Surpotentiel

Résumé

En électrochimie, le surpotentiel (appelée surtension en français) est la différence de potentiel (tension) entre le potentiel d'oxydoréduction d'une demi-réaction déterminé thermodynamiquement et le potentiel auquel la réaction redox est observée expérimentalement. Le terme est directement lié à l'efficacité de la tension dune cellule. Dans une cellule électrolytique, l'existence d'un surpotentiel implique que la cellule nécessite plus d'énergie que prévu thermodynamiquement pour que la réaction ait lieu. Dans une cellule galvanique, l'existence d'un surpotentiel signifie que moins d'énergie sera récupérée que l'étude thermodynamique de la réaction ne le prévoit. Dans chaque cas, l'énergie supplémentaire/manquante est perdue sous forme de chaleur. La quantité de surpotentiel est propre à chaque type de cellule et varie selon les conditions expérimentales - celle-ci varie également lors d'une même réaction. Le surpotentiel est déterminé expérimentalement en mesurant le potentiel

Source officielle

https://fr.wikipedia.org/wiki/Surpotentiel

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Energy Storage in the chemical bonds of water, a process accessible through electrochemical conversion, is a simple approach to compensate supply fluctuation common to sustainable energy production such as wind and solar. However, the water cycle is limited by the oxygen half-cell reaction hindered by large overpotentials and suitable electrocatalysts are necessary to enhance electrochemical conversion. For the tailored design of electrocatalysts, fundamental insight into catalytic processes as well as the interaction between individual components of the catalytic material is crucial. This thesis focuses on the investigation of transition metal electrocatalysts involving organic or purely inorganic components. The catalysts are structurally and chemically characterized before and after electrocatalysis by a complementary scanning tunneling microscopy and x-ray spectroscopy approach. The sample preparation in ultra-high vacuum is bridged to ambient electrochemical conditions by a home-built transfer system. The first part concentrates on Co, Cu and Fe stabilized within a porphyrin environment forming metal-organic coordination networks which are bifunctionally active towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Co sublimation onto a Fe/Cu metalloporphyrin network on a Au(111) substrate drives an irreversible on-surface transmetalation process with an exchange rate of ~50% at room temperature. Supporting DFT calculations suggest a redox transmetalation, in which Fe is reduced and expelled from the macrocycle while oxidized Co accommodates within the porphyrin cage. The disposability of a second coordination site in the form of a peripheric pyridyl group is not inhibiting the transmetalation. The bimetallic Co/Fe tetrapyridylporphyrin network catalyzes the ORR successfully. Both the local structural integrity of the self-assembled network as well as its chemical stability is confirmed for up to 10 electrocatalytic cycles. In contrast, the opposite reaction direction of OER is more aggressive leading to an entirely modified surface structure already after the first cycle. The applied potential induces the formation of Co/Fe oxides which are determined as actual catalytically active phase. Subsequently, in situ prepared oxide structures are explicitly investigated in the transfer system, starting from Co cluster. Exposure to 1 bar Ar atmosphere partially oxidizes Co, presumably due to adventitious oxygen or water. Aqueous solutions like water or 0.1 M NaOH promote the oxidation process of Co which goes along side with sintering of the cluster. Oxidized Co cluster successfully catalyze the OER reducing the overpotential of Au(111) by approximately 100 mV. Last the integrity of Fe3O4(001) in the transfer system was investigated, which could be utilized for stabilizing catalytically active single transition metal atoms. The reconstruction persists both Ar and water exposure, but disappears upon 0.1 M NaOH contact which is explained by the formation of a hydroxo complex. Preliminary experiments on Ag adatoms/Fe3O4(001) leave the ORR unaffected but x-ray spectroscopy confirms the stability of the adatoms after ORR. In conclusion, this thesis demonstrates the importance of catalyst characterization on the molecular scale. The influence of the potential and the electrolyte on catalyst structure should not be underestimated and might change the as-synthesized material to the actual electroactive composition.

EPFL2018

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Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Lichen Bai, Xile Hu, Lei Liu

Hydrogen (H-2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H-2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H-2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

WILEY2020

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Optimized hierarchical nickel sulfide as a highly active bifunctional catalyst for overall water splitting

Pengzuo Chen, Yun Tong

Rational design of non-noble metal electrocatalysts with high intrinsic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely impressive for sustainable electrocatalytic water splitting systems. However, it still remains a major challenge to engineer bifunctional performance. Here, we put forward a highly efficient water electrolyzer based on Ni3S2-based materials. The hierarchical structure of Ni3S2 can be well regulated for optimizing the HER catalytic activity. The best c-Ni3S2/NF electrode exhibits a much smaller overpotential of 220 mV to reach the current density of 100 mA cm(-2). Upon introducing Fe species onto the Ni3S2/NF electrode by a simple dipping/drying method, the intrinsic OER activity can be extremely improved. As a result, the Fe-c-Ni3S2/NF catalyst showed excellent catalytic activity for the OER, including an overpotential of 193 mV at 10 mA cm(-2), high specific current density and excellent stability. Post-characterization studies proved that the remaining S anions have an effective influence on improving the OER intrinsic activity. The assembled water electrolyzer also presented superior performance, such as a very low cell voltage of 1.50 V at 10 mA cm(-2) and excellent durability for 120 h in alkaline medium. This strategy provides a promising way to design highly active and low-cost materials for overall water electrolysis.

ROYAL SOC CHEMISTRY2021

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EPFL2018