Added subtitle

talk.tex

@@ -172,6 +172,7 @@ \section{How we can store Energy}
 \savebox\techpingu{\tikz{\pingu[left wing=wave,laptop left,lightsaber right=blue,headphone=accent,vr-headset,vr-headset hair,heart=accent]}}
 \begin{frame}
 	\frametitle{Existing energy storage methods}
+	\framesubtitle{A small selection}
 	\begin{wide}
 	\begin{columns}[c]
 		\begin{column}{0.5\textwidth}