The productivity can be affected by numerous factors, including the MEC design; however, the choice and profile of the substrate and feedstock might be underestimated, since the type and accessibility of carbohydrates have proven essential to the efficiency and operation of a MEC [84,85].

The material used for the anode can heavily affect the MEC’s performance. MEC has three active materials responsible for converting feedstock to biohydrogen, i.e., the anode, cathode and membrane. These active materials can be configured to accommodate a wide range of feedstock and microbial cultures. Most research suggests that the limiting parameter is coupled to the bacterial activity at the anode [86] or the catalysis and availability of protons at the cathode [87].

The common factor of anodes is the coupling of the anode to a more conductive metal, thereby allowing a concentrated flow of electrons, ensuring bonding and thriving microbes. The bonding is facilitated when the right functional groups are attached to the anode [88]. Attachment to the electrode mainly occurs via the adsorption or entrapment of the microbial species. Both can be affected by the anode’s surface properties, roughness and porosity [89]. Furthermore, a successful anode requires a strong and resilient construction with a developed surface area, still allowing for substrate flow and high proton and electron productivity [90,91]. However, biocompatibility is vital for achieving the required microbial adhesion. Therefore, the presence of molecular affinity, chemical bonding or electrostatic forces is necessary [88].

Carbon and metal-based anodes are commonly used in MECs as an optional mix in which the metal is used to conduct the current [57,92,93]. Carbon-based anodes comprise carbon felt [94,95,96], carbon cloth [97,98], graphite (as felt, plate or fiber) [99,100,101], carbon fiber, carbon brushes, carbon mesh, carbon paper, activated carbon (granules), graphite granules, etc. (as seen in Table 2). When investigating the differences between carbon and graphite materials, it should be noted that some studies state that graphite materials are carbon, as it is an allotrope of carbon. Purely metal-based anodes consists of stainless steel (mesh [102], sheets [103] and scrubbers [104]), nickel [105], silver [106], titanium [107], copper [108], etc.

Carbon felt exhibits a high porous structure while being an excellent electric conductor. Carbon felt is also a low-cost anode with a sizeable flexible surface area when considering its flat-plate surface [109,110]. Carbon cloth possesses high mechanical strength, but as it is a dense structure, clogging is a potential hazard [111]. Carbon cloth is also an expensive carbon-based anode, especially for larger-scale MECs. A carbon brush is a carbon-based anode with a large surface area per unit of volume. These brushes are often based on a titanium wire, which increases the conductivity compared with other carbon materials and increases the overall price. The carbon brush has been proposed as a next-generation anode since its open structure allows for a high surface area with a lower chance of clogging [90,111]. Carbon mesh can be used as an alternative to carbon cloth, which can be an expensive anode. Carbon mesh has a low cost but possesses many of the same properties as a carbon cloth. Carbon mesh is less prone to clogging due to its more open structure [112]. Carbon paper has a paper-like structure, which offers a slightly porous structure, but it has not been assessed to be an optimal solution for larger-scale operations, as the material is fragile [113,114]. Activated carbon granules have a large surface area and porous structure, promoting biofilm formation. Carbon granules are also a low-cost material but have the potential to clog. The conductivity of carbon granules is often relatively low, as the granules have a poor connection to the current collector. However, the granules have another property that can be exploited, as the structure allows for easier entrapment of pollutants, such as heavy metals [115].

Stainless steel is another material used as an anode; however, the material is constricted, as the surface area is reduced compared to carbon-based anodes. Stainless steel can be used with other materials such as graphene to increase biofilm formation [116]. Stainless steel is one of the cheapest materials used for anodes and has excellent electric conduction, exhibiting strong mechanical strength and corrosion resistance and having a long lifetime. Other materials such as nickel [105], silver [106], titanium [117] and copper [118] have also been tested as anode materials, but in their pure forms, these materials lack the ability of microbial adhesion. However, most can use a combination of materials, such as a carbon-based anodes modified with a nickel coating [105]. Both copper and nickel ions can be poisonous for microbes and are undesirable when released into, e.g., wastewater, which makes these materials less ideal, as it constrains the environmental aspect of the MEC design.

Most bacteria’s outer membrane consists of proteins and amino acids which are negatively charged. Thus if the anode material can be configured to be more positively charged by embedding nitrogen molecules in the structure by nitrogen doping, biofilm formation can be enhanced and promoted [124]. The chemical bonding between C-, N-, O- and S-containing functional groups can be exploited when optimizing the anode.

Y. Zhau et al. (2018) [109] investigated the modification of carbon felt anodes by double oxidant HNO₃/H₂O₂ and found it to decrease the inoculation period and to increase electron production for the anodes [109]. Another study by S. Rozenfeld et al. (2019) [86] investigated how to improve anode activity by utilizing two different anode materials: plasma-pretreated carbon cloth and stainless steel. The study found that plasma pretreated anodes, with the introduction of nitrogen doping atoms, increased -C-OH, -COOH, and =C=O groups, and this relationship was found to increase both biofilm formation and the current in biohydrogen production.

Already known surface modifications have proven to increase the productivity of electrolysis cells with methods as simple as utilizing the heating of the material with ammonia, giving currents of 2400 mW · m⁻² [125] and hydrogen production of 7.4 L H₂ · L⁻¹ · day⁻¹ [126].

When comparing anode materials with and without surface modification, increased productivity was found for surface-modified anodes, cf. Table 3. As surface modification substantially increases productivity, future studies should investigate optimal treatments allowing for high productivity for a long duration, which can feed into scalability for both the anodes and optimized surface modification.

For MEC to become commercially successful, the anode material must have a low cost with stable operation over time. The anode must be environmentally friendly and have strong mechanical strength [90,115]. The most commonly used anode material capable of complying with the success criteria is the carbon-based anode, which has been used in many studies [90,115,127]. Carbon-based anodes have the advantage of good attachment for electrogenic microbial species, and some exhibit a large surface area in a small volume due to their 3D structure. However, this can also be a problem, as the pores can be affected by clogging, thereby restricting flow. This decreases electron transfer and biohydrogen production, resulting in the usage of 2D carbon anodes.

Anodes are typically defined as 2D or 3D structures depending on the surface. Three-dimensional structures have been preferred in a laboratory scale and have shown the highest biohydrogen production and current density, as seen in Table 3. However, a study conducted by E. Blanchet et al. (2016) [128] mimicked an industrial MEC operation based on food waste and wastewater, where 3D (carbon felt) and 2D (carbon cloth) anodes were used for comparison. The study found that the 3D structure performed worse than the 2D structure, as there was no clogging in the 2D structure. This contradicts previous studies, suggesting that 3D anodes are not always the preferred choice [128]. This could especially be relevant for the upscaling process, as larger facilities are able to implement 2D anodes with a faster flow, ultimately creating the potential for higher production of protons and electrons. Therefore, 2D structures show an interesting approach when dealing with larger particle substrates, as the chance of clogging decreases.

An investigation into 2D structured anodes was conducted by D. Pocaznoi et al. (2012) [129], comparing the following anode materials: stainless steel, flat-plate graphite and carbon cloth. The study found that carbon-based materials increased biofilm formation compared with stainless steel. Comparing the flat-plate graphite with stainless steel, no difference in biofilm formation was found, but the stainless steel had twice the current density. Comparing the stainless steel with the carbon cloth, a higher current density was exhibited by the carbon cloth. The study concluded that the topography of stainless steel prevented it from being the preferred anode material, but surface treatment can be used to increase the current density [129]. Therefore, combining the electric properties of stainless steel with a biofilm-promoting carbon material can provide anodes with higher hydrogen productivity.

No single anode material has been able to satisfy all criteria for an excellent industrial and environmentally friendly anode while still being economically favorable. Carbon-based materials with a metal current collector outperform their solely metal-based counterparts, but few have provided sufficient biohydrogen production. Almost all studies have used surface modification on their anode in one form or another, and it seems generally accepted that surface modifications increase the performance of almost all materials, as shown in Table 3 [90,112,115,130]. Surface treatments are worth exploring, where treatments with low environmental effects and prices can bring the anodes closer to a viable industrial MEC. Additionally, surface treatment and combinations with other anode materials could make the metals a viable candidate in the future, especially considering the costs and operation times of anodes.

To drive the HER reaction in an MEC, a catalyst is needed, predominantly implemented as a catalyst coating/catalytic material or by using microbes to drive catalysis [39,43,144,145]. Like the anode, the cathode must exhibit different properties to be a viable candidate for an industrially applicable MEC. These properties include fast catalysis, low overpotential, a low cost and a long operation time [146].

Cathodes can be based on current-collecting materials or carbon-based materials, used as a basis for the structure and electron transportation. Materials used as an anode can be used as a base; the difference is denoted by incorporating a catalyst. Traditional catalysts, such as noble metals, e.g., platinum or palladium, incorporated as a coating can also be used due to stable catalytic activity. Platinum-based cathodes are well established and known for their good catalytic properties and low degree of overpotential [55,56]. However, using platinum has the drawbacks of a negative impact on the environment and a high cost; however, it is often used as a baseline for comparison [147]. Platinum can be a viable option for lab-scale MECs, but the use of complex substrates for larger operations would require protection from sulfide poisoning from platinum, which reduces its catalytic effectiveness [147,148]. Palladium-based catalysts have been explored as an alternative used as a coating, as palladium is more resilient and can be acquired at a lower cost [149].

Nickel has been proposed as an alternative to platinum, as it exhibits good catalytic activities and high corrosion resistance at a low cost [150,151]. Nickel is often used as an alloy, as it increases the active surface area and lowers the overpotential, as explained in a study by A. Jeremiasse et al. (2010) [150]. The study found that nickel foam decreased the overpotential and achieved production of 50 L H₂ · L⁻¹ · day⁻¹ [150]. Nickel foam also showed potential as a cathode base utilized with a platinum coating [152]. Nickel foam can be used as a versatile cathode at a low cost and can be a cheap alternative to platinum cathodes as a baseline comparison for future studies. This was investigated by Z. Yan et al. (2012) [153], who combined a low concentration of platinum and nickel nanoparticles and compared the performance to a Pt/C catalyst. The study found that the new nickel alloy was stable and could produce current densities in the same range at a quarter of the price [153]. This tendency aligns with other findings [154,155], as shown in Table 4, but the toxicity of nickel ions should be investigated before upscaling. Nickel foam has the advantage of easier scalability, as the material can be found in nickel metal hydride (NIMH) batteries produced on a large scale, and the material is therefore readily available.

Many of the highest-performing MECs have been constructed based on expensive catalysts, which are rarely viable in an industrial setting. However, research with biotic cathodes has started to increase HER productivity and current density, enabling the use of microbial species instead of a catalyst [68,90]. Biotic cathodes are an inexpensive catalyst and can be used for industrial settings with a low maintenance cost, as the biofilm driving the biotic catalysis is replenished by itself. However, using a biotic cathode, especially in a one-chamber configuration, also creates the possibility of cultivating methanogens. Methanogens can be eliminated or decreased by utilizing some of the following tactics: using a pure culture, exposing electrodes to oxygen before each batch, temperature shock, the addition of beryllium sulfide or by modifying the MEC design [68]. Furthermore, biocathodes have thus far not been reviewed as the highest-producing MEC [147], and thus, a cost–benefit analysis should be implemented to ensure their viability.

Evaluating the optimal cathode material and catalyst is a complex decision, but it should focus on cost, efficiency and scalability. The basis of the cathode has not been proven to be the deciding factor and does not affect hydrogen production in the same manner as the catalyst. As the base material constitutes a large fraction of the total cathode, investigation into the structural effect when scaling from small MECs to larger systems should be investigated, as larger MECs would require a more robust system. The focus aims to find new catalysts or to optimize previous catalysts to decrease costs and improve catalytic properties.

If a biotic cathode is utilized, this enables the production of pure biohydrogen, where methanogens can also be cultivated, leading to hydrogen loss. The presence of methanogens can be prevented, and it should not be the deciding factor. Nickel alloys provide a cheaper alternative to platinum catalysts coupled with HER-catalyzing microbial species. Nickel alloys outperform platinum-based alternatives and are becoming well studied at the laboratory-scale, as seen in Table 4. Thus far, biotic cathodes have experienced lower hydrogen productivity than platinum-based cathodes, and methods enabling higher productivity, either through biofilms, cultures or material optimization, must be explored.