## 1 Introduction  
Microbial photosynthesis (MES) is an economically emerging bio-electrochemical technology for transforming CO\({}_{2}\) and renewable electrical energy into chemicals and fuels [1, 2]. It could allow the storage and increase in value of intermittent renewable energies such as solar and wind [3]. In an MES reaction, the electroactive microorganisms (called electrophoresis) can utilize electrons (e\({}^{-}\)) from external conducting materials (i.e., electrodes) reducing energy to catalyze the conversion of CO\({}_{2}\)[4]. These electrotrophs achieve CO\({}_{2}\) conversion through several carbon-fixation pathways including the reductive pentose phosphate cycle (e.g., photo- or chemoattractotrophs), reductive tricarboxylic acid cycle (e.g., _Clostridium thiosulfatopulium_), and reductive acetyl-CoA pathway (e.g., _Clostridium ljungdahlii_) [1, 5]. _Rhodobacter sphaeroides_, a CO\({}_{2}\)-fixing chemoautotrophic bacterium, can catalyze both the production and consumption of hydrogen molecules (H\({}_{2}\)) [6]. In a recent investigation, MES-driven CO\({}_{2}\) uptake and H\({}_{2}\) production in _R. sphaeroides_ were found to occur simultaneously without additional organic carbon substrates [7].  
Extracellular electron uptake mechanisms of electrotrophs from electrodes rely on: (i) indirect electron uptake by H\({}_{2}\) or electron shuttles (i.e., natural and artificial redox mediators) or (ii) direct electron uptake via physical contact through electron-transfer proteins or apparatus [1,8]. Although the cathodically evolved H\({}_{2}\) can support electrotrophic growth of a microorganism, the direct supply of cathodic electrons to microorganisms is more sustainable and energy efficient in an MES reaction [8]. Direct electron uptake from electrodes occurs in range of more positive potential ranges (\(-0.4\) to \(-0.5\) V vs. SHE) than H\({}_{2}\)-mediated indirect electron uptake [4,8]. Meanwhile, using artificial redox mediators has drawbacks to application due to their chemical instability, toxicity to microbes, and difficulty in separating them from electrolytes [9]. A dense and well-developed cathodic biofilm is a key parameter for improving direct electron uptake rates and production rates in MES [10]. For example, an increased biofilm of _Sporomusa ovata_ on a graphene-functionalized carbon composite cathode enabled higher cathodic current consumption with higher acetate production compared with a bare cathode in an MES reactor [11]. Furthermore, accelerating robust biofilm formation enabled improvement in MES performance. [PERSON] et al. reported that a modified cathode with positively charged layers such as chitosan, cyanuric chloride, 3-aminopropyltriethoxysilane, and polyaniline showed enhanced electron consumption and acetate production rates of _S. ovata_ in an MES reactor [12]. This is because a cathode surface coated with positively charged layers is more suitable for interactions with negatively charged microorganisms [13].  
In this study, we performed the cathode modification for MES by _R. sphareoides_ to increase biproducts synthesis and Faradaic efficiency. In order to promote direct electron uptake, the graphite felt material was coated with a chitosan layer. The chitosan layer presents unique functionalities (co-existing polycationic and ne nucleophilic properties) that are suitable for constructing a rigid biofilm onto the electrode surfaces [13]. Additionally, the chitosan layer was modified with a carbodiimide compound to enable more substantial interaction between the _R. sphareoides_ cells and cathode surface (Figure 1).