Efficient pH-gradient-enabled microscale bipolar interfaces in direct borohydride fuel cells


The disparate pH requirements for borohydride oxidation and peroxide reduction in direct borohydride fuel cells (DBFCs) currently hinder their performance and efficiency. Here we develop a pH-gradient-enabled microscale bipolar interface (PMBI) that facilitates sharply different local pH environments at the anode and cathode of a DBFC. Using a recessed planar electrode in conjunction with transmission electron microscopy, we show that the PMBI maintained a sharp local pH gradient (0.82 pH units nm–1 on average) at the electrocatalytic reaction site. The PMBI configuration enabled enhanced performance in a DBFC compared with either all-anion- or all-cation-exchange configurations (330 mA cm–2 at 1.5 V and a peak power density of 630 mW cm–2 at 1.0 V, respectively). The high power densities obtained at voltages well above 1.0 V—achieved by virtue of the effective separation of anolyte and catholyte locally at the electrocatalytically active sites by the PMBI—provide a pathway to reduce fuel cell stack size for autonomous propulsion applications.

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Fig. 1: Scheme for the RPE system designed to simulate a PMBI.
Fig. 2: Evidence for strong and localized pH gradients.
Fig. 3: General scheme of the PMBI.
Fig. 4: Characterization of the PMBI.
Fig. 5: Impact of the interface on device performance.
Fig. 6: Comparison of DBFC and PEMFC.

Data availability

The data supporting the findings of this study are available from the corresponding author on reasonable request.


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The authors acknowledge with gratitude the Office of Naval Research (ONR grant no. N00014-16-1-2833) for funding this work. The authors acknowledge the Institute of Materials Science and Engineering for the use of Bruker ICO AFM, JEOL JEM-2000 FX TEM, and staff assistance and the Nano Research Facility within the Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis for access to SEM facilities. The authors acknowledge financial support from the McKelvery School of Engineering at Washington University in St. Louis. V.R. acknowledges with gratitude generous support from the Roma B. and Raymond H. Wittcoff Distinguished University Professorship.

Author information




Z.W. carried out the DBFC experiments, synthesized the ionomers, characterized the materials by SEM, FTIR, AFM and NMR, analysed the data, prepared the manuscript and aided in revisions. Z.W. and C.H. carried out the RPE experiments. C.H. carried out the TEM experiment. J.P. assisted with the experiments, manuscript revisions and data analysis. S.S. conceived the RPE experiments, carried out some SEM measurements and assisted with the RPE data analysis and manuscript revisions. V.R. conceived and supervised the project and played a primary role in data analysis, manuscript preparation and revision.

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Correspondence to Shrihari Sankarasubramanian or Vijay Ramani.

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Supplementary Discussion, Supplementary Methods, Supplementary Figures 1–11, Supplementary Tables 1–2, Supplementary References

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Wang, Z., Parrondo, J., He, C. et al. Efficient pH-gradient-enabled microscale bipolar interfaces in direct borohydride fuel cells. Nat Energy 4, 281–289 (2019). https://doi.org/10.1038/s41560-019-0330-5

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