The field of electrochemical carbon dioxide reduction has developed rapidly during recent years. At the same time, the role of the anodic half-reaction has received considerably less attention. In this Perspective, we scrutinize the reports on the best-performing CO 2 electrolyzer cells from the past 5 years, to shed light on the role of the anodic oxygen evolution catalyst. We analyze how different cell architectures provide different local chemical environments at the anode surface, which in turn determines the pool of applicable anode catalysts. We uncover the factors that led to either a strikingly high current density operation or an exceptionally long lifetime. On the basis of our analysis, we provide a set of criteria that have to be fulfilled by an anode catalyst to achieve high performance. Finally, we provide an outlook on using alternative anode reactions (alcohol oxidation is discussed as an example), resulting in high-value products and higher energy efficiency for the overall process.

A major goal within the CO 2 electrolysis community is to replace the generally used Ir anode catalyst with a more abundant material, which is stable and active for water oxidation under process conditions. Ni is widely applied in alkaline water electrolysis, and it has been considered as a potential anode catalyst in CO 2 electrolysis. Here we compare the operation of electrolyzer cells with Ir and Ni anodes and demonstrate that, while Ir is stable under process conditions, the degradation of Ni leads to a rapid cell failure. This is caused by two parallel mechanisms: (i) a pH decrease of the anolyte to a near neutral value and (ii) the local chemical environment developing at the anode (i.e., high carbonate concentration). The latter is detrimental for zero-gap electrolyzer cells only, but the first mechanism is universal, occurring in any kind of CO 2 electrolyzer after prolonged operation with recirculated anolyte.

Electronic supplementary material:The online version of this article (doi: 10.1007/s11144-017-1155-5) contains supplementary material, which is available to authorized users. Effect of Mo incorporation

No abstract

No abstract

The preparation and the thorough characterization of 40 wt% Pt electrocatalysts supported on Ti (1-x) M x O 2-C (M= W, Mo; x= 0.3-0.4) composite materials with enhanced stability and efficiency is presented. W-containing composite supported catalyst with different structural characteristics were compared in order to explore the influence of the nature of the W species on the electrocatalytic performance. The assessment of the electrochemical properties of the novel catalysts revealed a correlation between the degree of W incorporation, the hydrogen spillover effect and the stability against initial leaching which influences the activity and CO tolerance of the catalysts. A preparation route for Ti 0.7 Mo 0.3 O 2-C composite with high extent of Mo incorporation was developed. No significant difference was observed in the activity, stability and CO tolerance of the W-or Mo-containing composite supported Pt catalysts with almost complete incorporation of the oxophilic dopant. Better performance of the Pt/Ti 0.7 M 0.3 O 2-C (M= W, Mo) electrocatalysts in a single cell test device using hydrogen containing 100 ppm CO compared to the reference Pt/C and PtRu/C (Quintech) catalysts was also demonstrated.

The electrochemical peculiarities of novel 20 wt.% Pt electrocatalysts supported on Ti 0.6 Mo 0.4 O 2 -C composite materials in low-potential CO oxidation reaction (LPCOR) were investigated. The oxidation of CO on the Mo-containing Pt-based catalyst commences at exceptionally low potential values (ca. 100 mV). The results suggest that only CO adsorbed on specific Pt sites, where Pt and Mo atoms are in atomic closeness, can be oxidised below 400 mV potential. When the weakly bounded CO is oxidized, some hydrogen adsorption can take place on the released surface, although this amount is much smaller than in the case of a CO-free Pt surface. The Pt/Ti 0.6 Mo 0.4 O 2 -C catalyst loses its activity in LPCOR when Mo becomes oxidized (above ca. 400 mV). Accordingly, presence of Mo species in lower oxidation state than 6+ is supposed to have crucial role in CO oxidation. Nevertheless, rereduction of oxidized Mo species formed above 400 mV is strongly hindered when adsorbed CO species are still present. Note that CO ads species can be completely removed only above 550 mV. Oxidized Mo species can be re-reduced and the activity in the LPCOR can be restored if the platinum surface is CO-free. Clear correlation between the so-called "pre-

Mo overlayers were prepared on smooth polycrystalline platinum and platinized platinum electrode surfaces by in situ electrochemical deposition of molybdenum oxide at potential below 500 mV for modeling Mo-Pt electrocatalysts. Correlations were found between the applied potential and the amount of deposited Mo, which never exceeded a monolayer, thus Pt-Mo bonds stabilize the deposited Mo oxide. Electrochemical measurements suggested that Mo deposited from a Mo(VI) solution was reduced to the 4+ oxidation state. In line with the ex situ XPS findings a certain part (20-25%) of the initial Mo layer remained irreversibly adsorbed on the Pt/Pt electrode even after oxidation into the 6+ state at high potentials; this fractional monolayer cannot be dissolved even by prolonged cyclic polarization up to 1000 mV. It has been demonstrated that the irreversibly bound Mo partial monolayer is enough to change significantly the CO poisoning properties of the Pt surface. On this Mo:Pt (1:4) surface CO oxidation is initiated at extremely low potentials (ca. 100 mV). Moreover, only Pt modified by Mo(IV) species is active in low-potential CO oxidation reaction as after oxidizing the irreversibly adsorbed Mo to the 6+ state, CO oxidation is no longer observable. Nevertheless, the catalyst can be reactivated by reduction of molybdenum into the 4+ oxidation state. However, this reduction requires clean, CO-free Pt surface. If Pt is largely covered by CO, reduction of Mo(VI) into Mo(IV) does not occur and thus the low potential CO oxidation remains hindered.