Research Requirements to Move the Bar forward Using Aqueous Formate Salts as H2 Carriers for Energy Storage Applications

Abstract

In this perspective on hydrogen carriers, we focus on the needs for the development of robust active catalysts for the release of H2 from aqueous formate solutions, which are non-flammable, non-toxic, thermally stable, and readily available at large scales at reasonable cost. Formate salts can be stockpiled in the solid state or dissolved in water for long term storage and transport using existing infrastructure. Furthermore, formate salts are readily regenerated at moderate pressures using the same catalyst as for the H2 release. There have been several studies focused on increasing the activity of catalysts to release H2 at moderate temperatures, i.e., < 80 °C, below the operating temperature of a proton exchange membrane (PEM) fuel cell. One significant challenge to enable the use of aqueous formate salts as hydrogen carriers is the deactivation of the catalyst under operating conditions. In this work we provide a review of the most efficient heterogeneous catalysts that have been described in the literature, their proposed modes of deactivation, and the strategies reported to reactivate them. We discuss potential pathways that may lead to deactivation and strategies to mitigate it in a variety of H2 carrier applications. We also provide an example of a potential use case employing formate salts solutions using a fixed bed reactor for seasonal storage of energy for a microgrid application.

Using earth abundant materials for long duration energy storage: electro-chemical and thermo-chemical cycling of bicarbonate/formate

“…Thus, here we just discuss the facts that are potentially relevant for the advancement of bicarbonate/formate energy cycle using a heterogeneous catalyst as outlined by Grubel. 4 The reduction of bicarbonate can follow two overall routes, i.e. , the indirect reduction (eqn (21) and (22)) and direct reduction eqn (23).HCO 3 − (aq) + H + (aq) ⇄ CO 2(aq) + H 2 O (l) CO 2(aq) + H 2 O (l) + 2e − ⇄ HCOO − (aq) + OH − (aq) HCO 3 − (aq) + H 2 O (l) + 2e − ⇄ HCOO − (aq) + 2OH − (aq) …”

“…This advantageous feature of the formate/bicarbonate cycle, as well as the abundance of the salts and their low toxicity have renewed the interest for their study. 3,4…”

“…, eqn (32) and the reverse of eqn (31).CO 2 + * ⇄ CO 2 *CO 2 * + H* ⇄ HCO 2 *HCO 2 * + e − ⇄ HCO 2 − + *e − + H + + * ⇄ H*H 2 + * ⇄ 2H* Understanding catalyst deactivation is arguably one of the most pressing issues (besides elucidation of the reaction mechanism) to enable development of a large scale formate oxidation process. 4 There has not been any systematic investigation of deactivation of the catalyst used in the bicarbonate/formate system, but several factors have been discussed as deactivators in other systems, i.e. , pore blocking (or fouling), 46 changes in metal particle size, 47 and chemical poisons ( e.g.…”

“…6 Previous perspectives had discussed the conversion of mixtures of formic acid and formate salts, as well as the performances of several materials and the corresponding challenges to advance towards application. 3,4 More details of thermodynamics, reaction mechanisms, and of effects of reaction conditions on reaction have been covered in ref. 7 and 8.…”

“…2 On the other hand, several groups have suggested the use of alkali formate salts, i.e., cesium, potassium, and sodium formate, as an attractive alternative to store hydrogen. 3,4 While they do store less hydrogen on a volumetric basis, compared to conventional hydrogen carriers, formate salts provide several benefits for stationary storage applications. For instance, they are composed by earth abundant elements, they are non-toxic and non-flammable, and readily cycle between hydrogen lean and rich forms at temperatures below 80 °C and pressures <30 bar.…”