The production of hydrogen from hydrocarbon/water compounds can be classified into three categories, namely "brown", "blue", and "green". 27 Brown hydrogen is produced from H 2 O by steam reforming of natural gas (or coal) at about 800−900 °C. The enthalpy required for H 2 production is significantly Figure 4. (a) Schematic diagram of (A) methane-fed and (B) electrolysis-driven Haber−Bosch ammonia production. Reproduced with permission from ref 44. Copyright 2020 The Royal Society of Chemistry. (b) Dissociative pathway for N 2 activation (above) and stepwise nondissociative pathway for N 2 activation (below). Reprinted with permission from ref 46. Copyright 2019 John Wiley and Sons. (c−e) Various strategies to carry out ammonia synthesis at low temperature and low pressure: (c) The use of electrostatically polar surfaces to alleviate hydrogen poisoning challenges at low temperature. Reprinted with permission from ref 52. Copyright 2020 American Chemical Society. (d) Dual-site mechanism to tackle nitrogen activation and hydrogen dissociation separately. Reprinted with permission from ref 54. Copyright 2020 Springer Nature. (e) Li-polarized surface to stabilize intermediates to carry out the more energy favorable non-dissociative pathway. Reprinted with permission from ref 46. Copyright 2019 John Wiley and Sons. (f) A novel mechanochemical method to enable ammonia synthesis at 45 °C and 1 bar. Reprinted with permission from ref 55. Copyright 2020 Springer Nature. (g) Direct eNRR via absorption of N 2 onto the catalyst surface, followed by progressive proton and electron additions to produce a first, followed by a second molecule of ammonia. Reprinted with permission from ref 56. Copyright 2018 John Wiley and Sons. (h) Indirect electrochemical N 2 reduction to ammonia based on lithium as a mediator, forming Li 3 N as an intermediate, and reaction with H 2 O on a copper substrate. Reprinted with permission from ref 57.