Recent progress towards the electrosynthesis of ammonia from sustainable resources

Ammonia (NH3) is a key commodity chemical of vital importance for fertilisers. It is made on an industrial scale via the Haber Bosch process, which requires significant infrastructure to be in place such that ammonia is generally made in large, centralized facilities. If ammonia could be produced under less demanding conditions, then there would be the potential for smaller devices to be used to generate ammonia in a decentralized manner for local consumption. Electrochemistry has been proposed as an enabling technology for this purpose as it is relatively simple to scale electrolytic devices to meet almost any level of demand. Moreover, it is possible to envisage electrosynthetic cells where water could be oxidised to produce protons and electrons at the anode which could then be used to reduce and protonate nitrogen to give ammonia at the cathode. If this nitrogen were sourced from the air, then the only required infrastructure for this process would be supplies of water, air and electricity, the latter of which could be provided by renewables. Hence an electrosynthetic cell for ammonia production could allow NH3 to be generated sustainably in small, low-cost devices requiring only minimal facilities. In this review, we describe recent progress towards such electrosynthetic ammonia production devices, summarizing also some of the seminal literature in the field. Comparison is made between the various different approaches that have been taken, and the key remaining challenges in the electrosynthesis of ammonia are highlighted.