The Impact of a NiFe-Based Metal Alloy on CO₂ Conversion to CH₄ and Carboxylic Acids in a Microbial Electrosynthesis Cell

Abstract

This study assessed the effects of NiFe-based metal catalysts on CO₂ conversion to methane (CH₄) and carboxylic acids in microbial electrosynthesis (MES) cells. A NiFeBi alloy, when electrodeposited on a conductive bioring cathode, significantly decreased CH₄ production from 0.55 to 0.12 L (Lc d)⁻¹ while enhancing acetate production to 1.0 g (Lc d)⁻¹, indicating suppressed methanogenic activity and improved acetogenic activity. On the other hand, NiFeMn and NiFeSn catalysts showed varied effects, with NiFeSn increasing both CH₄ and acetate production and suggesting potential in promoting both chain elongation and CO₂ uptake. When these alloys were electrodeposited on a 3D-printed conductive polylactide (cPLA) lattice, the production of longer-chain carboxylic acids like butyrate and caproate increased significantly, indicating enhanced biocompatibility and nutrient delivery. The NiFeSn-coated cPLA lattice increased caproate production, which was further enhanced using an acetogenic enrichment. However, the overall throughput remained low at 0.1 g (Lc d)⁻¹. Cyclic voltammetric analysis demonstrated improved electrochemical responses with catalyst coatings, indicating better electron transfer. These findings underscore the importance of catalyst selection and cathode design in optimizing MES systems for efficient CO₂ conversion to value-added products, contributing to environmental sustainability and industrial applications.

1. Introduction

The growing concern over climate change and the need to reduce greenhouse gas emissions have spurred interest in innovative approaches to carbon capture and utilization. Among these, the conversion of carbon dioxide (CO₂) into valuable products such as methane (CH₄) and carboxylic acids through microbial electrosynthesis (MES) cells represents a promising solution [1]. MES harnesses the metabolic capabilities of electroactive microorganisms, which utilize electrons from an electrode to reduce CO₂, thereby providing an environmentally friendly pathway for resource recovery and the reduction in greenhouse gases. The efficiency of CO₂ conversion can be significantly enhanced by incorporating suitable catalysts on the cathode. Recent research has focused on the development of advanced metal catalysts that can enhance the cathode biofilm electron transfer [2,3,4,5,6,7,8,9,10,11,12,13], increase the number of available electroactive sites through increased catalyst porosity [13,14,15], and facilitate efficient electron mediators [16], which are necessary for effective CO₂ conversion. Among these catalysts, NiFe-based metal alloys have shown considerable promise. These alloys are known for their excellent electrochemical properties and catalytic efficiency, making them ideal candidates for facilitating the conversion of CO₂ to CH₄ and carboxylic acids in MES [4,17]. NiFe-based alloys are known for their robustness and catalytic efficiency, especially in electrochemical applications involving hydrogen evolution and oxygen reduction reactions [18].

The integration of metal catalysts, such as NiFe-type alloys, into MES not only improves conversion rates but also supports the stability and longevity of microbial biofilms, which are critical for the sustained operation of these systems. Studies have demonstrated that the presence of such catalysts enhances the adsorption and activation of CO₂ molecules on the electrode surface, leading to improved conversion efficiencies [2,8,19]. Furthermore, these catalysts could be tailored to optimize their interaction with microbial communities [17], thereby enhancing the overall electrosynthesis process.

Beyond their role in enhancing electron transfer, metal alloy catalysts also contribute to the broader goals of environmental sustainability and resource recovery. By converting CO₂ into CH₄ and/or carboxylic acids, MES not only reduces greenhouse gas emissions but also produces e-chemicals and e-fuels that can be reintegrated into industrial processes, thereby supporting a circular economy [7,17,20,21,22]. Nonetheless, it is essential to specifically target a particular compound [23] to improve product purity and minimize the need for an additional energy-demanding separation process. Therefore, designing effective catalysts that promote metabolic specificity is essential. In this study, it is hypothesized that appropriately designed NiFe-based catalysts could help achieve this objective. Moreover, Ni and Fe are essential cofactors in acetogenic enzymes, particularly in the enzymes involved in carbon fixation and acetate synthesis [24,25]. Previous studies [26] have demonstrated that certain transition metals facilitate the bioconversion of CO₂ to CH₄ and acetate by enhancing electron transfer from the electrode surface to electroactive microorganisms [4]. To ensure that the catalyst surface is both efficient and biocompatible, it is essential to select metals with low toxicity. Therefore, Bi [27], Mn [28], and Sn [29] were chosen as alloying elements with NiFe as they are known for their good biocompatibility. In addition, Mn can facilitate acetogenesis by increasing H₂ availability [30]. Finally, Sn is known to catalyze electrochemical CO₂ conversion to formate, thus providing a supplementary electron donor for microbial chain elongation [11].

This study aims to evaluate the impact of NiFe-based metal catalysts on the conversion of CO₂ to CH₄ and carboxylic acids (mainly acetate) in an MES reactor. It considers the electrochemical mechanisms underpinning these transformations and assesses the efficacy of various metal catalyst types in facilitating electron transfer within microbial biofilms towards acetate and/or CH₄. As such, this study aims to advance the development of more efficient and sustainable catalysts for improved CO₂ conversion to high-value e-chemicals such as medium-chain fatty acids (MCFAs), e.g., caproate, thereby paving the way for future applications in environmental management and industrial processes [6].

2. Results and Discussion

2.1. MES Operation with Bi-Coated Conductive Biorings

Recently, it has been shown that transition metal catalysts can have a significant impact on the microbial conversion of CO₂ and the metabolism of acetate to CH₄ in a microbial electrosynthesis cell [26]. Particularly, it was demonstrated that the injection of 0.1 g L⁻¹ of Bi⁺³ into an MES reactor resulted in approximately a threefold increase in CH₄ production. However, this increase was transient, and changes in operating conditions, such as increased current and CO₂ supply, could not prevent a decrease in the CH₄ production rate shortly after its production peaked. Hence, it was interesting to evaluate the impact of the Bi catalyst on the production of CH₄ in an MES reactor. The alloy ensured the consistent presence of Bi on the cathode surface. Hence, a NiFeBi metal alloy was in situ-electrodeposited on a conductive bioring cathode before reactor inoculation. Figure 1 shows that the production rate of CH₄ after more than 3 months of continuous testing decreased by 80% to 0.12 ± 0.01 L (L_c d)⁻¹ compared to MES without an alloy catalyst coating. The 0.55 ± 0.02 L (L_c d)⁻¹ CH₄ production rate recorded on the uncoated conductive bioring cathode is similar to the values reported in the literature [2,4]. Note that it has been demonstrated by previous work that the in situ electrodeposition of NiFe on carbon-based microbial supports significantly increased the production rate of CH₄ [4,31]. Therefore, it can be inferred that the converse result obtained in the presence of the NiFeBi catalyst is predominantly due to the presence of Bi in the alloy matrix, as the catalyst covered the surface of the cathode support. It is noteworthy that the energy consumption (EC) decreased by 35% to 7.8 ± 0.6 W h (L_H2)⁻¹ as an increase in electroactivity on the bioring surface with the NiFeBi catalyst coating led to a lower required applied voltage. Moreover, the Coulombic efficiency (CE) of the MES cell increased slightly from 70 ± 6% to 76 ± 5%, indicating an increase in the production of measurable conversion products and a reduction in activation losses. This is partly supported by Figure 1, which shows that while CH₄ production decreased, acetate and propionate production rates substantially increased to 1.0 ± 0.1 g (Lc d)⁻¹ and 0.13 ± 0.01 g (Lc d)⁻¹, representing more than 2-fold and 7-fold increases, respectively. Nevertheless, the total carbon recovery of the MES cell increased slightly from 83 ± 3% to 87 ± 2% which is statistically insignificant (t-test, p = 0.15). The concentrations of other short-chain carboxylic acids, such as butyrate, remained negligible and therefore were not considered.

The increased production of carboxylic acids due to the presence of different transition metal alloy catalysts has been reported in the literature [5,6,31]. This indicates that acetogenic microorganisms are more efficient in utilizing substrates in the presence of metal alloys, as stoichiometrically, it requires more CO₂ to form acetate than CH₄. Furthermore, it can be deduced that although the NiFeBi catalyst enhanced the production of volatile fatty acids (VFAs), the presence of Bi in the alloy matrix suppressed methanogenic activities. This is corroborated by reports indicating that an in situ-deposited NiFe catalyst significantly improves the CH₄ production rate [4,31]. One possible explanation is that the presence of Bi at the cathode surface inhibits the growth and metabolic activity of methanogenic microorganisms by either disrupting the activity of enzymes such as hydrogenases or interfering with electron transport mechanisms [32,33]. At the same time, acetogenesis is improved in the presence of Bi in the metal alloy. This observation suggests that CH₄ production is strongly dependent on the ability of hydrogenotrophic methanogens to reduce CO₂ in the presence of H₂ to CH₄. Thus, it can be stated that hydrogenotrophic methanogenesis is primarily the bioreaction mechanism governing the CH₄ production rate [8,20]. Another plausible explanation is that different metal alloy catalysts can specifically influence the diversity of the microbial biofilm population, as CH₄ production initially increased due to the dominant effect of improved electron transfer efficiency to the cathode. However, this impact decreases as the selection of microbial population on the catalyst surface takes precedence.

2.2. MES Operation with Carbon Felt-Coated NiFeMn and NiFeSn

Based on the results discussed above, it was expected that different transition metals, when alloyed with NiFe, may have varying impacts on the transformation products in CO₂ conversion. As a preliminary test of this hypothesis, Mn and Sn were alloyed with NiFe by in situ electrodeposition on a carbon felt electrode to form NiFeMn and NiFeSn catalysts. First, uncoated carbon felt was tested for its performance as a cathode material for CO₂ conversion in an MES reactor. This test lasted for 30 days (unless specified, each experiment was conducted for approximately the same duration). Afterwards, a similar MES system was set up and electrodeposited with the NiFeMn catalyst for 24 h at 200 mA of current, followed by the removal of the coating solution. This MES reactor underwent a comparable test to the reactor equipped with the uncoated carbon felt electrode. It can be seen from Figure 2A that at an operating current of 50 mA, the MES system with the NiFeMn coating demonstrated a 100% increase in acetate production and a 10% decrease in CH₄, thereby suggesting that the addition of Mn to the NiFe alloy has a direct effect on acetogenic and methanogenic microbial populations. This overall improvement in product formation is supported by an increase in CE from 85 ± 3% to 92 ± 7%, with a corresponding decrease in the EC value from 6.9 ± 0.2 to 5.1 ± 0.7 W h (L_H2)⁻¹. Further, the total carbon recovery increased by 15% to 86 ± 2%, showcasing higher CO₂ uptake.

It was interesting to investigate whether a further increase in the current of the uncoated MES reactor could augment CH₄ production in an attempt to understand the possible impact of increased H₂ availability @on microbial populations. It was observed from Figure 2B that increasing the current to 80 mA on the uncoated cathode resulted in more than a 2-fold increase in CH₄ production, as there was an increased utilization of H₂ (by approximately 19 ± 6%). Consequently, the acetate production level decreased by more than 100% to 0.10 ± 0.02 g (L_c d)⁻¹, indicating a clear shift in bioreaction pathways toward methanogenesis. Subsequently, the carbon felt cathode of a freshly prepared MES reactor was electrodeposited with NiFeSn to further elucidate the impact of metal alloy composition on the bioconversion of CO₂ to CH₄ and acetate at an operating current of 80 mA. From Figure 2B, it can be seen that the NiFeSn catalyst significantly improved the production of both acetate and CH₄ in comparison to the uncoated carbon felt electrode (t-test, p ≤ 0.002), unlike the NiFeMn catalyst (Figure 2A). Nonetheless, both catalyst coatings had comparable EC, CE, and carbon recovery values. This result implies that NiFeSn likewise facilitates increased CO₂ uptake. Moreover, one clear trend is the increase in the production rate of acetate in the presence of metal alloy catalysts, suggesting that these catalyst types promote the proliferation of acetogenic populations.

2.3. MES Operation with 3D Lattice Coated with NiFeMn and NiFeSn

It is expected that by using a carbon-based cathode support that provides an improved transport of ions and dissolved compounds, in comparison to carbon felt, the performance of a metal alloy electrodeposited cathode in an MES reactor with a well-developed microbial biofilm can be further improved in terms of the production of value-added molecules such as CH₄ and carboxylic acids. As such, a 3D-printed lattice structure of conductive polylactide (cPLA), which is designed to improve the delivery of nutrients and CO₂ to the attached biofilm, was used to electrodeposit Ni, Fe, Mn, and Sn, which are metals forming NiFeMn/cPLA and NiFeSn/cPLA cathodes. The results in Figure 3A show that the trends in the performance of the NiFeMn and NiFeSn catalysts deposited on cPLA and carbon felt were similar.

However, there was a further improvement in the performance of the MES reactor for both CH₄ and acetate production, as well as longer-chain carboxylic acids including butyrate and caproate (which were insignificant on carbon felt, p > 0.05), when the cPLA lattice was used as the cathode support. Despite showcasing similar EC, CE, and carbon recovery values at around 5.6 ± 0.8 W h (L_H2)⁻¹, 93 ± 5%, and 87 ± 4%, respectively, the NiFeSn/cPLA cathode outperformed NiFeMn/cPLA in the production of caproate, thus suggesting that the former may perhaps support the development and selection of chain-elongating bacteria. To avoid competition from methanogenic microbes in the CO₂ conversion to high-value products such as butyrate and caproate, the inoculum was enriched in acetogenic and chain-elongating populations through multiple culture transfers for this test. In each enrichment cycle, the culture bottles had H₂/CO₂ (4:1 vol/vol) headspace. 2-Bromoethanosulfonate (BES) was added to the enrichment bottles to suppress methanogenic populations, resulting in the complete suppression of CH₄ production.

It is evident from Figure 3B that, in the MES reactor with efficient microbial enrichment and a NiFeSn/cPLA cathode, the production of butyrate and caproate was significantly increased by more than two orders of magnitude (t-test, p ≤ 0.001). This is substantiated by the observed increase in Coulombic efficiency and carbon recovery (≥15%) in comparison to the non-enriched MES reactor with NiFeSn/cPLA. In contrast, the acetate production in the MES reactor with an uncoated cathode increased by approximately 100%, whereas butyrate and caproate concentrations remained relatively negligible. Consequently, its Coulombic efficiency and total carbon recovery remained similar, at around 72 ± 3% and 83 ± 5%, respectively, with the enriched culture. As such, it can be deduced that chain elongation was promoted in the enriched MES reactor composed of the NiFeSn/cPLA cathode. In previous studies [34,35,36], two key biochemical pathways were identified for chain elongation. The synthesis of MCFAs in an MES reactor is reported to occur primarily through reverse β-oxidation (RBO) [37] and the canonical fatty acid synthesis (FAS) [38] pathways, which are well-documented in the literature [36]. While FAS is integral to lipid biosynthesis across many organisms, the RBO pathway is predominantly utilized by specialized anaerobic bacteria for the production of MCFAs, especially during anaerobic fermentations.

The RBO pathway extends the carbon chain of carboxylic acids by two carbon units per cycle, using acetyl-CoA as the electron donor. Essentially, it reverses the β-oxidation pathway used for fatty acid degradation, involving four sequential enzymatic steps, namely, condensation, first reduction, dehydration, and second reduction. Further details of these steps can be found elsewhere [36].

A closer look at the result in Figure 3B suggests that the RBO pathway may govern the chain elongation biochemical reaction pathway. This is because, unlike in the uncoated cPLA lattice cathode, there was an increase in the production of acetate (C2) together with a substantial increase in butyrate (C4) and caproate (C6).

2.4. Cyclic Voltammetry

To examine the in situ electrochemical response of the MES system around the operating current and voltage, cyclic voltametric (CV) measurements were performed using a standard Ag/AgCl reference electrode, with the cathode materials and the Ti/IrO₂ anode serving as the working and counter electrodes, respectively. The MES system was poised in the potential window of −1.25 to −0.45 V vs. Ag/AgCl. The recorded representative cyclic voltammograms on the surface of the cPLA cathodes are given in Figure 4.

Note that the current densities were normalized based on the exposed area of the Nafion membrane. As shown in Figure 4A, the electrochemical response on NiFeMn/cPLA exhibits improved electrochemical performance, yielding higher current densities compared to the uncoated cPLA lattice at the same applied voltages. These observed electrochemical responses are substantiated by the lower energy consumption value (5.5 ± 0.8 W h (L_H2)⁻¹) obtained with the MES reactor containing the NiFeSn/cPLA cathode relative to that of the uncoated cathode (10.2 ± 0.7 W h (L_H2)⁻¹). The CV profile on the NiFeMn/cPLA cathode at the end of the MES test indicated a higher current density in comparison to the start of testing and the control uncoated cPLA cathode. The increased response with NiFeMn/cPLA at the end of the test compared to the beginning can be attributed to the development of an electroactive biofilm at the surface of the cathode. These results support the CH₄ production rate data shown in the previous section, which suggested that an improvement in the electron transfer efficiency on the cathode with the NiFeMn catalyst does not equate to improved CH₄ production, even though the bioreaction mechanism in MES is suggested to be dominated by hydrogenotrophic methanogenesis. Further, from Figure 4B, it is evident that the CV recorded on the NiFeSn/cPLA cathode had a statistically significant (t-test, p < 0.0001) improvement in its electrochemical response towards higher current densities in comparison to the uncoated cPLA cathode. This corroborates the improved bio-electrochemical activity achieved through metal alloy coatings, as confirmed by the lower energy consumption of 3.9 ± 0.3 W h (L_H2)⁻¹ obtained using the NiFeSn/cPLA cathode compared to the 7.0 ± 0.6 W h (L_H2)⁻¹ determined using the uncoated cPLA cathode. It is noteworthy that in Figure 4B, the reduction shoulder that is observed at 0.7 V vs. Ag/AgCl on the CV profile obtained on NiFeSn/cPLA, which is lower than that evident on the uncoated cPLA (1.0V vs. Ag/AgCl), could be attributed to the effect of microbial redox mediators [39] responsible for reducing the redox potential to a level suitable for the chain elongators that produce butyrate and caproate. Such a feature is expectedly absent on the NiFeMn cathode catalyst, which was an inefficient surface for caproate formation.

2.5. Cathode Surface Characterization

The surface of the cathodes was imaged by scanning electron microscopy, and the elemental composition of metal alloy cathodes was determined by X-ray diffraction at the end of testing in the MES reactor. The SEM/EDX results shown in Figure 5 confirmed the electrodeposition of NiFeMn and NiFeSn alloy catalysts on the surface of carbon felt and the cPLA lattice, respectively.

There was approximately 14 ± 1 wt% and 30 ± 2 wt% of Mn and Sn present on the cathode surfaces, which were similar to the initial values. Hence, this indicates the presence of stable alloy coatings for the test duration. In particular, the coarse homogenous surface morphology of the electrodeposited NiFeSn/cPLA lattice resembles that of metal coatings with biofilm as reported elsewhere [4,7,8]. The EDX results corroborate that the presence of metallic Mn, Sn, and Bi (11 ± 2 wt%) facilitated electron transfer as compared to the uncoated carbon felt or cPLA lattice. The elemental O and C revealed by EDX analysis are impurities and, as such, were not included in the composition determination of the alloy coatings.

3. Materials and Methods

3.1. Analytical Measurements

The effluent gas composition of the anode and cathode compartments of the MES reactor was determined using a gas chromatograph (GC) (HP 6890 GC, Hewlett-Packard, Palo Atlo, CA, USA). The effluent gas flow rate from each electrode compartment was measured using a gas counter (MilliGascounter MGC-1V3.4 PMMA, Ritter North America, Summerville, SC, USA). Additionally, the effluent gas flow measurements were corroborated using anode and cathode gas balance calculations based on the water electrolysis stoichiometry, as described elsewhere [20]. The concentrations of carboxylic acids, namely, acetate, propionate, butyrate, and caproate, were evaluated using a GC (Agilent 6890 N gas chromatograph, Agilent Technologies, Santa Clara, CA, USA). The complete description of these analytical methods can be found elsewhere [4].

To assess the performance of the MES reactor, the energy consumption (EC) and Coulombic efficiency (CE) were determined. EC is expressed per H₂ equivalent (in W h (L_H2)⁻¹). CE is given as the ratio of output charge to the total input charge (expressed in %) based on the major products (H₂, CH_4, acetate, etc.) formed at the MES reactor’s cathode compartment or that diffused through the membrane to the anode. All measurements were conducted in duplicate.

3.2. MES Cell Design and Operation

Two differently sized MES reactors were used in the experiments. Each cell type had a two-chamber design, consisting of a 50 or 250 mL anode chamber and a 150 or 750 mL cathode chamber, respectively, which represented a 5-fold scale up. The anode and cathode chambers were separated by a Nafion membrane (Nafion 211, IonPower, Tyrone, PA, USA), wrapped in protective nylon cloth. The larger MES reactor had a cathode chamber with conductive polypropylene plastic biorings which were manufactured in-house at the NRC. The 200 mL MES reactors used pieces of carbon felt (approximately 1.5 cm × 1.5 cm × 0.5 cm) of roughly 30 µm thick fibers with less than 0.1 mm separations or custom-made 3D-printed cPLA lattices with a 5 mm distance between structural elements (beams). All MES reactors had a Ti/IrO₂ mesh (Magneto Special Anodes, Netherlands) counter electrode. The cathode chambers were continuously fed with CO₂ (pure, 99.999%) at 2 L (L_c d)⁻¹. In addition, nutrient solution containing minerals and trace metals was fed to each cathode compartment, yielding a hydraulic retention time (HRT) of 8–10 days. The catholyte was composed of 1.34 g L⁻¹ K₂HPO₄, 1 g L⁻¹ KH₂PO₄, 0.7 g L⁻¹ NH₄C, 3.2 g L⁻¹ KCl, 0.5 g L⁻¹ yeast extract, and 4 mL L⁻¹ of trace metal solution. The catholyte pH was maintained at 6.5 by a pH controller that fed a 0.5 M H₃PO₄ solution. The detailed composition of the catholyte’s trace metal solution is reported elsewhere [4]. A phosphate-buffered solution (PBS) composed of 6 g L⁻¹ Na₂HPO₄ and 3 g L⁻¹ KH₂PO₄ was used as the anolyte. Both the catholyte and anolyte were recirculated at a rate of 40 L d⁻¹ using a peristatic pump to enable mixing. The catholyte temperature was maintained at 30–32 °C using a rope heater installed in the external recirculation line of the cathode chamber. The schematic of the reactor design is as depicted in a previous publication [26].

Unless specified, the cathode chambers of each MES reactor were inoculated either with homogenized anaerobic sludge sourced from an anaerobic reactor treating agricultural food wastes (Lassonde, Rougemont, QC, Canada) or microbial acetogenic enrichment, as specified in Section 2. The volume of inoculum comprised 20% of the cathode chamber. After inoculation, the developed biofilm on the electrode and the planktonic bacteria were self-reproducing, allowing for long-term MES operation. MES cells were typically operated for approximately 30 days. Additionally, several long-term experiments were conducted using MES cells, which were operated for up to 12 months. The reactor’s energy input was provided by a power supply (PW18–1.8AQ, Kenwood, Tokyo, Japan) operated in a constant current mode at an estimated current density of 15 A m⁻² with respect to the exposed membrane area.

To evaluate whether there is statistically significant variance in the production rates of CH₄ and acetate in the presence or absence of metal alloy catalysts, the mean values of these quantitative metrics were computed and analyzed using a t-test.

3.3. Electrochemical Measurements

The electrochemical behavior of the cathode material in the hydrogen evolution reaction (HER) region was examined through cyclic voltammetry (CV) using a three-electrode cell setup, where the cathode served as the working electrode, and a Ag/AgCl electrode (+0.199 V vs. reversible hydrogen electrode, RHE, CH Instrument, Bee Cave, TX, USA) acted as the reference electrode. A pair of 100 × 45 × 1 mm Ti/IrO₂ anode meshes or their equivalent in the case of the larger reactor functioned as the counter anodes. Recorded cyclic voltammograms were generated in a potential window of −0.25 V to −0.75 V vs. Ag/AgCl at a scan rate of 5 mV s⁻¹ using a potentiostat (Model 2273, Princeton Applied Research, Oak Ridge, TN, USA) operated by VersaStudio v2.6 software (Princeton Applied Research, USA).

3.4. Catalyst Electrodeposition and Surface Characterization

Metal alloy electrodeposition was performed in situ in a two-electrode chamber MES reactor featuring a cathode material (as mentioned above) and Ti/IrO₂ mesh anodes as described previously, without the need for a reference electrode. For the deposition of bismuth, manganese, and tin, the cathode compartment contained an electrolyte of 0.30 g L⁻¹ BiCl₃, 0.30 g L⁻¹ MnSO₄·H₂O, and 0.56 g L⁻¹ SnCl₂·2H₂O, respectively. The solutions were supplemented with an electrolyte mixture consisting of 20 g L⁻¹ H₃BO₃, 25 g L⁻¹ NH₄SO₄, 20 g L⁻¹ NaCl, 50 g L⁻¹ FeSO₄·7 H₂O, and 50 g L⁻¹ C₆H₅Na₃O₇·6H₂O. The electrodeposition process was conducted at a constant current of 200 mA for 24 h at a temperature of 37–40 °C. The electrodeposition parameters were maintained for all coating types to minimize the preferential advantage of one coating over another and to ensure nearly 100% coverage, as revealed by SEM analysis.

The surface of the electrodeposited cathode material was characterized by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, SEM/EDX, before and after testing in the reactor to determine the elemental composition of electrodeposited metal alloys and to follow morphological and compositional changes on the surface of the cathode materials.

4. Conclusions

This study demonstrated that transition metal alloy catalysts, when electrodeposited on carbon-based cathode supports, can significantly influence the microbial electrosynthesis of CH₄ and carboxylic acids from CO₂. The investigation revealed that the NiFeBi alloy suppressed methanogenesis while enhancing the production of acetate, highlighting the role of Bi in altering microbial activity. Conversely, NiFeMn and NiFeSn catalysts showed varied effects, with NiFeSn particularly enhancing the production of both CH₄ and carboxylic acids, highlighting its potential for promoting chain-elongating microbial populations. The use of a 3D-printed cPLA lattice as a cathode enabled the unobstructed transport of nutrients and products throughout the entire cathode volume, further enhancing the production of longer-chain carboxylic acids and indicating its superior biocompatibility and nutrient delivery capabilities. Cyclic voltammetric analysis corroborated the improved electrochemical responses of the NiFeMn and NiFeSn coatings, supporting their role in facilitating electron transfer. Surface characterization by SEM/EDX confirmed the successful electrodeposition of metal alloys, reinforcing their contribution to enhanced MES performance. Future research is recommended to include the microbial community structure and bacterial functions to examine microbial diversity and evaluate potential microbe/catalyst interactions to elucidate bioreaction mechanisms or pathways to different transformation product types. Overall, these findings provide insights into optimizing MES systems for efficient CO₂ conversion to value-added products and showcase the importance of catalyst selection and cathode design for enhancing microbial activity and electron transfer efficiency. This technology can be applied to various industrial processes, thereby reducing reliance on fossil fuels and mitigating greenhouse gas emissions. By converting CO₂ into valuable chemicals and fuels that create a circular carbon economy, this research work can potentially contribute to a more sustainable future.