Rebecca Jayne Thorne

Biophotovoltaics: oxygenic photosynthetic organisms in the world of bioelectrochemical systems

The field of bioelectrochemical system (BES) research includes a wide range of emerging technologies that utilise microbes to catalyze anodic and/or cathodic reactions within a fuel cell setup, and has developed greatly in the last 2–3 years. Although the vast majority of BESs utilise organic substrates as a fuel source (e.g. microbial fuel cells), several systems have been developed that are fuelled by light energy. In this review we focus on and contextualise a specific subset of light-harvesting BESs, which we have called biophotovoltaic systems (BPVs). BPVs utilise oxygenic photosynthetic organisms, such as microalgal and cyanobacterial species, to harvest light energy to generate current, critically, in the absence of an organic feedstock. Here we discuss the state-of-the-art for all light-harvesting BESs and present a novel classification system to illustrate how BPVs integrate into the broad fields of BES and photovoltaic research. We compare and contrast the present understanding of electron transfer pathways in systems that use heterotrophic microbes with those in cyanobacteria-based BPVs. Finally we present, for the first time, an estimate of the achievable power outputs of this emerging technology.

Surface morphology and surface energy of anode materials influence power outputs in a multi-channel mediatorless bio-photovoltaic (BPV) system

Bio-photovoltaic cells (BPVs) are a new photo-bio-electrochemical technology for harnessing solar energy using the photosynthetic activity of autotrophic organisms. Currently power outputs from BPVs are generally low and suffer from low efficiencies. However, a better understanding of the electrochemical interactions between the microbes and conductive materials will be likely to lead to increased power yields. In the current study, the fresh-water, filamentous cyanobacterium Pseudanabaena limnetica (also known as Oscillatoria limnetica) was investigated for exoelectrogenic activity. Biofilms of P. limnetica showed a significant photo response during light-dark cycling in BPVs under mediatorless conditions. A multi-channel BPV device was developed to compare quantitatively the performance of photosynthetic biofilms of this species using a variety of different anodic conductive materials: indium tin oxide-coated polyethylene terephthalate (ITO), stainless steel (SS), glass coated with a conductive polymer (PANI), and carbon paper (CP). Although biofilm growth rates were generally comparable on all materials tested, the amplitude of the photo response and achievable maximum power outputs were significantly different. ITO and SS demonstrated the largest photo responses, whereas CP showed the lowest power outputs under both light and dark conditions. Furthermore, differences in the ratios of light : dark power outputs indicated that the electrochemical interactions between photosynthetic microbes and the anode may differ under light and dark conditions depending on the anodic material used. Comparisons between BPV performances and material characteristics revealed that surface roughness and surface energy, particularly the ratio of non-polar to polar interactions (the CQ ratio), may be more important than available surface area in determining biocompatibility and maximum power outputs in microbial electrochemical systems. Notably, CP was readily outperformed by all other conductive materials tested, indicating that carbon may not be an optimal substrate for microbial fuel cell operation.

Porous ceramic anode materials for photo-microbial fuel cells

This study focuses on porous ceramics as a promising new type of anode material for photo-microbial fuel cells (p-MFCs). The anodes were made from titanium dioxide and chemical vapour deposition was used to coat them with a layer of fluorine doped tin oxide (FTO) to make them conducting. Chlorella vulgaris biofilms were grown in the millimetre sized pores of the ceramic electrodes, producing an extensive extra cellular matrix that was anchored directly to the electrode surface. In contrast algal cells grown on carbon felt appeared misshapen and lacked a continuous extra cellular matrix. A preliminary comparison of different anodes in p-MFCs showed that the power density was ∼16 times higher on a ceramic anode compared to the best performing carbon anode. Good power densities were also found for algae grown directly onto FTO coated glass, but in contrast to the ceramic anodes the biofilm did not adhere strongly to the planar surface and was easily removed or damaged.

An investigation of anode and cathode materials in photomicrobial fuel cells

Photomicrobial fuel cells (p-MFCs) are devices that use photosynthetic organisms (such as cyanobacteria or algae) to turn light energy into electrical energy. In a p-MFC, the anode accepts electrons from microorganisms that are either growing directly on the anode surface (biofilm) or are free floating in solution (planktonic). The nature of both the anode and cathode material is critical for device efficiency. An ideal anode is biocompatible and facilitates direct electron transfer from the microorganisms, with no need for an electron mediator. For a p-MFC, there is the additional requirement that the anode should not prevent light from perfusing through the photosynthetic cells. The cathode should facilitate the rapid reaction of protons and oxygen to form water so as not to rate limit the device. In this paper, we first review the range of anode and cathode materials currently used in p-MFCs. We then present our own data comparing cathode materials in a p-MFC and our first results using porous ceramic anodes in a mediator-free p-MFC.

Iron reduction by the cyanobacterium Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 uptakes iron using a reductive mechanism, similar to that exhibited by many other microalgae. Various bio-electrochemical technologies have made use of this reductive cellular capacity, but there is still a lack of fundamental understanding of cellular reduction rates under different conditions. This study used electrochemical techniques to further investigate the reductive interactions of Synechocystis cells with Fe(III) from the iron species potassium ferricyanide, with varying cell and ferricyanide concentrations present. At the lowest cell concentrations tested, cell reduction machinery appeared to kinetically limit the reduction reaction, but ferricyanide reduction rates were mass transport controlled at the higher cell and ferricyanide concentrations studied. Improving the understanding of the reduction of Fe(III) by whole cyanobacterial cells is important for improving the efficiencies of technologies that rely on this interaction.

Correction: Biophotovoltaics: oxygenic photosynthetic organisms in the world of bioelectrochemical systems

Correction for ‘Biophotovoltaics: oxygenic photosynthetic organisms in the world of bioelectrochemical systems’ by Alistair J. Mccormick et al., Energy Environ. Sci., 2015, DOI: 10.1039/c4ee03875d.

Interaction Between Anode Aggregate and Binder in the Sessile Drop Wetting Test

Coal Tar Pitch (CTP), recycled butts and Calcined Petroleum Coke (CPC) are the main raw materials used for anode fabrication. Over the last decades, considerable changes in CTP and CPC quality have been observed. Changes in metallurgical coke production technology have affected coal tar quality, and crude oil qualities have changed causing severe changes of CPC properties. This can have consequences for the mixing step during anode production, and the final anode performance. Hydro Aluminium has systematically followed these raw material changes and studied the effect on the interaction between coke and binders by applying a sessile drop measurement wetting test. The results are reviewed in terms of relevance for the anode production process. This article focuses on equipment, routines and sample preparation as they are crucial factors for obtaining reliable and repeatable results during these experiments. Wetting dependence on particle size and butt impurity level was found. Additionally, a method where hot pitch is added to a hot coke substrate is demonstrated.

Transition to a Low Carbon Economy; Impacts to Health and the Environment

As carbon dioxide (CO2) emissions and other greenhouse gases (GHGs) represent both a great environmental challenge in terms of climate change and a problem for human health and social issues, their emission must be mitigated. Technological pathways to a low carbon economy include improving resource application/energy intensity, or by cutting carbon intensity through promoting low carbon technologies such as renewable energy or carbon capture and storage (CCS). For these technologies to be a truly effective option to mitigate climate change, they must be sustainable and be protective of the environment and human health over the long-term. Whilst analysis of most initiatives focuses on direct GHG quantification, other benefits and potential impacts are often not considered, and indirect emissions are often overlooked. In this chapter, the benefits of adapting to a low carbon economy are assessed, and technological adaptation strategies discussed along with associated impacts to human health and the environment. In particular, CCS is focused upon and life cycle assessment (LCA) shown as a particularly useful tool to give a holistic view of the impacts relating to mitigation options. These are important decision criteria for policymakers, and should be dealt with as such.