# Sustainable Additives for the Production of Hydrogen via Sodium Borohydride Hydrolysis
## 4 Conclusions  
Sodium borohydrides are currently positioned among the most promising strategies for hydrogen storage in a solid state. However, the maximum hydrogen release and the reaction kinetics are challenges ahead to reach higher TRLs and, therefore, full industrial implementation. In this regard, additives represent an option to overcome some of the most frequently reported performance limitations. The present work contributes to making a step forward in the use of additives in low quantities, specifically at 1 wt%, studying the effects on the hydrolysis of NaBH\({}_{4}\) and describing the most relevant process variables. For this purpose, four additives with different viscosities and natures have been evaluated, analyzing the yields, hydrogen generation rates, lag time, and gravimetric densities.  
The results showed that the additive polyacrylamide (PAM) presented the highest yield, close to 100%, and a relatively high H\({}_{2}\) release rate, with a total value of 721 mL\(\cdot\)min\({}^{-1}\cdot\)gcat\({}^{-1}\). The time required to obtain the total amount of hydrogen (4000 s) was significantly longer in comparison to the rest of the additives tested. In contrast, sodium carboxymethyl cellulose (CMC) provided reasonable hydrogen yield values (85%), safe values of lag time (70 s), the highest value of hydrogen generation rate at 1374 mL\(\cdot\)min\({}^{-1}\cdot\)gcat\({}^{-1}\), and the lowest time for stabilization, only 300 s, positioning it as the most valuable additive under the studied reaction conditions. The other two additives, \(\beta\)-cyclodextrinrin and sodium dodecyl sulfate, achieved lower values in terms of H\({}_{2}\) release rate and yield but, in both cases, higher replicability and safety were obtained in comparison with the control test.  
Further efforts are still required to optimize the effects of additives such as polyacrylamide and sodium carboxymethyl cellulose for the use of sodium borohydrides in hydrogen storage and release. The employment of additives could be of great importance to achieve a safe, stable, and reproducible system for sodium borohydride hydrolysis.  
Conceptualization, A.O., A.M.F.R.P. and I.O.; Methodology, L.G.-C. and D.L.S.; Formal analysis, L.G.-C., D.L.S. and V.M.O.-M.; Investigation, L.G.-C., D.L.S., A.O., C.M.R., V.M.O.-M., A.M.F.R.P. and I.O.; Resources, A.O., C.M.R., A.M.F.R.P. and I.O.; Data curation, L.G.-C., D.L.S. and V.M.O.-M.; Writing--original draft, L.G.-C., D.L.S., A.O., C.M.R., V.M.O.-M., A.M.F.R.P. and I.O.; writing--review and editing, L.G.-C., D.L.S., A.O., C.M.R., V.M.O.-M., A.M.F.R.P. and I.O.; Supervision, A.O., A.M.F.R.P. and I.O.; Project administration, A.O., C.M.R., A.M.F.R.P. and I.O.; Funding acquisition, A.O., C.M.R., A.M.F.R.P. and I.O. All authors have read and agreed to the published version of the manuscript.  
This research received financial support from the LIFE program (LIFE19 ENV/ES/000143). This work has been conducted with financial support from the Spanish Ministry of Science and Innovation (project PID2021-123120 OB-I00). This research is also being supported by the Project \"HYLANTIC\"-EAPA_204/2016\", which is co-financed by the European Regional Development Fund in the framework of the Interreg Atlantic program.  
Not applicable.  
Data Availability Statement: Data available on request.  
The authors declare no conflict of interest.