Microfluidics for Electrochemical Energy Conversion and Storage: Prospects Toward Sustainable Ammonia Production

Abstract

Ammonia is a key chemical in the production of fertilizers, refrigeration and an emerging hydrogen-carrying fuel. However, the Haber-Bosch process, the industrial standard for centralized ammonia production, is energy-intensive and indirectly generates significant carbon dioxide emissions. Electrochemical nitrogen reduction offers a promising alternative for green ammonia production. Yet, current reaction rates remain well below economically feasible targets. This work examines the application of electrochemical microfluidics for the enhancement of the rates of electrochemical ammonia synthesis. The review is built on the introduction to electrochemical microfluidics, corresponding cell designs, and the main applications of microfluidics in electrochemical energy conversion/storage. Based on recent advances in electrochemical ammonia synthesis, with an emphasis on the critical role of robust experimental controls, electrochemical microfluidics represents a promising route to environmentally friendly, on-site and on-demand ammonia production. This review aims to bridge the knowledge gap between the disciplines of electrochemistry and microfluidics and promote interdisciplinary understanding and innovation in this transformative field.

Keywords: Ammonia; Electrochemistry; Energy conversion; Microreactors; Nitrogen reduction reaction.

Figure 1

The role of ammonia in the global economy: (a) Mass flows in the ammonia supply. Numerical values are given in Tg (2019 data). MAP, DAP, CAN, UAN, and AS are monoammonium phosphate, diammonium phosphate, calcium ammonium nitrate, urea ammonium nitrate, and ammonium sulfate, respectively. (b) Energy and emission intensities for key industrial products (2021 data). Adapted from Ref. [1], International Energy Agency, CC BY 4.0.

Figure 2

A scheme of a one‐electron electrochemical cell in an electrolytic mode. A source of direct electrical current drives a thermodynamically non‐spontaneous reduction and oxidation at the cathode and anode, respectively.

Figure 3

A polarization curve for an electrolytic cell.

Figure 4

The most common designs of electrochemical microfluidic cells: (a) the flow‐by architecture is based on the horizontal flow of streams over the electrode surface; (b) the flow‐through architecture relies on stream flow through the pore network of the porous electrode; (c) the air‐breathing architecture employees a gas diffusion electrode. Schemes inspired by Ref. [47].

Figure 5

Graphical summary of applications in all areas of electrochemical microfluidics: electrolyzers, fuel cells, and redox flow batteries.

Figure 6

Strategies for optimization of proton source concentration at the interface: (a) modulation of conductivity of the electrode substrate to increase the HER overpotential, (b) electrolyte design for hindered proton transfer from bulk to the interface, (c) suppressing proton diffusion to the interface through hydrogen bonding between an additive and proton source. Reprinted with permission from: (a) Ref. [99]. Copyright 2019 American Chemical Society, (b) Ref. [93]. Copyright 2019, The Authors, under exclusive license to Springer Nature Limited, (c) Ref. [98]. Copyright 2021, Wiley‐VCH GmbH.

Figure 7

Scheme of the NRR at the triphasic N₂‐electrolyte‐electrode interface in the bubble‐based microreactor, adapted and modified based on Ref. [104].