fuel cell

Alkaline fuel cells are low temperature fuel cells for which stationary applications, like cogeneration in buildings, are a promising market. To guarantee a long life, water and thermal management has to be controlled in a careful way. To understand the water, alkali and thermal flows, a model for an alkaline fuel cell module is developed using a control volume approach. Special attention is given to the physical flow of hydrogen, water and air in the system and the diffusion laws are used to gain insight in the water management. The model is validated on the prediction of the electrical performance and thermal behaviour. The positive impact of temperature on fuel cell performance is shown. New in this model is the inclusion of the water management, for which an extra validation is performed. The model shows that a minimum temperature has to be reached to maintain the electrolyte concentration. Increasing temperature for better performance without reducing the electrolyte concentration is possible with humidified hot air.     

Study of CoO as an anode catalyst for a membraneless direct borohydride fuel cell

In this paper, cobalt(II) oxide (CoO) has been used as an anode catalyst in a direct borohydride fuel cell (DBFC). The microstructure of CoO has been characterised by X-ray diffraction. The cell performance and short-term performance stability of the DBFC using the CoO as anode catalyst have been investigated. At the optimum conditions, the maximum power density of 80 mW cm⁻² has been achieved at 30 °C for this cell without using any precious metals and ion exchange membranes. Results from XRD, TEM, and XPS analysis confirm that the good performance of the fuel cell is attributed to the co-operation of CoO and CoB which formed from CoO during the operation.     

Electrooxidation of hydrogen peroxide and sodium borohydride on Ni deposited carbon fiber electrode for alkaline fuel cells

In this study, Ni deposited carbon fiber electrode (Ni/CF) prepared by electroless deposition method was examined for their redox process and electrocatalytic activities during the oxidation of hydrogen peroxide and sodium borohydride in alkaline solutions. The Ni/CF catalyst was characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), scanning electron microscopy (SEM) and electrochemical voltammetry analysis. The electrocatalytic activity of the Ni/CF for oxidation of hydrogen peroxide and sodium borohydride in alkaline solutions was investigated by cyclic voltammetry. The anodic peak current density is found to be three times higher on Ni/CF catalyst for sodium borohydride compared to that for hydrogen peroxide. Preliminary tests on a single cell of a direct borohydride/peroxide fuel cell (DBPFC) and direct peroxide/peroxide fuel cell (DPPFC) indicate that DBPFC with the power density of 5.9 mW cm⁻² provides higher performance than DPPFC (3.8 mWcm⁻²).     

Study of fuel efficiency in a direct borohydride fuel cell

In this study, direct borohydride fuel cells (DBFCs) potentialities are evaluated. These emerging systems make it possible to reach maximum powers of about 200 mW cm⁻² at room temperature and ambient air (natural convection) with high concentrated borohydride solutions. On the other hand, a part of borohydride hydrolyses during cell operating which leads to hydrogen formation and fuel loss: the practical capacity represents about only 18% of the theoretical one. In order to improve fuel efficiency, thiourea is tested as an inhibitor of the catalytic hydrolysis associated with BH₄⁻ electro-oxidation on Pt. The practical capacity is drastically improved: it represents about 64% of the theoretical one. Against, electrochemical performances (I–E curves) are affected by the presence of thiourea.     

Thermodynamic model for an alkaline fuel cell

Alkaline fuel cells are low temperature fuel cells for which stationary applications, e.g. cogeneration in buildings, are a promising market. In order to guarantee a long life, water and thermal management has to be done in a careful way. In order to better understand the water, alkali and thermal flows, a two-dimensional model for an Alkaline Fuel Cell is developed using a control volume approach. In each volume the electrochemical reactions together with the mass and energy balance are solved. The model is created in Aspen Custom Modeller, the development environment of Aspen Plus, where special attention is given to the physical flow of hydrogen, water and air in the system. In this way the developed component, the AFC-cell, can be built into stack configurations to understand its effect on the overall performance. The model is validated by experimental data from measured performance by VITO with their Cell Voltage Monitor at a test case, where the AFC-unit is used as a cogeneration unit.     

A boron-11 NMR study of the stability of the alkaline aqueous solution of sodium borohydride that is both an indirect fuel and a direct fuel for low-temperature fuel cells

Alkaline aqueous solution of sodium borohydride NaBH₄ (denoted SB-fuel) is an indirect fuel when it is used to generate H₂ by hydrolysis, with the as-generated H₂ feeding a fuel cell, and it is a direct fuel when it is an anodic fuel of a direct fuel cell. However, SB-fuel suffers from a major drawback: NaBH₄ spontaneously hydrolyzes. Our study falls within this context. We studied the instability, at the NMR scale and over 12 weeks, of a series of SB-fuels (initial NaBH₄ concentration from 3.65 to 31.22 wt%, NaOH concentration from 1 to 16 M, and temperature between ?15 and 60 °C) to find the conditions at which SB-fuel can be stored for weeks in relative safety. We found that SB-fuel with a NaOH concentration of ≥8 M is relatively stable under cold conditions (?15 and 4 °C). In these conditions, NaBH₄ is not prevented from hydrolyzing, but the reaction is significantly mitigated. Otherwise, our study highlights the gaps in our understanding of the SB-fuel, emphasizes SB-fuel is a new concept of fuel (it should not be seen as any current fuel), and points out the challenges for attaining higher technology readiness levels.     

A computational simulation of an alkaline fuel cell

An advanced one-dimensional, isothermal mathematical model for a single cell of an alkaline fuel cell (AFC) is presented. Advances in an expression for the volume average velocity and in correlations and parameters are achieved. The parameters and operating conditions of the model are based on the Obiter Fuel Cell, which is employed as a power source for NASA space shuttles. A stimulated result is obtained that shows a close agreement with some of the experimental data. Profiles of variables, local overpotential and local current density are also obtained as a function of cell voltage. An investigation of the influence of initial electrolyte concentration shows that the performance of the AFC is maximized at a concentration of 3.5 M. Finally, it is found that increasing the operating pressure steadily enhances cell performance.     

Preparation and characterization of Ni-plated polytetrafluoroethylene plate as an electrode for alkaline fuel cell

At about 50 wt% Ni content, Ni-plated polytetrafluoroethylene (Ni-PTFE) particles show conductivity of 300 S m⁻¹ when plated on 25 μm PTFE particles. For this study, Ni-PTFE particles were formed into the Ni-PTFE plate using heat treatment at 350 °C after 300 kg cm⁻² pressing. The Ni-PTFE plate displayed electrical conductivity and gas permeability. The plate was used as an electrode in an alkaline fuel cell (AFC). In terms of the current density, AFC using the Ni-PTFE electrode plated with Pt or Pd by immersion plating showed improved performance.     

Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review

Hydrogen generation from the hydrolysis of sodium borohydride (NaBH₄) solution has drawn much attention since early 2000s, due to its high theoretical hydrogen storage capacity (10.8 wt%) and potentially safe operation. However, hydrolysis of NaBH₄ for hydrogen generation is a complex process, which is influenced by factors such as catalyst performance, NaBH₄ concentration, stabilizer concentration, reaction temperature, complex kinetics and excess water requirement. All of these limit the hydrogen storage capacities of NaBH₄, whose practical application, however, has not yet reached a scientific and technical maturity. Despite extensive efforts, the kinetics of NaBH₄ hydrolysis reaction is not fully understood. Therefore, better understanding of the kinetics of hydrolysis reaction and development of a reliable kinetic model is a field of great importance in the study of NaBH₄ based hydrogen generation system. This review summarizes in detail the extensive literature on kinetics of hydrolysis of aqueous NaBH₄ solution.     

Performance of an alkaline direct ethanol fuel cell with hydrogen peroxide as oxidant

An alkaline direct ethanol fuel cell (DEFC) with hydrogen peroxide as the oxidant is developed and tested. The present fuel cell consists of a non-platinum anode, an anion exchange membrane, and a non-platinum cathode. It is demonstrated that the peak power density of the fuel cell is 130 mW cm⁻² at 60 °C (160 mW cm⁻² at 80 °C), which is 44% higher than that of the same fuel cell setup but with oxygen as the oxidant. The improved performance as compared with the fuel cell with oxygen as the oxidant is mainly attributed to the superior electrochemical kinetics of the hydrogen peroxide reduction reaction and the reduced ohmic loss associated with the liquid oxidant.     

The use of polypyrrole modified carbon-supported cobalt hydroxide as cathode and anode catalysts for the direct borohydride fuel cell

A polypyrrole modified carbon-supported cobalt hydroxide (Co(OH)₂-PPY-C) has been prepared by the impregnation-chemical method and used as the electrode catalyst in a direct borohydride fuel cell (DBFC). The microstructure of Co(OH)₂-PPY-C has been characterized by X-ray diffraction and transmission electron microscopy. The cell performance and short-term performance stability of the DBFC using the Co(OH)₂-PPY-C as catalysts have been investigated. A maximum power density of 83 mW cm^?2 has been achieved at 0.6 V under ambient conditions. The Co(OH)₂-PPY-C catalyst demonstrates a smaller value of polarization than the carbon-supported Co(OH)₂ catalyst. Results from electrochemical impedance spectrum analysis confirm that the polypyrrole addition to the cathode effectively decreases its resistance. During operation of the DBFC using Co(OH)₂-PPY-C as catalyst, the Co(OH)₂ tends to be converted into CoHO₂.     

Hydrogen generation utilizing alkaline sodium borohydride solution and supported cobalt catalyst

Supported Co catalysts with different supports were prepared for hydrogen generation (HG) from catalytic hydrolysis of alkaline sodium borohydride solution. As a result, we found that a γ-Al₂O₃ supported Co catalyst was very effective because of its special structure. A maximum HG rate of 220 mL min⁻¹ g⁻¹ catalyst and approximately 100% efficiency at 303 K were achieved using a Co/γ-Al₂O₃ catalyst containing 9 wt.% Co. The catalyst has quick response and good durability to the hydrolysis of alkaline NaBH₄ solution. It is feasible to use this catalyst in hydrogen generators with stabilized NaBH₄ solutions to provide on-site hydrogen with desired rate for mobile applications, such as proton exchange membrane fuel cell (PEMFC) systems.     

Apparatus for the production of hydrogen from sodium borohydride in alkaline solution

Extended application of hydrogen as energy carrier demands an economical, safe and reliable technology for storage. In particular, chemical hydrides appear as capable and promising to overcome the issues related to hydrogen safety and handling and to be considered competitive with respect to conventional fuels.