Iron phthalocyanine as a cathode catalyst for a direct borohydride fuel cell

A direct borohydride fuel cell (DBFC) is constructed using a cathode based on iron phthalocyanine (FePc) catalyst supported on active carbon (AC), and a AB₅-type hydrogen storage alloy (MmNi_3.55Co_0.75Mn_0.4Al_0.3) was used as the anode catalyst. The electrochemical properties are investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), etc. methods. The electrochemical experiments show that FePc-catalyzed cathode not only exhibits considerable electrocatalytic activity for oxygen reduction in the BH₄⁻ solutions, but also the existence of BH₄⁻ ions has almost no negative influences on the discharge performances of the air-breathing cathode. At the optimum conditions of 6 M KOH + 0.8 M KBH₄ and room temperature, the maximal power density of 92 mW cm⁻² is obtained for this cell with a discharge current density of 175 mA cm⁻² at a cell voltage of 0.53 V. The new type alkaline fuel cell overcomes the problem of the conventional fuel cell in which both noble metal catalysts and expensive ion exchange membrane were used.     

Manganese dioxide as a new cathode catalyst in microbial fuel cells

This study focused on manganese oxides with a cryptomelane-type octahedral molecular sieve (OMS-2) structure to replace platinum as a cathode catalyst in microbial fuel cells (MFCs). Undoped (ud-OSM-2) and three catalysts doped with cobalt (Co-OMS-2), copper (Cu-OMS-2), and cerium (Ce-OMS-2) to enhance their catalytic performances were investigated. The novel OMS-2 cathodes were examined in granular activated carbon MFC (GACMFC) with sodium acetate as the anode reagent and oxygen in air as the cathode reagent. The results showed that after 400 h of operation, the Co-OMS-2 and Cu-OMS-2 exhibited good catalytic performance in an oxygen reduction reaction (ORR). The voltage of the Co-OMS-2 GACMFC was 217 mV, and the power density was 180 mW m^?2. The voltage of the Cu-OMS-2 GACMFC was 214 mV and the power density was 165 mW m^?2. The internal resistance (R_in) of the OMS-2 GACMFCs (18 ± 1 Ω) was similar to that of the platinum GACMFCs (17 Ω). Furthermore, the degradation rates of organic substrates in the OMS-2 GACMFCs were twice those in the platinum GACMFCs, which enhance their wastewater treatment efficiencies. This study indicated that using OMS-2 manganese oxides to replace platinum as a cathodic catalyst enhances power generation, increases contaminant removal, and substantially reduces the cost of MFCs.     

Ag/C nanoparticles as an cathode catalyst for a zinc-air battery with a flowing alkaline electrolyte

The cyclic voltammetry indicated that the oxygen reduction reaction (ORR) proceeded by the four-electron pathway mechanism on larger Ag particles (174 nm), and that the ORR proceeded by the four-electron pathway and the two-electron pathway mechanisms on finer Ag particles (4.1 nm), simultaneously. The kinetics towards ORR was measured at a rotating disk electrode (RDE) with Ag/C electrode. The number of exchanged electrons for the ORR was found to be close to four on larger Ag particles (174 nm) and close to three on finer Ag particles (4.1 nm). The zinc-air battery with Ag/C catalysts (25.9 nm) was fabricated and examined.     

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.     

Cobalt nickel boride nanocomposite as high-performance anode catalyst for direct borohydride fuel cell

Similar to MXene, MAB is a group of 2D ceramic/metallic boride materials which exhibits unique properties for various applications. However, these 2D sheets tend to stack and therefore lose their active surface area and functions. Herein, an amorphous cobalt nickel boride (Co–Ni–B) nanocomposite is prepared with a combination of 2D sheets and nanoparticles in the center to avoid agglomeration. This unique structure holds the 2D nano-sheets with massive surface area which contains numerous catalytic active sites. This nanocomposite is prepared as an electrocatalyst for borohydride electrooxidation reaction (BOR). It shows outstanding catalytic activity through improving the kinetic parameters of BH₄^? oxidation, owing to abundant ultrathin 2D structure on the surface, which provide free interspace and electroactive sites for charge/mass transport. The anode catalyst led to a 209 mW/cm² maximum power density with high open circuit potential of 1.06 V at room temperature in a miniature direct borohydride fuel cell (DBFC). It also showed a great longevity of up to 45 h at an output power density of 64 mW/cm², which is higher than the Co–B, Ni–B and PtRu/C. The cost reduction and prospective scale-up production of the Co–Ni–B catalyst are also addressed.     

Investigation of amorphous CoB alloy as the anode catalyst for a direct borohydride fuel cell

Co-B amorphous alloy powders have been synthesized by chemical reduction of cobalt chloride with potassium borohydride in an aqueous solution. We find that this alloy can be used as an anode catalyst for a direct borohydride fuel cell (DBFC). This catalyst exhibits excellent electrocatalytic activity. An essential power output of 220 mW cm⁻² has been achieved at 15 °C, and a life test last for 160 h with no attenuation has been observed. The amorphous structure of the CoB alloy is still stable after the life test.     

A direct borohydride fuel cell employing a sago gel polymer electrolyte

The electrochemistry of a direct borohydride fuel cell based on a gel polymer electrolyte was studied. Sago is a type of natural polymer, was employed as the polymer host for the electrolyte. An electrolyte with a composition of sago + 6 M KOH + 2 M NaBH₄ was prepared and evaluated as a novel gel polymer electrolyte for a direct borohydride fuel cell system because it exhibited a high electrical conductivity of 0.270 S cm⁻¹. The rate at which oxygen was consumed at the cathode can be related to the electric current by comparing the calculated number of electrons reacted per molecule of oxygen for different currents supplied to the fuel cell. From the oxygen consumption data, it was deduced that four electrons reacted per molecule of oxygen. The performance of the fuel cell was measured in terms of its current–voltage, discharge and open circuit voltage measurements. The maximum power density obtained was 8.818 mW cm⁻² at a discharge performance of ∼230 mA h and nominal voltage of 0.806 V. The open circuit voltage of the cells was about 0.900 V and sustained for 23 h.     

Direct use of alcohols and sodium borohydride as fuel in an alkaline fuel cell

The performance of an alkaline fuel cell (AFC) was studied at different electrolyte concentrations and temperatures for the direct feeding of methanol, ethanol and sodium borohydride as fuels. Potassium hydroxide is used as the electrolyte in the alkaline fuel cell. The anode was prepared by using Pt black, carbon paper and Nafion dispersion. Nickel mesh was used as the current collector. A standard cathode made of manganese dioxide/carbon paper/Ni-mesh/Teflon dispersion (Electro-Chem-Technic, UK) was used for testing the fuel cell performance. The experimental results showed that the current density increases with increase in KOH concentration. Maximum current densities of 300, 270 and 360 A m⁻² were obtained for methanol, ethanol and sodium borohydride as fuel respectively with 3 M KOH electrolyte at 25 °C. The cell performance decreases with further increase in the KOH concentration. The current density of the alkaline fuel cell increases with increase in temperature for all the three fuels. The increase in current density with temperature is not as high as expected for sodium borohydride. These results are explained based on an electrochemical phenomenon and different associated losses.     

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.     

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.     

LaNi0.8Co0.2O3 as a cathode catalyst for a direct borohydride fuel cell

A perovskite-type oxide LaNi_0.8Co_0.2O₃ is prepared as a direct borohydride fuel cell (DBFC) cathode catalyst. Its electrochemical properties are studied by cyclic voltammetry. The results demonstrate that LaNi_0.8Co_0.2O₃ exhibits excellent electrochemical activity with respect to the oxygen reduction reaction (ORR) and good tolerance of BH₄⁻ ions. Maximum power densities of 114.5 mW cm⁻² at 30 °C and 151.3 mW cm⁻² at 62 °C are obtained, and good stability (300-h stable performance at 20 mA cm⁻²) is also exhibited, which shows that such perovskite-type oxides as LaNi_0.8Co_0.2O₃ can be excellent catalysts for DBFCs.     

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.     

Amorphous NiB alloy decorated by Cu as the anode catalyst for a direct borohydride fuel cell

In this study, amorphous NiB alloy decorated by Cu is prepared by chemical reduction. Moreover, the effect of Cu addition for the electrocatalytic activity of borohydride (BH₄⁻) oxidation is studied. The physical characteristics of NiB or NiBCu_x nanoparticles are confirmed by transmission X-ray diffraction (XRD) and scanning electron microscopy (SEM). The physical characterization results demonstrate that the NiB or NiBCu_x nanoparticles have an amorphous structure and NiBCu_x nanoparticles have improved dispersity. The borohydride oxidation activity of the as-prepared catalysts is investigated by cyclic voltammetry (CV), linear scan voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Results indicate that the NiBCu_x/C nanoparticles have higher electrocatalytic activity for borohydride oxidation than NiB/C, and the borohydride oxidation current of NiBCu_x/C nanoparticles initially increases and then decreases with the increase in Cu content. The optimum molar content of copper in the six prepared catalysts is 2.2%. It is proposed that the addition of Cu is conducive to sodium borohydride adsorption, thereby improving the electro-oxidation activity for borohydride. Hence, amorphous NiB alloy decorated by Cu can enhance the electro-oxidation performance for borohydride.     

A cobalt polypyrrole composite catalyzed cathode for the direct borohydride fuel cell

A cobalt polypyrrole carbon (Co-PPY-C) composite has been attempted for use as a cathode catalyst in a direct borohydride fuel cell (DBFC). A Co-PPY-C composite has been fabricated in laboratory and characterized by the field emission scanning electron microscopy, transmission electron microscopy, as well as X-ray photoemission spectroscopy. Fabricated Co-PPY-C catalyst demonstrates good short-term durability and activity which are comparable to those obtained from the Pt/C catalyst. A maximum power density of 65 mW cm⁻² has been achieved at ambient conditions. This research concludes that metallo-organic coordination compounds would be potential candidates for use as cathode catalysts in the DBFC.     

Efficient nickel catalyst with preferred orientation and microsphere for direct borohydride fuel cell

Ni catalysts with preferred orientation and microsphere were fabricated by a facile one-step pulsed voltage electrodeposition (PVE) method, while the crystal plane orientation and catalytic activity for borohydride oxidation reaction (BOR) were researched systematically. The results indicate that adjusting pulse anode potential can effectively regulate the morphology and crystal plane preferred orientation of Ni catalysts. The optimal Ni-PVE_0.15 catalyst exhibits the highest catalytic activity, stability, and fuel efficiency to BOR. The improvement of catalytic activity of Ni and fuel efficiency is attributed to the more exposure of the (220) crystal plane, which blocks the competing hydrogen evolution reaction (HER), increases the intrinsic activity of the catalytic sites to BOR, and strengthens the adsorption of escaping hydrogen to further electrooxidation. Meanwhile, the no-cracks feature of the microsphere increases the active sites and avoids the shedding of catalysts. The derived insights are important for the design of efficient nickel catalysts for practical applications of direct borohydride fuel cells.     

An intermediate-temperature direct ammonia fuel cell with a molten alkaline hydroxide electrolyte

This paper describes the development and testing of a direct ammonia fuel cell utilizing a molten alkaline hydroxide electrolyte at temperatures between 200 and 450 °C. The advantages of a molten hydroxide fuel cell include the use of a highly conductive and very low-cost electrolyte, inexpensive base metal electrocatalysts, a wide operating temperature range, fuel flexibility, and fast electrode kinetics. The direct use of ammonia in such a fuel cell, even at temperatures as low as 200 °C, is made possible due to the very chemically aggressive nature of the melt. A test cell was constructed using a KOH–NaOH eutectic mixture and produced approximately 40 mW cm⁻² of power at 450 °C while operating on a stream of pure ammonia fed to the anode and compressed ambient air fed to the cathode.     

A liquid electrolyte alkaline direct 2-propanol fuel cell

Prototype alkaline direct 2-propanol fuel cells (AD2PFCs) using commercial Pt/C electrodes and hardware, and a liquid electrolyte, were constructed and compared to the 3-dimensional current-time-potential profiles for the 3-electrode oxidation of 2-propanol. A substantial current maximum occurs at low potentials and is attributed to a change in the mechanism of 2-propanol oxidation. This mechanism change influenced the stability of the AD2PFC; when the cell was polarized to a lower cell voltage limit of 0.5 V, stable and relatively high power densities are achieved. When the cell was polarized to a lower cell voltage limit of 0 V, unstable and only marginally higher power densities were observed. A maximum power density of 22.3 mW mg_Pt⁻¹ was achieved, and most of the cell polarization occurred at the cathode.     

A direct borohydride fuel cell based on poly(vinyl alcohol)/hydroxyapatite composite polymer electrolyte membrane

A new poly(vinyl alcohol)/hydroxyapatite (PVA/HAP) composite polymer membrane was synthesized using a solution casting method. Alkaline direct borohydride fuel cells (DBFCs), consisting of an air cathode based on MnO₂/C inks on Ni-foam, anodes based on PtRu black and Au catalysts on Ni-foam, and the PVA/HAP composite polymer membrane, were assembled and investigated for the first time. It was demonstrated that the alkaline direct borohydride fuel cell comprised of this low-cost PVA/HAP composite polymer membrane showed good electrochemical performance. As a result, the maximum power density of the alkaline DBFC based on the PtRu anode (45 mW cm⁻²) proved higher than that of the DBFC based on the Au anode (33 mW cm⁻²) in a 4 M KOH + 1 M KBH₄ solution at ambient conditions. This novel PVA/HAP composite polymer electrolyte membrane with high ionic conductivity at the order of 10⁻² S cm⁻¹ has great potential for alkaline DBFC applications.     

Modelling of a porous flowing electrolyte layer in a flowing electrolyte direct-methanol fuel cell

The flowing electrolyte-direct methanol fuel cell is a developing technology that may have practical uses in the future. Its main advantage over a direct methanol fuel cell is that it limits methanol crossover using a flowing electrolyte layer. The flowing electrolyte layer (or flowing electrolyte channel) involves an ion-conducting fluid that allows protons to be transported from the anode to the cathode, and flows through a porous material to wash away crossed-over methanol. In this study, the flowing electrolyte layer is modelled as a porous domain in ANSYS CFX. General flow behaviour and the effects of volume flux, channel thickness, and porous material properties are investigated. It is found that the flow has a flattened velocity profile with thin boundary layers that are virtually independent of volume flux and channel thickness. The pressure drop is mainly dependent on the volume flux and the permeability. It is recommended that cell performance could be improved by using a flowing electrolyte channel that is thinner, and selecting a sufficiently high volume flux and a sufficiently permeable porous material to achieve an optimal combination of pressure drop and methanol removal characteristics.