7.3.2. Cathode Material

The necessity to create either CH4 or H2 dictates reactions at the cathode, and this is dependent on the need and potential to manufacture and utilize hydrogen or methane on-site. The rate at which H2 is consumed is determined by the amount of methanogenic activity present. The hydrogen will very probably be consumed throughout the reaction if the device is operated as a single chamber without a membrane, and the biogas generated will be in form of CH4 [80]. Temperature has a direct influence on methanogen activity, with temperatures exceeding 35 ◦C considerably boosting methanogenic activity [120]. Hydrogenotrophic methanogens are more prevalent in MECs where CH4 is produced, according to the study. Hydrogenotrophic methanogens produce CH4 through the intermediate synthesis of H2. The CH4 synthesis route reveals that the cathode material's hydrogen evolution capacity is a critical design factor. As a result, the cathode serves as both a biocatalyst and an electrocatalyst, enhancing hydrogen evolution reactions (HER) by increasing electrode–microbe electron transfer [121]. The presence of hydrogen-scavenging bacteria in waste streams necessitates the use of membranes if pure hydrogen is required. Multiple investigations have shown that membrane systems can achieve hydrogen purity above 98% [122]. Corrosion resistance, good conductivity, high specific surface area, biocompatibility, and outstanding mechanical qualities are all required of successful cathode materials [123]. Furthermore, cathodic materials must minimize significant hydrogen evolution overpotentials. Cathode fabrication for industrial application must be low-cost, utilizing easily accessible materials and conventional production procedures, for the largescale deployment to be practicable. Metals have been investigated because they conduct electricity more efficiently than carbon-based materials [124], and they have greater biocompatibility, as well as cathode potential, which prevents corrosion. Platinum has the strongest HER activity, which leads to improved H2 evolution [125]. Platinum, on the other hand, has some disadvantages, including being expensive and having substantial mining environmental effects; as a result, the invention of new metallic electrode materials is required [126]. Stainless steel and nickel have performed well as nonprecious metals [127].

Stainless steel is a typical material for electrode construction because it is a relatively inexpensive metal. When it comes to hydrogen production, stainless steel with a large specific surface area can be as effective as a platinum catalytic electrode containing carbon. Because of its high conductivity as a transient metal, stainless-steel mesh is thought to have outstanding ohmic resistance and electron transport resistance. Meshes and brushes made of stainless steel have a low cost and excellent performance, making them an ideal cathode made of a non-precious metal for further evaluation and scale-up operations. The findings are consistent with the use of meshes and wool in pilot-scale systems to produce a low-cost, high-surface-area cathode with a low cost and large surface area. Nickel, like other non-platinum metals, has high corrosion resistance, as well as high hermetic electron transfer activity. Nickel is also more corrosion-resistant than stainless steel, which is important for an electrode because it must be long-lasting in order to be commercially viable. Hydrogenotrophic methanogens play a role in the enhanced performance, implying that nickel's high HER activity relative to other materials helps it perform better [128]. HER activity must be a key focus of study to maximize the efficiency of cathodes. Stainless steel is now commonly used. Due to its availability, a pilot study has shown it to be the ideal cathode material for scale-up. Cost and machinability are two factors to consider. On a pilot scale, a comparison was made between nickel and stainless steel. It would be wise to experiment with nickel cathodes to determine if the improved performance justifies the additional expense. Various cathode materials are described in Table 3.

#### *7.4. Effect of Electrolyte pH*

Because the HER at the cathode depends on electrolyte pH and has the most crucial impact on overall performance of MECs. High overpotentials can occur owing to a difference in redox potentials between anode and cathode chambers; it was observed that more cation instead of proton percolates through the cation-exchange membrane. Consequently, the cathode becomes alkaline while the anode becomes acidic. Theoretically, 59 mV of voltage loss is incurred due to a difference in pH level of 1 between the anode and cathode. Microbial activities are pH-dependent; microbes are highly sensitive to surrounding pH, and its variability may cause modifications in microbial respiration and, consequently, extracellular electron transfer. In fact, because microbes are mostly active at neutral pH, most MEC studies are conducted at pH 7. Moreover, many other parameters (ion transfer, conductivity, substrate oxidation, etc.) are directly or indirectly associated with pH. Researchers have reported that low cathode and high anode pH improved hydrogen production [129–131]. Protons accumulate under high pH, thereby increasing the electrogen proliferation due to conducive environment. Research suggests that periodic polarity reversal can be used to stabilize pH in two-chambered MECs [132]. An electrolyte, including a weak acid, operates as an electric charge at high pH, increasing MEC characteristics, and the deprotonation process may increase the conductivity of the electrolyte while lowering the impedance between the anode and cathode. However, we must evaluate the possible impacts of weak acid catalysis and solution resistance for a lower pH electrolyte to determine whether the reactor can function more effectively. However, certain experimental findings revealed that the presence of phosphate species and some weak electrolyte acids, as a charge carrier for improving conductivity, had a beneficial impact on a stainless-steel brush cathode and also reduced the Pt/C cathode's overpotential. Merrill et al. [133] found that lowering the pH improves MEC performance by lowering solution resistance and cathode overpotential. Munoz et al. [134] found that using phosphate as an electrolyte may increase the rate of hydrogen generation and current density in MECs. Yossan et al. [135] investigated five kinds of catholytes in MECs, namely, deionized water, tap water, NaCl solution, acidified water, and a phosphate buffer. Due to its greater buffer capacity, a 100 mM phosphate catholyte in a MEC exhibited the best rate of hydrogen generation. As a result, phosphate is the most often utilized electrolyte in MECs.
