Direct borohydride fuel cells (DBFCs) are a possible high energy source because a complete borohydride oxidation reaction (BOR) can provide up to 8 electrons per BH₄^- anion that may result in high energy density. However, complete electrooxidation in a DBFC is challenging to achieve for a number of reasons. The BOR (BO₂^-(aq) + 6H₂O + 8e^- ↔ BH₄^-(aq) 8OH^-, E^o=-1.24 vs. SHE) has a complex reaction mechanism that can result in a number of intermediate species (BH_x­_,ads or BH_x­OH_, x = 1 to 3), hydrogen gas (H₂), adsorbed hydrogen (H_ads) and solid borates.¹ Hydrolysis of BH₄^- (BH₄^-(aq) + 2H₂O → BO₂^-(aq) + 4H₂) can occur, reducing the availability of BH₄^- for complete oxidation and the resulting H₂ may compete with intermediate species for electrooxidation. The escape of reaction species, including unutilized H₂, from the electrocatalyst surface prior to complete electrooxidation results in incomplete BOR and varied concentrations as the fuel proceeds along the flow field towards the outlet.² It is therefore desirable to have catalyst layers capable of utilizing all reaction species and intermediates that need to be oxidized along with having a limited affinity for heterogeneous hydrolysis of BH₄^-­. To date, no single electrocatalyst has been found to effectively utilize all BOR species alone and any cell design will likely require variable and/or graded catalyst compositions along the flow channel as well as electrocatalysts capable of H₂ electrooxidation.

In this study, we examine single and graded catalyst compositions in a well-characterized liquid flow hydrogen peroxide DBFC (H₂O₂-DBFC) design.³ Either a single carbon-supported catalyst, mixture or in-plane gradient of catalysts are deposited directly onto bipolar graphite plate (BGP) electrodes at the anode by ultrasonic spray deposition. Linear single flow channels are used at the cathode and anode separated by a Nafion^® 117 membrane prepared for Na⁺ exchange. Performance and reaction mechanisms of the electrocatalysts are tested under variable experimental conditions using chronoamperometry (CA), rotating disk electrode (RDE) voltammetry, electrochemical impedance spectroscopy (EIS) and H₂ escape measurements.

The polarization curves of the DBFC with anodes composed of carbon supported Pt (Pt/C) and/or Pd (Pd/C) can be improved by creating a gradient of the electrocatalysts (Figure 1, for ~22 ^ºC and a low BH₄^- concentration (10mM)).⁴ Pt/C has the best performance at high cell potentials (low overpotentials), while Pd/C has superior performance at lower cell potentials (high overpotentials). Cell performance improves at lower cell potentials when catalysts are graded, where a mixture of the catalysts perform similarly to Pt/C alone. The experiments carried out at varied cell conditions give further insight into BOR mechanisms at the anode catalyst layer using this DBFC configuration. This presentation will discuss the performance of such graded electrodes under more relevant fuel cell conditions of 60 ^ºC and 100 mM NaBH₄.

Figure 1: Polarization curves of Pd/C (a, black), Pt/C (a, red), mixed Pt/C and Pd/C (b, blue) and graded Pd/C to Pt/C (b, gold).⁴

References

1. P.-Y. Olu, A. Bonnefont, M. Rouhet, S. Bozdech, N. Job, M. Chatenet and E. Savinova, Electrochemica Acta, 179, 637 (2015).
2. R. O. Stroman and G. S. Jackson, Journal of Power Sources, 247, 756 (2014).
3. R. O. Stroman, G. S. Jackson, Y. Garsany and K. Swider-Lyons, Journal of Power Sources, 271, 421 (2014).
4. R. M. E. Hjelm, Y. Garsany, R. W. Atkinson III, R. O. Stroman, K. Swider-Lyons, C. Lafforgue and M. Chatenet, ECS Transactions, 2017. 80(8): p. 1033-1042.