*4.2. Electrocatalysis*

In addition to photocatalysis, the synthesis of ammonia by the electrocatalysis process is also currently being explored. In a traditional proton-conductive electrolyte cell, gaseous H2 passes through the anode where it is changed to protons (H+) while the nitrogen reduction reaction occurs at the cathode [101]. The H<sup>+</sup> then diffuses into the cathode, where it forms NH3 in combination with the dissociated N. The following equations can describe the reaction:

$$\text{CH}\_2 \rightarrow \text{OH}^+ + \text{6e}^- \tag{6}$$

$$\text{N}\_2 + \text{6H}^+ + \text{6e}^- \rightarrow \text{ 2NH}\_3 \tag{7}$$

$$\text{N}\_2 + \text{3H}\_2 \rightarrow 2\text{NH}\_3 \tag{8}$$

The problem, though, is such cell configuration had to work at low temperatures where the kinetics of reaction were sluggish [102]. Moreover, an electrochemical cell is more advantageous when operating at higher temperatures since higher rates of reaction can be achieved in the same electrode area, and hydrazine development can be prevented [103]. As a result, proton (H+) conductivity solid-state materials that operate at a temperature above 500 ◦C were developed [102].

Based on the working temperature, electrocatalytic ammonia synthesis can be broken down into high (higher than 500 ◦C), intermediate (between 100 ◦C and 500 ◦C) and low (lower than 100 ◦C) [36]. In the high-temperature electrocatalysis process, most of the studies occupy solid electrolytes with perovskite as the material in reactor configuration. Among the studies, quite high ammonia rates were reported by Wang et al. [104,105] using doped Ceria-Ca3(PO4)2-K3PO4 composite electrolyte in combination with a Ag-Pd electrode. Ammonia yields up to 9.5 × <sup>10</sup>−<sup>9</sup> mol s<sup>−</sup>1cm−<sup>2</sup> can be achieved using N2/H2 as feedstock and up to 6.95 × <sup>10</sup>−<sup>9</sup> mol s<sup>−</sup>1cm−<sup>2</sup> using N2/natural gas. In 2009, the novel Solid State Ammonia Synthesis (SSAS) configuration using the Ag-Ru/MgO cathode developed by Skodra and Staukides [106] was able to directly use water as a source of hydrogen. Other than the configurations mentioned above, the oxygen-ion (O2−) conductor has also been shown to be used for ammonia synthesis in SSAS where both processes of electrolysis and ammonia synthesis occur at the cathode but have suffered very low ammonia production rates [106,107].

In intermediate electrocatalysis processes with operating temperatures ranging from 500 ◦C to 100 ◦C, molten salts are typically used as electrolytes. Murakami et al. [108] made the earliest study in this temperature range in 2003, using a molten salt mixture electrolyte and porous nickel as electrodes. Other sources of hydrogen, such as water steam [109,110], hydrogen chloride [111], methane [112] and hydrogen sulphide [113], have also been tested. More recently, Licht et al. [114] used a similar configuration for the experiment with NaOH/KOH as electrolyte and nickel as the electrode but added a Fe2O3 nanoparticle catalyst to the molten salt. The maximum forming rate of ammonia <sup>1</sup> × <sup>10</sup>−<sup>8</sup> mols<sup>−</sup>1cm−<sup>2</sup> could be achieved by this setup, although it is much lower compared to the works of Murakami's group.

In low-temperature electrochemical ammonia synthesis below 100 ◦C, Nafion and Sulfonate Polysulfone (SPSF) are commonly used as proton electrolyte conductors [103]. Kordali et al. [115] in 1999 reported a novel configuration that could synthesise ammonia below 100 ◦C using a combination of the Nafion membrane and KOH solution. The anode was Pt, while the anode was carbon cloth on which Ru had been deposited. The hydrogen source was either hydrogen gas or water. A summary of the selected electrochemical system by previous studies is presented in Table 8.


**Table 8.** A summary of the selected electrocatalytic ammonia synthesis by previous studies.


**Table 8.** *Cont.*

Note: SCY denotes ScCe0.95Yb0.05O3-<sup>α</sup>, BCN denotes Ba3(Ca1.18Nb1.82)O9-δ, BCZN denotes Ba3CaZr0.5Nb1.5O9−δ, BCNN denotes Ba3Ca0.9Nd0.28Nb1.82O9−δ, BCS denotes BaCe0.9Sm0.1O3−<sup>δ</sup>, BCGS denotes BaCe0.8Gd0.1Sm0.1O3−<sup>δ</sup>, BZCY denotes BaZr0.7Ce0.2Y0.1O3−δ, LSGM denotes La0.9Sr0.1Ga0.8Mg0.2O3−<sup>α</sup>, LCZ denotes La1.95Ca0.05Zr2O7−<sup>δ</sup>, LCC denotes La1.95Ca0.05Ce2O7−δ, BCG1 denotes BaCe0.8Gd0.2O3−<sup>δ</sup>, BCY1 denotes BaCe0.85Y0.15O3−<sup>α</sup>, BCY2 denotes Ba0.98Ce0.8Y0.2O3−<sup>α</sup>, CYO denotes Ce0.8Y0.2O1.9, CSO denotes Ce0.8Sm0.2O1.9, CGO denotes Ce0.8Gd0.2O1.9, CLO denotes Ce0.8La0.2O1.9, LCGM denotes La0.9Ca0.1Ga0.8Mg0.2O3−α, LSGM denotes La0.9Sr0.1Ga0.8Mg0.2O3−<sup>α</sup>, LBGM denotes La0.9Ba0.1Ga0.8Mg0.2O3−<sup>α</sup>, BCG2 denotes BaCe0.85Gd0.15O3−α, BCZS denotes BaCe0.7Zr0.2Sm0.1O3−<sup>α</sup>, BCC denotes BaCe0.9Ca0.1O3−<sup>α</sup>, LSFC denotes La0.6Sr0.4Fe0.8Cu0.2O3−<sup>δ</sup>, SBCF denotes SmBaCuFeO5+δ, SBCC denotes SmBaCuCoO5+δ, SSC denotes Sm0.5Sr0.5CoO3−<sup>δ</sup>, SSN denotes Sm1.5Sr0.5NiO4, SFCN denotes SmFe0.7Cu0.1Ni0.2O3.
