The role of HBF4 in electro-catalysis: Arsenic contamination and anion adsorption

► In situ nanogravimmetry was used to follow atypical effect of HBF4 on Pt|HClO4 system. ► The anomalous mass change profile is due to arsenic present in one of the HBF4 used. ► For Pt|HClO4, HCOOH two different effects were observed due to arsenic contamination. ► At low potentials, the current increase was due to arsenic in one of the HBF4 used. ► At high potentials, the current decrease was caused by anion adsorption.