Microbial fuel cells (MFCs) produce electricity as a result of the microbial metabolism of organic substrates, hence they represent a sustainable approach for energy production and waste treatment. If the technology is to be implemented in industry, low cost and sustainable bioelectrodes must be developed to increase power output, increase waste treatment capacity, and improve service intervals. Although the current application of abiotic electrode catalysts, such as platinum and electrode binders such as Nafion leads to greater MFC performance, their use is cost prohibitive. Novel bioelectrodes which use cost effective and sustainable materials are being developed. These electrodes are developed with the intention to reduce start-up time, reduce costs, extend life-span and improve core MFC performance metrics (i.e. power density, current density, chemical oxygen demand (COD) reduction and Coulombic efficiency (CE)). Comparison of different MFC systems is not an easy task. This is due to variations in MFC design, construction, operation, and different inocula (in the case of mixed-culture MFCs). This high intra-system variability should be considered when assessing MFC data, operation and performance. This review article examines the major issues surrounding bioanode and biocathode improvement in different MFC systems, with the ultimate goal of streamlining and standardising improvement processes.
Ba(Ti,Fe)O 3 is a useful system for the exploration of multiferroic properties as a function of composition and variation in structure, based upon a model of intersubstitution of the B site cation. Nanocrystals of Ba(Ti,Fe)O 3 could be used as building blocks for composite multiferroic materials, provided ferroic properties are recognizable at this length scale and Ti and Fe serve as ideal models for the case of d 0 versus d n in a ferroic perovskite. A series of iron-substituted barium titanate nanocrystals (BaTi 1−x Fe x O 3 ) were synthesized at 60 °C using a hybrid sol−gel chemical solution processing method. No further crystallization/ calcination steps were required. The as-prepared nanocrystals are fully crystalline, uniform in size (∼8 nm by TEM), and dispersible in polar organic solvents, yielding nanocrystal/alcohol formulations. Complete consumption of the reactant precursors ensures adequate control over stoichiometry of the final product, over a full range of x (0, 0.1 to 0.75, 1.0). Pair distribution function (PDF) analysis enabled in-depth structural characterization (phase, space group, unit cell parameters, etc.) and shows that, in the case of x = 0, 0.1, 0.2, 0.3, BaTiO 3 , and BaTi 1−x Fe x O 3 nanocrystals, it is concluded that they are tetragonal noncentrosymmetric P4mm with lattice parameters increasing from, e.g., c = 4.04 to 4.08 Å. XPS analysis confirms the presence of both Fe 3+ (d 5 ) and Fe 4+ (d 4 ), both candidates for multiferroicity in this system, given certain spin configurations in octahedral field splitting. The PDF cacluated lattice expansion is attributed to Fe 3+ (d 5 , HS) incorporation. The evidence of noncentrosymmetry, lattice expansion, and XPS conformation of Fe 3+ provides support for the existence of multiferroicity in these sub-10 nm uniform dispersed nanocrystals. For x > 0.5, Fe impacts the structure but still produces dispersible, relatively monodisperse nanocrystals. XPS also shows an increasing amount of Fe 4+ with increasing Fe, suggesting that Fe(IV) is evolving as charge compensation with decreasing Ti 4+ , while attempting to preserve the perovskite structure. A mixture of Fe 3+ /Fe 4+ is thought to reside at the B site: Fe 4+ helps stabilize the structure through charge balancing, while Fe 3+ may be complimented with oxygen vacancies to some extent, especially at the surface. The structure may therefore be of the form BaTi 1−x Fe x O 3-δ for increasing x. At higher concentrations (Fe > 0.5) the emergence of BaFeO 3 and/or BaFe 2 O 4 is offered as an explanation for competing phases, with BaFeO 3 as the likeliest competing phase for x = 1.0. Because of the good dispersibility of the nanocrystals in solvents, spin coating of uniform 0−3 nanocomposite BaTi 1−x Fe x O 3 /polyvinylpyrrolidone thin film capacitors (<0.5 μm) was possible. Frequency dependent dielectric measurements showed stable dielectric constants at 1 MHz of 27.0 to 22.2 for BaTi 1−x Fe x O 3 samples for x = 0−0.75, respectively. Loss tangent
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