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Stainless steel made to rust: a robust water-splitting catalyst with benchmark characteristics
(2015)
The oxygen evolution reaction (OER) is known as the efficiency-limiting step for the electrochemical cleavage of water mainly due to the large overpotentials commonly used materials on the anode side cause. Since Ni–Fe oxides reduce overpotentials occurring in the OER dramatically they are regarded as anode materials of choice for the electrocatalytically driven water-splitting reaction. We herewith show that a straightforward surface modification carried out with AISI 304, a general purpose austenitic stainless steel, very likely, based upon a dissolution mechanism, to result in the formation of an ultra-thin layer consisting of Ni, Fe oxide with a purity >99%. The Ni enriched thin layer firmly attached to the steel substrate is responsible for the unusual highly efficient anodic conversion of water into oxygen as demonstrated by the low overpotential of 212 mV at 12 mA cm−2 current density in 1 M KOH, 269.2 mV at 10 mA cm−2 current density in 0.1 M KOH respectively. The Ni, Fe-oxide layer formed on the steel creates a stable outer sphere, and the surface oxidized steel samples proved to be inert against longer operating times (>150 ks) in alkaline medium. In addition Faradaic efficiency measurements performed through chronopotentiometry revealed a charge to oxygen conversion close to 100%, thus underpinning the conclusion that no “inner oxidation” based on further oxidation of the metal matrix below the oxide layer occurs. These key figures achieved with an almost unrivalled-inexpensive and unrivalled-accessible material, are among the best ever presented activity characteristics for the anodic water-splitting reaction at pH 13.
The development of base metal electrodes that can act as active and stable oxygen generating electrodes in water electrolysis systems, especially at low pH levels, remains a challenge. The use of suspensions as electrolytes for water splitting has until recently been limited to photoelectrocatalytic approaches. A high current density (j=30 mA/cm2) for water electrolysis has been achieved at a very low oxygen evolution reaction (OER) potential (E=1.36 V vs. RHE) using a SnO2/H2SO4 suspension-based electrolyte in combination with a steel anode. More importantly, the high charge-to-oxygen conversion rate (Faraday efficiency of 88% for OER at j=10 mA/cm2 current density). Since cyclic voltammetry (CV) experiments show that oxygen evolution starts at a low, but not exceptionally low, potential, the reason for the low potential in chronoamperometry (CP) tests is an increase in the active electrode area, which has been confirmed by various experiments. For the first time, the addition of a relatively small amount of solids to a clear electrolyte has been shown to significantly reduce the overpotential of the OER in water electrolysis down to the 100 mV region, resulting in a remarkable reduction in anode wear while maintaining a high current density.
The development of non-precious metal-based electrodes that actively and stably support the oxygen evolution reaction (OER) in water electrolysis systems remains a challenge, especially at low pH levels. The recently published study has conclusively shown that the addition of haematite to H2 SO4 is a highly effective method of significantly reducing oxygen evolution overpotential and extending anode life. The far superior result is achieved by concentrating oxygen evolution centres on the oxide particles rather than on the electrode. However, unsatisfactory Faradaic efficiencies of the OER and hydrogen evolution reaction (HER) parts as well as the required high haematite load impede applicability and upscaling of this process. Here it is shown that the same performance is achieved with three times less metal oxide powder if NiO/H2 SO4 suspensions are used along with stainless steel anodes. The reason for the enormous improvement in OER performance by adding NiO to the electrolyte is the weakening of the intramolecular O─H bond in the water molecules, which is under the direct influence of the nickel oxide suspended in the electrolyte. The manipulation of bonds in water molecules to increase the tendency of the water to split is a ground-breaking development, as shown in this first example.
For a good reason, water splitting is the most pioneering energy storage technology. However, particularly water electrolysis still has a shadow existence compared to currently used methods for mass production of hydrogen. All known materials currently exploited as anodes for electrocatalytically initiated water-splitting suffer from high overpotentials and substantial mass loss during long term operation in acidic media. Low electrode stability affects operating and maintenance costs and together with high overpotentials directly lowers the overall efficiency of electrocatalytically driven splitting of water. In circumventing these problems, scientists and engineers are currently modifying electrode materials. We chose a completely different path and modified the electrolyte. An electrolysis set up, that consists of a Ni42 stainless steel anode and of hematite which is suspended in high concentration in sulfuric acid and acts as the electrolyte, exhibits oxygen evolution electrocatalysis at extremely low potential (1.26 V vs. RHE; 0.5 M H2SO4, j = 30 mA cm−2). If implemented in a suitable electrolyzer, an ultralow cell voltage of 1.6 V and an almost quantitative charge to oxygen + hydrogen conversion rate can be achieved. Remarkably, the negligible mass loss of the anode which consists exclusively of non-platinum group metals (non-PGM) during 100 h of operation. Experiments aimed at clarifying the mechanism suggest that Fe2O3 is converted to a Fe(II)/Fe(III) oxide species on the cathode which is then reconverted to Fe2O3 upon release of molecular oxygen when touching the anode. As a result, the oxygen-evolving centers are likely to be on the oxide particles rather than on the electrode. This proposed mechanism would explain the low potential of the OER electrode (+1.26 V vs. RHE at j = 30 mA cm−2) that could not be explained convincingly by an assumed direct oxidation of water molecules.