Sustainability

Sustainability

chaired by Guido Mul, Mark Huijben & Gerrit Brem

Abstracts

Integrating a Copper Oxide Top Photoabsorber in Tandem with a Si Microwire Array for Unassisted Solar Water Splitting, Pramod Patil Kunturu (MnF/MCS)

Photoelectrochemical devices aimed at splitting water into H2 and O2, constitute a challenging route to the conversion and storage of solar energy. Here, we present a tandem device with p-Cu2O as a top photoabsorber and a Si microwire array with a radial pn junction as a bottom photoabsorber. The p-Cu2O film was grown by electrodeposition on the pn-Si substrate. Introducing a radial p/n junction in the Si microwires enables maximum photovoltage of the Si substrate. Electrodeposition of Cu2O ensures a conformal layer over these high aspect structures. Si microwire arrays with p-Cu2O have thus been successfully prepared, and their performance was compared to that of planar devices. We optimized layer thickness and wire geometry by simulating and in experiments, to minimize the overall reflectivity and optimize the absorption and charge collection. Application of protection layers aids in the charge separation and maximizes the photovoltage of the underlying p-Cu2O photon absorber and of the tandem couple with Si. We show that the presented tandem design produces both high photocurrent and photovoltage in unassisted solar water splitting. Overall, the microwire array ensures effective harvesting of photons and effective photocurrent collection, a large surface area for catalytic reactions.

Pulsed electrochemical synthesis of formate, Martijn J.W. Blom (PCS/SPT)

Lead cathodes show decreasing Faradaic Efficiency (FE) in electrochemical conversion of CO2 to formate, favoring hydrogen, within minutes of operation in bicarbonate electrolyte. We demonstrate by Raman spectroscopy that surface PbCO3 is reductively converted to metallic Pb at -1 to-1.3 V vs RHE, potentials required for formate Faradaic Efficiency (FE). Periodic anodic polarization is a means to maintain time-averaged high FE towards formate, temporarily (re)generating the PbCO3 surface layer. An anodic polarization time of 0.1-1s appears sufficient to obtain a Pb-surface providing a continuously high average formate FE of ~50%, if the cathodic polarization time is maintained small, preferably as small as 0.1 s. Pulsed electrochemical synthesis is thus a viable operation method for Pb-catalyzed electro-reduction of CO2 to formate.

Development of a Heat battery, Mina Shahi (TE)

There is a growing need for flexibility for the use of local and/or sustainable energy sources which is caused by the natural fluctuations in the supply of these forms of energy. Compact thermal energy storage strongly contributes to the desired flexibility and energy savings while maintaining a secure supply and level of comfort. Thermochemical materials as the heart of the so-called ‘Heat Battery’ have the potential to store 5 to 10 times as much heat per volume as a storage system based on water with a relevant temperature difference. In addition, there are no heat losses when storing heat in TCM, which makes the application particularly suitable for long-term storage. At TFE Department we are aiming to dive one step deeper in the matter by finding the principles guiding the process of optimizing material configurations for optimal use in active components of the heat battery.

Advanced Thin Film Technology for Interface Engineering of Next-generation Batteries, Ron Hendriks (IMS-NECS)

Existing battery applications are limited by inadequate cycle life and inherently poor safety features. Spinel LiMn2O4, is a promising cathode material due to its relatively high operating voltage (4.1V) and good energy density (148 mAh/g) combined with low cost and absence of direct environmental or safety hazards. However, LiMn2O4 does suffer from limited cycle life as excessive capacity fading occurs during battery cycling due to dissolution of Mn into the acidic electrolyte.

Perfect control on the interfacial properties between the electrodes and electrolyte is needed but remains a great challenge. Detailed understanding of the electrochemical behavior of specific crystal facets of battery materials can only be obtained through advanced thin-film technology: synthesizing a single type of crystal orientation interfacing the electrolyte.

We show that LiMn2O4 thin-films expose predominantly <111> crystal facets, the lowest energy state surface for this spinel structure, resulting in dramatic differences in surface morphology with pyramidal, rooftop or flat features for respectively (100), (110) and (111) oriented LiMn2O4 thin-films. Interestingly, the (100)-oriented films exhibited the highest capacities, (dis)charging rates up to 33C, and good cyclability over a thousand cycles, demonstrating enhanced cycle life without excessive capacity fading as compared to polycrystalline studies.