*5.1. Direct Fuel Cell*

Fuel cells are currently intensively examined as a breakthrough candidate for carbonfree power generation. The device provides a highly efficient conversion directly from chemical to electricity and has a low environmental footprint. In an early study, ammonia, which has 17% hydrogen by weight, was proposed for use in PEM fuel cells. Though, due to the low operating temperature of the PEM fuel cell, the thermodynamics decomposition of ammonia cannot occur [183]. On top of that, ammonia is lethal to the Nafion membrane

utilised in PEM fuel cells. Thus, external cracking reactors are required to completely convert ammonia into hydrogen, giving an extra energy input and additional costs [184].

Based on electrolyte and reaction, a direct ammonia-fed fuel cell can be divided into three major systems, alkaline fuel cell (AFC), alkaline membrane fuel cell (AMFC) and solid oxide fuel cells (SOFC). The discussion of each configuration is provided below. Ammonia has been reported as a feed for a fuel cell as early as the 1960s based on the alkaline fuel cell developed by Francis Thomas Bacon [185]. The cells use alkaline electrolytes such as potassium hydroxide (KOH) and platinum cathodes. Most recently, Hejze et al. [186] reported the potential of molten hydroxide (NaOH/KOH) as an electrolyte. Unfortunately, the use of KOH and NaOH is not favourable for air-intake fuel cells since it reacts with CO2 to form K2CO3 and Na2CO3 and degrades the performance of the alkaline electrolyte.

Recently, alkaline membrane fuel cells (AMFCs) gained attention from the fuel cell society due to the compatibility with CO2. As reported by Unlu et al. [187], CO2 introduced in the cathode has a positive effect on improving fuel cell performance. In a recent development, room temperature AMFC has been developed by Lan and Tao [185]. Compared to fuel cells based on acidic polymer electrolytes, low-cost non-precious catalysts, including MnO2, silver or nickel, may be used for AMFCs [37]. Moreover, Pt/C, PtRu/C and Ru/C were recently investigated AMFCs and can also be used as anodes [188].

Other types of ammonia fuel cells, namely SOFC, are initially developed to prevent NOX formation [189]. However, the number of scientists who studied SOFC becomes more intense caused by the potencies of the cell to operate at high temperature, thus overcoming the disadvantage suffered by PEMs. At high temperatures, ammonia can be directly decomposed into hydrogen, normally ranging between 500 and 1000 ◦C, and hence the need for an external cracking reactor is negated. In addition, there was no evidence of ammonia having a bad effect on the ceramic electrolytes used in SOFCs [190]. Nonetheless, because of the fragility of porcelain materials, SOFCs are usually not appropriate for transport use [191].

Research on SOFC fuel cells can be separated into Oxygen Ion-Conducting Electrolytes (SOFC-O) and Hydrogen Ion-Conducting Electrolyte (SOFC-H), which is also known as proton-conducting electrolytes. The schematic of SOFC-O and SOFC-H fuel cells is illustrated in Figure 7.

**Figure 7.** Schematic representation of (**a**) SOFC-O and (**b**) SOFC-H [27].

The SOFC-O operating principle lies in the transportation of oxygen anions across the electrolyte while the charge carrier in SOFC-H is a proton [192]. For both types, ammonia is fed into the anodic site, where it thermally decomposes into nitrogen and hydrogen [193]. In SOFC-O, the oxygen in the cathode compartment is reduced into oxygen ions at the cathode–electrolyte interface and transported across the solid electrolyte, which then reacts with hydrogen electrochemically to produce water [27,194]. The reactions that occur at the anode and cathode are stated below:

$$\text{Anode}: \text{H}\_2 + \text{O}^{2-} \rightarrow \text{H}\_2\text{O} + 2\text{e}^- \tag{9}$$

$$\text{Cathode}: \frac{1}{2} \text{O}\_2 + 2\text{e}^- \rightarrow \text{O}\_2\text{}^- \tag{10}$$

The SOFC-O electrolytes tend to be built based on solid ceramics with metal oxide. YSZ is most widely used because of its high ionic conversion, which facilitates the efficient movement of oxygen anions through the electrolyte [195]. These solid electrolytes also show strong chemical and thermal stability, which is crucial for the treatment of high temperatures. Samarium doped ceria (SDC)-based electrolytes also sparked interest due to its capability to have high ionic conductivities at lower temperatures [196].

In SOFC-H, the hydrogen in the anode compartment is oxidised into a proton which is then transported across solid electrolyte into the cathode [197]. This later reacts with oxygen to produce water. The reactions that occur at the anode and cathode are given below:

$$\text{Anode}: \text{H}\_2 \rightarrow 2\text{H}^+ + 2\text{e}^- \tag{11}$$

$$\text{Cathode}: \text{O}\_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}\_2\text{O} \tag{12}$$

An SOFC-H system electrolyte is selected based on the conductivity of a proton as well as chemical and mechanical stability. The extraordinarily high proton conductivities of doped BaCeO3(BCO) and BaZrO3(BZO) have been shown over a wide 300 to 1000 ◦C temperature range [198].

In addition to the qualities of the employed material, electrolyte thickness has a direct effect on fuel cell performance. When a thinner electrolyte has been used, the internal resistance of SOFC decreases. However, reducing electrolyte thickness can affect the mechanical strength and consequently stability over the long term [198]. Table 10 summarises the preceding SOFC fuel cell work that turns ammonia into electricity.

**Table 10.** Summarization of previous research on SOFC for ammonia synthesis.


Note: YSZ denotes yttria-stabilized zirconia, SDC denotes samarium doped ceria, LSM denotes La0.5Sr0.5MnO3, SSC denotes Sm0.5Sr0.5Co3−<sup>δ</sup>, BSCF denotes Ba0.5Sr0.5Co0.8Fe0.2O3−<sup>δ</sup>, BCGP denotes BaCe0.8Gd0.19Pr0.01O3-δ, BCG denotes BaCe0.8Gd0.2O3-δ, BCE denotes BaCe0.85Eu0.15O3, LSC denotes La0.5Sr0.5CoO3−<sup>δ</sup>, BZCY denotes BaZr0.1Ce0.7Y0.2O3-δ, BSCF denotes Ba0.5Sr0.5Co0.8Fe0.2O3-δ, CGO denotes Ce0.8Gd0.2O1.9, BCN denotes BaCe0. 9Nd0.1O3−δ.

> In addition to all of the above forms, microbial fuel cells (MFC) are also seen as an alternate technique for generating electricity directly from ammonia. MFC uses microorganisms in the oxidation process for the conversion of chemical energy from bio-degradable material, for example, ammonia contaminated wastewater. The electrons flow from the anodic side of the external circuit to the cathode, where they combine with the proton and oxygen to form water [198]. The schematic diagram of MFC is shown in Figure 8.

**Figure 8.** Schematic diagram of MFC [213].

According to Li et al. [214], MFC has been deemed a potential technique for treating wastewater while producing energy, but low power, high cost and reactor scalability issues severely limit its advancement. In addition to wastewater treatment, MFC technology has been proposed as a feasible alternative for air cleaning by removing ammonia from the environment. Yan and Liu [215], in 2020, found Sn-doped V2O5 nanoparticles to be a good catalyst for the rapid removal of ammonia in the air using photo-electrocatalysis (PEC) MFCs.
