Instability of sulfate and selenate solid acids in fuel cell environments

The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO₄, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at ∼175°C under inert conditions. The overall decomposition process can be expressed as 2CsHSO₄ → Cs₂SO₄ + H₂O + SO₃ with Cs₂S₂O₇ appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO₄ + 4H₂ → Cs₂SO₄ + 4H₂O + H₂S. In the absence of fuel cell catalysts, this reduction reaction is slow; however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H₂S. In the case of M₃H(XO₄)₂ compounds, where M = Cs, NH₄, or Rb and X = S or Se, a similar reduction reaction occurs: 2M₃H(XO₄)₂ + 4H₂ → 3M₂XO₄ + 4H₂O + H₂X. In an operational fuel cell based on CsHSO₄, performance degraded with time, presumably as a result of H₂S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160°C.