energies Review Rehydrogenation of Sodium Borates to Close the NaBH4-H2 Cycle: A Review Helder X. Nunes 1,†, Diogo L. Silva 1,† , Carmen M. Rangel 2,* and Alexandra M. F. R. Pinto 1,* 1 Transport Phenomena Research Center, Chemical Engineering Department, Faculty of Engineering, University of Porto, Dr. Roberto Frias s/n, 4200-465 Porto, Portugal;
[email protected] (H.X.N.);
[email protected] (D.L.S.) 2 National Laboratory of Energy and Geology, Estrada da Portela, 1649-038 Lisboa, Portugal * Correspondence:
[email protected] (C.M.R.);
[email protected] (A.M.F.R.P.) † These authors contributed equally to this work. Abstract: In 2007, the US Department of Energy recommended a no-go on NaBH4 hydrolysis for onboard applications; however, the concept of a NaBH4-H2-PEMFC system has the potential to become a primary source for on-demand power supply. Despite the many efforts to study this technology, most of the published papers focus on catalytic performance. Nevertheless, the development of a practical reaction system to close the NaBH4-H2 cycle remains a critical issue. Therefore, this work provides an overview of the research progress on the solutions for the by-product rehydrogenation leading to the regeneration of NaBH4 with economic potential. It is the first to compare and analyze the main types of processes to regenerate NaBH4: thermo-, mechano-, and electrochemical. Moreover, it considers the report by Demirci et al. on the main by-product of sodium borohydride hydrolysis. The published literature already reported efficient NaBH4 regeneration; however, the processes still need more improvements.