## 3 Results and Discussion
### Optimisation of pH  
The overall methane production in the 25 days of this \(-\)0.70 V potentiostatically operated experiment was around 432 mL (17.7 mmol) with an average methane concentration of about 91.4% (395 mL or 16.1 mmol; Figure 4). While the reaction was continuously fed with W\({}_{\rm aA}\) (feed pH \(\sim\)7.8) and operated potentiostatically at \(-\)0.70 V, the effluent pH can be seen increasing from 8.2 to 8.7. Correspondingly, a fall in methane production rates (MPRs) from about 5.28 mmol/L\({}_{\rm(reactor)}\)-day to its lowest value of 2.62 mmol/L\({}_{\rm(reactor)}\)-day is observed (Figure 5). Thereafter, a change of feed to W\({}_{\rm aB}\) (pH of \(\sim\)7.0) generated a significant impact on the effluent pH and thereby the MPRs. The effluent pH was gradually brought down to 8.1 and this in turn improved the MPR significantly to 9.05 mmol/L\({}_{\rm(reactor)}\)-day (Figure 5).  
Figure 4: Cumulative methane production and methane percentage during optimisation of the pH experiment.  
Figure 3: Electrochemical methane yield and current production with respect to cathode potential.  
EMY during the first half of the experiment starts at 87.7%, reaches the lowest value of 70% and stabilises at around 77% (Figure 6). However, during the second half of the experiment, it rises to 225% at an exponential rate. This clearly shows the impact of pH on methane production. High ranges of pH \(\sim\)8.7 have been reported to eliminate or reduce the heterotrophic methanogenesis activity [29]. This means that during the first half of the experiment, a majority of methane production was autotrophic and during the second half, a large portion of methane production was heterotrophic. This suggests that the pH directly affects only heterotrophic methanogenesis. This can also be confirmed from the values of current generated (or electrons supplied) in the last week of the experiment that have remained constant, while the MPR and EMY have increased significantly. This emphasizes that the impact of pH was not significant on the electrochemical CO\({}_{2}\) reduction to methane.  
Figure 5: Methane production rate and effluent variation with feed pH.  
Figure 6: Electrochemical methane yield and currents production with respect to feed pH.  
Another observation that contributes to this hypothesis is the methane concentration in biogas. It can be observed in Figure 4 that the methane percentage is continually maintained around 92-93% for the first 20 days. As the feed pH is decreased, the increased heterotrophic activity leads to a slight decrease in the methane concentration to about 91%. This can be attributed to the production of heterotrophic biogas that contains relatively higher concentrations of CO\({}_{2}\)[28]. These observations provide a new argument that electrochemical CO\({}_{2}\) reduction does depend on pH indirectly, as it reduces the heterotrophically generated CO\({}_{2}\). Moreover, it can be concluded that electrochemical methane production is not directly dependent on the pH but more dependent on the biofilm cover and the applied potential. An open circuit experiment is carried out to determine the share of methane production through each of the two production pathways.