To achieve artificial photosynthesis to convert sunlight, water and carbon dioxide into fuel - as plants do - researchers not only need to identify materials for efficient photoelectrochemical water splitting, but also need to understand why a certain material may or may not work. Now, scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have pioneered a technique that uses nanoscale imaging to understand how local nanoscale properties affect a material's macroscopic properties. Their study, "Nanoscale imaging of charge carrier transport in water-splitting anodes," has just been published in Nature Communications. The principal researchers are Johanna Eichhorn and Francesca Toma of Berkeley Lab's Chemical Sciences Division. "This technique relates a material's morphology to its function and provides insights into charge transport mechanisms, or how charges move within a material at the nanoscale," said Toma, who is also a researcher at the Joint Center for Artificial Photosynthesis, Energy Innovation center. Artificial photosynthesis aims to produce energy-dense fuel using only sunlight, water and carbon dioxide as inputs. The advantage of this approach is that it does not compete with food stocks and does not create or reduce greenhouse gas emissions. Photoelectrochemical water splitting systems require specialized semiconductors that use sunlight to split water molecules into hydrogen and oxygen. Bismuth vanadate has been identified as a promising material for photoanodes, which provide charge to photochemically transform water in electrochemical cells. "This material is a case where the efficiency should be good in theory, but in experimental tests you actually observe very low efficiency," Eichhorn said. "The reasons are not fully understood." The researchers used photoconductive atomic force microscopy to map the current at every point in the sample with high spatial resolution. This technique has been used to analyze local charge transport and photoelectric properties of solar cell materials, but is not known to have been used to understand nanoscale charge carrier transport limitations in photoelectrochemical materials. Eichhorn and Toma collaborated with scientists at Molecular Foundry, Berkeley Lab's nanoscale science research facility, to make these measurements through Foundry's user program. They found differences in properties related to the material's nanoscale morphology. "We found that the way the charges are used is not uniform across the sample, but rather there is heterogeneity," Eichhorn said. "When we do water splitting, these performance differences can affect its macroscopic properties - the overall yield of the sample." To understand this characteristic, Toma gave the example of a solar panel. "Let's say the group's efficiency is 22 percent," she said. "But can you do it at the nanoscale, at every point of the panel, and it would give you 22 percent efficiency? This technology allows you to say, yes or no, especially with photoelectrochemical materials. If the answer is no , that means there's less activity on your material. In the best case, it just reduces your overall efficiency, but if you have a more complex process, your efficiency can be reduced a lot." Right A better understanding of how bismuth vanadate works will also allow researchers to synthesize new materials that can drive the same reaction more efficiently. This research builds on previous research by Toma and others, in which she was able to analyze and predict the mechanisms that define the chemical stability of (photo)photoelectrochemical materials. Toma said these results bring scientists closer to achieving efficient artificial photosynthesis. "Now we know how to measure localized photocurrent in these materials, which have very low electrical conductivities," she said. "The next step is to put all of this in a liquid electrolyte and do the same thing. We have the tools. Now we know how to interpret the results, and how to analyze them, which is an important first step forward."

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