Real-time DEMS monitoring of gas evolution in a hybrid integrated EAOP reactor for electrochemical valorization of polluted water toward green chemical feedstocks

• BDD drives rapid mineralization (>95% COD removal) with detectable CO 2 evolution, while DSA favors selective electroconversion into short-chain carboxylic acids. • Online DEMS monitoring shows CO 2 formation only with BDD, whereas DSA suppresses gaseous carbon release, confirming fundamentally different anodic mechanisms. • H 2 generated at the Ni–Fe cathode reaches >99% purity and 98–100% Faradaic efficiency, independent of anode type or organic load. • The system couples water treatment with simultaneous H 2 production and chemical valorization. Decentralized electrochemical technologies based on hybrid electrochemical advanced oxidation processes (EAOPs), which integrate contaminated water treatment with the simultaneous green chemical feedstocks and hydrogen (H 2 ) production, are a promising strategy within the framework of the current energy transition. In this context, the real-time evolution of gases released in a hybrid EAOP reactor operated in batch mode under galvanostatic conditions (70 mA cm -2 , 60 min, acidic pH conditions) was investigated, for first time, by coupling the system to a differential electrochemical mass spectrometer (DEMS). In parallel, by-products electrogenerated in solution during electrooxidation (EO) were analyzed by high-performance liquid chromatography (HPLC), assessing the influence of the anode type, boron-doped diamond (BDD) and dimensionally stable anode (DSA), as well as the initial organic load (0, 20, and 50 mg L -1 ). The use of BDD promoted rapid mineralization of the organic matter, achieving chemical oxygen demand (COD) removals above 95% and leading to the emission of O 2 and CO 2 in the gas phase. Conversely, the use of DSA favored the electroconversion and accumulation of the organic load into short-chain carboxylic acids (SCCAs), with overall Faradaic efficiencies (FE) of 0.8-1.5%. Meanwhile, dry H 2 co-electrogenerated in the cathodic compartment using a Ni-Fe mesh cathode exhibited high purity (>99%) and FE of 98-100%. Moreover, the H 2 production rate showed a synergistic dependence on the anodic process, associated with electronic and protonic fluctuations arising from the different oxidation pathways of the simultaneously electrooxidized organic load. Finally, this win-win strategy enabled to understand the key role of anodic materials via real-time DEMS monitoring of gas evolution as well as both wastewater purification and production of green H 2 processes.