| dc.description.abstract | The sluggish oxygen evolution reaction (OER) has been a major barrier for advancing green hydrogen production from water splitting. Whilst the overwhelming majority of work has been devoted towards buffered pure water electrolytes, a growing interest in the field is now emerging towards utilizing buffered seawater as a natural electrolyte feed to the electrolyzer. Investigation of highly electro-active, stable, cost-effective, and earth-abundant electrocatalysts in saline electrolytes is seen as a prerequisite to the commercial realization of saline water electrolysis. We undertook the design challenge of the electrocatalysts to be developed by rationally integrating key features that have been proven detrimental to both activity and stability of anodic materials in water electrolysis. In doing so, we initially fabricated crystalline spinel Co3O4 through the highly scalable solution combustion synthesis (SCS) to act as the catalyst’s core and Co-source. The highly crystalline core acts as a conductive substrate and serves as a template for growing the active electrochemical surface area. Further, we developed highly modulated S,B-(CoFeCr) and S,B-(CoFeV) oxyhydroxide shells atop the spinel core, with a high ratio of surface Co2+. Non-metallic dopants were employed to both synergistically enhance activity through surface modulation and offer a highly hydrophilic surface that activates higher performance during near-neutral water oxidation – when H2O adsorption is the initial OER step. We performed an array of surface chemistry and textural characterization techniques including HRTEM, FESEM, EDS, XRD, Elemental Mapping, and Raman spectroscopy to effectively investigate the highly active catalysts developed. Pre- and post-OER XPS analyses were undertaken to reveal the changes in surface chemistry of the attained oxyhydroxides after prolonged OER operation. The as-prepared S,B-(CoFeCr)OOH and S,B-(CoFeV)OOH electrocatalysts deposited on glassy carbon electrodes (GCE) required low overpotentials of 174 and 242 mV to achieve current density of 10 mA cm-2, respectively, in alkaline saline (1M KOH + 0.5M NaCl) electrolyte, with low Tafel slopes of 45.3 and 51.2 mV dec-1. The reported materials are amongst the most active based on our literature survey and have exhibited high stabilities in harsh electrolytic regimes. Moreover, we attained polarization curves for these electrocatalysts in near-neutral pH (1M HCO3−/CO32− + 0.6M NaCl; pH = 8) and neutral pH (1M PB + 0.6M NaCl; pH = 7) saline electrolytes. Chronoamperometry (CA) studies were performed in neutral saline electrolytes in order to ensure chlorine corrosion resistance and quantify chlorine evolution reaction (CER) products. To attain higher performance and more accurately test for stabilities in realistic seawater mimicking environments, the optimum trimetallic oxyhydroxides were deposited on conductive and highly porous nickel foam (NF). S,B-(CoFeCr)OOH@NF and S,B-(CoFeV)OOH@NF achieved 50 hours of chronopotentiometric stabilities under neutral saline electrolyte and 1.71 V (vs. RHE) of applied potential. Further, the materials exhibited suppressed CER products formation rates of 0.014 and 0.044 mg (Lh)-1, respectively, for the Cr and V doped oxyhydroxides. We correlate the higher CER suppression of the Cr analogue to stronger localized Crδ+(OOH)δ– which activates stronger electrostatic shielding from anionic Cl– and aids in H2O dissociation kinetics at neutral pH operation. This study provides a systematic approach towards a rational design of highly active and stable electrocatalysts. | |
