Poly(4‐vinylpyridine)‐modified copper(I) oxide photocathodes, bare or decorated with palladium nanoparticles, can be used for the visible‐light‐driven selective reduction of CO2, mostly to methanol and carbon monoxide (in the absence of Pd cocatalyst) or formic acid and carbon monoxide (in the presence of Pd cocatalyst). The photocathode materials, which are composed of hierarchically deposited (onto transparent fluorine‐doped conducting glass electrode) Cu2O (inner layer) and poly(4‐vinylpyridine) (P4VP) polyelectrolyte film (outer‐layer), with or without dispersed Pd nanoparticles, have been fabricated and examined using electrochemical methodology, atomic force microscopy, and various spectroscopic techniques, including the Raman approach. While the physicochemical characteristics of Cu2O, including the oxide identity and its semiconducting properties, are not largely affected by the interfacial modification with P4VP, the photostability of the hybrid photocathode is significantly improved. The P4VP‐modified Cu2O exhibits reasonable durability during photoelectrochemical reduction of CO2 upon illumination with sunlight in semi‐neutral medium (Na2SO4). While Cu2O can be identified using Raman spectroscopy as the sole bulk component of the semiconducting film, the XPS data imply that the Cu2O surface is likely to be partially reduced during prolonged operation. Introduction of palladium cocatalyst seems to improve distribution (collection and transport) of photoelectrons at the photoelectrochemical interface and affects the CO2‐electroreduction mechanism.

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Poly(4-vinylpyridine)-modified copper(I) oxide photocathodes, bare or decorated with palladium nanoparticles, can be used for the visible-light-driven selective reduction of CO2, mostly to carbon monoxide (in the absence of Pd co-catalyst) and methanol (in the presence of Pd co-catalyst). The photocathode materials, which are composed of hierarchically deposited (onto transparent fluorine-doped conducting glass electrode) Cu2O (inner layer) and poly(4-vinylpyridine), P4VP, polyelectrolyte film (outer-layer), with or without dispersed Pd-nanoparticles, have been fabricated and, subsequently, examined using electrochemical methodology, atomic force microscopy and various spectroscopic techniques, including Raman approach. While the physicochemical characteristics of Cu2O, including the oxide identity and its semiconducting properties, are not largely affected by the interfacial modification with P4VP, the photostability of the hybrid photocathode is significantly improved. The P4VP-modified Cu2O photocathodes exhibit reasonable durability during photoelectrochemical reduction of CO2 upon illumination with sunlight in semi-neutral medium (Na2SO4). On the whole, the photoelectrochemical reduction of CO2 proceeds at potentials starting from ca 0.55 V (vs RHE), which are substantially (almost 800 mV) more positive, in comparison to those characteristic of the conventional electrochemical reduction in the dark. Introduction of palladium co-catalyst seems to improve distribution (collection and transport) of photoelectrons at the photoelectrochemical interface and affects the CO2-electroreduction mechanism.

Conversion of carbon dioxide is a process of particular scientific interest in recent years, due to the need to reduce the level of this greenhouse gas in the atmosphere by its storage and reduction. Another profit of the CO2 reduction is the possibility of producing useful chemical compounds from such a widespread in the environment source of carbon. Electrochemical and photoelectrochemical reduction of carbon dioxide are optional methods of obtaining fuel materials from CO2. However, because of the molecule stability and by that, high overpotentials of its electroreduction, and moreover because of the competitive hydrogen evolution reaction – efficient and selective catalytic systems active in the CO2 conversion process are sought. There has been proposed a bioelectrocatalytic system for conversion of carbon dioxide, containing bacterial biofilm (of the species Yersinia enterocolitica) at the electrode surface. Biofilms are robust matrices, stable in a wide range of environmental conditions, able to operate at room temperature and under atmospheric pressure, and appear to be attractive for use in electrocatalytic processes. They easily create well developed three-dimensional structures on various types of surfaces, including diverse materials used in the construction of electrodes. Biofilms hydrated matrix allows the electrolyte ions to easily move at the electrocatalytic interface – as a result, biofilms resemble ion-conducting gel-type systems, supporting redox processes. The structure and properties of biofilms make it possible to immobilize catalytically active molecules in the microbial layer grown on the electrode surfaces, and therefore there has been proposed a catalytic system with the organometallic ruthenium (II) complex dispersed in the biological layer, active in the process of carbon dioxide conversion. Ruthenium (II) complex is immobilized in the biofilm matrix by successive modification of the liquid medium (Luria-Bertani medium) for culturing bacteria with a solution of this complex compound. In addition, a biological matrix is used (along with the ruthenium (II) complex molecules dispersed in its layer) as a protective coating, stabilizing the unstable p-type semiconductor – copper (I) oxide. The proposed catalytic system present activity in the photoelectrochemical reduction of carbon dioxide and stability under experimental conditions.

Most of the bacterial species form biofilms, in which microorganisms are attached to a surface and they are held together by extracellular polymeric substances that they produce. They tend to grow almost everywhere both on living or non-living surfaces. Biofilms are able to propagate charge within their structures and to transfer effectively electrons at interfaces, as well as they could exhibit electrocatalytic properties (e.g. in Microbial Fuels Cells). The application of microbes provides better flexibility: experiments with fuel cells can be operated at normal conditions (temperatures and pressures). Wide variety of microbial metabolic pathways gives the possibility to use aggregates of bacteria in diverse processes. Proposed electrochemical studies using bacterial biofilms (in the form of thin coatings on the glassy carbon electrodes) can be considered as an attempt to find efficient methods of using the energy produced by microorganisms and converting it to electricity. The ultimate goal of the present research has been to determine whether it is possible under laboratory conditions to perform electrocatalytic processes using the hybrid (composite) layers composed of aggregates of bacteria in pristine or modified forms. A biofilm formed by a strain of Yersinia enterocolitica (Y. enterocolitica) is characterized by a high physicochemical stability over a wide pH range (4-10) and temperatures (0-40°C).The subject of interest is a fairly complex reaction, electroreduction of carbon dioxide. There has been growing interest in the search of electrocatalytic anf photoelectrochemical systems capable of efficient conversion of carbon dioxide into fuels and utility chemicals. Our previously performed studies have clearly shown that the Y. enterocolitica biofilm itself has no activity with respect to reduction of CO2, however it acts as a good matrix for the catalytic (e.g. noble metal or metaloorganic) centers [1,2], because it affects the reaction mechanism and appears to decrease overpotential of the electroreduction processes. The conducted research shows that the composite materials containing bacterial biofilms can be successfully used to construct systems that have an electrocatalytic reactivity in the reduction of carbon dioxide. In particular, the influence of the biological matrix on the catalytic activity of different transition metal nanoparticles (Pd, Pt, Ru, PtRu) in the carbon dioxide conversion process will be compared. Here, the successful system based on platinum nanoparticles deposited on the biological carrier, Y. enterocolitica biofilm, supported onto conductive polymer (polyaniline) and utilizing multi-walled carbon nanotubes should be mentioned. We will also address the possibility of dispersing the organometallic ruthenium (II) complex in the biological layer (biofilm). Indeed, the ruthenium (II) complex has been immobilized in the biofilm matrix by successive modification of the liquid medium (Luria-Bertani medium) for culturing bacteria with a solution of the complex compound. In addition, the biological matrix was used (along with the ruthenium (II) complex molecules dispersed in its layer) as a protective coating, stabilizing the unstable p-type semiconductor - copper (I) oxide. The proposed hybrid co-catalytic system showed activity during the photoelectrochemical reduction of carbon dioxide and stability under semi-neutral experimental conditions. Finally, we are going to address the design of the above-mentioned catalytically active systems emphasizing the need to control the structure of the studied hybrid materials (in addition to their stability). Among important issues is the viability of bacteria in the biological membrane as well as elucidation of the role of the bacterial biofilm during the carbon dioxide reduction. [1] Seta E., Lotowska W., Rutkowska I.A., Wadas A., Raczkowska A., Nieckarz. M., Brzostek K., Kulesza P.J., Australian Journal of Chemistry 69 (2016) 411-418. [2] Lotowska W., Rutkowska I.A., Seta E., Szaniawska E., Wadas A., Raczkowska A., Brzostek K., Kulesza P.J., Electrochimica Acta 213 (2016) 314-323.