The most recent investigations of operating conditions in a forward-bias bipolar-membrane zero-gap electrolyser using a silver cathode catalyst for the reduction of CO 2 to CO at low temperatures and near-ambient pressures are reported. First, the CO 2 electrolyser performance was investigated as a function of cathode feed humidification and composition. The highest CO partial current density was 127 mA cm −2 , which was obtained at an iR-corrected cell voltage of 2.9 V, a cathode feed humidification of 50%RH, CO 2 feed concentration of 90% and a CO Faradaic efficiency of 93%. The cells were tested continuously for 12 h at 3 V and 8 h at 3.4 V cell voltage to investigate system stability. While Faradaic efficiencies were maintained during the measurements at 3.0 V, a shift in selectivity was observed at 3.4 V, while a deterioration in current densities occurred in both cases. Using a specially designed electrochemical cell with an integrated reversible hydrogen reference electrode, it was found that the cathode catalyst is the main responsible for the observed loss in performance. It was furthermore determined via post-mortem SEM and EDX investigations that cathode deterioration is caused by catalyst agglomeration and surface poisoning.

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We report on an electrochemically driven CO 2 separation process employing commercial anion exchange membranes to directly remove CO 2 from a dilute gas mixture and transport it across a cell. This methodology exploits the carbonation behavior of alkaline membrane systems to react CO 2 with hydroxide ions generated through the hydrogen evolution reaction and form (bi)carbonate ions. Electrochemically pumped (bi)carbonate then evolves as CO 2 on the anode side through the hydrogen oxidation reaction with H 2 . The resulting mixture of CO 2 and residual H 2 could be utilized for downstream valorization processes. Cell polarizations with 0.1−100% CO 2 in N 2 as the feed gas were performed with current densities of up to 50 mA• cm −2 , and CO 2 concentrations were monitored using online gas analysis. Further experiments examining the effect of pumping against pressure and concentration gradients were performed, along with Pd wire reference electrode experiments to discern cathode and anode overpotentials. Additional fundamental techno-economic considerations are presented to explore the cost dynamics of the system and the relevant targets for cell operation. The results show the complex interactions between cell input and performance parameters, as well as some of the critical limitations that must be overcome to allow for process scale-up to become viable.

The large-scale deployment of polymer electrolyte water electrolysis (PEWE) is largely limited by the use of O 2 evolution reaction (OER) catalysts based on scarce and expensive iridium in PEWE anodes. The ensuing need for betterperforming, Ir-based OER catalysts requires an improved understanding of the relation between these materials' activity and their physicochemical operando properties. To shed light on this matter, here, we employed operando modulation excitation X-ray absorption spectroscopy to determine the oxidation state of surface Ir in a range of Ir oxides with different surface compositions, crystal structures, and OER activities. Our results reveal that, irrespectively of these diverging properties, the surface Ir in all catalysts systematically undergoes a linear, potential-driven oxidation that stabilizes once a +5 state is reached. The completion of this surface oxidation process is then showed to correlate with the onset of O 2 evolution, thus strongly hinting at the involvement of Ir in oxidation states ≥+5 in the OER and indirectly discarding those mechanisms that do not consider such states as a part of the reaction sequence.

Understanding the deactivation mechanisms affecting the state-of-the-art, Ir oxide catalysts employed in polymer electrolyte water electrolyser (PEWE-) anodes is of utmost importance to guide catalyst design and improve PEWE-durability. With this motivation, we have tried to decouple the contributions of various degradation mechanisms to the overall performance losses observed in rotating disk electrode (RDE) tests on three different, commercial Ir oxide catalysts (pure or supported on Nb2O5). Specifically, we investigated whether these performance decays stem from an intrinsic deactivation of the catalysts caused by alterations in their oxidation state, crystalline structure, morphology and/or Ir-dissolution, and also assessed possible decreases in the catalyst loading caused by the delamination of the materials over the course of these OER-stability tests. Additionally, we also examined recently reported artifacts related to the use of RDE voltammetry for such measurements and found that neither these nor the above mechanisms (or combinations thereof) can cause the totality of the observed performance losses. Beyond these uncertainties, complementary PEWE-tests showed that this apparent RDE-instability is not reproduced in this application-relevant environment.

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The price of electricity produced from renewable sources has sharply sunk in the last decade, making these a cheaper choice than newly built thermal power plants in almost all parts of the world [1]. The missing link towards the full decarbonisation of the energy sector is a crossover vector that can account for the intrinsic intermittency of these renewable energy sources. Hydrogen seems to be the best candidate for this, particularly if produced using water electrolysis powered by excess energy from renewable sources. In particular, the load capability and high power density of polymer electrolyte water electrolysis (PEWE) renders this technology an excellent match to these renewables’ sporadic nature [2]. However, the need for scarce Ir to catalyze the sluggish oxygen evolution reaction (OER) taking place in PEWE anodes hinders the large scale application of this technology [3]. Therefore, a better understanding of the OER mechanism on Ir-based electrocatalysts is needed to guide catalyst design and decrease the efficiency losses related to this reaction. Over the last decades, the OER mechanism on Ir-based materials has been extensively investigated through operando/in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [4-7]. Despite enormous efforts, there is still a lack of consensus regarding the oxidation state of Ir under OER conditions and the nature of the OER-active sites. Therefore, in this work we employed operando modulation excitation XAS to study the OER mechanism on several calcined and uncalcined Ir-oxide catalysts. By combining this modulation excitation approach with phase sensitive analysis, we enhanced the sensitivity of XAS towards oxidation state changes that are otherwise undiscernible on low surface area materials, such as calcined IrO2. To derive more precise information from these phase resolved spectra, standard materials with Ir oxidation states ≥ +5 were also characterized. The comparison of the demodulated, operando spectra of a given material with the corresponding difference between its low-potential spectrum and the standard compounds’ spectra indicates that all oxides reach a maximum Ir oxidation state of +5 under OER conditions. In addition, multivariate curve resolution (MCR) analysis was employed to extract kinetic information from these period averaged spectra. This analysis unveiled that the oxidation of the surface iridium proceeds at the same rate as its reduction in the case of highly OER-active materials, while reduction is slower than oxidation for the less OER-active samples. In summary, this contribution presents novel insights on the Ir oxidation under potential control, and in doing so also features the capabilities of modulation excitation XAS and MCR to elucidate kinetics of electrochemical reactions. References [1] IRENA (2021), Renewable Power Generation Costs in 2020, International Renewable Energy Agency [2] M. Carmo et al., International Journal of Hydrogen Energy, 38 (2013) 4901-4934. [3] M. Bernt et al., J Phys Chem Lett, 9 (2018) 3154-3160. [4] V. A. Saveleva et al., J Phys Chem Lett, 9 (2018) 3154-3160. [5] V. Pfeifer et al., Chem Sci, 7 (2016) 6791-6795. [6] D. F. Abbott et al., Chemistry of Materials, 28 (2016) 6591-6604. [7] A. Minguzzi et al., ACS Catalysis, 5 (2015) 5104-5115.