Nano-Scale Characterization Capabilities for Photoelectrochemical Water Splitting
LaboratoryNational Renewable Energy Laboratory (NREL)
Capability ExpertMowafak Al-Jassim, Chun-Sheng Jian
Node Readiness Category2: Photoelectrochemical (PEC)
A set of atomic force microscopy (AFM)-based nano-probes, namely electrochemical AFM (EC-AFM), scanning electrochemical potential microscopy (SECPM), Kelvin probe force microscopy (KPFM), and scanning spreading resistance microscopy (SSRM), for in-situ and ex-situ characterizations of PEC materials and devices.
Based on AFM, nano-probes detect various electrical signals between the AFM probe and sample, and map the corresponding electrical or electrochemical properties in nanometer resolutions. EC-AFM, SECPM, KPFM, and SSRM are four techniques directly related to PEC characterizations and available at the Microscopy and Imaging Group at NREL. EC-AFM and SECPM are in-situ nano-imaging techniques that investigate the PEC activity in a working device. KPFM and SSRM are ex-situ techniques that measure PEC-related material/device properties.
EC-AFM: Based on an AFM operational in water, EC-AFM probes in-situ electrochemical current flowing through the probe apex and local photocathode area underneath the probe. A metal probe serves as electrochemical anode. Inhomogeneous current map directly results from PEC reactions. This technique can image active/inactive or enhanced/reduced PEC activity of an inhomogeneous device, such as grain boundary versus grain interior in a polycrystalline thin-film PEC device.
SECPM: Based on an AFM operational in water, SECPM images the electrical potential in-situ on a PEC electrode surface or in electrical double layer with a PEC device in operation. The electrical potential distribution directly relates to inhomogeneous PEC reaction by affecting charge carrier transport and charge donation/extraction at the electrode/liquid interface.
KPFM: Based on an AFM operational in inert air, KPFM probes the Coulombic force between the AFM probe and sample, and maps electrostatic potential on a sample surface in nm-resolutions. This ex-situ technique can map the potential in two ways. One is to map the inhomogeneous potential on a cathode surface, which is a direct result of electronic work function and thus affect the charge exchange at the electrode/liquid interface. The other is to measure potential barriers at the interface between the photo-absorber and passivation layer by measuring on cross-section of cathode, which is critical to understand the electronic role of the passivation layer in the state of the art PEC devices, such as GaInP.
SSRM: Based on an AFM operational in an inert atmosphere, SSRM probes ex-situ local spreading resistance to map the local resistivity of a photocathode, in nm-resolutions. Electrical conductivity of a cathode with an organic/inorganic passivation layer of insulating/semi-insulating is important for photo-carrier transport.
These capabilities have resolutions ranging from 10 nm to sub-100 nm, depending on the electrical signal probed, probe size, and specific PEC samples. The sample sizes can range from sub-mm to several cm. In a relatively larger sample size (cm), selected survey allows for statistical imaging and analysis. A large sample conductivity range from conductive to semi-insulating (~105 Wcm) is allowed for all these techniques.
Very limited reports can be found on related nm-scale characterization of PEC materials and devices using these techniques. Historically, these nano-probes have been developed and applied in photovoltaic studies at NREL. The characterization results on single- and poly-crystalline PV materials and devices have resulted in more than 100 publications, making significant impact in the PV community, and playing leading roles in the field of nanoelectrical characterization in PV studies. Based on the similarity between the PEC and PV absorber materials and their multiple layer structure of the devices, the PV nanoelectrical characterization capabilities can be applied to PEC either directly or after certain minor modifications/development. Scientists and expertise in these techniques are well established with an international track record. Such renowned expertise could play a critical role in the successful application to PEC. This pioneer work will be unique in combining the electrical/electrochemical and nm-resolutions, and the results are direct device performance in nm-scale.
These nm-probes are available at the Microscopy and Imaging Group at NREL. KPFM, and SSRM are ready to be applied to PEC; EC-AF and SECPM will be ready after minor developments. We have setup SECPM to image the electrical potential on WOx PEC films and obtained initial results showing the non-uniformity of the potential (see the image). Combining NREL's existing nanoelectrical characterization and HydroGEN technology, both are state of the art, will make the R&D of HydroGEN highly "scientifically-guided" based on the iteration of device fabrication and characterization. Fast pace of device understanding and fabrication is critical for the R&D success, and expected to result in high-impact publications. Another important aspect of these nm-scale capabilities at NREL is artifact-free characterization by immediately transferring the PEC devices to the characterization set-up without long-distance sample transport, which is critical in these AFM-based measurements. These capabilities are user-friendly, and we can host researchers from outside NREL.
The nano-probe does not only directly image local nm-scale PEC activity for in-situ water splitting, but also measures ex-situ the electrical properties in inner material/device to understand the electrical mechanisms behind the PEC performance. The nm-electrical and electrochemical mapping will bridge the knowledge gap between the atomic structure and the macroscopic PEC performance. The characterization results will provide guidance for material/device R&D and accelerate the advanced water splitting material development.
An SECPM image taken on an WOx PEC thin film in water with 0.5 M Na2SO4, showing the highly inhomogeneous potential distribution in fine resolution of nm-scale. Brightness and darkness correspond qualitatively to high and low local potentials. The image size is 2μm×2μm.