From potato peels to power: Evaluation and scale-up simulation of hydrochar as catalyst for water splitting

• Residues as raw material for synthesizing OER catalyst over green hydrothermal route. • Potato peels hydrochar showed best OER results due to its physicochemical properties. • Scaling 1:10 the electrochemical cell caused negligible OER performance variations. • CFD simulations adjusted the electrochemical set up properly. • Manufacturing issues compromised the stability of the larger electrode (10 cm 2 ) Electrochemical water splitting process is a promising alternative to face energy shortage by synthesizing H 2 . Nevertheless, the process has a negative energy balance unless catalysts be added. This study is focused on using natural residues hydrothermally converted into so-called hydrochars as catalysts on the oxygen evolution reaction (OER), the water splitting rate-limiting step. These residues were evaluated through cyclic voltammetries, impedance spectroscopy and calculating Tafel slopes. Moreover, the physicochemical properties of hydrochar were determined: active surface area, point of zero charge (PZC), FTIR, Raman, XPS and SEM-EDS. Computational fluid dynamics (CFD) analyses were done to evaluate the behaviour of the gas bubbles on this electrochemical process. Potato peels hydrochar performance metrics were comparable to IrO 2 benchmark catalyst, obtaining 10, 50 and 100 mA/cm 2 by applying low overpotentials (352, 386 and 397 mV, respectively), which obtained viable faradaic efficiencies of 89.1, 91.2 and 93.6 %, respectively. The good performance of this hydrochar is explained by its acid PZC of 4.25 and the high content of defects (Raman) and functional groups such as C=C, or C-OH (FTIR, XPS). Scale-up experiments were carried out by increasing the electrode size 1:10 and 1:100. The catalytic electrode stability resulted competitive even in the scaled-up reactor, surpassing 90 h with detriments below 16 % at 100 mA/cm 2 . CFD results show a reasonably similarity with experimental tests and the capability to capture the intricate dynamics of multiphase electrolytic reactors. This study demonstrates that a metal-free hydrochar-based catalyst can be useful for OER, showing scale-up feasibility and CFD-validated performance.