Iron-based electrocatalysts for oxygen reduction in microbial fuel cells

The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts as good candidates for platinum substitution.