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Electrochemical
Electrochemical Production (Electrolysis)
The fact that protons and electrons were required for the reaction to be completed suggested that ammonia may be generated electrochemically. To do this, numerous research teams evaluated the performance of aqueous electrochemical cells that generated NH3 from H2O and N2. However, the fact that these cells required to function at low temperatures where reaction kinetics were slow presented a challenge.
Solid state materials with significantly high proton (H+) conductivity at high temperatures (500–1000 °C) were found in 1981 by Iwahara and colleagues. Experimental evidence of the electrochemical synthesis of ammonia from its constituent parts was presented in 1998 using a high-temperature solid electrolyte cell of this type (Fig. 1).
The procedure was easy. H+ was created as gaseous hydrogen passed over the proton conducting cell's anodic electrode. The generated protons were electrochemically carried to the cathode by applying the correct voltage, where they interacted with gaseous nitrogen to make ammonia. Because the high pressure need was balanced by the expenditure of electrical energy, the cell operated at atmospheric pressure.
In an effort to increase reaction rates and reduce electric energy consumption, various research teams have researched the Solid State Ammonia Synthesis (SSAS) over the past 20 years. Amar et al., Giddey et al., and Garagounis et al. recently examined the key findings from research published prior to 2013.
The current review provides an update on the development of liquid and solid electrolyte cells used in the electrochemical production of ammonia. The experimental studies are separated into three groups based on the operational temperature range: high temperature (T > 500 °C), intermediate temperature (500 °C > T > 100 °C), and low temperature (T 100 °C).
In most of the investigations, solid electrolytes were used in reactor-cell designs similar to the one in Fig. 1. The electrolyte substance used in the majority of these instances was perovskite. There have also been reports of materials with fluorite or pyrochlore structures being used as electrolytes. One study used a composite electrolyte made of YDC and calcium-potassium phosphate and reported relatively high ammonia rates. The electrochemical reactions at the two electrodes can be expressed as follows when, as in Fig. 1, the electrolyte is a proton conductor:
Together, reactions (3) and (4) provide an overall reaction that is identical to reaction (1). Gaseous H2 and N2 were the most prevalent reactants at the anode and cathode, respectively.
The industrial process's preparation of the hydrogen input gas, more specifically its purification, accounts for a sizeable portion of the entire cost. Natural gas is the main source of hydrogen production. The latter contains substances that can contaminate the industrial catalyst even in minute concentrations. H2 must therefore undergo substantial purification.
This prerequisite is not met by the electrochemical synthesis (Fig. 1) because only protons (H+) are transported through the solid electrolyte. Furthermore, using gaseous H2 is not required. Evidently, any substance containing hydrogen might be utilized.
Thus, the viability of SSAS from steam and nitrogen was demonstrated in 2009 utilizing the solid electrolyte cell of Fig. 2 and an Ag-Ru/MgO catalyst (cathode).
Similar studies followed on Pt and Ag electrodes [23]. Wang et al. [33] studied the reaction at 650◦C on a Ag-Pd cathode, with a YDC-Ca3(PO4)2, K3PO4 composite electrolyte, but used natural gas (CH4) as a hydrogen source. The maximum ammonia rate they observed was 6.95 × 10−9 mol s−1 cm−2, which is one of the highest at these temperatures.
Fig. 3 shows schematically how an oxygen-ion (O2−) conductor can be used for SSAS. Ammonia is synthesized from gaseous nitrogen and steam according to the following reactions:
Electrochemical Synthesis of Ammonia (docx)
Electrochemical synthesis of Ammonia a low temperature a low pressure approach (docx)
Electrochemical synthesis of Ammonia in solid electrolye cells (docx)