Transition metal phthalocyanine-modified shungite-based cathode catalysts for alkaline membrane fuel cell

The alkaline anion exchange membrane fuel cell (AEMFC) is one of the green solutions for the growing need for energy conversion technologies. For the first time, we propose a natural shungite based non-precious metal catalyst (NPMC) as an alternative cathode catalyst to Pt-based materials for AEMFCs application. The Co and Fe phthalocyanine (Pc)-modified shungite materials were prepared via pyrolysis and used for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) studies. The most promising ORR performance was observed in alkaline media for FePc-modified and acid-leached shungite-based NPMC material. The catalysts were also evaluated as cathode materials in a single cell AEMFC and peak power densities of 232 and 234 mW cm^?2 at 60 °C using H₂ and O₂ gases at 100% RH were observed for CoPc- and FePc-modified acid-treated materials, respectively.     

Study of operating conditions and cell design on the performance of alkaline anion exchange membrane based direct methanol fuel cells

Direct methanol fuel cells using an alkaline anion exchange membrane (AAEM) were prepared, studied, and optimized. The effects of fuel composition and electrode materials were investigated. Membrane electrode assemblies fabricated with Tokuyama^® AAEM and commercial noble metal catalysts achieved peak power densities between 25 and 168 mW cm⁻² depending on the operating temperature, fuel composition, and electrode materials used. Good electrode wettability at the anode was found to be very important for achieving high power densities. The performance of the best AAEM cells was comparable to Nafion^®-based cells under similar conditions. Factors limiting the performance of AAEM MEAs were found to be different from those of Nafion^® MEAs. Improved electrode kinetics for methanol oxidation in alkaline electrolyte at Pt-Ru are apparent at low current densities. At high current densities, rapid CO₂ production converts the hydroxide anions, necessary for methanol oxidation, to bicarbonate and carbonate: consequently, the membrane and interfacial conductivity are drastically reduced. These phenomena necessitate the use of aqueous potassium hydroxide and wettable electrode materials for efficient hydroxide supply to the anode. However, aqueous hydroxide is not needed at the cathode. Compared to AAEM-based fuel cells, methanol fuel cells based on proton-conducting Nafion^® retain better performance at high current densities by providing the benefit of carbon dioxide rejection.     

A novel cathode for alkaline fuel cells based on a porous silver membrane

Porous silver membranes were investigated as potential substrates for alkaline fuel cell cathodes and as an approach for studying pore size effects in alkaline fuel cells. The silver membrane provides both the electrocatalytic function, mechanical support and a means of current collection. Relatively high active surface area (∼0.6 m² g⁻¹) results in good electrochemical performance (∼200 mA cm⁻² at 0.6 V and ∼400 mA cm⁻² at 0.4 V) in the presence of 6.9 M KOH. The electrode fabrication technique is described and polarization curves and impedance measurements are used to investigate the performance. The regular structure of the electrodes allows parametric studies of the performance of electrodes as a function of pore size. Impedance spectra have been fitted with a proposed equivalent circuit which was obtained following the study of impedance measurements under different experimental conditions (electrolyte concentration, oxygen concentration, temperature, and pore size). The typical impedance spectra consisted of one high frequency depressed semi-circle related to porosity and KOH wettability and one low-frequency semi-circle related to kinetics. A passive air-breathing hydrogen-air fuel cell constructed from the membranes in which they act as mechanical support, current collector and electrocatalyst achieves a peak power density of 50 mW cm⁻² at 0.40 V cell potential when operating at 25 °C.     

Microbial fuel cell performance with non-Pt cathode catalysts

Various cathode catalysts prepared from metal porphyrines and phthalocyanines were examined for their oxygen reduction activity in neutral pH media. Electrochemical studies were carried out with metal tetramethoxyphenylporphyrin (TMPP), CoTMPP and FeCoTMPP, and metal phthalocyanine (Pc), FePc, CoPc and FeCuPc, supported on Ketjenblack (KJB) carbon. Iron phthalocyanine supported on KJB (FePc-KJB) carbon demonstrated higher activity towards oxygen reduction than Pt in neutral media. The effect of carbon substrate was investigated by evaluating FePc on Vulcan XC carbon (FePcVC) versus Ketjenblack carbon. FePc-KJB showed higher activity than FePcVC suggesting the catalyst activity could be improved by using carbon substrate with a higher surface area. With FePc-KJB as the MFC cathode catalyst, a power density of 634 mW m⁻² was achieved in 50 mM phosphate buffer medium at pH 7, which was higher than that obtained using the precious-metal Pt cathode (593 mW m⁻²). Under optimum operating conditions (i.e. using a high surface area carbon brush anode and 200 mM PBM as the supporting electrolyte with 1 g L⁻¹ acetate as the substrate), the power density was increased to 2011 mW m⁻². This high power output indicates that MFCs with low cost metal macrocycles catalysts is promising in further practical applications.     

Electrochemical and fuel cell evaluation of Co based catalyst for oxygen reduction in anion exchange polymer membrane fuel cells

Co based catalyst were evaluated for oxygen reduction (ORR) in liquid KOH and alkaline anion exchange membrane fuel cells (AAEMFCs). In liquid KOH solution the catalyst exhibited good performance with an onset potential 120 mV more negative than platinum and a Tafel slope of ca. 120 mV dec⁻¹. The hydrogen peroxide generated, increased from 5 to 50% as the electrode potential decreased from 175 to −300 mV vs. SHE.In an AAEMFC environment, one catalyst (GP2) showed promising performance for ORR, i.e. at 50 mA cm⁻² the differences in cell potential between the stable performance for platinum (more positive) and cobalt cathodes with air and oxygen, were only 45 and 67 mV respectively. The second catalyst (GP4) achieved the same stable power density as with platinum, of 200 and 145 mW cm⁻², with air at 1 bar (gauge) pressure and air (atm) cathode feed (60 °C), respectively. However the efficiency was lower (i.e. cell voltage was lower) i.e. 40% in comparison to platinum 47.5%.     

Non-precious metal catalysts synthesized from precursors of carbon, nitrogen, and transition metal for oxygen reduction in alkaline fuel cells

Non-precious metal catalysts (NPMCs) synthesized from the precursors of carbon, nitrogen, and transition metals were investigated as an alternate cathode catalyst for alkaline fuel cells (AFCs). The procedures to synthesize the catalyst and the post-treatment were tailored to refine its electrocatalytic properties for oxygen reduction reaction (ORR) in alkaline electrolyte. The results indicated that the performance of NPMCs prepared with carbon-supported ethylenediamine-transition metal composite precursor and subjected to heat-treatment shows comparable activity for oxygen reduction with Pt/C catalyst. The NPMC exhibits an open circuit potential of 0.97 V and a maximum power density of 177 mW cm⁻² at 50 °C when tested in anion exchange membrane (AEM) fuel cells.     

Carbon-supported Pd-Pt cathode electrocatalysts for proton exchange membrane fuel cells

A series of carbon-supported Pd-Pt alloy (Pd-Pt/C) catalysts for oxygen reduction reaction (ORR) with low-platinum content are synthesized via a modified sodium borohydride reduction method. The structure of as-prepared catalysts is characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The prepared Pd-Pt/C catalysts with alloy form show face-centered-cubic (FCC) structure. The metal particles of Pd-Pt/C catalysts with mean size of around 4-5 nm are uniformly dispersed on the carbon support. The electrocatalytic activities for ORR of these catalysts are investigated by rotating disk electrode (RDE), cyclic voltammetry (CV), single cell measurements and electrochemical impedance spectra (EIS) measurements. The results suggest that the electrocatalytic activities of Pd-Pt/C catalysts with low platinum are comparable to that of the commercial Pt/C with the same metal loading. The maximum power density of MEA with a Pd-Pt/C catalyst, the Pd/Pt mass ratio of which is 7:3, is about 1040 mW cm⁻².     

An improved cathode for alkaline fuel cells

The use of nickel foam as an electrode substrate in alkaline fuel cells (AFCs) has been investigated for bipolar cells incorporating an electrically conducting gas diffusion layer (GDL). Improved performance, compared to a previous design, was obtained by adding an extra active layer (AL) composed of manganese (IV) oxide (MnO₂) deposited onto carbon black. This new cathode design performed significantly better (130 mA cm⁻² at 0.8 V and 25 °C) than the previous design (35 mA cm⁻² under the same condition), especially at higher potential. It has been shown that the GDL is a key component of the gas diffusion electrode for both performance and durability, especially with liquid electrolytes. Electrochemical impedance spectroscopy was used to study the various losses in the cathode as a function of electrolyte concentration, oxygen concentration and cell voltage. The typical impedance spectra consisted of one high frequency depressed semi-circle which is believed to be related to porosity and KOH wettability, and one low frequency semi-circle related to electrode kinetics.     

Non precious metal catalysts for the PEM fuel cell cathode

Low temperature fuel cells, such as the proton exchange membrane (PEM) fuel cell, have required the use of highly active catalysts to promote both the fuel oxidation at the anode and oxygen reduction at the cathode. Attention has been particularly given to the oxygen reduction reaction (ORR) since this appears to be responsible for major voltage losses within the cell. To provide the requisite activity and minimse losses, precious metal catalysts (containing Pt) continue to be used for the cathode catalyst. At the same time, much research is in progress to reduce the costs associated with Pt cathode catalysts, by identifying and developing non-precious metal alternatives. This review outlines classes of non-precious metal systems that have been investigated over the past 10 years. Whilst none of these so far have provided the performance and durability of Pt systems some, such as transition metals supported on porous carbons, have demonstrated reasonable electrocatalytic activity. Of the newer catalysts, iron-based nanostructures on nitrogen-functionalised mesoporous carbons are beginning to emerge as possible contenders for future commercial PEMFC systems.     

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.     

Performance of solid alkaline fuel cells employing anion-exchange membranes

For the performances of solid alkaline fuel cells (SAFCs) using anion-exchange membranes (AEMs), anion-exchange membranes were prepared via chloromethylation and amination of polysulfone and membrane-electrode assemblies (MEAs) were fabricated using the AEMs as an electrolyte, the ionomer binder prepared by the AEMs and Pt/C and Ag/C electrocatalysts as an anode and a cathode, respectively. Anion-exchange membranes were aminated by a mixing amine agent of trimethylamine (TMA) as a monoamine and various diamines such as N,N,N′,N′-tetramethylmethanediamine (TMMDA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N′-tetramethyl-1,3-propandiamine (TMPDA), N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBDA) and N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA). Amination using various diamines enabled to investigate the effect of the length of alkyl chain of the diamines on membrane properties such as ion conductivity and thermal characteristics. The AEMs aminated by the amination agent of mixing TMA and TMHDA (with longer alkyl chain) showed better hydroxyl ion conductivity and thermal stability than those aminated by a diamine. The H₂/air SAFC performance of the MEA with 0.5 mg cm⁻² Pt/C at the anode and the cathode, respectively, was comparable to one with 0.5 mg cm⁻² Pt/C at the anode and 2.0 mg cm⁻² Ag/C at the cathode, i.e., approximately 28–30 mW cm⁻² of the peak power density range.     

Synthesis and characterization of the Au-modified Pd cathode catalyst for alkaline direct ethanol fuel cells

Three kinds of Au-modified Pd catalysts with the alloy, core–shell and physically-mixed structures are prepared with carbon nanotubes as the support. The structural details and the particle sizes of the prepared catalysts are characterized by X-ray diffraction and transmission electron microscopy. The experimental results indicate that the Au-modified Pd catalysts show smaller particle sizes with narrower particle size distributions than the mono-Pd catalyst does. The catalytic activities of the prepared catalysts for oxygen reduction and ethanol oxidation are examined by electrochemical measurements. Compared with the mono-Pd catalyst, the Au-modified Pd catalyst with the physically-mixed structure exhibits increased catalytic activity for oxygen reduction, but decreased catalytic activity for ethanol oxidation in alkaline media. The cell performance with the Au-modified Pd as the cathode catalyst yielded a peak power density of as high as 185 mW cm⁻², which is about 1.4 times higher than that with the mono-Pd cathode.     

Imidazolium-functionalized polysulfone hydroxide exchange membranes for potential applications in alkaline membrane direct alcohol fuel cells

A series of imidazolium-functionalized polysulfones were successfully synthesized by chloromethylation-Menshutkin two-step method. PSf-ImOHs show the desired selective solubility: insoluble in alcohols (e.g., methanol and ethanol), and soluble in 50 vol.% aqueous solutions of acetone or tetrahydrofuran, implying their potential applications for both the alcohol-resistant membranes themselves and the ionomer solutions in low-boiling-point water-soluble solvents. PSf-ImOH also possesses very high thermal stability (T_OD: 258 °C), higher than quaternary ammonium and quaternary phosphonium functionalized polysulfones (T_OD: 120 °C and 186 °C, repsectively). Ion exchange capacity (IEC) of PSf-ImOH membranes ranges from 0.78 to 2.19 mmol g⁻¹ with degree of chloromethylation from 42% to 132% of original chloromethylated polysulfone. As expected, water uptake, swelling ratio, and hydroxide conductivity increase with IEC and temperatures. With 2.19 mmol g⁻¹ of IEC, the PSf-ImOH 132% membrane exhibits the highest hydroxide conductivity (53 mS cm⁻¹ at 20 °C), higher than those of all other reported polysulfone-based HEMs (1.6–45 mS cm⁻¹) and other imidazolium-functionalized HEMs (19.6–38.8 mS cm⁻¹). In addition, PSf-ImOH membranes have low methanol permeability of 0.8–4.7 × 10⁻⁷ cm² s⁻¹, one order of magnitude smaller than that of Nafion212 membrane. All these properties indicate imidazolium-functionalized polysulfone is very promising for potential applications in alkaline membrane direct alcohol fuel cells.     

Cobalt oxyphosphide as oxygen reduction electrocatalyst in proton exchange membrane fuel cell

The cobalt oxyphosphides supported on carbon black were prepared using incipient wetness method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The possibility of their application as the electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) was investigated and the electrocatalytic activities were evaluated by the electrochemical measurements and single cell test, respectively. The electrocatalyst presents attractive catalytic activity towards ORR and good stability in acid media and exhibits an onset potential for oxygen reduction as high as 0.69 V (RHE) in H₂SO₄ solution. The maximum power density obtained in a H₂/O₂ PEMFC is 57 mW cm⁻² with Co₄P₂O₉/C loading of 1.13 mg cm⁻². No significant performance degradation is observed over 50 h of continuous fuel cell operation. The combination of heteroatom P with nanostructured oxides with high stability, excellent functionality and low cost which are prerequisites for large-scale applications, probably provide a new solution for the critical challenge of finding effective cathode materials for PEMFC.     

Ag nanoparticles on mesoporous carbon support as cathode catalyst for anion exchange membrane fuel cell

Silver nanocatalyst (40 wt%) is deposited on commercial mesoporous carbon support material (Ag/C) using two different wet chemical methods, to obtain high electrochemically active surface area. The catalyst materials are characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetric analysis and are evaluated toward the oxygen reduction reaction (ORR) in alkaline media employing the rotating disk electrode method. It is worth noting that the Ag/C leads to oxygen reduction through a direct four-electron pathway in alkaline medium. The silver catalyst on mesoporous carbon exhibits relatively higher mass activity for ORR (38 A g⁻¹) compared to that with Vulcan carbon (32 A g⁻¹) at −0.2 V vs SCE at room temperature. Anion exchange membrane fuel cell shows maximum power density of 310 mW cm⁻² with Ag/C cathode catalyst using H₂ and O₂ gases at 65% RH conditions at 65 °C.     

Iron and cobalt containing electrospun carbon nanofibre-based cathode catalysts for anion exchange membrane fuel cell

The use of Pt-based cathode catalyst materials hinders the widespread application of anion exchange membrane fuel cells (AEMFCs). Herein, we present a non-precious metal catalyst (NPMC) material based on pyrolysed Fe and Co co-doped electrospun carbon nanofibres (CNFs). The prepared materials are studied as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts in alkaline and acidic environments. High activity towards the ORR in alkaline solution indicated the suitability of the prepared NPMCs for the application at the AEMFC cathode. In the AEMFC test, the membrane-electrode assembly bearing a cathode with the nanofibre-based catalyst prepared with the ionic liquid (IL) (Fe/Co/IL–CNF–800b) showed the maximum power density (P_max) of 195 mW cm⁻², which is 78% of the P_max obtained with a commercial Pt/C cathode catalyst. Such high ORR electrocatalytic activity was attributed to the unique CNF structure, high micro-mesoporosity, different nature of nitrogen species and metal-N_x active centres.     

A facile synthesis of Pd/C cathode electrocatalyst for proton exchange membrane fuel cells

A carbon supported palladium (Pd/C-NaBH₄-NH₃) catalyst was synthesized via modified sodium borohydride reduction method using ammonia as the complexing reagent. The Pd/C catalysts were characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Rotating disk electrode (RDE), cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and single cell measurements were employed to evaluate the activities of the catalysts. The as-prepared catalysts with face-centered cubic (fcc) structure are uniformly dispersed on the carbon supports. Twinned and polycrystalline structures are observed in the HRTEM image of Pd/C-NaBH₄-NH₃. The results indicate that the Pd/C-NaBH₄-NH₃ catalyst shows high activity for the oxygen reduction reaction. Single cell with Pd/C-NaBH₄-NH₃ as the cathode displays a maximum power density of 508 mW cm⁻². The favorable performance of the Pd/C-NaBH₄-NH₃ catalyst may be attributed to the uniformly dispersed nanoparticles and more crystalline lattice defects.     

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem^® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.     

Anion-exchange membranes composed of quaternized-chitosan derivatives for alkaline fuel cells

A series of quaternized-chitosan derivatives (QCDs) with various degrees of quaternization was synthesized using glycidyltrimethylammonium chloride as a main quaternized reagent. These QCDs were then processed into hydroxide—form quaternary ammonium salts with aqueous potassium hydroxide solutions. The resultant hydroxide—form QCD gels were further crosslinked into anion-exchange membranes using ethylene glycol diglycidyl ether. The crosslinking density, crystallinity, swelling index, ion exchange capacity, ionic conductivity and thermal stability of the crosslinked membranes were subsequently investigated. It was found that properties of crosslinked membranes were modulated mainly by the degree of quaternization and crosslinking density of membranes. Some membranes exhibited promising characteristics and had the potential for applications in alkaline polymer electrolyte fuel cells in considering their integrative properties.