\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning} 
\tikzset{
    %Define standard arrow tip
    >=stealth',
    % Define arrow style
    pil/.style={->,thick}
}

\begin{document}
  \begin{tikzpicture}
      \node (a)[vertex] at (0,0) {};
      \node (b)[vertex] at (1,1) {};
      \node (c)[vertex] at (2,1) {};
      \node (d)[vertex] at (3,0) {};
      \node (e)[vertex] at (4,1) {};
      \node (f)[vertex] at (5,1) {};
      \node (g)[vertex] at (6,0) {};
      \node (h)[vertex] at (5,-1) {};
      \node (i)[vertex] at (4,-1) {};
      %\node (j)[vertex] at (3,0) {};
      \node (k)[vertex] at (2,-1) {};
      \node (l)[vertex] at (1,-1) {};
      \node (m)[vertex] at (2,2) {};
      \node (n)[vertex] at (3,2) {};

      \foreach \from/\to in {a/b,b/c,c/d,d/k,k/l,l/a}
        \draw[selected edge] (\from) -- (\to);

      \foreach \from/\to in {a/b,b/c,c/d,d/e,e/f,f/g,g/h,h/i,i/d,d/k,k/l,l/a}
        \draw[line width=2pt] (\from) -- (\to);
      \draw[line width=2pt] (b) -- (m);
      \draw[line width=2pt] (m) -- (n);
  \end{tikzpicture}
\end{document}