Macroporous monolithic Magnéli-phase titanium suboxides as anode material for effective bioelectricity generation in microbial fuel cells

Abstract

A macroporous monolithic ceramic anode material based on Magnéli-phase titanium suboxides, fabricated by a facile method, is able to effectively generate bioelectricity in microbial fuel cells (MFCs). Owing to their highly active surface area and efficient extracellular electron transfer from electricigens to the anode, MFCs can achieve a peak biocurrent time of 37 h with a maximum power density of 1541 AE 18 mW m À2 .Microbial fuel cells (MFCs) are able to convert chemical energy from organic substances into electricity by an exoelectrogenic biolm grown on the anode, which offers a sustainable technology to recover energy from organic waste. 1 In MFCs, the electrons stored in organic matter are rst released during the microbial metabolism, followed by transfer to the anode through the extracellular electron transfer (EET) process. The EET can proceed via direct contact of c-type cytochromes on the bacterial outer membrane, 2 long-distance transport by conductive bacterial pili (i.e. nanowires), 3 and indirect mediated transfer via electron shuttles.4 As EET constitutes one of the most signicant steps that limit the magnitude of power produced in an MFC, there is an increasing interest in developing effective anode materials and structures for electroactive biolm attachment and power generation.Much effort has been invested in exploring anode materials for the enrichment of exoelectrogenic bacteria. These anode materials should have an open-cell macro-and mesoscale porous structure to allow a multi-dimensional colonization and to increase the anolyte-biolm-anode interfacial area for mass transfer and electron transport.5 For materials containing 2D and 3D microscale pores, the rapid microbial growth will clog the pores and hinder the inner colonization and substrate diffusion.6 Moreover, the long-term mechanical strength and stability of conventional carbon-based electrodes may be another concern as a consequence of interfacial corrosion and oxidation of the anode material.7 Based on these considerations, it will be highly desirable to develop macroporous, highly conductive and chemically stable electrode materials for efficient attachment of electrochemically active microorganisms and viable electricity generation in MFC systems.Titanium dioxide (TiO 2 ) is a well-known semi-conductor, which has been widely used in photocatalytic reactions. Interestingly, the electronic properties of TiO 2 can be drastically transformed by establishing oxygen deciencies within the crystalline lattice, affording what is known as Magnéli-phase titanium suboxides, with a generic formula of Ti x O 2xÀ1 (4 # x # 10).8 Magnéli-phase titanium suboxides have a crystal structure with an oxygen deciency for every xth layer, resulting in shear planes where 2D chains of octahedra become face-sharing to accommodate the deciency in oxygen. This unique structure leads to a combination of outstanding electrical conductivity similar to that of metals and a great corrosion resistance close to that of ceramic materials.9 For example, Ti 4...

Different Reactivity of Rutile and Anatase TiO2 Nanoparticles: Synthesis and Surface States of Nanoparticles of Mixed‐Valence Magnéli Oxides

“…Titanium oxides Ti n O 2nÀ1 (4 n 10), so-called titanium MagnØli phases,w ere discovered in the 1950s [1] and since the 2010s have experiencedarenewal of interesto wing to their high conductivity and good stability,r elatedt ot he mixed Ti 4 + /Ti 3 + valences and to their oxidic nature,r espectively.T hese properties have prompted studies on MagnØli phases in recenty ears as conductive scaffolds for use under harsh conditions as cathodes for aprotic lithium-air [2][3][4] and lithium-sulfur batteries, [5][6][7][8] anodesf or alcohol oxidation, [9,10] microbial fuel cells, [11] and water remediation membranes. [12,13] Superconductivity has also been recently evidenced in an epitaxially grown MagnØli phase.…”

“…For instance, many authors attribute the enhancedc yclability of Ti 4 O 7 -sulfur electrodes for Li-S batteries to the chemical bonding between sulfur atoms and Ti atoms at the surface of the Ti 4 O 7 electrodes. Although X-ray photoelectron spectroscopy( XPS) has shown that Ti 4 + and Ti 3 + coexista tt he surface of MagnØli materials, [4,6,9,11,15] only two reports suggest surface oxidation of Ti 4 O 7 materials, [2,8] but precise evaluation of surface states is lacking, so that the conduction ands orption properties of the surface with thicknesses of af ew layers remain mostly ab lack box. For theser easons, increased surface-to-volume ratio is sought through nanostructuring, whiche mphasizes the importance of deep scrutiny of the compositiona nd reactivity of MagnØli oxide surfaces.…”

“…The first pathway relies on carbothermal reduction of TiO 2 at about 800-1100 8C. [3,4,6,11,13] In this route, crystal growth is difficult to restrain andl eads to materials with submicrometer length scale. [5,7,8,[18][19][20][21] These residues confine the oxide during the transformation of TiO 2 into MagnØli phases and enable recovery of nanostructured materials.…”

“…It unavoidably produces carbon residues, which accountf or about 2-20 wt %o ft he products. [6,11,13] Attempts to limit grain growth by silica templating [3,4,15] encountered only limited success, leading mostly to 100-300nms pheres [3,4] after dissolution of the silica coating. However,c arbonaceous moieties are present at the surface of the MagnØli material.…”

“…[3,4,6,11,13] In this route, crystal growth is difficult to restrain andl eads to materials with submicrometer length scale. [6,11,13] Attempts to limit grain growth by silica templating [3,4,15] encountered only limited success, leading mostly to 100-300nms pheres [3,4] after dissolution of the silica coating. Herein,w ep ropose an alternative methodt op roduce carbon-free MagnØli nanoparticles ( Figure 1) by reduction of a low-costc ommercial TiO 2 product, so-called P25, am ixture of nanoparticles of anatase and rutile TiO 2 polymorphs.…”